Polymer organogels stabilized HIPE organogels and applications in oil fields
by
Tao Zhang (MEng, BEng)
Submitted in fulfilment of the requirements for the degree of
Doctor of Philosophy
Deakin University
April, 2016
I
ACKNOWLEDGEMENTS
I would like to acknowledge the following people for their encouragement,
support and contributions to the fulfilment of my research and thesis:
First of all, I would like to thank my supervisor Professor Qipeng Guo for his
academic guidance and supervision which is absolutely vital to the progress of
my research. I appreciate his support and effects throughout my PhD research,
without these the research shown here would not have been possible.
I really appreciate Deakin University for providing a Deakin University
Postgraduate Research Scholarships (DUPRS) to support my PhD study. Thanks
to Australian Synchrotron (AS) for using small/wide angle X-ray scattering
facilities. I would also like to thank Dr Nigel Kirby and Adrian Hawley at AS
for their technical support and valuable advices. I would like to extend my
gratitude to Dr Timothy C. Hughes at CSIRO Materials Science and Engineering
who helped me using the TA ARES rheometer.
I would like to thank all the colleagues in the polymers research group at Institute
for Frontier Materials (IFM), Deakin University, and they are Zhiguang Xu,
Shuhua Peng, Shuying Wu, Renyan Xiong, Gigi George, Yuanpeng Wu,
Anbazhagan Palanisamy, Mashihullah Jabarulla Khan, Ping’an Song, Haoguan
Gui, Seyed Mohsen Seraji, and Bijan Nasri-Nasrabadi. I really learned a lot from
all of them and especially, I would like to thank Zhiguang Xu, Shuhua Peng and
Shuying Wu for their assistance and suggestions for my experiments and life. I
would also like to extend my appreciate to Zengxiao Cai and Zhifeng Yi at IFM
for assistance in confocal experiment. I appreciate the comfortable and friendly
working atmosphere created at IFM.
I am grateful to Professor Xungai Wang and other technical and administrative
staffs at IFM, in particular Marion Wright, Rob Pow, Lin Zhang, Leanne Costa,
Helen Woodall, Ben Allardyce, Rosey Van Driel, Robert Lovett, John Robin,
II
Sandy Benness and those who have assisted me in research-related and various
administrative affairs.
Thanks to Dr Yunsheng Ding and Dr Zongquan Wu at Hefei University of
Technology, China for inspiring me to do a PhD at Deakin University and their
encouragement and advices during my PhD.
Last but not the least, I would like to express my appreciations and thanks to my
parents, brother and sister in China for looking after my son and their endless
love, support, patience and advice. I also appreciate my wife for her selfless love,
great encouragement and understanding.
III
PUBLICATION LISTS
Journal Articles
1) Tao Zhang, Zhiguang Xu, Yuanpeng Wu and Qipeng Guo. Industrial &
Engineering Chemistry Research, DOI: 10.1021 /acs.iecr.6b00039.
“Assembled block copolymer stabilized high internal phase emulsion
hydrogels for enhancing oil safety”.
2) Tao Zhang, and Qipeng Guo. Chemical Communications 2016, 52, 4561-
4564. “Isorefractive high internal phase emulsion organogels for light
induced reactions”.
3) Tao Zhang, Zhiguang Xu and Qipeng Guo. Physical Chemistry Chemical
Physics 2015, 17, 16033-16039. “Phase inversion of ionomer-stabilized
emulsions to form high internal phase emulsions (HIPEs)”.
4) Yuanpeng Wu, Tao Zhang, Zhiguang Xu and Qipeng Guo. Journal of
Materials Chemistry A 2015, 3, 1906-1909. “High internal phase emulsion
(HIPE) xerogels for enhanced oil spill recovery”.
5) Tao Zhang, Yuanpeng Wu, Zhiguang Xu and Qipeng Guo. Chemical
Communications 2014, 50, 13824-13827. “Hybrid high internal phase
emulsion (HIPE) organogels with oil separation properties”.
6) Tao Zhang and Qipeng Guo. RSC Advances 2014, 4, 35055-35058.
“Polyoxometalate-based hybrid organogels prepared from a triblock
copolymer via charged-driven assembly”.
7) Tao Zhang and Qipeng Guo. Chemical Communications 2013, 49, 11803-
11805. “High internal phase emulsion (HIPE) organogels prepared from
charge-driven assembled polymer organogels”.
8) Tao Zhang and Qipeng Guo. Chemical Communications 2013, 49, 5076-
5078. “A new route to prepare multiresponsive organogels from a block
ionomer via charge-driven assembly”.
IV
Patent
Qipeng Guo and Tao Zhang. “Absorbent material”, Australian Provisional
Patent (2014904762, 2014), International Patent Application
(PCT/AU2015/050736, pending).
V
Conferences
1) Tao Zhang, Qipeng Guo, “Multiresponsive organogels from block ionomer
via charge-induced assembly”, 34th Australian Polymer Symposium,
Darwin, Australia, 7-10th July 2013.
2) Tao Zhang, Yuanpeng Wu, Zhiguang Xu, Qipeng Guo, “Hybrid high
internal phase emulsion (HIPE) organogels for oil-water separation”, RACI
VIC Polymer Technologies Workshop, Melbourne, 26-27th June, 2014.
3) Yuanpeng Wu, Tao Zhang, Zhiguang Xu, Qipeng Guo, “A facile approach
to Fe3O4/SEBS magnetic elastomers”, RACI VIC Polymer Technologies
Workshop, Melbourne, 26-27th June, 2014.
VI
TABLE OF CONTENTS
ACKNOWLEDGEMENTS I
PUBLICATION LISTS III
TABLE OF CONTENTS VI
LIST OF FIGURES XI
LIST OF SCHEMES XVII
LIST OF TABLES XVIII
LIST OF ABBREVIATIONS AND SYMBOLS XIX
ABSTRACT XXI
Chapter One General Introduction 1
1.1 Thesis objective 1
1.2 The outline of the work 2
Chapter Two Literature Review 3
2.1 Introduction 3
2.2 HIPEs 4
2.2.1 Surfactants as HIPE stabilizers 5
2.2.2 Particles as HIPE stabilizers 8
2.2.3 Organogels as HIPE stabilizers 17
2.2.4 Developments in the applications of these HIPE stabilizers 18
2.2.5 Challenges 19
2.3 Polymer organogels 20
2.3.1 Addition of physical crosslinking agents 20
2.3.2 Polymers with physical cross-linkable points 21
2.3.3 Introduction of LMWOGs into polymers 24
2.3.4 Self-assembly of block copolymers 26
2.3.5 Organogels via ionic interactions 28
2.4 Materials for oil recovery 31
2.4.1 Oil spills 31
2.4.2 Phase-selective gelators 33
2.4.3 Absorbents 35
2.5 Summary 40
VII
2.6 Reference 41
Chapter Three A New Route to Multiresponsive Organogels from
a Block Ionomer via Charge-Driven Assembly
51
3.1 Abstract 51
3.2 Introduction 52
3.3 Experimental section 53
3.3.1 Materials and preparation of samples 53
3.3.2 Rheological measurements 53
3.3.3 Fourier-transform infrared (FTIR) spectroscopy 54
3.3.4 Small-angle X-ray scattering (SAXS) 54
3.4 Results and discussion 54
3.4.1 The preparation of SSEBSs 54
3.4.2 The formation of organogels 55
3.4.3 The ionic interaction between SSEBS and PS-b-P2VP 57
3.4.4 The microphase separation in organogels 60
3.4.5 The responsiveness of polymer organogels to acid, base and
salt
64
3.5 Conclusions 67
3.6 References 67
Chapter Four HIPE Organogels Prepared from Charge-Driven
Assembled Polymer Organogels
70
4.1 Abstract 70
4.2 Introduction 71
4.3 Experimental section 72
4.3.1 Materials and preparation of samples 72
4.3.2 Fourier-transform infrared (FTIR) spectroscopy 73
4.3.3 Rheological experiments 74
4.3.4 Conductivity measurement 74
4.3.5 Confocal microscopy 74
4.3.6 Small-angle X-ray scattering (SAXS) 74
4.4 Results and discussion 75
VIII
4.4.1 The formation of organogels 75
4.4.2 Charge-driven assembled polymer organogels 76
4.4.3 Responsiveness of charge-driven assembled organogels 79
4.4.4 The formation of HIPE organogels 80
4.4.5 Morphologies of HIPE organogels 81
4.5 Conclusions 85
4.6 References 86
Chapter Five Hybrid HIPE Organogels with Oil Separation
Properties
88
5.1 Abstract 88
5.2 Introduction 89
5.3 Experimental section 90
5.3.1 Materials and preparation of samples 90
5.3.2 Rheological experiments 92
5.3.3 Fourier-transform infrared (FTIR) spectroscopy 92
5.3.4 Conductivity measurement 92
5.3.5 Confocal microscopy 92
5.3.6 Optical microscopy 93
5.3.7 Contact angle measurements 93
5.3.8 Small-angle X-ray scattering (SAXS) 93
5.3.9 Scan electron microscopy (SEM) 93
5.3.10 Oil absorption evaluation 93
5.4 Results and discussion 94
5.4.1 The formation of charge-driven assembled organogels 94
5.4.2 The formation of HIPE organogels 97
5.4.3 HIPE xerogels 101
5.4.4 Oil water separation performance 104
5.5 Conclusions 105
5.6 References 105
Chapter Six Reactive Polymer Organogels Stabilized HIPE
Organogels with Oil Separation Properties
108
IX
6.1 Abstract 108
6.2 Introduction 109
6.3 Experimental section 110
6.3.1 Materials and preparation of samples 110
6.3.2 Rheological experiments 112
6.3.3 Conductivity measurement 112
6.3.4 Small-angle X-ray scattering (SAXS) 113
6.3.5 Confocal microscopy 113
6.3.6 Scan electron microscopy (SEM) 113
6.3.7 Contact angle measurements 113
6.3.8 Oil absorption capacity 113
6.3.9 Absorption rate 114
6.3.10 Recovery rate 114
6.4 Results and discussion 114
6.4.1 SSEBS/benzoxazine organogels 114
6.4.2 SSEBS/benzoxazine HIPE organogels 118
6.4.3 SSEBS/benzoxazine HIPE xerogels 120
6.5 Conclusions 125
6.6 References 125
Chapter Seven HIPE Xerogels for Enhanced Oil Spill Recovery 127
7.1 Abstract 127
7.2 Introduction 129
7.3 Experimental section 129
7.3.1 Materials and preparation of samples 129
7.3.2 Characterization of HIPE xerogels 130
7.3.3 Characterization of oil absorption performance 130
7.4 Results and discussion 131
7.4.1 Formation of SSEBS/DAB-8 HIPE xerogels 131
7.4.2 Absorption capacity 132
7.4.3 Absorption rate and recovery rate 134
7.5 Conclusions 139
7.6 References 140
X
Chapter Eight Conclusions and Perspective for Future Work 141
8.1 General conclusions 141
8.2 Perspective for future work 142
XI
LIST OF FIGURES
Figure 2.1 Chemical structures of LMWOGs as HIPE stabilizer. 18
Figure 2.2 Chemical structures of diamidopyridine-containing
copolymer and crosslinking agent.
21
Figure 2.3 Structures of isotactic and syndiotactic polystyrene. 22
Figure 2.4 Chemical structure of PPE.
Figure 2.5 Chemical structure of MEH-PPV. 23
Figure 2.6 Chemical structures of polymer gelators containing L-
Lysine derives.
24
Figure 2.7 Chemical structures of P2+ and EDTA4-. 26
Figure 2.8 Chemical structure of polystyrenylphosphonium
polymer.
28
Figure 2.9 Chemical structure of polyelectrolyte-surfactant
complex.
30
Figure 2.10 SAXS profiles of polymer oganogel formed by ionic
interactions. Copyright 2011, ACS publishing.
30
Figure 2.11 Phase-selective gelation and diesel recovery from a two
phase system. Reproduced with permission from
Ref.131. Copyright 2010, Wiley-VCH Verlag GmbH &
Co. KGaA, Weinheim.
34
Figure 2.12 Pumping oil from water surface with hydrophobic
porous materials. Reproduced from Ref. 159 with
permission from The Royal Society of Chemistry.
36
Figure 2.13 The preparation of phase-selective xerogels for oil spill
recovery.152 Copyright 2013, ACS publishing.
37
Figure 2.14 PolyHIPEs for oil absorption.157 Reproduced from
Ref. 157 with permission from The Royal Society of
Chemistry.
37
Figure 2.15 Superhydrophobic surface of the MTMS–DMDMS
marsh for oil water separation. Reproduced with
permission from Ref.158. Copyright 2013, Wiley-VCH
Verlag GmbH & Co. KGaA, Weinheim.
39
XII
Figure 2.16 Digital photographs of water and lubricating oil droplets
on coated PU sponge156. Reproduced from Ref. 156
with permission from The Royal Society of Chemistry.
40
Figure 3.1 Dynamic moduli G’ (filled) and G” (hollow) of
organogels (a) with [-SO3H]/[2VP] = 1/1 as a function of
strain and, (b) with [-SO3H]/[2VP] = 1/1 1/2 and 1/3 as a
function of oscillatory shear frequency.
56
Figure 3.2 Dynamic moduli G’ (filled) and G” (hollow) of
SSEBS1/PS-b-P2VP, SSEBS2/PS-b-P2VP and
SSEBS4/PS-b-P2VP organogels with [-SO3H]/[2VP] =
1/1 as a function of oscillatory shear frequency.
57
Figure 3.3 FTIR spectra of SSEBS1/PS-b-P2VP gels in the range of
(a) 1700 - 1540 cm-1 and (b) 1250-880 cm-1.
58
Figure 3.4 FTIR spectra of SSEBS1/PS-b-P2VP mixtures in the
range of 1700 - 1560 cm-1.
59
Figure 3.5 (a) SAXS profiles for SSEBS1/PS-b-P2VP organogels
with different [-SO3H]/[2VP] ratios, (b) SAXS curve for
gel with [-SO3H]/[VP] = 1/2. The full line in (b)
represents the fitting curve according to a core-shell
model with a hard-sphere radius of 12.5 nm.
60
Figure 3.6 SAXS profiles for SSEBS1/PS-b-P2VP solids dried from
the corresponding organogels with [-SO3H]/[2VP] =
1:0.5, 1:1, 1:2 and 1:3.
63
Figure 3.7 SAXS profiles for (a) SSEBS1/PS-b-P2VP, SSEBS2/PS-
b-P2VP, and SSEBS4/PS-b-P2VP organogels, and (b)
SSEBS3/PS-b-P2VP, SSEBS4/PS-b-P2VP SSEBS5/PS-
b-P2VP organogels with [-SO3H]/[2VP] = 1:1.
64
Figure 3.8 Dynamic moduli G’(filled) and G”(hollow) of organogels
with 0.5% of acid, amine and salt as a function of
oscillatory shear frequency.
65
XIII
Figure 4.1 Chemical structures of PPI dendrimers DAB-4, DAB-8
and DAB-16.
72
Figure 4.2 Elastic moduli G’ and viscous moduli G” of
SSEBS/DAB-4, SSEBS/DAB-8 and SSEBS/DAB-16
organogels as a function of oscillatory shear frequency.
75
Figure 4.3 FTIR spectra of SSEBS/PPI dendrimers organogels after
evaporation of solvents in the range of (a) 3000 - 3600
cm-1 and (b) 1500 - 1650 cm-1.
76
Figure 4.4 SAXS profiles for SSEBS solution, SSEBS/DAB-4 and
SSEBS/DAB-16 gels.
77
Figure 4.5 SAXS profiles for (a) SSEBS3 with different sulfonation
degrees /DAB 8 organogels (b) SSEBS/DAB 4
organogels with different polystyrene contents.
78
Figure 4.6 Dynamic moduli G’(filled) and G”(hollow) of organogels
from SSEBS/DAB-4 with 0.5% of acid, amine and salt as
a function of oscillatory shear frequency.
79
Figure 4.7 Confocal image of HIPE gel from SSEBS/DAB-4 with
0.25 M NaCl added, excited by laser with a wavelength
of 405 nm for the pyrene labelled organic phase, the scale
bar: 100 m.
81
Figure 4.8 Dynamic moduli G’ and G” of HIPE organogels prepared
from SSEBS/DAB-4 with different NaCl concentrations
as a function of oscillatory shear frequency.
82
Figure 4.9 SAXS profiles of HIPE organogels (a) with
SSEBS/DAB-4, SSEBS/DAB-8 and SSEBS/DAB-16
with 0.5 M NaCl, (b) SSEBS/DAB-4 with 0.25, 0.5 and 1
M NaCl.
83
Figure 4.10 SAXS profiles of HIPE organogels (a) with
SSEBS/DAB-4, SSEBS/DAB-8 and SSEBS/DAB-16
with 0.5 M NaCl, (b) SSEBS/DAB-4 with 0.25, 0.5 and 1
M NaCl.
84
Figure 5.1 TEM imagine of –NH2 modified Fe3O4 nanoparticles. 91
XIV
Figure 5.2 Dynamic moduli G’ and G” for (a) SSEBS1/Fe3O4
organogel with 1 and 10 rad s-1 as a function of strain (b)
SSEBS1, SSEBS2 and SSEBS3/Fe3O4 organogels as a
function of oscillatory shear frequency.
94
Figure 5.3 FT-IR spectra of SSEBS1, mixtures with SSEBS1/-NH2
modified Fe3O4 nanoparticles = 30/1 and 10/1 (v/v) in
the range of 1500–1650 cm-1.
96
Figure 5.4 SAXS profiles for SSEBS1/Fe3O4 hybrid organogels
with SSEBS/Fe3O4 = 20/3, 20/2 and 20/1.
96
Figure 5.5 Confocal image of hybrid HIPE organogels, excited by
laser with a wavelength of 405 nm for the pyrene
labelled organic phase, scale bar: 50 m.
97
Figure 5.6 Optical micrographs (Mag. × 100) of the hybrid HIPE
organogels with (a) 80%, (b) 84% and (c) 88% of the
internal phase.
98
Figure 5.7 G’ and G” of SSEBS1/Fe3O4 HIPE organogels with
volume fractions of the internal phase at 75, 80, 84 and
88 % as a function of oscillatory shear frequency.
99
Figure 5.8 G’ and G” of SSEBS1/Fe3O4 HIPE organogels with (a)
different Fe3O4 nanoparticles at and (b) different salt
concentrations.
100
Figure 5.9 SEM micrographs of macroporous materials formed
from HIPE organogel with an 88% internal phase at
different magnifications.
101
Figure 5.10 (a) Optical micrograph (Mag. ×500) of macroporous
materials formed from HIPE organogel with an 88%
internal phase and (b) the optical photo of the HIPE
xerogels with a magnet.
102
Figure 5.11 SAXS profiles of SSEBS1, SSEBS2 and SSEBS3/Fe3O4
HIPE xerogels.
103
Figure 5.12 Photograph of a water droplet on the surface of porous
materials taken during contact measurements.
103
XV
Figure 5.13 Oil absorption capacity of hybrid materials formed from
hybrid HIPE organogels.
104
Figure 6.1 1H-NMR spectrum of the synthesized benzoxazine in
CDCl3.
111
Figure 6.2 Dynamic moduli G’ (filled) and G” (hollow) of 1OG,
2OG and 3OG as a function of oscillatory shear
frequency
115
Figure 6.3 (a) Dynamic moduli G’ (filled) and G” (hollow) as a
function of oscillatory shear frequency and (b) stress
relaxation for control 1OG, polymerized at 75 °C for
2 and 4 hours 1OG.
116
Figure 6.4 SAXS profiles of (a) SSEBS solution, 1OG, 2OG and
3OG; (b) 2OG under 75 °C for 0, 2 and 4 hours.
118
Figure 6.5 A typical confocal image of HIPE organogels, excited
by laser at a wave length of 405 nm for the pyrene
labelled organic phase, scale bar: 50 m.
119
Figure 6.6 (a) Dynamic moduli G’ (filled) and G” (hollow) as a
function of oscillatory frequency and (b) stress
relaxation for fresh prepared, polymerized at 75 °C
for 2 and 4 hours 1HIPEOG.
120
Figure 6.7 SEM micrographs of 1HIPEXG, 2HIPEXG and
3HIPEXG.
121
Figure 6 8 Photographs of water droplets on the surfaces of (a)
1HIPEXG, (b) 2HIPEXG and (c) 3HIPEXG taken
during contact measurements.
122
Figure 6 9 Absorption rates of toluene and engine oil for
1HIPEXG, 2HIPEXG and 3HIPEXG.
123
Figure 6.10 Oil absorption capacity of 1HIPEXG, 2HIPEXG and
3HIPEXG.
124
Figure 6.11 Recovery rate of 1HIPEXG for engine oil. 124
XVI
Figure 7.1 (a) Confocal image of HIPE organogels; (b) SEM
image of HIPE xerogels.
131
Figure 7.2 Separation of diesel from water by HIPE xerogels.
The diesel was dyed with oil red o.
132
Figure 7.3 Absorption capacities of the HIPE xerogels for
various organic solvents and oils.
133
Figure 7.4 Weight gain by the HIPE xerogels in toluene and
diesel versus time.
135
Figure 7.5 Absorption capacities of HIPE xerogels based on
SEBS with 28 mol % of polystyrene blocks for
organic solvents and oils.
136
Figure 7.6 Absorption capacities of HIPE xerogels based on
SEBS with 68 mol % of polystyrene blocks for
organic solvents and oils.
136
Figure 7.7 Absorption-squeeze process of HIPE xerogels
separating diesel from water.
136
Figure 7.8 Absorption capacities of HIPE xerogels and spilled oil
recovery with diesel absorption–squeezing-out cycle
number at (a) 20 °C, (b) 0 °C and (c) 45 °C.
138
Figure 7.9 Optical images of a droplet of (a) distilled water, (b)
0.5 M NaCl aqueous solution and (c) seawater on the
surface of HIPE xerogels.
138
Figure 7.10 Water absorption of HIPE xerogels in distilled water,
0.5M NaCl solution and sea water.
139
XVII
LIST OF SCHEMES
Scheme 2.1 O/w HIPEs stabilized by CCS polymers with dual
responsiveness. Reproduced from Ref. 23 with permission
from The Royal Society of Chemistry.
14
Scheme 2.2 The formation of polyhedron structure with CNCs
stabilized HIPEs.
15
Scheme 2.3 Maps of oil spill locations. 33
Scheme 3.1 Scheme of nanoscaled gel based on SAXS fitting, radius
of gyration RG = 35 nm, radius of hard sphere RHS = 12.5
nm and distance between microdomains L = 29.5 nm.
62
Scheme 4.1 The microstructures of polymer organogels stabilized
HIPE organogels.
85
XVIII
LIST OF TABLES
Table 2.1 Mainly used surfactants for HIPEs. 6
Table 2.2 Top 20 major spills. 32
Table 3.1 The SSEBS prepared from various SEBS. 55
Table 7.1 Comparison of absorption capacities of several
absorbents.
134
XIX
LIST OF ABBREVIATIONS AND SYMBOLS
APS Ammonium persulfate
AM Acrylamide
APTES 3-Aminopropyltriethoxysilane
BPO Benzoyl peroxide
CCS Core cross-linked star
CTAB Cetyltrimethylammonium bromide
DVB Divinylbenzene
DCE 1,2-Dichloroethane
DCPD Dicyclopentadiene
DPEHA Dipentaerythritol penta/hexa-acrylate
EB poly(ethylene-ran-butylene)
EHA 2-Ethylhexyl acrylate
EGDMA Ethylene glycol dimethacrylate
FTIR Fourier transform infrared
GPC Gel permeation chromatography
GMA Glycidyl methacrylate
HIPE High internal phase emulsion 1H NMR 1H Nuclear magnetic resonance
IPA Isopropyl alcohol
IPN Interpenetrating polymer network
MEH-PPV Poly(2-methoxy-5-(2-ethylhexyloxy)-1,-4-phenylenevinylene)
MBAM N,N'-Methylenebisacrylamide
PDMAEMA poly(N,N-dimethylaminoethylmethacrylate)
PEI Polyetherimide
PS-b-P2VP polystyrene-block-poly(2-vinylpyridine)
PPI Polypropylenimine
PMMA-b-P2VP poly(methyl metacrylate)-block-poly(2-vinylpyridine)
XX
PCL-b-P2VP poly( -caprolactone)-block-poly(2-vinyl pyridine)
PFDMA Poly(perfluoroalkyl methacrylate)
PAA Poly(allyamine)
PMMA Poly(methyl methacrylate)
PBLG Poly( -benzyl-L-glutamate)
PPE Poly(2,5-dinonyl-p-phenylene ethylene)
PF6 Hexafluorophosphate
ROMP Ring-opening metathesis polymerization
SAXS Small-angle X-ray scattering
SEBS Polystyrene-b-poly(ethylene-ran-butylene)-b-polystyrene
SEM Scanning electron microscopy
SSEBS Sulfonated SEBS
SMA Poly(styrene-alt-maleic anhydride)
SPS Sulfonated polystyrene
SANS Small-angle neutron scattering
TEM Transmission electron microscopy
THF Tetrahydrofuran
TMPTA Trimethylolpropane triacrylate
TEM Transmission electron microscopy
TFSI Bis(trifluoro methanesulfonyl )imide
XXI
ABSTRACTThis research thesis reports upon charge-driven assembled polymer
organogels stabilized high internal phase emulsion (HIPE) organogels and their
application in oil spill recovery.
Charge-driven assembled polymer organogels were fabricated with SSEBS
and a diblock copolymer PS-b-P2VP as a model component. The –SO3H groups
of SSEBS react with pyridine groups of PS-b-P2VP to form ionic complexes
and because these ionic complexes are solvophobic, they aggregate in organic
solvents to form spherical cores which are connected by the solvophilic EB
blocks of SSEBS to form three dimensional networks. The mechanical
properties of these polymer organogels are affected by stoichiometry and type
of SSEBS. The as-formed polymer organogels can become liquids because of
the ionic strength of the ionic complexes are affected by the addition of acid,
amine or salt.
The amphiphilicity of polymer organogels can be tuned with the changes of
the alkaline components into PPI dendrimers, -NH2 modified Fe3O4 particles or
benzoxazine molecules. The ionic complexes are hydrophilic and can stabilize
water droplets in organogels. The volume fraction of the dispersed aqueous
phase can exceed 74 %, leading to the formation of HIPE organogels.
Microphase separation was observed both in the pre-formed polymer organogels
and in the HIPE organogels, and the morphology of the HIPE organogels is
mainly dependent on SSEBS. The mechanical strengths of the HIPE organogels
can be tuned through the salt concentration, while the HIPE morphologies only
vary slightly.
Hybrid HIPE organogels were formed from hybrid organogels which were
prepared from SSEBS and –NH2 modified Fe3O4 nanoparticles via charge-
driven assembly. Macroporous materials were produced from these HIPE
organogels by removal of solvents via freeze drying, and these materials exhibit
magnetic responsiveness due to the existence of Fe3O4 nanoparticles, which
facilitates the collection of these macroporous materials. Reactive polymer
organogels from SSEBS and benzoxazine molecules were found to be able to
stabilize HIPE organogels and after reaction, these HIPE organogels were strong
enough to be dried at room temperature. After reaction of benzoxazine in HIPE
XXII
organogels, three-dimensional macroporous polymers were obtained by direct
removal of the dispersed aqueous phase at room temperature. The obtained
macroporous materials were studied for oil water separation, and experimental
results confirmed the excellent performances including high absorption rate and
relatively large absorption capacity to a variety of organic solvents and oils,
simple recovery method by squeezing and re-usability.
A model study was carried out to investigate the application of macroporous
materials from SSEBS/PPI dendrimer HIPE organogels (HIPE xerogels) for
enhanced oil spill recovery. The HIPE xerogels absorbed diesel from the water–
oil mixture in 20–30 seconds. The absorption capacity of the HIPE xerogels
ranged from 20 to 32 g/g for different kinds of oils, and the oils were recovered
simply by being squeezed out, with a recovery rate of around 80%. The HIPE
xerogels were used at least 40 times without an obvious deterioration in oil
separation properties and could be used from 0 to 45 °C. The research
demonstrated conclusively that these novel xerogels are suitable for practical
use in oil spill reclamation and wastewater treatment.
1
Chapter One
General Introduction
1.1 Thesis objective High internal phase emulsions (HIPEs) are emulsions with a total volume
fraction of the internal/dispersed phased being over 74%, and they are widely
used as end products and for the preparation of porous materials with various
applications. HIPEs are usually stabilized by amphiphilic stabilizers from
surfactants, particles all the way to gels particles. Gels have been confirmed as
efficient HIPE stabilizers, but the mechanism to stabilize HIPEs are not clear.
Moreover, in comparison to gels from low molecular-weight gelators, polymer
organogels exhibit several advantages, but they are less explored as HIPE
stabilizers.
Ionic interactions are one of the most common interactions in the world.
They are dynamic and can be affected by several external interactions such as
solvents and pH. Ionic interactions can induce various assembly and it is known
as charge-driven assembly. Charge-driven assembled polymer organogels can
be formed and the amphiphilicity of these organogels can be tuned by choosing
suitable components. These polymer organogels can be used to stabilize HIPEs,
and the morphology of the charge-driven assembled polymer organogels may
change during the stabilization of HIPEs, which may provide some information
of the microstructures of HIPEs.
The research described in this thesis aims to develop new HIPE organogels
stabilized by charge-driven assembled polymer organogels and their
applications in oil fields. The ionic complexes between the –SO3H groups of
sulfonated polystyrene-b-poly(ethylene-ran-butylene)-b-polystyrene (SSEBS)
and oppositely charged components form solvophobic cores, and these cores are
connected and stabilized by middle EB blocks of SSEBS to form charge-driven
assembled polymer organogels. These ionic cores could absorb and stabilize
water to form HIPE organogels. After the removal of solvents from various
2
HIPE organogels, hydrophobic HIPE xerogels were obtained. These xerogels
are excellent candidates for oil water separation and spill recovery considering
their high absorption capacity, easy recovery and reusability.
1.2 The outline of the work Chapter 2 is a literature review over HIPEs, polymer organogels and oil spill
recovery with an emphasis on gel-stabilized HIPE gels. This review describes
the HIPEs formation and application, polymer organogels and various methods
for oil spill recovery.
In Chapter 3, charge-driven assembled polymer organogels from SSEBS
and polystyrene-b-poly(2-vinylpyridine) (PS-b-P2VP) are discussed. A model is
proposed to describe the formation of polymer organogels via charge-induced
assembly and the responsiveness of the assembled polymer organogels to acid,
amine and salt.
Chapter 4 discusses the formation of HIPE organogels from polymer
organogels which were formed from SSEBS and polypropylenimine (PPI)
dendrimer. The salt concentration, co-solvent, and PPI were investigated for
mechanical strength and morphology of HIPE organogels.
Chapter 5 describes hybrid HIPE organogels obtained from hybrid polymer
organogels which are from SSEBS and –NH2 modified Fe3O4 nanoparticles. It
is noted that obtained macroporous materials from HIPE organogels showed
excellent performance in oil water separation, and these materials were collected
with the help of magnetic field.
In Chapter 6, we have investigated reactive HIPE organogels from reactive
polymer organogels based on SSEBS and benzoxazine by charge induced
assembly. These HIPE organogels could be transformed into macroporous
materials at room temperature and the obtained materials could be used for oil
water separation.
In Chapter 7, HIPE xerogels were obtained from SSEBS/PPI dendrimer
HIPE organogels by simple freeze drying, and as a type of novel porous
materials, they are confirmed to be an excellent candidate for oil spill recovery.
Chapter 8 presents the general conclusion and potential future research topic
related to this thesis.
3
Chapter Two
Literature Review
2.1 Introduction Emulsions consist of immiscible liquids forming a continuous phase and an
internal/dispersed phase stabilized by emulsifiers. Lissant classified emulsions
exhibiting an internal phase fraction over 70% by volume as High Internal Phase
(Ratio) Emulsions (HIPEs).1 However, HIPEs are widely accepted as emulsions
with a total volume fraction of the internal phase over 74.05 % which is the
maximum packing efficiency of perfect spheres in a container.2 HIPEs usually
exhibit high viscosity due to the high volume fraction of the dispersed phase and
thus they are also called gel emulsions. HIPEs have attracted considerable
attentions from both academic fields and industrial community because they
may act as end products in foods,3 control release of drugs,4 petroleum gels for
fuel safety5 and serve as templates in the preparation of macroporous materials
with a low density.6 These macroporous materials are known as polyHIPEs and
they are widely applied to liquid separation,7-10 tissue engineering,11 sensor12 all
the way to drug delivery.13
To prepare HIPEs, the emulsifiers or stabilizers are of great importance to
maintain the stability of these emulsions. The stability of emulsions is mainly
classified into two categories, the stability of dispersion and the stability of
dispersed droplets.14 Two main mechanisms which explain the droplets stability
are coarsening and coalescence. Coarsening means the diffusion of the internal
phase from small droplets to large droplets driven by Laplace pressure. The sizes
of small droplets continue to decrease till their disappearance resulted from the
increase of interfacial pressure. The sizes of the big droplets get larger by
receiving molecules from the neighbouring smaller droplets. The diffusion rate
of the dispersed phase is determined by the interracial barrier created by these
stabilizers. Droplets tend to coalescence to reduce the total interfacial area when
they approach each other. To avoid the coalescence, stabilizers should provide
4
enough repulsion between the interfaces of the dispersed droplets. The repulsion
is varied by the properties and concentration of stabilizers. Flocculation is
usually to characterize the stability of dispersion, and it tends to occur in
adhesive emulsions, where droplets aggregate and subsequently a sharp decrease
in the stability. Due to the suppression of Brownian motions of droplets by the
high viscosity of HIPEs, coalescence is less likely to occur in HIPEs. The highly
immiscibility of the dispersed phase and the continuous phase hinders the
coarsening. As a result, the stability of HIPEs is dependent on the inter-droplets
forces.
HIPEs have been successfully fabricated many years ago and all of them
were stabilized by surfactants until the occurrences of solid particles stabilized
HIPEs in 2007.15 From then on, particles stabilized HIPEs, especially soft
particles stabilized HIPEs have been developed speedily in recent years. With
the development of HIPE stabilizers, a variety of HIPEs with new properties and
applications have been prepared as well. This chapter reviews mainly on the
preparation and applications of HIPEs stabilized by recently developed
stabilizers. It is noted that soft particles stabilized HIPEs usually exhibit a flow-
free behavior, indicative of the formation of HIPE hydrogels or organogels
according to their types of the continuous phase.16, 17 This chapter also presents
the routes to prepare polymer organogels which were confirmed to be a novel
and cheap HIPE stabilizer in this thesis. Polymer organogels stabilized HIPEs
were found to be an excellent candidate for oil spill recovery and for clarification,
this chapter also simply describes the recent development of materials for oil
spill recovery.
2.2 HIPEs
To prepare stable HIPEs, the most important thing is their stabilizers.
Currently, HIPE stabilizers can be mainly classified into three categories and
they are surfactants, particles and bulk organogels. Surfactants are the traditional
and the most commonly used stabilizers for the preparation of HIPEs. Particles
are the fastest developed HIPE stabilizers in recent years. A great breakthrough
was achieved by applying solid particles as HIPE stabilizers in 2007.15
Subsequently soft particles were successfully used to stabilize HIPEs18 and from
5
then on, increasing number of stabilizers were developed for the stabilization of
HIPEs. HIPE organogels stabilized by bulk organogels also achieved great
success though they are rare and the mechanism of stabilization is still unclear.
2.2.1 Surfactants as HIPE stabilizers
Surfactants are the most commonly used HIPE stabilizers. Careful choice is
required in the selection of the surfactant. According to Bancroft rule, the liquid
in which the surfactant is mainly dissolved tends to form the continuous phase
of an emulsion. Thus to prepare stable HIPEs, it is required that the surfactant
dissolves solely in the continuous phase to prevent phase inversion at high
internal phase fraction.19, 20 As a result, there are only a limited number of
surfactants suitable for HIPEs though the number is increasing. The mainly used
surfactants for HIPEs are listed in Table 2.1. It can be seen from the table that
nonionic surfactants are usually used to stabilize w/o HIPEs and that Span 80 is
the most common one. Recently, a cationic surfactant, CTAB has been reported
to stabilize w/o HIPEs. Besides surfactants, copolymers are also investigated to
form HIPEs as stabilizers,8, 10, 11, 21 and block copolymers consisting of
poly(ethylene glycol) and poly(propylene glycol) blocks are the most studied,
including commercially available Pluronic L-121, Pluronic F-68, P123 and
Synperonic PEL 121. It is noted that there are some drawbacks using surfactants
as HIPE stabilizers. One is the inefficiency of surfactants, where a high
concentration of surfactants is required to stabilize HIPEs,4, 22-24 and the
surfactant concentrations can be as high as 5 – 50 w/v % based on the continuous
phase. Till now, the surfactant concentrations in most studies are higher than 2.5%
though one example has been reported at 0.25%. With the developments of
monomer, the continuous phase and the dispersed phase, such as ionic liquids,
more HIPEs have been prepared with novel surfactants. These new surfactants
can be obtained from chemical synthesis or combination of multiple surfactants
as HIPE stabilizers.
Another shortcoming of surfactant stabilized HIPEs is that usually the
surfactants should be removed after freezing the structures of HIPEs due to the
relative high toxicity of surfactants. One alternative method to reduce toxicity is
6
the incorporation of the surfactant (Span-80) covalently into polyHIPEs by
ROMP of cyclooctene in the continuous phase.25
Table 2.1 Mainly used surfactants for HIPEs.
HIPE stabilizers Continuous phase Surfactant concentration
Ref
1 Span 80 4-Vinylbenzyl chloride and DVB
40% 26
Styrene and DVB 20%,30%, 33%
27-
30
Butyl methacrylate, porogen, DVB
/ 31
1 equivalent of PB, 1.5 equivalent of bromo-compound, 5 mol% of tetrakis(triphenylphosphine) palladium(0)
~10% 32
4-Vinylbenzylphthalimide, 4-Vinylbenzylamine, DVB
~28.5% 33
2 Pluronic L-121 EGDMA, DCPD, initiator 30% 34
7.5%, 0.25 to 10 %
35,
36
Pluronic F-68 Dextran-methacrylate, Irgacure 2959, APS and water
20% 37
P123 1,1-Dichloroethene 10% 38
Synperonic PEL 121
EGDMA, GMA and EHA, toluene
20%~27% 39
M2 ((H2IMes)(PCy3) Cl2Ru(3-phenyl-indenylid-1-ene), H2IMes ¼ N,N-bis(mesityl) 4,5- dihydroimidazol-2-yl, PCy3 ¼ tricyclohexylphosphine) and water
10 % 40
3 Polyoxyethylene-polyoxypropylene-polyoxyethylene
1-octyl-3-methylimidazolium hexafluorophosphate
3 % 41
4 CITHROL DPHS-SO-(MV)
Lauryl methacrylate 1,14-tetradecanediol dimethacrylate
2.5 – 15 %, 15%
42
7
5 PGPR 4125 EGDMA and initiator 10 - 30 % 13
PFDMA and BPO 5% 11
6 Synperonic A7 Water, chitosan and genipin 10% 43
7 Hypermer 2296 Styrene and DVB / 44
8 Sugar-based surfactants
AM, MBAM, initiator and water 5 % 45
9 Hypermer B246 Trithiol, TMPTA or DPEHA and chloroform
2.5% 38,
46
4-Vinylbenzylphthalimide, 4-Vinylbenzylamine, DVB
~14%
10 Mixture of Span 80, Span 85, Tween 85 and Pluronic L121
Methacrylic acid, styrene, DVB / 47
Span 80: sorbitan monooleate; Span 85: sorbitan trioleate; Tween 85:
polyoxyethylene sorbitan trioleate; Pluronic L-121: poly(ethylene glycol)-
block-poly(propylene glycol)-block-poly(ethylene glycol); Synperionic
PEL121: poly(ethylene glycol)–block-poly(propylene glycol)-block-
poly(ethylene glycol); Hypermer B246: triblock copolymer of poly(12-
hydroxystearic acid) and poly(ethylene glycol); PGPR 4125: polyglycerol
polyricenoleate.
As mentioned before, one major application of HIPEs is to prepare porous
materials or polyHIPEs. In these porous materials, surfactants could be removed
during the washing and drying processes, which may not facilitate the surface
modifications. One method to overcome this issue is by incorporating
surfactants covalently to these polyHIPEs, and the first example was shown by
ring-opening metathesis polymerization of cyclic olefinic monomers and a
surfactant with double bond(s).25 In this system, the surfactant works as a chain
transfer agent. As the ring-opening polymerization is very fast, it is believed that
the oil phase is polymerized first and the surfactants incorporates later in at the
oil-water surface. Results show that the surface properties changed with covalent
linkage of surfactant onto the surface of polyHIPEs, which may provide new
functionality to these porous materials.
8
It should be pointed out that the recently reported CCS polymers exhibit
similar structures of particles and similar properties in pH and salt driven phase
inversion21-23 although they exhibit the property of reducing surface tension.
Therefore we classify the CCS polymers as polymer particles and will discuss
later.
2.2.2 Particles as HIPE stabilizers
Particles stabilized emulsions are also called Pickering emulsions, and for
Pickering emulsions, it is required a balance of interfacial energies of the
following three interfaces: solid-oil, solid-water and water-oil, respectively s-o
s-w and w-o. To prepare water in oil emulsion, it requires partial wettability of
solid particles. i.e., the spreading coef cient of water S(w/o) is negative and the
adhesion energy of water EAdh(w/o) is positive:
S(w/o) = s-o o-w s-w <0 (1)
EAdh(w/o) = s-o + o-w s-w >0 (2)
An equivalent method is to study the wetting of the solid with oil inside a
water medium in which the negative spreading coef cient S(o/w) of oil and the
positive adhesion energy EAdh(o/w):
S(o/w) = EAdh(w/o) = s-w o-w s-o <0 (3)
EAdh(o/w) = S(w/o) = s-w + o-w s-o > 0 (4)
As too hydrophilic surface of solid particles could be totally wet by water,
the solid particles would be dispersed in the aqueous phase of the emulsion as
they do not adsorb on the interfaces between the aqueous phase and oil phase.
For the same reason, very hydrophobic particles would be totally wet by the oil.
For partial wetting conditions, the contact angles of particles in water, w, and
in oil, o, are described by the Young’s law ( o = w) listed below:
(5)
or
(6)
The adsorption of these partial wetting solid particles on the interfaces is
very strong. The free energy of adsorption (the free energy needed for desorption
of one particle either into the continuous phase or the dispersed phase) is related
9
to the size of the solid particles and the interfacial tensions. For spherical
particles with radius of R, it means
adsF = R2 o-w(1 cos( w)) for w < 90° (7)
adsF = R2 o-w(1 cos( w)) for w > 90° (8)
Particles stabilized HIPEs are also named Pickering HIPEs. HIPEs are also
emulsions and thus particles should satisfy the above conditions to obtain stable
emulsions. These particles can be inorganic or organic but they exhibit quite
different performances when used to support HIPEs.
Solid particles
The volume fraction of the dispersed phase has been limited to certain
degree although Pickering emulsions have been found and studied for a quite
long time. According to the thermodynamic model, Kralchevsky and co-workers
predicted that phase inversion would happen in Pickering emulsions when the
volume fraction of the internal phase increases to 50%.48 After consideration of
kinetic factors, phase inversion is usually occurred at a volume fraction of
70%.49, 50 It has been demonstrated by experiments that phase inversion in
Pickering emulsions happens between 65% and 70%, where the majority phase
will convert to be the continuous phase. Therefore, the fabrication of HIPEs
solely stabilized by particles seems impossible. Menner et al. first reported that
carbon nanotubes could stabilize HIPEs with 60% of internal phase.51 Akartuna
et al. reported the preparation of porous materials from Pickering HIPEs with
72 -78% of the internal phase where 35% of particles were used.52
A breakthrough is the successful application of organic acid modified solid
nanoparticles as HIPE stabilizers, and the volume fraction of the dispersed phase
in these Pickering HIPEs reached over 74% with a low concentration of particles.
These nanoparticles are usually modified by oleic acid.15, 53 Results show that a
low concentration of oleic acid (2.5 wt %) is required to modify titanium
nanoparticles for the stabilization of HIPEs with an internal phase of 80%
against phase inversion and coalescence. This is the first example of preparation
of HIPEs with a surfactant-free method. The dispersed phase can be increased
to 92% by changing the titanium nanoparticles into silica nanoparticles.
10
Magnetic nanoparticles were studied systematically to stabilize emulsion
considering the effects of oil-water volume fraction, particle concentration as
well as oil polarity on the type, stability, composition and morphology of
emulsions.54 Three-phase contact angle of these nanoparticles to nonpolar and
weakly polar is close to 90°, while to strongly polar oils is far below 90°. The
nanoparticles can stabilize emulsions formed from nonpolar or weakly polar
solvents and water, but they cannot stabilize strongly polar solvent water
mixtures. All the emulsions are of oil-in-water type and the average size of
droplets increases with the increase of oil fraction. It is found that not all the
nanoparticles are absorbed to droplet interfaces, and the fraction of absorption
decreases with the increase of oil fraction. The droplet sizes decrease with the
increase of concentration but the concentration has no obvious effects on the
emulsion stability.
Hybrid porous materials can be obtained with polymerisable monomers as
the continuous phase, and these porous materials may provide useful
information about the absorption of particles at the interface between the
continuous phase and the dispersed phase. This study was carried out with oleic
acid-modified iron oxide nanoparticles stabilized HIPEs.55 From the magnetic
porous materials, it was found that the nanoparticle either arranged at the
interface of oil and water or partially dispersed into the oil phase according to
the amount of oleic acid. The ability to migrate to the interface showed great
effects on the droplet size distribution of emulsions and the maximum internal
phase fraction.
Except particles, one dimensional nanomaterials (tubes) and two
dimensional nanomaterials (sheets) are also investigated as stabilizer for HIPEs,
where carbon tubes and graphene oxides are most studied.56-58 Based on their
stabilized HIPEs, various porous composites have been studied.
To satisfy the stabilization of HIPEs, solid particles are usually modified by
organic components. It should be pointed out that these organic components are
surfactants in certain aspects. As a result, there are certain interactions between
these organic components modified particles and traditional surfactants. One of
the interactions is the antagonist effects, which has been exampled with oleic
acid modified Fe3O4 and hydrophobic Hypermer 2296 as a model system.59 It is
11
known that partially hydrophobized Fe3O4 can be applied to prepare stable
HIPEs, and phase separation occurs to these emulsions after addition of
surfactants at a low concentration. The phase separation is ascribed to the
obvious increase of hydrophobicity of nanoparticles surface, because these
hydrophobic surfactant has preferential affinity of the surfactant for the
nanoparticles surfaces. W/o emulsions could be obtained mainly by surfactant
molecules at a higher surfactant concentration. The conclusion was confirmed
by the results from the polymerized emulsions.
Except antagonist interaction, synergistic effect was also found between
hydrophobic silica nanoparticles (H30) and nonionic surfactant Span 85 as
model system.60 These HIPEs can be prepared with an extreme high volume
fraction of the internal phase at 98.5%, and the ratios of the surfactants and
particles affect the sizes of the internal phase. A high concentration of H30
induces the increase of droplet sizes, while increasing the concentration of
surfactants drives the smaller sizes of the dispersed droplets. The synergism
between the particle and surfactants are vital to the stabilization of HIPEs.
Polymer particles
Synthesized polymer particles
A great milestone is the successful application of cross linked microgel
particles as HIPE stabilizers. As mentioned before, copolymers are inefficient
as HIPE stabilizers, but the microgels showed high efficiency. The first example
was made by To Ngai and co-workers who applied PNIPAM-co-MAA microgel
particles as stabilizer to form HIPEs,18 and these HIPEs are Pickering emulsions
too, although the stabilizers are not solid. The work showed that polymeric
particles can serve as stabilizer to support HIPEs and that these microgel
particles had a variety of advantages over traditional solid particles. These
HIPEs are oil-in-water (o/w) emulsions in contrast to the water-in-oil (w/o)
emulsions stabilized by traditional solid particles. These microgel particles can
be of high efficiency by tuning the chemical structure and composition of gels.
It is reported that a low concentration of these microgels at 0.05% is enough to
stabilize Pickering HIPEs, which is quite efficient in comparison to traditional
surfactants where a high concentration of 5 – 50% of the continuous phase is
12
required. The concentration of microgel particles does not affect the structures
of the dispersed phase. As the microgel particles have a cross-linked structure
and a high molecular weight, macroporous materials could be obtained from
these HIPEs by direct evaporation of solvents without chemical reactions.
Therefore these HIPEs provide a facile method to prepare macroporous
materials. Applying PS-co-MAA microgel particles as stabilizer,61 o/w
emulsions were obtained, and phase inversion occurs to these emulsions to form
w/o HIPE emulsions by increasing salt concentration or decreasing pH values.
These w/o HIPE emulsions are strong enough without liquid-like flow behavior
when the concentration of these particles increases to 5% of the continuous
phase. Microgel particles are one kind of colloidal particles, and thus these
emulsions are actually colloidal HIPE gels.
The performances of HIPE stabilizer can be improved by tuning the
chemical composition of microgel particles.16, 17, 62, 63 Some successful examples
are shown by Bon and co-workers who have incorporated functional groups into
microgel particles for the formation of HIPE.16, 17 These microgels were
synthesized by copolymerization of some monomers with UPy groups and these
groups can form strong and highly directional hydrogen bonds. These microgels
are cross-linked by strong physical interaction (hydrogen bonding interaction)
instead of chemical cross-linkage. It is believed that these microgels transfer
across the water-oil interface in the continuous phase through hydrogen bonding.
Due to the existence of the strong interaction, these microgel particles are able
to stabilize w/o HIPE gels at a low concentration of 0.4% based on the
continuous phase, which is much lower than that of PS-co-MAA microgel
particles. These physically cross-linked gel particles are suitable to stabilize
HIPE hydrogels by changing the chemical composition of microgels, and these
hydrogels are strong enough to be moldable16 and thermosensitive. Unlike PS-
co-MAA particles, there is no permanent chemical cross-linkage in these
particles, but they are still stable because the hydrogel bonding between these
functional groups are strong enough even in water.
Chemical cross-linked hybrid particles were investigated to stabilize HIPEs,
where paraffin oil was used as the dispersed phase with a volume fraction of
83.3%.64 These hybrid particle are prepared from aminolysis of SMA with
13
APTES via the direct chemical self-assembly and in situ condensation of APTES.
It is noted that the hybrid particles are pH-sensitive and this the as prepared
HIPEs are able to demulsify with the addition of alkaline solution.
Star and branched polymers
Except polymer particles, some polymers with specific structures such as
core cross-linked star (CCS) and branched polymers were also investigated as
HIPE stabilizers.21-23 CCS polymer consisting of PDMAEMA were applied to
stabilize HIPEs at a high oil fractions of 80 – 89% at a relative wide range of pH
at 2 – 12.23 As the star polymer is sensitive to pH, the properties of emulsions
such as oil droplet size, long term stability and rheology were pH-dependent.
Complete demulsification of HIPEs occurred after addition of base. The CCS
polymer are pH sensitive, and phase inversion from ordinary emulsions to
HIPEs are studied with changing pH. It was observed that ordinary w/o
emulsions were obtained with a pH range from 12.1 – 9.3. Double emulsions
oil-in-water-in-oil (o/w/o) and water-in-oil-in-water (w/o/w) were observed at
pH 8.6 and 7.5 respectively. Further decreasing the pH to the range of 6.4 to 0.6,
HIPE gels were obtained, realising the phase inversion with pH changes. Further
study showed that a salt and temperature sensitive CCS polymer poly(MEAx-
co-PEGAy) may affect the properties of the corresponding HIPEs although
almost no effect of presence and absence of salt was found on the maximum oil
fraction. Addition of kosmotropes increase demulsification efficiency, and
addition of chaotropes declined demulsification efficiency or increase the
thermal stability of the CCS-stabilized HIPEs.
CCS polymers have a number of arms connected to a small central core, and
these polymers are readily soluble/dispersible in water as individual polymer
without formation of aggregates. CCS polymers can be considered as a bridge
between colloidal particles and linear polymers; they showed certain degree of
flexibility due to the existence of polymer arms and certain degree of
compactness because of the highly cross-linked core. The sizes of these star
polymers range from 10 -50 nm and are strongly absorbed in oil/water interface,
which facilitate the formation of HIPEs. These CCS polymers can rearrange
14
their confirmation because of their flexibility and adapt the most favourable
conformation to minimize interfacial energy.
Scheme 2.1 O/w HIPEs stabilized by CCS polymers with dual responsiveness.
Reproduced from Ref. 23 with permission from The Royal Society of
Chemistry.
In comparison with surfactants or solid particles as HIPEs stabilizers,
polymers may exhibit high molecular weight and sometimes many functional
groups, and thus they may entangle or reacts with the polymerisable continuous
phase to form polyHIPEs with the surface modifications. One example has
shown the possibility with diblock copolymer as HIPE stabilizers to modify the
surface properties of polyHIPEs.65 These HIPEs were prepared with amphiphilic
block copolymer as stabilizer, and after polymerization these block copolymers
were incorporated into polymer foams by either physical entanglement or
chemical reactions, leaving the hydrophilic block on the pore surface. The one-
pot polymerization allows the full surface modification of these porous materials.
Combined with the facts that the foam morphologies can be controlled by
emulsion parameters including shear rate, initiator and volume fraction of
aqueous phase, these porous materials are believed for many applications.
Nature and modified products
The application of porous materials for tissue engineering requires that
materials are bio-compatible. With the better understanding of stabilizer for
15
HIPEs, it is possible to fabricate emulsions stabilized by environmentally
friendly bio-based materials, and till now, a variety of natural materials such as
cellulose nanocrystals, starch and lignin have been developed as stabilizer.
Recently, HIPEs are also successfully prepared from these bio-based materials.
The first example of oil-in-water HIPEs are stabilized by cellulose nanocrystals
(CNCs) which are colloidal particles.66 It is believed that CNCs particles can be
absorbed irreversibly at the water-oil interface to produce stable emulsions. The
stable HIPE hydrogels were formed with 0.1 % of CNCs particles when the
volume fraction of the internal phase was increased over 90%. In order to
prepare HIPEs, two steps are required, first prepare Pickering emulsions
stabilized by CNCs, and second increase the dispersed phase to over 74%. The
formation process was considered. As the amount of CNCs is a constant, the
increase of the dispersed phase means the drop of the surface coverage. At the
initial stage of the formation. The surface coverage is quite high, actually
forming multilayer at the interface. The increase of the average drop occurs until
the volume fraction of the dispersed phase below 40%. However, the surface
coverage is not sufficient and a new reorganization happens, forming lager
droplets. The size of droplets continuously increases until it exceeds the close
packing limit, and then droplet deformation occurs with the further increasing
the dispersed phase.
Scheme 2.2 The formation of polyhedron structure with CNCs stabilized HIPEs.
HIPEs can be applied as end products and thus all components from food-
grade materials are quite interesting. By careful choice, HIPE gels using food-
grade components as the continuous phase, the dispersed phase and stabilizer
have been achieved.67 An example was that HIPE emulsions were obtained with
polyglycerol polyricinoleate as a surfactant and then incorporating
16
hydrocolloids (a combination of locust bean gum: carrageenan and locust bean
gum: xanthan gum) into the aqueous phase at high temperature. After cooling
the system, HIPE gels were formed due to the fact that hydrocolloid
combinations are thermoreversible and low temperature setting. Two
considerations are required to choose a system for fabrication of HIPE gels via
this method. The first one is to produce a stable continuous oil phase and then
choosing thermoreversible hydrocolloids to gelate the aqueous phase. The
second one is that all the hydrocolloids should have no properties of surfactant
to avoid phase inversion of emulsions.
Bovine serum albumin (BSA) protein nanoparticles were also studied as
HIPE stabilizers.68 A stable o/w HIPE were obtained with BSA protein
nanoparticles as stabilizer, and by chemical crosslinking the particles, a gel
could be formed in the continuous phase to freeze the emulsion’ microstructures.
Then meso-macroporous protein scaffold were obtained after removal of the
dispersed oil in the scaffold. As the protein are biocompatible, it is believed that
the protein scaffolds are suitable for a wide range of biomedical applications.
Controlled phase inversion
As mentioned before, phase inversion tends to occur to particles stabilized
emulsions upon increasing the volume fraction of the disperse phase, which is a
drawback in the preparation of HIPEs. It has been reported that the phase
inversion of emulsions can be manipulated using soft particles instead of solid
particles as stabilizers by changing the oil/water ratio, hydrophilic/hydrophobic
particles in a mixture, pH value of the aqueous phase and the temperature.
However, most of these studied focused on the emulsion with an equal volume
of the two immiscible liquids. Applying PS-co-MAA microgel particles as
stabilizer61, Ngai first realized the inversion of oil-in-water emulsions to water-
in-oil HIPEs by increasing salt concentration or decreasing pH values. These
w/o HIPE emulsions are strong enough without liquid-like flow behavior when
the concentration of these particles increases to 5% of the continuous phase.
These emulsions are actually HIPE gels.
Except microgels particles, core cross-linked star (CCS) polymers are also
used as stabilizers, and it is found that pH induced phase inversion happened in
17
these CCS stabilized emulsions.22 The water-in-oil emulsions were transferred
into oil-in-water HIPEs by varying the pH values of the aqueous phase.
Moreover, the stabilizer concentration is as low as 0.5% of the continuous phase,
which is about 10 times lower than the PS-co-MAA microgels particles.
The phase inversion from common emulsions to HIPEs was experimentally
achieved by varying the preparation conditions.69 The emulsions systems were
studied with toluene and water as the dispersed phase and the continuous phase
with silica nanoparticles as stabilizers. Stable HIPEs could be produced from
two approaches. The first one is by applying a mid-amount of nanoparticles from
0.3-0.5% and modest shear time ranging 1-3 minutes, and the second one is
using a high concentration of particles at 1.0% and a high shear rate of 34 000-
43 000 per second. The author believed that the flow property of the phase
determines the formation of HIPEs. Oil dispersed phase are formed with
nanoparticles trapped at oil water interfaces or well dispersed by long time
mixing.
2.2.3 Organogels as HIPE stabilizers
Organogels are three-dimensional network containing organic solvents.
They are usually prepared from low molecular-weight organogelators
(LMWOGs) and some of them can be prepared from polymers. LMWOGs are
more common while polymers gelators are believed to be less expensive.
Recently increasing number of papers have reported HIPEs stabilized by
organogels though there are only a limited number of organogels reported.
HIPEs stabilized by organogels were developed at the same time with those
using microgels particles as stabilizers but they are less developed, perhaps
because the lack of powerful technology to study the mechanism of the
stabilizing HIPEs.
Organogels from LMWOGs as HIPE stabilizers were mainly investigated
by Yu Fang’s group.6, 40, 70 These LMWOGs are always cholesterol
derivatives,70 which form transparent organogels in a variety of organic solvents
from n-alkanes (7 n 14) to fuels. Addition of water into the preformed organogels
leads to the formation of mixture gels and even HIPE gels, where the water phase
in the w/o HIPE organogels can be up to 92%. HIPEs with polymerisable
18
monomers as the continuous phase have been stabilized by a cholesterol
derivative.6 It is noted that the storage modulus and yield stress of the HIPE
emulsions decrease with the increase of the volume fraction of the dispersed
phase, which is quite different from these of the classic HIPEs. After
polymerization, macroporous materials with a low density are obtained. These
macroporous materials are also called polyHIPEs, and polyHIPEs obtained from
LWMOGs stabilized HIPEs are similar to these traditional polyHIPEs in the
separation of oil from water.40
O
OHN
NO
HN
NH
O
OO
O
Fe
O
OHN
NHO
HN
OO
O
Figure 2.1 Chemical structures of LMWOGs as HIPE stabilizer.
2.2.4 Developments in the applications of these HIPE stabilizers
As mentioned above, traditional HIPEs are always stabilized by surfactants
and solid particles which limit HIPEs as template to prepare macroporous
materials directly. With the developments of HIPE stabilizers as reviewed above,
polymer particle and organogels endow novel functions to these HIPEs and
related porous materials.
With the development of microgel particles as stabilizer for fabrication of
HIPE gels, some novel applications of HIPE have been developed.18, 71, 72 One
is the preparation of macroporous materials by directly removal of solvents in
HIPEs,18 which has been mentioned above. Another is in the preparation of
porous inorganic materials with different length scale. The process consists of
19
preparation of HIPEs with microgel particles and inorganic particles as
stabilizers, freeze-drying of these as-prepared HIPEs and finally removal of
polymeric particles and meanwhile chemically crosslinking inorganic particles
through calcinations.71, 72 As both the microgel particles and inorganic particles
can function as stabilizer for HIPEs, this method also refers to dual templating
synthesis of porous materials. Based on this method, the obtained porous silica
materials have orders on three scales, containing macropores at 10 - 30 m,
interconnecting windows at 3 - 5 m, and nanoporous walls at about 80 nm.71
Since sizes of the disperse droplets and microgel particles can be adjusted, it is
believed that the macro- and nano-pore dimensions can be controlled directly
and independently. When the functional inorganic particles such as TiO2
particles are used,72 the obtained porous TiO2 materials are suitable for practical
application as the macropores allow the mass transfer and the smaller pores have
a large surface area for host-guest interaction, selectivity and catalytic or ion-
exchange properties.73
2.2.5 Challenges
Although great progress has been made on the preparation of HIPE
stabilizers, there are still a variety of challenges which require chemists, physics
and engineers to work together to overcome.
With the development of HIPE stabilizers, increasing number of HIPEs can
be prepared with different applications, combined with the incorporation of new
chemistry and phase components such as ionic liquids, it is believed that the
investigation and application of HIPEs will be booming. Though great
advancements have been made in the preparation of HIPE stabilizers, there are
still a variety of work to do involving both in the chemistry and physics.
LMWOGs are usually difficult to synthesize and thus expensive. Non-
covalently cross-linked polymer organogels are usually less expensive and have
several advantages over those from low molecular weight gelators. They should
be able to stabilize HIPEs by carefully tuning the structures of polymers.
Further the physics of these HIPE stabilizers. The mechanism of these
stabilizers is still unknown especially for organogels stabilized HIPEs, perhaps
because the limit technology to study the transition processing. Due to these
20
reasons, it can be found that most papers about HIPE stabilizers are reported in
communications. For a most important aspect of application, the effect of HIPE
stabilizers on the structures of polyHIPEs still requires exploitation.
2.3 Polymer organogels
This section of the literature review focusses on the formation of organogels
from polymers, which are rare in comparison to organogels from low molecular
weight organogelators (LMWOGs).74 Specifically, we focus on the preparation
of physically cross-linked networks in organic solvents from polymers. As
electrostatic interaction plays an important role in my study, charge-induced
organogels from low molecular weight organogelators and in rare cases from
polymers are also reviewed here. Polymer organogels are generally classified
into three categories according to their formation methods: the addition of
crosslinking agents, the formation of physical crosslinking points through
conformational changes and the self-assembly of polymer segments.
2.3.1 Addition of physical crosslinking agents
Polymer organogels can be formed via addition of certain low molecular-
weight molecules which may form physical interactions with polymer chains to
obtain three-dimensional networks in organic solvents. There are some systems
reported although they are really rare.75-77 Carretti et al.75 reported polymer
organogels form PAA cross-linked by carbon dioxide (CO2) in alcohols ROH
(R=C1-C8), dimethyl sulfoxide, dichloromethane and 1-methyl-2-pyrrolidone. It
is well known that PAA does not contain any solvents by itself, but unique
organogels were formed by bubbling CO2 into its alcohols or 1-methyl-2-
pyrrolidone solutions. Studies showed that two new groups allylammonium and
allylcarbamate were formed from the reaction between PAA and CO2, and it is
believed that the two groups serve as cross-linkable points for the formation of
three-dimensional networks.
Another example is reported by Thibault and co-workers76 who have
reported the formation of organogels from a diamidopyridine-containing
copolymer and a small molecule thymine (shown in Fig. 2.2). The functional
groups on diamidopyridine and thymine can form specific three-point hydrogen
21
bonds which is much strong than the common hydrogen bond. Moreover, the
three-point hydrogen bonds are directional, which can stabilize the conformation
of polymer chains. As a result, three-dimensional networks were formed. As
hydrogen bonds are not stable at high temperature, it was observed that the
organogels melt at about 50 °C and the organogels were re-formed on the
cooling process. The sol-gel transition is totally reversible.
* 20
Cl
*10 10
O
NNH
O
NH
OO O
nO O
N N
HN NH
O O
O O
Copolymer crosslinking agent Figure 2.2 Chemical structures of diamidopyridine-containing copolymer and
crosslinking agent.
2.3.2 Polymers with physical cross-linkable points
In solvents, polymers can display various conformations and thus polymers
do not form three-dimensional networks in organic solvents, because there is no
suitable cross-linkable points between the polymers to stabilize their structures.
However, in some cases, certain polymers can form organogels through helical
confirmations and - interactions. Helical conformations and - interactions
often act as physical crosslinking points and help to form three-dimensional
networks.
Polystyrene is one of the commonly used polymer, which can form a few
stereoisomers. It is generally known that atactic polystyrene can be dissolved in
a variety of solvents without gel formation. However, highly stereoregular
polystyrenes can form three-dimensional networks in certain solvents. For
example, isotactic polystyrene has been reported to form organogels in decalin.78
Syndiotactic polystyrenes have been observed to form organogels in chloroform,
carbon tetrachloride and benzene. It is believed that these organogels are
stabilized by helical structures.79 However, it seems that solvents affect the
mechanism of organogels formation. For instance, although syndiotactic
polystyrene form organogels in a mixed solvents of 1,2-dichloroethylene and 1-
22
chlorotetradecane like that in chloroform or benzene, the driving force is
believed to be host-guest interaction.80 Moreover, it has been reported that
syndiotactic polystyrene forms physical organogel in tetrahydrofuran, benzene,
p-xylene, carbon tetrachloride and 1,2-dichloroethane with a few different
solvent by formation of molecular complex.81
** n ** n
Isotactic polystyrene Syndiotactic polystyrene Figure 2.3 Structures of isotactic and syndiotactic polystyrene.
Another well-studied stereoregular polymer is PMMA, which can form
organogels too. Thermoreversible organogels have been obtained from
stereoregular PMMA in bromobenzene, chlorobenzene and toluene, and chain
trajectory was investigated by SANS.82 The results show that the chains were
rigid due to the formation of helical structures. Theoretical fits of the scattering
curves were agreeable with a double-stranded helical conformation.
Polyesters were also used to form gels in organic solvents.83-85 For example,
poly(3-hydroxybutyrate-co-3-hydroxyvalerate),84 which is known as a
biodegradable and biocompatible polyester, forms a thermos-reversible
organogel in toluene. The polyester was totally dissolved in toluene at 90 oC,
and organogels were formed during the cooling process. The behaviours are also
accounted to helical structures formed in polymer backbones. The helical
conformations induce three dimensional networks and the formation of
organogels.
Synthetic polypeptides are a kind of polymers which can form gels in
organic solvents.86-90 The most studied polypeptide is PBLG, which forms
helical structures in a variety of organic solvents, such as toluene and benzyl
alcohol.87 Block copolymers containing PBLG block may form gels in certain
organic solvents. It is noticed that block copolymers containing PBLG block and
with poly(ferrocenylsilane),86 polystyrene or poly(ethylene glycol) monomethyl
ether have been prepared and that organogels were observed in toluene. The
23
gelation concentration is quite low for these block copolymers, and it is reported
that the concentrations were only 0.2% for PFS38-PBLG95, 0.3% for PS93-
PBLG76 and 0.5% for PEG227-PBLG56.86 Further study by transmission electron
microscopy (TEM) showed that three dimensional networks were formed from
these block copolymers by forming nanofibers with a diameter of about 20 nm.
The formation mechanism is different to that of some polymers since PBLG is
a rigid segment. The segment can form rigid helical rod structures which inhibit
chain entanglement, and strong interaction between groups stabilizes these
nanofibers.
- interactions are also strong interactions used as supramolecular
crosslinks,91 and a variety of polymer organogelators were prepared based on
the interaction. A typical polymer organogelator is poly(2,5-dinonyl-p-
phenylene ethylene) (PPE, chemical structures shown in Figure 2.4) which
forms organogels in toluene though - interactions.92 It was found that PPE
polymer was dissolved into toluene with full extended dimensions, and polymer
aggregates were formed with large flat structures on cooling. Organogels were
formed when these aggregates were large enough to arrest organic solvents.
C9H19
C9H19
**n
Figure 2.4 Chemical structure of PPE.
- interactions are strongly dependent on solvents and thus a good method
to control the formation of gels is to change the solvent. For example, Wang et
al.93 have prepared MEH-PPV, chemical structure shown in Figure 2.5
organogels by changing the composition of solvents. MEH-PPV was dissolved
well in 1,3-dichlorobenzene to form extended conformation. However, the
morphology changed to a coil-like conformation after addition of nonane. The
polymer solution changed into organogels with 40% of nonane added. These
24
changes result from nonane which may act as a cross-linking agent for the side
chains of MEH-PPV.
**
O
O
n
Figure 2.5 Chemical structure of MEH-PPV.
As mentioned above, polymer organogels can be formed by - interactions,
and in some cases, polymer organogels can also be formed by combination of
several driving forces. For example, gels from poly (pyridinium-1,4-
diyliminocarbonyl-1,4-phenylene methylene)94 are induced by - interaction,
hydrogen bonding, ion- interaction and electrostatic interactions. The existence
of the interactions renders the polymer some unique properties. The polymer can
trap different kinds of solvents from water, organic solvents all the way to ionic
liquids by changing its counteranion. The polymer can arrest water with chloride
as counterion, while the polymer contains organic solvents when the chloride
was replaced by other ions. For example, the polymer formed gels in methanol
with 50g L-1 when the ion is TFSI, and the polymer formed gels in ionic liquids
such as 1-ethyl-3-methylimidazolium tetrafluoroborate and 1-butylpyridinium
tetrafluoroborate with PF6 as the counterion.
2.3.3 Introduction of LMWOGs into polymers
Introduction of LMWOGs into polymers is a useful strategy to produce
organogels from polymers, providing gelation ability of LMWOGs is
maintained.95-100 The LMWOGs can serve as supramolecular crosslinkable
points in polymers to form crosslinking points, and the polymer backbone
provides space for solvents. Many polymer gelators can be easily obtained
though this method as a variety of commercially available polymers can be used.
25
In general, there are two methods to introduce LMWOGs into polymers;
copolymerization and polymer modification. In order to copolymerize
LMWOGs, some reactive groups, such as vinyl groups and epoxy groups are
required.
Ihara and co-workers100 have reported glutamide-containing polystyrene
and polyacrylate copolymers. The copolymers have a gelling ability in the mixed
solvents of benzene and cyclohexane/ethanol in the ratio of 9:1. The gelation
ability could be tuned by copolymerization ratio (n/m), and the minimum gel
concentration decreases with the increasing n/m. In addition, the
copolymerization of the L-glutamate with methyl methacrylate induced the
appearance of new circular dichroism band at 245 and 215 nm, indicating the
copolymerization improves a chiral orientation through aggregation. It is
believed that these organogels are stabilized by the formation of highly-oriented
aggregates.
It is generally known that Low weight molecular mass containing L-lysine
moiety are good organogelators.101 By incorporating L-lysine derivatives into
suitable polymer chains, the newly formed polymers were used to trap organic
solvents. Suzuki and co-workers96, 97 have studied a series of polymer
organogelators from isocyanate-terminated l-lysine-LMWOGs. The isocyanate-
terminated molecules react with alcohol to form urethane and amine.
C12LysNHPPG, C12LysPDMS and C12LysNHPEG were synthesized from
amino-terminated PPG, PDMS and PEG, respectively. C12LysOPEG was
prepared from PEG. C12LysNHPPG show gelation ability for oil, such as
soybean oil, linseed oil and triolein, though C12LysNHPEG does not form gels
in these oils. C12LysPDMS formed in gels in myristic isopropyl ester and cyclic
siloxanes, which are often used in cosmetics but cannot be trapped by
C12LysNHPPG or C12LysNHPEG. Results also show that C12LysOPEG has
good gelling ability in many solvents, which demonstrates that the linking mode
between the LMWOG segment and polymer plays a significant role in the gel
formation.
26
OCNHN
HN
OOO
C12H25
HN C11H23
O
NH
HN
HN
OOO
C12H25
HN C11
H23O
RNH
OOHN
HN
OOOC12H25
HNC11H23
O
NH
ONH33 N
HO
NH43 O
O44 Si
OSi
OSi NH
HN
C12LysNHPPG37
C12LysNHPEG C12LysOPEG C12LysPDMS
Figure 2.6 Chemical structures of polymer gelators containing L-Lysine
derives.
The gelling ability of polymer organogelators depends on the gelation
properties of the LMWOGs segment, and polymer organogelators with l-valine
or isoleucine segments show better gelling abilities in comparison with l-lysine
gelators.102 The results also indicated that an overly-strong interaction decreases
the gelling ability even through gelation requires hydrogen bonding interaction.
As mentioned before, polymer backbones also affect gelation properties.
The effect of polymer backbones on the gelation properties has been studied in
recent years. The results show that the addition of polymer into LMWOGs
improves the solubility of gelators in organic solvents and the gelling ability of
LMWOGs. For example, the minimum gel concentration of L-IleNCO (a kind
of LMWOG) is 10 g/L for dodecane, 20 g/L for squalane and 20 g/L for PEG400.
For L-IlePEG 2000, the minimum gel concentration is 2 g/L for dodecane, 5 g/L
for squalane and 1 g/L for PEG400.99 Polymers are relative cheap in comparison
to LMWOGs, and thus incorporating LMWOGs into polymers to form
organogels is a good method to decrease the cost of gelators.
2.3.4 Self-assembly of block copolymers
A polymer can form organogelators via helical structures and -
interactions, and polymers modified by LMWOGs can also be used to form gels
in organic solvents. Moreover, in rare cases, organogels can result from block
copolymers containing LMWOGs. In recent years, with the development of
control polymerisation, block copolymers have been reported to form gels in
non-aqueous solutions by self-assembly.103-112
27
Block copolymers are macromolecules consisting of at least two chemically
distinct blocks which are linked by covalent bonds. If the blocks are
incompatible, they can form nanophase separated structures from spheres on a
body-(or face) cantered cubic lattice, cylinders on a hexagonal lattice,
bicontinuous channels or alternating lamellae. The phase separation is
determined by two factors, one; enthalpically, which drives block to stretch to
reduce repulsive contacts and two; entropically which affects block elasticity to
minimize chain deformation. The completion of the two factors leads to the
microphase separation. 113, 114
Based on this theory, micelles or microphase separation may occur if the
one block can be well-dissolved but another cannot. The two end blocks may
enter different micro-domains when a triblock copolymer is used at a high
concentration. In other words, different micro-domains can be connected by the
middle blocks, leading to the formation of three-dimensional networks.103, 115-117
Michelle E. Seitz et al.118 have systematically studied the effect of polymer
concentration on micelle morphology and mechanical response of AB and ABA
in B-selective solvents. The equilibrium phase map and micelle volume fraction
have been calculated by self-consistent field theory. The results showed that gel
modulus and energy dissipation increased upon the spherical micelle transiting
to the cylindrical one. Kinetic trapping was obtained from gel relaxation time
and equilibrium phase map and, based on the trapping, cylindrical morphologies
were obtained at dramatically lower polymer fractions.
Usually, high temperature improves the motion of the end blocks, and
destroys the micro-domains formed by end blocks, leading to the dissolution of
organogels. Therefore, most organogels reported are dissolved at high
temperature and the gel-sol transition is reversible upon cooling and heating.119,
120 However, Yuri Jeong et al.105 reported reverse thermal organogels in
chloroform. They synthesized poly(ethylene glycol)-poly(L-alanine-co-L-
phenyl alanine) (PEG-PAF) and found that the block copolymer dissolved in
chloroform at low temperature, but formed gels when the temperature was
increased to 20 oC. Chloroform is not a good solvent for PAF but it is a good
solvent for PEG, and as a result, PEG-PAF forms micelle in the solvent. As
28
temperature increases, the solubility of PEG decreases, and inter-micellar
aggregation forms and gelation occurs.
2.3.5 Organogels via ionic interactions
In recent years, ionic interaction and charge-containing molecules have
attracted considerable attention both in academic and industrial fields. The
interaction has been applied in the formation of organogels, and the development
started from small molecules to polymers.94, 119, 121-125
In 2008, Michel Wathier and Mark W. Grinstaff121 first reported ionic
network from ionic liquids, and the network were formed by reacting between
dichloride salt of the dicationic phosphonium (P2+: 2Cl-) and silver salts of
ethylenediaminetetraacetic acid (EDTA4-). The ionic network is stabilized by
ionic interaction, and results showed novel supramolecular ionic network could
be prepared from multicationic and multianionic molecules.
P P
NN
OO
O
O
O O
O
O
Figure 2.7 Chemical structures of P2+ and EDTA4-.
Based on the network from ionic liquids, M. Ali Aboudzadeh et al.124
developed a simple route to an ionic network from commercially available
(di/tri)carboxylic acids and (di/tri)alkyl amines by ionic and hydrogen
interaction. The ionic polymers displayed a sharp rheological transition and a
great jump in ionic conductivity between 30 and 80°C. The materials also
demonstrated self-healing properties from the combined properties of
supramolecular polymers and ionomers. They also studied the relationship
between chemical structures and properties.123 The ionic networks were from
citric acid and aliphatic diamines, and results showed that ionic network from
different diamines were almost same in their solid and liquid states, but the
network-liquid transition temperatures were affected by strength of ionic bonds
and the mobility of the aliphatic pendant groups.
29
Polymers may contain some acidic or basic groups, it is expected that
polymers can serve as cationic or anionic molecules. Guilhem Godeau et al.122
used a polystyrenylphosphonium as cationic molecules to form ionic networks
with a variety of carboxylic acid. The equilibrium modulus, storage modulus
and loss modulus are dependent on chemical structures of acids and the numbers
of ionic interactions. When the acid is mono fatty carboxylate, the complex can
bounce like a rubber ball, and a bilayer structure forms. Polymers can be also
used as anionic molecules. Xinrong Lin et al126 studied networks from
poly(acrylic acid and phosphonium monocations and dications. The results
show rheological and thermal properties and are greatly dependent on the alkyl
chain length. The ionic network can fully reassemble after disruption by heat or
shear without sacrificing the mechanical properties.
**
P
n
OH
O
7 6
Figure 2.8 Chemical structure of polystyrenylphosphonium polymer.
Liu and co-workers have investigated thermoreversible gels from
polyelectrolyte-surfactant complex poly(N,N-dimethyl-n-octadecylammonium
p-styrenesulfonate)(PSS-DMODA) in toluene, styrene, toluene and o-xylene.127
This work is quite different from the above ionic networks, since it is quite stable
in organic solvents. The concentration was studied in the range of 2.5~ 20 w/v %
and the gel transition temperature was dependent on polymer concentration,
molecular weight and gelled solvent. Scanning electron microscopy (SEM)
confirmed the formation of network structures which result from ionic clustering.
30
SO3H N C18H37
n
Figure 2.9 Chemical structure of polyelectrolyte-surfactant complex.
Figure 2.10 SAXS profiles of polymer organogel formed by ionic interactions.
Copyright 2011, ACS publishing.
Ionic networks can be produced not only from polymer / low molecular
weight molecules but also polymer / polymer. For example, Atsushi Noro et al128
reported the preparation of polymer gel by simple mixing of carboxyl-
terminated telechelic poly(ethyl acrylate) or carboxyl-terminated telechelic
polydimethylsiloxane with PEI. All the polymers used are liquid at room
temperature, and after complexation, the polymer gel shows a reversible gel-to-
sol transition at a temperature of 23 oC. The polymer gels are robust with elastic
modulus G’ about 1 MPa. The same group125 also found that the nanostructural
separation occurred in polymer gels when carboxyl-terminated telechelic
polydimethylsiloxane and PEI were used. The results from FT-IR show that the
31
ionic interactions between the two polymers play an important role in the
nanostructural separation.
As properties and functions of organogels are highly dependent on their
microstructures, the study of microstructures is also very important. As
previously mentioned, although the structures of organogels from low molecular
weight molecules have been well studied, the knowledge about structures of
organogels from macromolecules via self-assemble is limited.
In summary, organogels could be prepared from polymers through the
following methods, physical crosslinking agents, polymers with physical cross-
linkable points, incorporating functional groups into polymers and self-
assembly of block copolymers. The dissolution temperature of polymer
organogelators is usually lower than that of the corresponding LMWOGs. The
dissolution temperature is very important factor for applications in the industrial
field. It is known that good LMWOGs require strong interactions and as a result,
their dissolution temperature is high.129 Organogelators have to be dissolved,
and dissolving temperature depends on the strength of the intermolecular
interactions. The introduction of polymer backbones improves the motion and
solubility of LMWOGs, leading to the decrease of dissolving temperature of
LMWOGs, which is good for industrial applications.74
2.4 Materials for oil spill recovery
2.4.1 Oil spills Oil spills happened around the world from Mexico, Australia all the way to
China, and top 20 major spills are listed in Table 2.2. Frequent oil spills are
becoming one of the major issues to the environments and society. The oil spills
result in serious/irreversible damage to the marine environment, the ecosystem,
and finally the human health. For example, the raise of fuel prices and finally
increase the prices of many aspects of life after oil spills; the cargo vessels must
wait to have the spilled oil cleaned; fishing and tourism have to be closed after
the oil spills. Thus the spilled oils must be removed from water surfaces quickly
and efficiently.
32
Table 2.2 Top 20 major spills.
Shipname Year Location Spill Size
(tonnes)
1 ATLANTIC
EMPRESS 1979 Off Tobago, West Indies 287,000
2 ABT SUMMER 1991 700 nautical miles off Angola 260,000
3 CASTILLO DE BELLVER
1983 Off Saldanha Bay, South Africa 252,000
4 AMOCO CADIZ 1978 Off Brittany, France 223,000
5 HAVEN 1991 Genoa, Italy 144,000
6 ODYSSEY 1988 700 nautical miles off Nova Scotia, Canada 132,000
7 TORREY CANYON 1967 Scilly Isles, UK 119,000
8 SEA STAR 1972 Gulf of Oman 115,000
9 IRENES SERENADE 1980 Bay, Greece 100,000
10 URQUIOLA 1976 La Coruna, Spain 100,000
11 HAWAIIAN PATRIOT
1977 300 nautical miles off Honolulu 95,000
12 INDEPENDENTA 1979 Bosphorus, Turkey 94,000
13 JAKOB MAERSK 1975 Oporto, Portugal 88,000
14 BRAER 1993 Shetland Islands, UK 85,000
15 AEGEAN SEA 1992 La Coruna, Spain 74,000
16 SEA EMPRESS 1996 Milford Haven, UK 72,000
17 KHARK 5 1989 120 nautical miles off Atlantic coast of Morocco 70,000
18 NOVA 1985 Off Kharg Island, Gulf of Iran 70,000
33
19 KATINA P 1992 Off Maputo, Mozambique 67,000
20 PRESTIGE 2002 Off Galicia, Spain 63,000
35 EXXON VALDEZ 1989 Prince William Sound, Alaska, USA 37,000
131 HEBEI SPIRIT 2007 South Korea 11,000
www.itopf.com/information-services/data-and statistics/statistics/
Scheme 2.3 Maps of oil spill locations.
Currently there are some technologies commonly applied for cleaning these
spilled oil, and these technologies include combustion, skimmers, solidifiers,
dispersants and bioremediation. Combustion is usually suitable for a large
amount of oils and all the oils are wasted. Skimmers are designed to recover the
spilled oils and they are effective on high-viscosity oil. But their adverse
effectives on the environment are ignored. Polymer solidifiers were patented for
oil spill clean-up, but it is believed that that they are not very suitable to gelate
the viscosity oils. In recent years, several new materials have been developed to
recover oil spills and these novel materials are phase-selective gelator and
macroporous materials.
2.4.2 Phase selective gelators
It is obviously advantageous to transform these spilled oils (liquids) to
solids, as solids are easy to collect. As mentioned before, low-molecular weight
gelators have been well developed to form organogels. These gelators are called
phase selective gelators, and they have been investigated in the past few years.67,
34
130-136 Santanu Bhattacharya and Yamuna Krishnan-Ghosh first reported phase
selective organogels formed from oil water mixtures.130 The gelator is an amino
acid derivative, N-lauroyl-L-alanine, and it could trap either commercial fuels or
organic solvents selectively from water surface. This gelator could be
solubilized either in solution of ethanol or by heating. It is believed that the
ability of self-assembly to form three-dimensional networks is required for
phase selective gelators. Meanwhile, the presence of lipophilic alkyl chains is
also necessary as it could exclude water. However, the requirements of heating
or using solvents during containing oils on water surface might limit its real-life
application.
Figure 2.11 Phase-selective gelation and diesel recovery from a two phase
system. Reproduced with permission from Ref.131. Copyright 2010, Wiley-
VCH Verlag GmbH & Co. KGaA, Weinheim.
The recovery of oils and recycling of gelators were first studied with low-
molecular-weight phase-selective oil gelators prepared from mannitol and
sorbitol.131 These gelators could gelate a variety of oil and organic solvents with
minimum gelation concentrations from 1.5 to 5%. These formed organogels are
thermo-reversible with gelation transition temperatures at 82-125 °C and 38-
69 °C. The organogels were quite strong and they could even support the water
35
phase. Quantitative recovery of oils from these organogels has been realised by
simple vacuum distillation, and the recovery process is shown in Figure 2.11.
The gelators are easy to be synthesized and can be reused for many times. In the
process of gelation of oil, water soluble co-solvent was used to dissolve these
gelators, which may increase the cost of oil collection.
Another example of phase-selective organogels is based on mannitol.132 The
mannitol-based gelators showed excellent abilities in the gelation of oils from
oil and water mixture, the recovery of absorbed oil and re-usability of the
gelators. These gelators showed high efficiency in the gelation of oil, where a
low concentration of 0.75wt % of oil was reported to form enough strong
organogels. The gelators could gelate oil from pure water surface and even
various aqueous situation. One obvious advantage is that the solvents used to
dissolve these gelators are water immiscible, which could avoid to pollute water
during the oil clean-up procedure.
These gelators have achieved great progress for the real-life application of
oil spill recovery, but there still exists some drawbacks which limit their
widespread application. The drawbacks include the requirements of a large
amount of energy during the recovery of oil from organogels, organic
solvents/heating for the preparation of gelator solution and high concentration
of gelator to obtain enough mechanical strength for collection.
2.4.3 Absorbents
Except phase-selective organogels, various porous materials are also
studied for oil spill recovery. One of these porous materials is membranes, and
functionalised membranes including meshes137, 138 and fabrics139, 140 have
attracted a great deal of attention due to their low cost and high separation
efficiency.141-143 One example is the micro-porous foam membrane for oil spill
recovery.144 As the foam membranes prepared from natural materials, they
showed natural abundance, less-no toxicity stability under a variety of testing
conditions, easy preparation and disposability. Porous membranes are also
reported to separation oil water emulsions.140, 145-150 Based on our understanding,
the application of porous membranes for oil water emulsions separation is
promising as these emulsions usually exhibit low viscosity. While the
36
application of porous membranes for oil spill recovery can be extremely difficult
as both collection and filtration of oil spills are hard.
Figure 2.12 Pumping oil from water surface with hydrophobic porous
materials. Reproduced from Ref. 159 with permission from The Royal Society
of Chemistry.
Porous hydrophobic and oleophilic materials can be efficient absorbents for
oil spill recovery, and these materials can be aerogels,151 xerogels,152 nanowire
membranes,153 porous boron nitride,154 carbon based sponges,155 and functional
polymer sponges.156 They are promising because they can absorb thin and low
viscosity oil completely from water surface. These materials focus on the
sorption capacity, absorption rate, recovery rate and reusability.
One type of absorbents studied for oil spill recovery is phase selective
xerogels.152 One xerogels was prepared from 12-Hydroxystearic acid (12-HSA),
which is well known to be an efficient low molecular weight gelator and can
gelate numerous organic solvents with low gelation concentrations ranging from
0.5% to 2.3% according to solvent types. After evaporation of solvents in these
formed organogels, phase selective xerogels were obtained from 12-HSA
organogels. These xerogels are confirmed to be an effective sorbent materials
which can absorb apolar spilled liquids in aqueous environments. The formation
condition of phase selective xerogels affected the oil absorption efficiency,
where the xerogels from 12-HSA-acetronitrile organogels are efficient than
those from 12-HAS pentane organogels because of the presence of highly
37
branched fibrillar networks. These xerogels are thermo-reversible, and thus be
used as for oils spill recovery and for reusability to form new xerogels for oil
containment and cleanup. The organogels are formed by the trapping solvents in
the microspace in three dimensional networks, the size of pores in these xerogels
should be quite small, and thus they may have a low absorption capacity and
absorption rate. Similar to phase-selective organogels, the reclamation of oils
form organogels requires a large amount of energy.
Figure 2.13 The preparation of phase-selective xerogels for oil spill
recovery.152 Copyright 2013, ACS publishing.
Figure 2.14 PolyHIPEs for oil absorption.157 Reproduced from Ref. 157 with
permission from The Royal Society of Chemistry.
38
PolyHIPEs are a kind of porous materials with macroporous structures
prepared form HIPE templating, and they have been investigated for oil water
separation.157 The HIPEs are stabilized by a low-molecular mass gelators with
polymerisable monomers as the continuous phase and water as disperse phase.
After polymerization of the continuous phase and removal of the water phase
and residue of organic phase, the obtained polyHIPEs were used for oil
absorption. It is noted that these polyHIPEs could absorb a variety of oils and
organic solvents, and that they showed quite high absorption capacity. Moreover,
these polyHIPEs are able to be used for several times. These porous materials
have been confirmed to be an excellent absorbent for oil spill recovery
considering their high absorption capacity and rate, easy preparation and recycle.
However, it should point out that the preparation of the stabilizer can be
complicated and thus the stabilizers should be of high price.
With the development of macroporous materials, some porous materials are
designed for oil spill recovery at harsh conditions. The example is the
marshmallow-like macro-porous gels (MTMS–DMDMS) which were formed
from various alkoxysilanes.158 The copolymerization of various types of
monomers endows a variety of functionalities to these porous materials, as it
offers flexible design of surface chemical properties. The obtained macro-
porous gels exhibited high hydrophobicity and could absorb oils from water
surface. The absorbed oil could be recovered by simple squeezing out. It is noted
that the elastic properties could be preserved over a large temperature range from
liquid N2 all the way to 320 °C, and thus these gels could be used in harsh
environments.
39
Figure 2.15 Superhydrophobic surface of the MTMS–DMDMS marsh for oil
water separation. Reproduced with permission from Ref.158. Copyright 2013,
Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.
Hydrophobic/oleophilic porous materials could be obtained by surface
modification of porous materials. One example is carbon nanotubes (CNTs)
reinforced polyurethane (PU) foam.156 After the modification of these PU
sponges, superhydrophobic and superolephilic materials were obtained, and
these materials showed high water repellence, high selective absorption capacity
to oils. It is reported that these absorbed oils could be recovered by simple
squeezing out, and that the porous materials could be used for 150 times.
These absorbents still exhibit some disadvantages though they are
promising for oil spills recovery. These drawbacks are the high cost of these
absorbents and transport. Considering the shortage of fossil fuels in the future,
the recovery of oil from absorbents is also important. Although the oil recovery
can be achieved by squeezing and distillation, these time-consuming and lengthy
procedures with low durability and require high cost of equipment are
troublesome for their real and commercial application. With the development of
efficient sorbents for recovering spilled oils at various conditions, some
technologies also have been developed to facilitate the application of these
sorbents. A promising method is developed by Shuhong Yu et al. recently, and
the method involves pumping spilled oils from porous hydrophobic/oleophilic
40
materials.159 The new model oil collection equipment could continuously collect
oil from oil water mixture with high efficiency and speed. It is believed that this
method saves sorbents, labour and operation time. As a result, the cost for oil
recovery would decrease sharply. This method has been confirmed in other
hydrophobic materials.155
Figure 2.16 Digital photographs of water and lubricating oil droplets on coated
PU sponge156. Reproduced from Ref. 156 with permission from The Royal
Society of Chemistry.
In conclusion, the reclamation of these spilled oils is of high importance, as
it prevents damaging the environments and saves the precious energy and lives.
Among many technologies developed for oil spill recovery, mechanical method
by sorbents is promising especially with the development of new technology.
The development of sorbents with high efficiency and low price is always
required. The porous materials with macropores are ideal as the pores with large
sizes facilitate the absorption of viscous oils. To decrease the price of recovering
spilled oils, it is required that these sorbents should be reusable and of high
absorption capacity and rates.
2.5 Summary All in all, the formation of HIPEs are reviewed, and it has been found that
bulk polymer organogels are not studied to stabilize HIPEs. Several routes for
the preparation of organogels from polymers have been summarized. It is found
that polymer organogels exhibit a variety of advantages over organogels from
low molecular weight gelator, and these advantages include a low price, easy
preparation and unique properties. These HIPEs are proposed for oil spill
41
recovery and oil water separation since macroporous materials are still required
to be developed for real applications although a variety of porous materials have
been developed for these purposes.
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51
Chapter Three
A New Route to Multiresponsive Organogels from a Block Ionomer via Charge-Driven Assembly
3.1 Abstract We report a novel route to prepare multiresponsive organogels from a triblock
ionomer and an oppositely charged component via charge-driven assembly. The
triblock ionomer is SSEBS with different polystyrene ratios and sulfonation
degrees, and the oppositely charge component is a diblock copolymer PS-b-
P2VP. The formation and responsivenesses of the polymer organogels were
investigated by tube-inversion method and rheological measurement. The
interaction between –SO3H groups of SSEBS and pyridine groups of PS-b-P2VP
was confirmed to be ionic interactions by FTIR. The morphology was studied
with SAXS, and it is believed that the formed ionic complex aggregates to form
spherical cores, which are connected by the middle block of the triblock ionomer
to form organogels. The organogels are responsive to acid, amine and salt as
these chemicals can affect the strength of ionic complexes.
This chapter is reproduced from the article: T. Zhang and Q. Guo. Chem. Commun. 2013, 49,
5076. Reprinted with permission from RSC, copy right 2013.
AUTHORSHIP STATEMENT
1. Details of publication and executive author
Title of Publication Publication details
A new route to prepare multiresponsive organogels from a blockionomer via charge driven assembly
Chemical Communications2013, 49, 5076 5078.
Name of executive author School/Institute/Division ifbased at Deakin; Organisationand address if non Deakin
Email or phone
Qipeng Guo Institute for Frontier Materials [email protected]
2. Inclusion of publication in a thesis
Is it intended to include this publication in a higherdegree by research (HDR) thesis?
Yes If Yes, please complete Section 3If No, go straight to Section 4.
3. HDR thesis author’s declaration
Name of HDR thesis author ifdifferent from above. (If thesame, write “as above”)
School/Institute/Division if basedat Deakin
Thesis title
Tao Zhang Institute for Frontier Materials Polymer organogelsstabilized HIPE organogelsand applications in oil fields
If there are multiple authors, give a full description of HDR thesis author’s contribution to thepublication (for example, how much did you contribute to the conception of the project, the design ofmethodology or experimental protocol, data collection, analysis, drafting the manuscript, revising itcritically for important intellectual content, etc.)I contributed about 75 % to the conception of the project. I designed the methodology andexperimental protocol, collected the data, analysed the data and drafted the manuscript.
I declare that the above is an accurate description ofmy contribution to this paper, and the contributionsof other authors are as described below.
Signatureand date
10/12/2015
4. Description of all author contributions
Name and affiliation of author Contribution(s) (for example, conception of the project, designof methodology or experimental protocol, data collection,analysis, drafting the manuscript, revising it critically forimportant intellectual content, etc.)
Tao Zhang at Institute for FrontierMaterials
Contributed about 75 % to the conception of the project.Designed the methodology and experimental protocol,collected the data, analysed the data and drafted themanuscript.
Prof. Qipeng Guo at Institute forFrontier Materials
He is my supervisor. He contributed about 25 % to theconception of the project and revised the paper critically.
5. Author Declarations
I agree to be named as one of the authors of this work, and confirm:i. that I have met the authorship criteria set out in the Deakin University Research
Conduct Policy,ii. that there are no other authors according to these criteria,iii. that the description in Section 4 of my contribution(s) to this publication is
accurate,iv. that the data on which these findings are based are stored as set out in Section 7
below.
If this work is to form part of an HDR thesis as described in Sections 2 and 3, I furtherv. consent to the incorporation of the publication into the candidate’s HDR thesis
submitted to Deakin University and, if the higher degree is awarded, thesubsequent publication of the thesis by the university (subject to relevantCopyright provisions).
Name of author Signature* Date
Tao Zhang 10/12/2015
Prof. Qipeng Guo 10/12/2015
6. Other contributor declarations
I agree to be named as a non author contributor to this work.
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Prof. Qipeng Guo at Institute forFrontier Materials
Contributed about 25 % to theconception of the project andrevised the paper critically. 10/12/2015
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52
3.2 Introduction Multiresponsive organogels have attracted considerable attention recently
in both academic and industrial fields for their widespread applications from oil
technology to drug delivery, since they are able to respond to external triggers.1,
2 A promising class of responsive organogels is physically crosslinked polymer
networks, including highly stereoregular polystyrene, poly(methyl methacrylate)
as well as poly(ester)s obtained through the formation of helical structures,3-5
end functional polymers via hydrogen bonding,6 -conjugated polymers via -
stacking,7 polymers modified by supramolecular crosslinking agents,8
polyelectrolyte-surfactant complex through ionic clustering,9 and self-
assembled triblock copolymers.10-14 The transient networks are usually either
only responsive to temperature or time-consuming to form. To satisfy a wide
range of potential applications, fast formation of multireponsive organogels
from polymers is needed.
Previous studies performed by our group have demonstrated that interaction
between acidic and alkaline polymers leads to precipitation in organic solvent.15,
16 In order to prevent the macroscopic separation, diblock copolymers containing
a solvophilic block are used. As a result, microphase separation structure forms
with a solvophobic core stabilized by a soluble block.17, 18 When triblock
copolymers containing two acidic/alkaline end blocks are used instead of
diblock copolymers and when polymer concentration is high enough, the two
end blocks will enter different solvophobic cores stabilized by the middle
soluble block. Thus the cores can be connected with each other by the middle
block, resulting in formation of gel. Since the ionic interaction between the end
blocks of triblock copolymer and the opposite charged polymer can be affected
by external environments, gels formed are usually responsive. Although this idea
has been recently applied in preparation of hydrogel,19, 20 to the best of our
knowledge, preparation of organogels has not been reported based on the idea.
Herein we report multiresponsive organogels based on charge-driven
assembly between a block ionomer and a diblock copolymer in organic solvents
for the first time. The block ionomer, namely SSEBS, consists of two acidic
sulfonated polystyrene blocks and a neutral solvophilic EB block, and the
diblock copolymer PS-b-P2VP contains an oppositely charged poly(2-
53
vinylpyridine) (P2VP) block. The work demonstrates a novel approach to
multiresponsive organogels through charge-driven assembly, and the resultant
organogels are responsive to acid, amine and salt.
3.3 Experimental section 3.3.1 Materials and preparation of samples
Materials. PS-b-P2VP (PS(57000)-b-P2VP(57000), Mw/Mn = 1.08) was
purchased from Polymer Source, Inc., while SEBS with different molecular
weights and polystyrene contents were obtained from Sigma-Aldrich Co.
Synthesis of SSEBS. SSEBS was prepared by sulfonation of SEBS with
freshly prepared acetyl sulfonate according to literatures.21, 22 Typically, acetyl
sulfonate was prepared by adding 7.63 mL of acetic anhydride and 2.8 mL of
concentrated sulphuric acid to 20 mL of DCE, respectively (Please note the
solution should be cooled to 0 °C during the preparation). 30 g of SEBS was
dissolved into 300 mL of DCE till completely dissolved, and then required
amount of acetyl sulfonate was added to the solution under nitrogen protection.
After reacting at 50 °C for about 4 hours, 10 mL of isopropanol was injected to
stop the reaction. The solution was dropped into boiling water slowly to remove
DCE, and the obtained SSEBS was washed with cold deionized water till the pH
became neutral. The SSEBS was dried at room temperature under vacuum.
Preparation of organogels. Polymer organogels were prepared by
neutralization of SSEBS with PS-b-P2VP. SSEBS was dissolved into a mixed
solvent of toluene and methanol (98/2, v/v) to obtain 5 % (w/w) SSEBS solution,
and then neutralized with 5 % (w/w) PS-b-P2VP solution in toluene. After
addition of PS-b-P2VP solution into SSEBS solution, the mixture was stirred
acutely. Gels with different -SO3H groups/2-vinylpyridine ([-SO3H]/[2VP])
ratios were prepared by addition of different amount of PS-b-P2VP solution into
1 mL of SSEBS solutions.
3.3.2 Rheological measurements
Gels were prepared by mixing together two separated solutions containing
5 % (w/w) of SSEBS and 5 % (w/w) of PS-b-P2VP. After stirred for 25 seconds,
the gels were loaded on rheometer and measured at 25 °C. Strain sweep at 1 and
54
10 rad s-1 were carried out before the frequency sweep with a shear strain of 1 %.
The gap was set at 52 m during all the measurements.
3.3.3 Fourier-transform infrared (FTIR) spectroscopy
FTIR spectra were recorded with a Bruker Vertex 70 FTIR spectrometer.
To prepare samples for FTIR measurements, THF solution of SSEBS was
dropped onto KBr disk. For SSEBS/PS-b-P2VP organogels, newly formed
organogels after mixing the solutions of SSEBS in toluene/methanol (98/2, w/w)
and PS-b-P2VP in toluene were dropped on KBr disks. The solvents were
evaporated in fume hood for 1 day and the disks were further dried under
vacuum at 40 °C before measurement. The spectra were obtained by an average
of 64 scans in the wave number range of 600–4000 cm-1 at a resolution of 4 cm-
1.
3.3.4 Small-angle X-ray scattering (SAXS)
SAXS experiments were conducted at the Australian Synchrotron on the
small/wide-angle X-ray scattering beam-line. The samples were put into 1.0 mm
quartz capillaries. The background correction was carried out by measuring the
scattering of an empty capillary and correcting for sample absorption.
3.4 Results and discussion 3.4.1 The preparation of SSEBSs
The triblock ionomer SSEBS was prepared from a triblock copolymer
SEBS, and polystyrene ratios of SEBS as well as sulfonation degrees of
the corresponding SSEBSs are listed in Table 3.1. The sulfonation degrees
of SSEBS were determined by titration, and calculated by the following
equation.
Sulfonation degree = moles of sulfonic acid / moles of styrene ×100%.
It should note that the sulfonation degrees of SSEBS are an average
value due to the existence of deviation in titration. Moreover, it has been
report that sulfonation degrees vary with calculation methods.23
The triblock ionomers SSEBS can be dissolved into certain solvents
including THF, toluene/methanol, xylene/methanol, toluene/DMF or
55
xylene/DMF. Toluene and xylene are the good solvents for SEBS, but after
grafting –SO3H groups onto the polystyrene blocks, they cannot be dissolved
into these solvents, because of the existence of hydrogen bonding between –
SO3H groups. The hydrogen interaction becomes weak with the incorporation
of methanol or DMF into solvents, and finally SSEBS can be dissolved totally.
The viscosities of SEBS1 solutions (5 % in toluene/methanol) are much higher
than the SEBS2 and SEBS3 solutions, which may be ascribed to the
entanglement of the middle EB blocks of SSEBSs at the high concentration.
Table 3.1 The SSEBS prepared from various SEBS.
SEBS Polystyrene contents SSEBS Sulfonation degrees
1 SEBS1 28% SSEBS1 17.9%
2 SEBS2 42% SSEBS2 13.3%
3 SEBS3 68% SSEBS3 11.6%
4 SSEBS4 14.3%
5 SSEBS5 19.1%
3.4.2 The formation of organogels
The organogels formed in 20~30 seconds upon mixing individual solutions
of two components, which was confirmed by the tube inversion method.24 All
organogels are slightly yellowish without visible heterogeneity, indicating that
no macrophase separation occurs. The fast reaction is understandable
considering the high reaction rate between -SO3H groups of SPS blocks and
pyridine groups of PS-b-P2VP. Experiments also showed that organogels were
also formed from the reaction of SSEBS with PEI, PPI dendrimers, benzoxazine
molecules, –NH2 modified inorganic nanoparticles and other block copolymers
including PMMA-b-P2VP, PCL-b- P2VP and so on. In this chapter, diblock
copolymer PS-b-P2VP was chosen as the oppositely charge component to
facilitate the investigation of the morphology.
The formation of organogels was further confirmed by rheological
measurement. Dynamic frequency/strain measurements of organogels were
carried out after mixing SSEBS1 and PS-b-P2VP solutions in 20-30 seconds.
The results for SSEBS1/PS-b-P2VP organogels are shown in Figure 3.1. Figure
56
3.1(a) shows the elastic moduli (G’) and viscous moduli (G”) of SSEBS1/PS-b-
P2VP organogels with [-SO3H]/[2VP] = 1/1 as a function of strain at 1 rad s-1
and 10 rad s-1, and it can be seen that the extent of the linear viscoelastic regime
ranges to about 100%. The large value of the strain may result from stretch-
ability of the middle EB blocks in toluene. Figure 3.1 (b) demonstrates the G’
and G” of SSEBS1/PS-b-P2VP organogels as a function of shear frequency. For
SSEBS1/PS-b-P2VP organogels with [-SO3H]/[2VP] ratios 1/1, 1/2 and 1/3, the
G’ are higher than the corresponding G” at all frequency from 1 to 100 rad s-1,
indicating the rubber-like behavior in the range.6 All the G’ increase with
increasing amount of PS-b-P2VP. For [-SO3H]/[2VP] = 1/1 organogel, the
elastic modulus is only about 300 Pa at 1 rad s-1, while it increases to over 1400
Pa for organogel with [-SO3H]/[2VP] = 1/3. It is also worth noticing that all the
SSEBS1/PS-b-P2VP organogels are “viscoelastic gels”, since G’ and G” do not
show a real plateau in the experimental frequency range but a slight increase
with frequency. Therefore, they may have a crossover in certain experimentally
inaccessible range and a finite relaxation time is often observed.25
100 101 102
102
103
1 rad s-1
10 rad s-1
Dyn
amic
Mod
ulus
(Pa)
Strain (%)
G' G"
(a)
1 10 100
102
103
1 / 1 1 / 2 1 / 3
Dyn
amic
Mod
ulus
(Pa)
Frequency (rad s-1)
G' G"[SO3H]/[2VP]
(b)
Figure 3.1 Dynamic moduli G’ (filled) and G” (hollow) of organogels (a) with
[-SO3H]/[2VP] = 1/1 as a function of strain and, (b) with [-SO3H]/[2VP] = 1/1
1/2 and 1/3 as a function of oscillatory shear frequency.
57
The SSEBS1/PS-b-P2VP, SSEBS2/PS-b-P2VP and SSEBS4/PS-b-P2VP
organogels were also studied by frequency measurements, and the results are
shown in Figure 3.2. All the G’ are higher than the corresponding G”, indicating
the formation of three-dimensional networks in organic solvents. Therefore,
polymer organogels were obtained from all the triblock ionomers and diblock
copolymer. It is noted that with the increase of polystyrene contents in the
SSEBSs, the strengths of organogels decrease obviously. The decrease of the
organogel strengths could be partly ascribed to the high viscosity of the
corresponding SSEBSs solutions.
100 101 102100
101
102
103
SSEBS1/PS-b-P2VP SSEBS2/PS-b-P2VP SSEBS4/PS-b-P2VP
Dyn
amic
Mod
ulus
(Pa)
Frequency (rad s-1)
G' G"
Figure 3.2 Dynamic moduli G’ (filled) and G” (hollow) of SSEBS1/PS-b-P2VP,
SSEBS2/PS-b-P2VP and SSEBS4/PS-b-P2VP organogels with [-SO3H]/[2VP]
= 1/1 as a function of oscillatory shear frequency.
3.4.3 The ionic interaction between SSEBS and PS-b-P2VP
As a type of block ionomer containing -SO3H groups on the two end blocks,
SSEBSs may form organogels in certain solvents.26 Considering this, the control
experiment of mixing SSEBS solutions with same amount of toluene shows that
SSEBS cannot form organogels in the mixed solvents. PS-b-P2VP is a diblock
copolymer, which may form organogels at certain circumstance.27 So the
58
experiment of blending PS-b-P2VP in same mixed solvents has been carried out,
and result demonstrates that no gel formed. So it is safe to assume that the
interaction between SSEBS and PS-b-P2VP plays a significant role in the
formation of organogels.
1680 1640 1600 1560
Abs
orba
nce
(a.u
.)
Wavenumber (cm-1)
1/0
1/0.5
1/1
1/2
1/3
0/11590
16251637
(a) [SO3H]/[2VP]
1200 1120 1040 960 880
Abso
rban
ce (a
.u.)
Wavenumber (cm-1)
0/1
1/3
1/2
1/1
1/0.5
1/0
993
1033
1038
(b) [SO3H]/[2VP]
Figure 3.3 FTIR spectra of SSEBS1/PS-b-P2VP gels in the range of (a) 1700 -
1540 cm-1 and (b) 1250-880 cm-1.
To reveal the interaction between SSEBS and PS-b-P2VP in the organogels,
FTIR spectroscopy was applied to organogel after evaporation of solvents. In
experiment, SSEBS1/PS-b-P2VP organogels were used and the results are given
in Figure 3.3. It is well-known that the characteristic stretching absorptions of
pyridine rings on P2VP are at 1590 and 993 cm-1, which are also the most
affected bands after reaction with acid.28 According to Figure 3.3(a), the
intensity of carbon-nitrogen stretching vibration of unprotonated pyridine rings
at 1590 cm-1 decreases with the addition of SSEBS. Meanwhile, the new
absorption bands at 1625 and 1637 cm-1 appear, which is the characteristic
absorptions of protonated pyridine ring on P2VP. The disappearance of pyridine
absorption and the appearance of new peaks can be attributed to the formation
of pyridnium replacing pyridine. This is also confirmed by another characteristic
absorption at 993 cm-1 in Figure 3.3(b), which becomes less pronounced upon
59
the addition of -SO3H groups. Meanwhile, a peak at 1038cm-1, which can be
assigned to symmetric stretching vibration of -SO3H,29 has been replaced by a
peak at 1033 cm-1. Therefore the evidence of ionic bonding between pyridine
groups on PS-b-P2VP and -SO3H groups of SPS blocks is evident.
FTIR results also show that although new peaks at 1625 and 1637 cm-1
become more pronounced with the addition of increasing amount of SSEBS
added, the absorptions at 1590 cm-1 and 993 cm-1 can still be observed even [-
SO3H]/[2VP] ratio reaches to 1/0.5, which demonstrates that there are still free
pyridine groups in the gel. This reasonably explains the rheological results. As -
SO3H groups and pyridine groups cannot react completely according to
stoichiometry, the more addition of PS-b-P2VP means more ionic bonds formed
between SSEBS and PS-b-P2VP, and thus elastic modulus increases.
1680 1640 1600 1560
Abs
orba
nce
(a.u
.)
Wavenumber (cm-1)
1/0
0/1
1/2
1/1
2/1
1637 cm-11625 cm-1
[S03H]/[2VP]
Figure 3.4 FTIR spectra of SSEBS1/PS-b-P2VP mixtures in the range of 1700 -
1560 cm-1.
The FTIR control experiment of the mixtures of SSEBS1 and PS-b-P2VP
was also conducted. In the control experiment, SSEBS1/PS-b-P2VP mixture
was prepared by neutralization of SSEBS with PS-b-P2VP. SSEBS was
dissolved into THF to obtain 5 % (w/w) SSEBS solution, and then neutralized
with 5 % (w/w) PS-b-P2VP of THF solution. After addition of PS-b-P2VP
60
solution into SSEBS1 solution, the mixture was stirred acutely. After removal of
solvents, the mixture was sandwiched in KBr for FTIR measurement. The results
are shown in Figure 3.4, and the results confirmed ionic interaction between
SSEBS1 and PS-b-P2VP, which is same as that from SSEBS1/PS-b-P2VP
organogels. So apparently, the gelation should be induced by ionic interaction.
3.4.4 The microphase separation in organogels
To elicit the structures of the gels, SAXS measurements were performed for
all the samples at room temperature, and their profiles are shown in Figure 3.5(a).
In order to avoid solvent effects on the morphology, the same solvents were used
for all samples, and the SAXS data were corrected for background scattering.
0.5 1.0 1.5 2.0
Inte
nsity
(a.u
.)
q 1/nm
1/3
1/2
29.5 nm
29.5 nm
1/1
1/0.5
27.9 nm
31.2 nm1/0
(a)SO3H /[2VP]
0.2 0.4 0.6 0.8 1.0
q (1/nm)
Inte
nsity
(a.u
.)(b)
SO3H /[2VP] = 1/2
Figure 3.5 (a) SAXS profiles for SSEBS1/PS-b-P2VP organogels with different
[-SO3H]/[2VP] ratios, (b) SAXS curve for gel with [-SO3H]/[VP] = 1/2. The full
line in (b) represents the fitting curve according to a core-shell model with a
hard-sphere radius of 12.5 nm.
It has been reported that microphase separation occurs in SSEBS in both
solid state and swelled in paraffinic oil.26 While in our experiment, there is no
obvious peak for SSEBS solution, demonstrating no microphase separation in
SSEBS1 solution. This result confirmed that SSEBS was well dissolved into
61
these mixed solvents. However, well-defined peaks are observed for
SSEBS1/PS-b-P2VP gel, and q values are dependent on the amount of PS-b-
P2VP added. The average distance between neighboring domain is 31.2 nm for
gel with [-SO3H]/[2VP] = 1/0.5. For the gel with [-SO3H]/[2VP] = 1/1, the peak
becomes more obvious and the average distance between two neighboring
domains can be estimated to be 27.9 nm. This average distance increases to 29.5
nm when [-SO3H]/[2VP] increases to 1/2, and the peak becomes narrower,
indicating more ordered structures in the gel. A further increase [-SO3H]/[2VP]
to 1/3 does not increase the average distance any more, and the peak becomes
wider, demonstrating less ordered nanostructures in the organogel comparing
with that for [-SO3H]/[2VP] = 1/2.
Quantitative information on size and distribution of the spherical
microdomains was obtained by fitting the SAXS data to a core-shell model using
a structure factor of hard spheres and a form factor of star polymers.26 The
measured SAXS scattering curves were corrected for background scattering.
Fitting was done using the SASfit program downloaded from
http://kur.web.psi.ch/sans1/SANSSoft/sasfit.html. Toluene is good solvent for
poly(ethylene-ran-butylene) (EB) block and polystyrene blocks, while
protonated poly(2-vinylpyridine) (P2VP) becomes less soluble. As a result,
protonated P2VP blocks and SPS blocks tend to form cores, EB blocks and PS
block forming corona to stabilize solvophobic cores. Each P2VP block in gel
with [-SO3H]/[2VP] reacting with 10 SPS blocks can be calculated from
molecular weights of SPS and P2VP blocks, and sulfonation degree of SEBS.
EB blocks on SSEBS are like branches emanating from reacted P2VP block, and
with the aggregation of the reacted P2VP blocks, the number of EB branches
increases. The structure is similar to that of star polymers. For star polymer,
Dozier30 has developed a scattering function as:
With
In which I0, RG, á, í and î stand for scale parameter, radius of gyration,
scale parameter for fractal, exponential damping length in mass fractal and Flory
exponent, 3/5 in good solvent, 1/2 in theta solvent (i.e. ì = 2/3 to 1) term
62
respectively.
For gel from charge-driven assembly, hard sphere shapes are always
found.19, 20 So scattering function for hard sphere model is used as:
Where RHS and fp stand for radius of hard sphere and volume fraction,
respectively.
Scheme 3.1 Scheme of nanoscaled gel based on SAXS fitting, radius of gyration
RG = 35 nm, radius of hard sphere RHS = 12.5 nm and distance between
microdomains L = 29.5 nm.
The neighbouring microdomains are symmetrical, and the relationship
2(RHS - dc) = L - (RG - RHS) can be obtained. As a result, radius of core dc = 9 nm
can be calculated (Scheme 3.1). Figure 3.5(b) shows the fitting curve obtained
with a hard-sphere radius of 12.5 nm for [-SO3H]/[2VP] = 1/2 gel, which is well
agreeable with the experimental data. On average about 10 SPS blocks reacted
with each P2VP block through ionic interaction between -SO3H groups and
pyridine groups. After the reaction, the newly formed ionic complexes become
less soluble and tend to form cores, and the radius (dc) of the core is 9 nm. EB
blocks and PS blocks form shells to connect and stabilize the solvophobic cores.
The microphase separation occurs not only in the state organogels but also
in the solid dried from these organogels. From the SAXS profiles listed in Figure
3.6, microphase separation can be found in all the SSEBS1/PS-b-P2VP solids
63
dried from organogels with different [-SO3H]/[2VP] ratios. Due to the poor
resolution of the high order, it is impossible to obtain the microstructures of the
solids. The poor resolution is always observed in neutralized block ionomer
SSEBS systems.
The assumption of the charged parts and the middle EB blocks forming the
cores and shells respectively was confirmed by the SAXS results from these
samples by the evaporation of solvents of the corresponding SSEBS1/PS-b-
P2VP organogels. The distances between the neighboring domains ranges from
16.4 to 19.3 nm, which is smaller than those of the corresponding organogels.
This because these polymer organogels got de-swollen with the evaporation of
solvents and in microstructures, the bridges between charged cores shrink and
shortened. As a result, the distances between the charged cores decreased. The
results further confirmed the assumption. Moreover, all the SAXS peaks become
wide after the transition of polymer organogels into dried gels, indicating less
ordered structures were formed during the drying process. This can be explained
by the fact that these polymer organogels were dried at room temperature, which
is not the suitable temperature for the polymer reaching balance.
1:0.5
18.6 nmDried from organogel
1:1
16.4 nm
1:2
19.3 nm
0.0 0.5 1.0 1.5 2.0
1:3
Inte
nsity
(a.u
.)
q (nm-1)
17.0 nm
Figure 3.6 SAXS profiles for SSEBS1/PS-b-P2VP solids dried from the
corresponding organogels with [-SO3H]/[2VP] = 1:0.5, 1:1, 1:2 and 1:3.
64
The morphology of polymer organogels from SSEBS with different
sulfonation degrees and EB blocks fractions are also investigated and the results
are listed in Figure 3.7. Figure 3.7(a) shows SSEBS with different EB block
fractions and similar sulfonation degrees, and as mentioned before, the
sulfonation degrees may be affected by titration. Thus it is possible to study the
effect of EB block fractions on the morphology of polymer organogels, and it
can be calculated from the first peak location that the average distance between
the neighboring domains increases with the decreasing of EB fraction. From
Figure 3.7(b), more ordered structures can be obtained with the increase of
sulfonation degrees in SSEBS. According to the previous modeling, it is
believed that one P2VP block reacts with less SPS blocks of SSEBS with more
–SO3H groups on each SPS block, and as a result, the cores become evenly and
ordered.
SSEBS4
Organogels [SO3H]/[2VP]=1:1
SSEBS2
0.0 0.5 1.0 1.5 2.0
SSEBS1
Inte
nsity
(a.u
.)
q (nm-1)
SSEBS3
Organogels[SO3H]/[2VP] = 1
SSEBS 4
0.0 0.5 1.0 1.5 2.0
SSEBS 5
Inte
nsity
(a.u
.)
q (nm-1)
Figure 3.7 SAXS profiles for (a) SSEBS1/PS-b-P2VP, SSEBS2/PS-b-P2VP, and
SSEBS4/PS-b-P2VP organogels, and (b) SSEBS3/PS-b-P2VP, SSEBS4/PS-b-
P2VP SSEBS5/PS-b-P2VP organogels with [-SO3H]/[2VP] = 1:1.
3.4.5 The responsiveness of polymer organogels to acid, base and salt
The formation of polymer gel is driven by ionic interaction, external
65
stimulates such as pH and salt which can affect the strength of ionic interaction
and tune the properties of the gels.19, 20 However SSEBS/PS-b-P2VP gels formed
in nonpolar organic solvent, in which acid, base and salt usually do not dissolve.
Thus common acids, bases and salts are not suitable for dissolving the gels. In
our experiments, acetic acid, triethylamine and their salt were selected to tune
the properties of the gels, since all of them are liquid at room temperature.
The responsiveness of organogel with [-SO3]/[2VP] = 1 was investigated by
rheological measurements when a very small amount of acid, base or salt was
added to pre-formed organogels. Dynamic frequency measurements of
organogels with 0.5% (v/v) acid, base or salt were also carried out on a
rheometer (ARES, TA), and the measurements were usually carried out after
mixing the pre-formed organogels and acid, base or salt in 10-20 seconds. The
results are shown in Figure 3.8. From the results, it is observed that after addition
of 0.5% (v/v) of acid, amine or salt to the gel, though the elastic modulus (G’)
remains larger than the corresponding viscous modulus (G”) in each sample, the
corresponding moduli decreased dramatically compared with the pre-formed
organogel. Indeed, it is observed that elastic modulus for the pre-formed
organogel is about 300 Pa at 1 rad s-1, while the modulus turns to be less than
100 Pa after addition of acid or salt and less than 200 Pa for the addition of
triethylamine.
1 10 100101
102
103
G' G''acidbasesalt
Dyn
amic
Mod
ulus
(Pa)
Frequency (rad s-1) Figure 3.8 Dynamic moduli G’(filled) and G”(hollow) of organogels with 0.5%
of acid, amine and salt as a function of oscillatory shear frequency.
66
When more acid, base or salt was added to the pre-formed SSEBS1/PS-b-
P2VP gels, for example, upon addition of 2% (v/v) of acetic acid, triethylamine
or their salt, the gels transform into a fluid which can flow freely like water.
As the organogels are stabilized by less soluble cores from ionic complexes
formed between -SO3H groups and pyridine groups, any factor changing the
strength of the ionic complex will affect the properties of the organogels. Acetic
acid, triethylamine, and their salt competitively react with either -SO3H groups
or pyridine groups, which weakens the strength of ionic interaction between SPS
and P2VP. As a result, moduli of gels decrease. When the ionic complex
becomes weak enough, the three-dimensional network lapses, leading to the free
flow of organogels.
These charge-driven assembled organogels were also investigated initially
for oil spill recovery. Herein, 20% of SSEBS solutions and 5% of PEI were
added to a toluene-water mixture individually, and after stirring for a few
seconds organogels were formed as expected. This showed that the polymer
organogels can be formed even with the existence of water, and that the gelation
of these polymer organogels are phase-selective. These organogels were not
totally transparent any more, with a small amount of tiny water droplets trapped
in these polymer gels, perhaps because the ionic domains are somewhat
hydrophilic. These organogels floated on the water surface, and the as-obtained
organogel were strong enough to be collected mechanically. With addition of
acid, alkaline or salt, the organogel transformed into liquids, and the absorbed
toluene can be recovered by vacuum distillation. The SSEBS could be re-used
again by dissolving into toluene. In experiments, xylene, diesel, engine oil and
gasoline were used to replace toluene, and similar results were obtained.
In comparison with the reported organogels for oil spill recovery, the
charge-driven assembly polymer organogels are obvious cheap than these phase
selective organogels from low molecular weight gelators. The addition of
external substances triggers the solid organogels into liquids, which facilitate
transportation and heat transfer during distillation. However, there are a variety
of drawbacks for the practical application. The viscosity of SSEBS solutions are
too high, which may not mix viscous crude oil well. The polymer organogels
67
also absorb a certain amount of water as the ionic domains can be hydrophilic,
which affects the quality of oils absorbed. Moreover, as a small amount of co-
solvents is required to dissolve SSEBS and these solvents are left in the water,
which may limit the real applications of this polymer organogels for oil spill
recovery.
3.5 Conclusions A new approach has been developed for the preparation of multiresponsive
organogels through charge-driven assembly of two organic solvent-soluble
components: a block ionomer consisting of a solvophilic middle block and two
acidic end blocks and oppositely charged components. Charge-driven assembled
polymer organogels have a high reaction rate, and could be formed in 20-30
seconds upon mixing the two individual solutions. The reacted SPS blocks and
P2VP blocks assemble to form spherical domains which are connected by EB
blocks to form organogels. The organogels can respond to acid, amine and salt
as these chemicals weakens the strengths of ionic interactions in these polymer
organogels. The polymer organogels can be formed with the existence of water,
showing phase selective gelation property, but some drawbacks still limit the
real applications for oil spill recovery. The multiresponsiveness and the
tunability in type and length of each of the individual component endow a
variety of properties of these novel organogels.
3.6 References 1 M. Suzuki and K. Hanabusa, Chem. Soc. Rev., 2010, 39, 455.
2 N. M. Sangeetha and U. Maitra, Chem. Soc. Rev., 2005, 34, 821.
3 C. Daniel, D. Alfano, G. Guerra and P. Musto, Macromolecules, 2003, 36,
1713.
4 A. Saiani and J.-M. Guenet, Macromolecules, 1997, 30, 966.
5 A. Pich, N. Schiemenz, V. Boyko and H.-J. P. Adler, Polymer, 2006, 47,
553.
6 A. Noro, M. Hayashi, A. Ohshika and Y. Matsushita, Soft Matter, 2011, 7,
1667.
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7 K. T. Kim, C. Park, G. W. M. Vandermeulen, D. A. Rider, C. Kim, M. A.
Winnik and I. Manners, Angew. Chem., 2005, 117, 8178.
8 M. Suzuki, S. Owa, H. Shirai and K. Hanabusa, Macromol. Rapid Commun.,
2005, 26, 803.
9 Y. Liu, A. Lloyd, G. Guzman and K. A. Cavicchi, Macromolecules, 2011,
44, 8622.
10 M. Nguyen-Misra and W. L. Mattice, Macromolecules, 1995, 28, 1444.
11 K. J. Henderson and K. R. Shull, Macromolecules, 2012, 45, 1631.
12 V. K. Kotharangannagari, A. Sánchez-Ferrer, J. Ruokolainen and R.
Mezzenga, Macromolecules, 2012, 45, 1982.
13 J. R. Quintana, E. Hernáez and I. Katime, J. Phys. Chem. B, 2001, 105, 2966.
14 M. E. Seitz, W. R. Burghardt, K. T. Faber and K. R. Shull, Macromolecules,
2007, 40, 1218.
15 N. Hameed, J. Liu and Q. Guo, Macromolecules, 2008, 41, 7596.
16 J. Huang, X. Li and Q. Guo, Eur. Polym. J., 1997, 33, 659.
17 N. V. Salim, N. Hameed and Q. Guo, J. Polym. Sci., Part B: Polym. Phys.,
2009, 47, 1894.
18 N. Hameed, N. V. Salim and Q. Guo, J. Chem. Phys, 2009, 131, 214905.
19 M. Lemmers, J. Sprakel, I. K. Voets, J. v. d. Gucht and M. A. C. Stuart,
Angew. Chem., 2010, 122, 720.
20 J. N. Hunt, K. E. Feldman, N. A. Lynd, J. Deek, L. M. Campos, J. M. Spruell,
B. M. Hernandez, E. J. Kramer and C. J. Hawker, Adv. Mater., 2011, 23,
2327.
21 R. A. Weiss, A. Sent, C. L. Willis and L. A. Pottick, Polymer, 1991, 32,
1867.
22 S. Wu, S. Peng, N. Hameed, Q. Guo and Y.-W. Mai, Soft Matter, 2012, 8,
688.
23 L. Zhang, B. C. Katzenmeyer, K. A. Cavicchi, R. A. Weiss and C.
Wesdemiotis, ACS Macro Letters, 2013, 2, 217.
24 A. R. Hirst and D. K. Smith, Langumir, 2004, 20, 10851.
25 Z. Chu and Y. Feng, Chem. Commun., 2011, 47, 7191.
26 X. Lu, W. P. Steckle, Jr. and R. A. Weiss, Macromolecules, 1993, 26, 6525.
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27 I. W. Hamley, J. A. Pople, C. Booth, Y.-W. Yang and S. M. King, Langmuir,
1998, 14, 3182.
28 O. Ikkala, J. Ruokolainen, G. t. Brinke, M. Torkkeli and R. Serimaa,
Macromolecules, 1995, 28, 7088.
29 J. Wang, W. H. de Jeu, P. Müller, M. Möller and A. Mourran,
Macromolecules, 2012, 45, 974.
30 W. D. Dozier, J. S. Huang and L. J. Fetters, Macromoelcules, 1991, 24,
2810.
70
Chapter Four
HIPE Organogels Prepared from Charge-Driven Assembled Polymer Organogels
4.1 Abstract We reported for the first time that HIPE organogels were stabilized by pre-
formed polymer organogels, and the polymer organogels were prepared from a
triblock ionomer, SSEBS, and PPI dendrimers via charge-induced assembly. The
formations of polymer organogels and HIPE organogels were studied by tube-
inversion method and rheological measurements. The microphase separations in
the polymer organogels and HIPE organogels were investigated by SAXS. The
morphology of these polymer organogels are determined by the type of PPI
dendrimers and sulfonation degrees as well as polystyrene contents of SSEBS.
The morphologies of the HIPE organogels are mainly affected by polystyrene
contents of SSEBS, with little changes with salt concentration, type of PPI
dendrimers and sulfonation degrees of SSEBS.
This chapter is reproduced from the article: T. Zhang and Q. Guo. Chem. Commun. 2013, 49,
11803. Reprinted with permission from RSC, copy right 2013.
AUTHORSHIP STATEMENT
1. Details of publication and executive author
Title of Publication Publication details
High internal phase emulsion (HIPE) organogels prepared fromcharge driven assembled polymer organogels
Chemical Communications2013, 49, 11803 11805.
Name of executive author School/Institute/Division ifbased at Deakin; Organisationand address if non Deakin
Email or phone
Qipeng Guo Institute for Frontier Materials [email protected]
2. Inclusion of publication in a thesis
Is it intended to include this publication in a higherdegree by research (HDR) thesis?
Yes If Yes, please complete Section 3If No, go straight to Section 4.
3. HDR thesis author’s declaration
Name of HDR thesis author ifdifferent from above. (If thesame, write “as above”)
School/Institute/Division if basedat Deakin
Thesis title
Tao Zhang Institute for Frontier Materials Polymer organogelsstabilized HIPE organogelsand applications in oil fields
If there are multiple authors, give a full description of HDR thesis author’s contribution to thepublication (for example, how much did you contribute to the conception of the project, the design ofmethodology or experimental protocol, data collection, analysis, drafting the manuscript, revising itcritically for important intellectual content, etc.)I contributed about 85 % to the conception of the project. I also designed the methodology andexperimental protocol, collected the data, analysed the data and drafted the manuscript.I declare that the above is an accurate description ofmy contribution to this paper, and the contributionsof other authors are as described below.
Signatureand date
10/12/2015
4. Description of all author contributions
Name and affiliation of author Contribution(s) (for example, conception of the project, designof methodology or experimental protocol, data collection,analysis, drafting the manuscript, revising it critically forimportant intellectual content, etc.)
Tao Zhang at Institute for FrontierMaterials
Contributed about 85 % to the conception of the project.Designed the methodology and experimental protocol,collected the data, analysed the data and drafted themanuscript.
Prof. Qipeng Guo at Institute forFrontier Materials
He is my supervisor. He contributed about 15 % to theconception of the project and revised the paper critically.
5. Author DeclarationsI agree to be named as one of the authors of this work, and confirm:i. that I have met the authorship criteria set out in the Deakin University Research
Conduct Policy,
ii. that there are no other authors according to these criteria,iii. that the description in Section 4 of my contribution(s) to this publication is
accurate,iv. that the data on which these findings are based are stored as set out in Section 7
below.
If this work is to form part of an HDR thesis as described in Sections 2 and 3, I furtherv. consent to the incorporation of the publication into the candidate’s HDR thesis
submitted to Deakin University and, if the higher degree is awarded, thesubsequent publication of the thesis by the university (subject to relevantCopyright provisions).
Name of author Signature* Date
Tao Zhang 10/12/2015
Prof. Qipeng Guo 10/12/2015
6. Other contributor declarations
I agree to be named as a non author contributor to this work.
Name and affiliation of contributor Contribution Signature* anddate
Prof. Qipeng Guo at Institute forFrontier Materials
Contributed about 15 % to theconception of the project andrevised the paper critically. 10/12/2015
* If an author or contributor is unavailable or otherwise unable to sign the statement ofauthorship, the Head of Academic Unit may sign on their behalf, noting the reason for theirunavailability, provided there is no evidence to suggest that the person would object to beingnamed as author
7. Data storage
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71
4.2 Introduction
High internal phase emulsions (HIPEs) are two-phase systems with the total
fraction of the dispersed / internal phase being over 74% by volume.1 HIPEs are
usually high viscous due to the low fraction of the continuous phase, and they
are also called gel emulsions.2 HIPEs are widely used in foods,3 chemical
industry,4 medicine5 and in preparation of materials having porous structures or
low-density.6, 7 Because of the fascinating properties and widespread
applications, development of new route to prepare HIPEs has attracted
considerable attention.
HIPEs are usually stabilized by surfactants and particles. As stabilizers,
surfactants are commonly used despite their low efficiency. Indeed, 5-50% (w/v)
of the volume of the continuous phase is needed to form HIPEs.8, 9 Solid particles
of a few nanometres to micrometres have also been used to stabilize HIPEs,
which are known as Pickering emulsions.10, 11 However, phase inversion may
occur in HIPEs stabilized by particles, although a variety of inorganic particles
functioning as stabilizers have been reported from silica, titanium oxide to clay.
Recently, organic gels have attracted considerable attention as a stabilizer
to form HIPEs.12 Ngai and co-workers reported the preparation of HIPE
organogels and hydrogels from poly(methacrylic acid)-based microgel particles,
representing a kind of physically crosslinked gels without flow behaviour.13-15
HIPE organogels and hydrogels were also prepared from microgels which
formed via hydrogen bonding interaction by incorporating functional groups
onto polymer chains.16, 17 Fang and co-workers reported preparation of HIPE
organogels from bulk organogels based on low molecular weight gelators and
found that the mechanical strength of the HIPE organogels was dominated by
the continuous phase.6,14
Herein we demonstrate for the first time that HIPE organogels can be
prepared from polymer organogels which are usually inexpensive in comparison
with organogels prepared from low molecular weight organogelators.18 In our
previous study, we developed a new route to prepare multiresponsive organogels
by charge-induced assembly of a triblock ionomer, sulfonated polystyrene-
block-poly(ethylene-ran-butylene)-block-polystyrene (SSEBS), and a diblock
copolymer polystyrene-block-poly(2-vinyl pyridine) (PS-b-P2VP).19
73
with -SO3H groups on SSEBS1, SSEBS2 and SSEBS 3, respectively based on
titration. In experiments, SSEBS1 was used without special clarification.
Preparation of organogels. The procedure for the preparation of organogels
was similar to that described in our previous paper.19 In brief, 5 % (w/v) SSEBS
was obtained by dissolving SSEBS into mixed solvents of toluene and methanol
(98/2, v/v), and then the solution was neutralized with 2 % (w/v) PPI dendrimers
of toluene solution. After addition of PPI dendrimer solution into SSEBS
solution, the mixture was stirred acutely. All the organogels were prepared at
stoichiometric ratio with [-SO3H]/[NH2+N] = 1 by addition of different amounts
of PPI dendrimer solutions into 1 mL of SSEBS solutions.
Preparation of HIPE gels. HIPE organogels were prepared by shearing a
mixture of salt aqueous solutions (4.5 mL) with different NaCl concentrations,
THF (1 mL) and organogel (1 mL) for about 2 minutes with a Vortex mixer at
3,400 rpm. The fraction of water phase in HIPE organogels was determined by
control experiment. The control experiment was carried out as follows: After
stirring the mixture of salt aqueous solutions (450 mL), THF (100 mL), toluene
(98 mL) and methanol (2 mL) for 10 minutes, the mixture was separated into
water and organic solvents with separating funnel, and then the volumes of water
and organic solvents were determined by a measuring cylinder. The fraction (fw)
of water phase was calculated by fw = Vw/(Vw + V0) where Vw and V0 stand for
the volumes of water phase and organic solvent phase, respectively.
4.3.2 Fourier-transform infrared (FTIR) spectroscopy
FTIR spectra were obtained with a Bruker Vertex 70 FTIR spectrometer.
To prepare samples for FTIR measurement, THF solution of SSEBS was
dropped onto KBr disk. For SSEBS/DAB-4, SSEBS/DAB-8 and SSEBS/DAB-
16 organogels, newly formed gels after mixing the solutions of SSEBS in
toluene/methanol (98/2, w/w) and PPI dendrimer in toluene were poured on KBr
disks. The solvent was evaporated in fume hood first and then the disks were
further dried under vacuum at 50 °C prior to measurement. PPI dendrimers were
dropped onto KBr disks and measured directly. The spectra were recorded by
the average of 32 scans in the standard wave number range of 600–4000 cm-1 at
a resolution of 4 cm-1.
74
4.3.3 Rheological experiments
Rheological experiments were taken on a TA DHR 3 rheometer with cone-
plate geometry at 25 °C. A cone with a diameter of 40 mm and a tilt angle of 2o
was used, and gap width was set to be 52 um for pre-formed polymer organogels
and 200 um for HIPE organogels. A solvent trap was used to minimize the effect
of evaporation. Frequency sweeps with an angular frequency from 0.1 to 100
rad s-1 were performed at a strain of 5 %. For accuracy, the data from 0.5 to 100
rad s-1 were used.
4.3.4 Conductivity measurement
The conductivity of the organogels was determined on a SevenEasy
conductivity meter (Mettler-Toledo GmbH, Switzerland) at room temperature.
The conductivities of water and organic solvents were also studied for
comparison, where water and organic solvents used were from the control
experiment described above.
4.3.5 Confocal microscopy
Confocal imaging was performed on a laser scanning confocal microscope
(Leica SP5, Leica Microsystems CMS GmbH, Germany). For observation,
samples of HIPE organogels were prepared by mixing SSEBS/DAB-4,
SSEBS/DAB-8 or SSEBS/DAB-16 organogels with 5% (w/v) pyrene in
water/THF (4:1, v/v) on a Vortex mixer for 2 min, and then the so-formed gels
were transferred onto glass slides. A laser with wavelength of 405 nm was used
to excited pyrene in organic phase.
4.3.6 Small-angle X-ray scattering (SAXS)
SAXS experiments were conducted at the Australian Synchrotron on the
small/wide-angle X-ray scattering beamline. For SSEBS solution,
SSEBS/DAB-4 and SSEBS/DAB-16 organogels, the samples were put into 1.0
mm quartz capillaries. The background correction was carried out by measuring
the scattering of an empty capillary and correcting for sample absorption. For
75
HIPE organogels, the samples were put into a multiwall plate, and the
background correction was with same solvent used.
4.4 Results and discussion 4.4.1 The formation of organogels
In this work, the pre-formed organogels were formed from triblock ionomer
SSEBS and PPI dendrimers. PPI dendrimers of generation 1 (DAB-4),
generation 2 (DAB-8) and generation 3 (DAB-16) were used. The organogels
were prepared by mixing SSEBS solution and PPI dendrimer solutions with an
equimolar amount of sulfonic acid groups and amine moiety, the organogels
were formed in 20-30 seconds upon mixing as confirmed by the tube-inversion
method.20 All the organogels were slightly yellowish without visible
heterogeneity, indicating that no macrophase separation occurred in the
organogels.
100 101 102101
102
103
DAB-4 DAB-8 DAB-16
Dyn
amic
Mod
ulus
(Pa)
Frequency (rad s-1)
G' G"
Figure 4.2 Elastic moduli G’ and viscous moduli G” of SSEBS/DAB-4,
SSEBS/DAB-8 and SSEBS/DAB-16 organogels as a function of oscillatory
shear frequency.
The formation of organogels was further confirmed by rheological
measurement, and the results for dynamic frequency measurements are shown
76
in Figure 4.2. For all organogels, the elastic moduli (G’) are higher than the
corresponding viscous moduli (G”) at all frequencies ranging from 0.5 to 100
rad s-1, demonstrating the rubber-like behaviour in the range. It can be seen that
both the elastic and the viscous moduli increase with the molecular weight of
PPI dendrimers, indicating that the mechanical strength of the organogels can be
tuned by the type of PPI dendrimer.
4.4.2 Charge-driven assembled polymer organogels
3000 3200 3400 3600
Abs
orba
nce
(a.u
.)
Wavenumber (cm-1)
DAB-4
DAB-8
DAB-16
SSEBS/DAB-4
SSEBS/DAB-8
SSEBS/DAB-16
SSEBS
33553283
(a)
1500 1550 1600 1650
Abs
orba
nce
(a.u
.)
Waveneumber (cm-1)
DAB-4
DAB-8
DAB-16
SSEBS/DAB-4
SSEBS/DAB-8
SSEBS/DAB-16
SSEBS156015411507
(b)
Figure 4.3 FTIR spectra of SSEBS/PPI dendrimers organogels after evaporation
of solvents in the range of (a) 3000 - 3600 cm-1 and (b) 1500 - 1650 cm-1.
FTIR spectroscopy was applied to study the ionic interaction between
SSEBS and PPI dendrimers in organogels after evaporation of solvents. Figure
4.3 presents the FTIR spectra of these organogels. Figure 4.3(a) shows two
characteristic peaks at 3283 and 3355 cm-1, attributable to symmetric and
asymmetric stretching vibrations of -NH2 in the PPI dendrimer (DAB-4, DAB-
8 and DAB-16), respectively.21 These two peaks are completely suppressed in
the SSEBS/DAB-4, SSEBS/DAB-8 or SSEBS/DAB-16 organogels, indicative
of disappearance of the amine groups due to formation of the protonated amine
moieties that exhibit characteristic peaks observed at 1507, 1541 and 1560 cm-1
in Figure 4.3.22 This is in accordance with the ionic interactions observed
77
between sulfonic acid groups and amine moieties in previous studies on blends
of triblock ionomer SSEBS.1,23 Therefore, the formation of gels was induced by
ionic interaction between SSEBS and PPI dendrimers.
0.0 0.5 1.0 1.5q (nm-1)
SSEBS
SSEBS/DAB-4Inte
nsity
(a.u
.) SSEBS/DAB-1620.9 nm
16.2 nm
Figure 4.4 SAXS profiles for SSEBS solution, SSEBS/DAB-4 and
SSEBS/DAB-16 gels.
The morphologies of the organogels were investigated by small-angle X-
ray scattering (SAXS) at room temperature, and the SAXS profiles are shown in
Figure 4.4. Although there is no obvious peak observed in the figure for SSEBS
solution, well-defined peaks are noted for SSEBS/DAB-4 and SSEBS/DAB-16
organogels, demonstrating that microphase separation occurred in the gels.
Since the first-order peak is broad and the resolution of the higher order peak
becomes poorer with the increase of ionic interaction for oil-swelling SSEBS
gels,24 it is impossible to clarify the exact morphology of the organogels.
According to the composition of the organogels and similar results reported,19,
23-27 it is believed that the ionic microdomains were stabilized by solvophilic
block of SSEBS in the gels. Based on the q values, the average distances between
neighbouring domains were calculated to be 20.9 and 16.2 nm for SSEBS/DAB-
4 and SSEBS/DAB-16 organogels, respectively. This result shows that the
morphology of polymer organogels is affected by the type of PPI dendrimer.
The peak for SSEBS/DAB-4 organogel is much narrower than that of
78
SSEBS/DAB-16 organogels, indicating the existence of more ordered structures
in the SSEBS/DAB-4 organogels. From the SAXS and FTIR results, it is
believed that these polymer organogels are formed from charge-driven assembly.
SSEBS3, DAB 8
11.6%
23.5 nm
28.3 nm
(a)
14.3%
0.0 0.5 1.0 1.5 2.0
19.1%
Inte
nsity
(a.u
.)
q (nm-1)
29.1 nm
DAB 4
28%, 17.95%
29.1 nm(b)
42%, 13.3%
22.5 nm
0.0 0.5 1.0 1.5 2.0
68%, 11.6%In
tens
ity (a
.u.)
q (nm-1)
19.7 nm
Figure 4.5 SAXS profiles for (a) SSEBS3 with different sulfonation degrees
/DAB 8 organogels (b) SSEBS/DAB 4 organogels with different polystyrene
contents.
The morphology of those charge-driven assembled polymer organogels
from different SSEBSs were also investigated and the results are presented in
Figure 4.5. Figure 4.5(a) reveals the effects of sulfonation degrees of SSEBSs
on the morphology of SSEBS/DAB 8 organogels, and it can be observed that
the first peak becomes narrow with the increase of sulfonation degrees, showing
more ordered structures occurred in these organogels from SSEBSs with higher
sulfonation degrees. This result indicates that PPI dendrimers reacts with –SO3H
groups on the same sulfonated polystyrene block. It also can be seen that with
the increase of sulfonation degrees of SSEBSs, the distances between
neighboring domains increase, which confirms that PPI dendrimers reacts with
the same sulfonated polystyrene block. Figure 4.5(b) shows the effects
polystyrene contents of SSEBSs on the morphology of polymer organogels, it is
noted that the distances between the neighboring distances increase obviously
79
with the decrease of polystyrene contents in SSEBSs, which verifies the EB
blocks forming solvophilic bridges to connect and stabilize solvophobic ionic
cores. These results confirmed that the structures of polymer organogels are
similar to those of SSEBS/PS-b-P2VP organogels; the interaction between –
SO3H groups of SSEBS and PPI dendrimers drives the formation of ionic
complex, and the ionic complexes aggregate to form solvophobic cores. These
cores are connected by the middle EB blocks of SSEBS to form three
dimensional networks or organogels.
4.4.3 Responsiveness of charge-driven assembled organogels
To confirm that the organogels were stabilized by the presence of ionic
microdomains, acetic acid, triethylamine and their salt were added to the pre-
formed organogels, as they could affect the strength of ionic interaction. In our
experiments, when 2 % (v/v) of acetic acid, triethylamine and their salt were
added to the pre-formed organogels, the gels transformed into a liquids,
indicating the breakdown of networks.
100 101 102
100
101
102
gel acid amine ionic liquid
Dyn
amic
Mod
ulus
(Pa)
Frequency (rad s-1)
Figure 4.6 Dynamic moduli G’(filled) and G”(hollow) of organogels from
SSEBS/DAB-4 with 0.5% of acid, amine and salt as a function of oscillatory
shear frequency.
80
The responsiveness of the pre-formed organogels was quantitatively
investigated by rheological measurement, and the results of dynamic frequency
measurements for pre-formed organogels with 0.5% (v/v) of acetic acid,
triethylamine and their salt are shown in Figure 4.6. It is noted that both elastic
moduli G’ and viscous moduli G” decrease with the addition of acid to the pre-
formed gels although the G’ remain higher than the corresponding G”, indicating
the existence of the three-dimensional network. A crossover appears between
the G’ and the corresponding G” for the organogels with amine or salt, showing
liquid-like behaviour in the low frequency before the crossover. These results
further verifies that the mechanism for the formation of SSEBS/PPI dendrimers
organogels is similar to that of SSEBS/PS-b-P2VP organogels.19 The ionic
complexes form charged microdomains, and the domains are connected and
stabilized by the middle blocks of SSEBS to form gels.
4.4.4 The formation of HIPE organogels
The ionic complexes are hydrophilic, and the charged domains in the
complexes should be able to dissolve or partially dissolve in water. It was
expected that water droplets could be dispersed in these charge-driven
assembled polymer organogels. However, upon addition of water to the pre-
formed organogels, a precipitate formed. In order to avoid precipitation,
tetrahydrofuran (THF) was added. THF is a good solvent for SSEBS and also
miscible with toluene and water, which may decrease the interaction between
sulfonated polystyrene blocks. In experiments, new white gels were formed
when a water-THF (4:1, v/v) mixture was added to the pre-formed organogels,
and the gel formation was confirmed by the tube inversion method and
rheological measurement. The volume fraction of water phase could be as high
as 80 %, which was studied by control experiments with the same solvents.
Although white gels can be obtained by simple blending of SSEBS/DAB-4
organogels with water-THF mixture, it is impossible to form gels from
SSEBS/DAB-8 and SSEBS/DAB-16 organogels without addition of NaCl. Our
experiments showed that the HIPE organogels can also be obtained by adding
other salts such as KCl or Na2SO4.From the results, it is reasonable to believe
an electrolyte plays an important role in the formation of the gels. The electrolyte
81
is believed to suppress the double layer of SSEBS and facilitate the absorption
of gels on the interface between aqueous phase and organic phase.
4.4.5 Morphologies of HIPE organogels
To study the structure of newly formed gels, conductivity of the gels was
measured. The results showed that the conductivity of newly formed white gels
was 0 S cm-1, so the gels had a structure of water droplets dispersed in the
organic continuous phase according to results from control experiments, which
showed that the conductivities of water phase and organic phase were 102.4 and
0 S cm-1, respectively. As the volume fraction of the internal phase was over
74 %, high internal phase organogels formed.
The structure of the high internal phase organogel was further investigated
by confocal microscopy. The confocal image of the gel in Figure 4.7 confirmed
the formation of water-in-oil organogels, with toluene (pyrene labelled) as the
continuous organogels phase with water droplets entrapped. The size of water
droplets varied from several to tens of micrometres, which was in the size range
of emulsion, and thus HIPE organogels formed.
Figure 4.7 Confocal image of HIPE gel from SSEBS/DAB-4 with 0.25 M NaCl
added, excited by laser with a wavelength of 405 nm for the pyrene labelled
organic phase, the scale bar: 100 m.
82
The effect of salt concentration on mechanical properties of HIPE
organogels was studied by rheological measurement. Figure 4.8 presents
SSEBS/DAB-4 HIPE organogels with different NaCl concentrations, and it is
observed that G’ always exceed G” at all frequencies ranged from 0.5 to 100 rad
s-1, indicating that HIPE organogels are formed. Both G’ and G” of HIPE
organogels decrease with the increase of salt concentration. To our knowledge,
this result is quite different from that reported from conventional HIPE
organogels, where a high concentration of the electrolyte leads to the formation
of strong HIPE organogels.13 This unusual result could result from salt screen
effect.28, 29 With the increase of salt concentrations in the water phase, the
electrostatic interaction between -SO3H groups on SSEBS and amine moieties
on PPI decreases, resulting in the decrease in the modulus of HIPE gels.
100 101 102
102
1 M 0.5 M 0.25 M
Dyn
amic
Mod
ulus
(Pa)
Frequency (rad s-1)
G' G"
Figure 4.8 Dynamic moduli G’ and G” of HIPE organogels prepared from
SSEBS/DAB-4 with different NaCl concentrations as a function of oscillatory
shear frequency.
To reveal the morphology of HIPE organogels, SAXS experiments were
carried out at room temperature and the results are shown in Figure 4.8. In
SSEBS organogels, the relaxation behaviour of charged domains controls the
83
morphology of gels.24 After addition of water to the pre-formed SSEBS
organogels, water would dissolve or swell ionic domains, which decreases the
relaxation time and increases the mobility of charged blocks. As a result, the
morphology of HIPE organogels would change. From Figure 4.9(a), it can be
seen that well-defined peaks occur for all the HIPE organogels, demonstrating
the persistence of microphase separation. The average distances between
neighbouring domains of the HIPE organogels in Figure 4.9(a) are considerably
increased compared to the pre-formed organogels (Figure 4.4). As the ionic
domains are hydrophilic after addition of water, the interaction between
endblocks on triblock ionomer decreases and the balance between endblocks and
midblock determines the phase separation, leading to the nearly unchanged
morphology of all the HIPE organogels. The SAXS profiles in Figure 4.9(b)
demonstrate that the morphologies of HIPE organogels are little affected by
different salt concentrations, indicating that salt concentrations do not dominate
the microphase separation in HIPE organogels though they affect the formation
and mechanical strength of HIPE organogels.
0.0 0.5 1.0 1.5q (nm-1)
SSEBS/DAB-4
SSEBS/DAB-8
SSEBS/DAB-16
22.4 nm
24.4 nm
Inte
nsity
(a.u
.)
22.4 nm(a)
0.0 0.5 1.0 1.5q (nm-1)
0.25 M
0.5 M
23.4 nm
(b)
Inte
nsity
(a.u
.)
1 M
23.4 nm
22.4 nm
Figure 4.9 SAXS profiles of HIPE organogels (a) with SSEBS/DAB-4,
SSEBS/DAB-8 and SSEBS/DAB-16 with 0.5 M NaCl, (b) SSEBS/DAB-4 with
0.25, 0.5 and 1 M NaCl.
84
Figure 4.10 SAXS profiles of HIPE organogels (a) with SSEBS/DAB-4,
SSEBS/DAB-8 and SSEBS/DAB-16 with 0.5 M NaCl, (b) SSEBS/DAB-4 with
0.25, 0.5 and 1 M NaCl.
The microphase separation in HIPE organogels were also investigated for
SSEBS with different sulfonation degrees or polystyrene contents, and the
results are shown in Figure 4.10. From Figure 4.10(a), it can be seen that the
profiles are almost same for HIPE organogels with SSEBS containing different
sulfonation degrees, indicating the morphology of these HIPE organogels are
not dependent on the sulfonation degrees. This result is quite different from that
from the pre-formed polymer organogels, indicating that the existence of water
decreases the ionic interactions between SSEBS and PPI dendrimers and that the
detanglement of triblock ionomer occurs during the formation of HIPE
organogels.
Although the morphology of HIPE organogels are not obvious affected by
the sulfonation degrees of SSEBS, it is affected by the polystyrene contents of
SSEBS, and this can be seen from Figure 4.10(b). With the increase of
polystyrene content in the SSEBSs, the average distance of the neighboring
domains decreases, which verifies that the middle EB blocks forms the bridges
to connect and stabilize the solvophobic ionic cores. The result also indicates
that the three dimensional networks is preserved in the HIPE organogels.
Based on the above results, it is believed that the water enters the ionic
11.6%
SSEBS 68%, DAB4 0.5M
22.0 nm
14.3%
0.0 0.5 1.0 1.5 2.0
19.1%
Intn
esity
(a.u
.)
q(nm-1)
DAB 8, 0.25M
24.4 nm
42%, 13.3%
28%, 17.95%25.4 nm
0.0 0.5 1.0 1.5 2.0
68%, 19.1%
Inte
nsity
(a.u
.)
q (nm-1)
22.0 nm
85
domains of the pre-formed polymer organogels, leading the detanglement of
charged ionomer chains, and the polymer organogels breaks into small gels
particles to stabilize water droplets dispersed in the organic phase during the
shearing. Scheme 4.1 shows that the gel particles are absorbed on the interfaces
between the continuous phase and the dispersed phase to stabilize the HIPE
organogels. As the HIPE organogels are stabilized by particles, and thus they
are Pickering emulsions. For Pickering water-in-oil (high internal phase)
emulsion, it requires partial wettability of the particles, i.e. the spreading
coef cient of water S(w/o) is negative and the adhesion energy of water
EAdh(w/o) is positive:
S(w/o) = -EAdh(o/w) = p-o o-w s-w < 0 (1)
EAdh(w/o) = -S(o/w) = p-o + o-w – s-w > 0 (2)
where p-o p-w and w-o stand for interfacial energies of the following three
interfaces: particle-oil, particle-water and water-oil, respectively. Base on above
theory, HIPE organogels could be prepared by tuning the sulfonation degree and
polystyrene content of SSEBE as well as oppositely charge components.
Scheme 4.1 The microstructures of polymer organogels stabilized HIPE
organogels.
4.5 Conclusions We have presented the first example of HIPE organogels prepared from
polymer organogels. The polymer organogels are formed from a triblock
ionomer SSEBS and PPI dendrimer by charge-induced assembly. Upon addition
of a water-THF mixture into the organogels, the charged domains in the pre-
formed organogels stabilize water droplets, resulting in the transition from
organogels to HIPE organogels. It is believed that the pre-formed polymer
86
organogels breaks into small gels particles absorbed on the interfaces between
the dispersed phase and the continuous phase to stabilize HIPE organogels.
4.6 References 1 K. J. Lissant, B. W.Peace, S. H. Wu and K. G.Mayhan, J. Colloid Interface
Sci., 1974, 47, 416.
2 A. Menner, R. Powell and A. Bismarck, Macromolecules, 2006, 39, 2034.
3 V. L. Sok Line, G. E. Remondetto and M. Subirade, Food Hydrocolloids,
2005, 19, 269.
4 A. Imhof and D. J. Pine, Nature, 1997, 389, 948.
5 S. Rocca, S. Muller and M. J. Stébé, J. Control. Release, 1999, 61, 251.
6 P. J. Colver and S. A. F. Bon, Chem. Mat., 2007, 19, 1537.
7 X. Chen, K. Liu, P. He, H. Zhang and Y. Fang, Langmuir, 2012, 28, 9275.
8 J. M. Williams, Langmuir, 1991, 7, 1370.
9 A. Barbetta and N. R. Cameron, Macromolecules, 2004, 37, 3188.
10 V. O. Ikem, A. Menner and A. Bismarck, Angew. Chem. Int. Ed. Engl., 2008,
47, 8277.
11 A. Menner, V. Ikem, M. Salgueiro, M. S. P. Shaffer and A. Bismarck, Chem.
Commun., 2007, 4274.
12 S. Zhang and J. Chen, Chem. Commun., 2009, 16, 2217.
13 G. Sun, Z. Li and T. Ngai, Angew. Chem., Int. Ed., 2010, 49, 2163.
14 Z. Li, T. Ming, J. Wang and T. Ngai, Angew. Chem., Int. Ed., 2009, 48,
8490.
15 Z. Li and T. Ngai, Langmuir, 2010, 26, 5088.
16 Y. Chen, N. Ballard, F. Gayet and S. A. F. Bon, Chem. Commun., 2012, 48,
1117.
17 Y. Chen, N. Ballard and S. A. F. Bon, Chem. Commun., 2013, 49, 1524.
18 M. Suzuki and K. Hanabusa, Chem. Soc. Rev., 2010, 39, 455.
19 T. Zhang and Q. Guo, Chem. Commun., 2013, 49, 5076.
20 A. R. Hirst and D. K. Smith, Langumir, 2004, 20, 10851.
21 L. González, A. Ladegaard Skov and S. Hvilsted, J. Polym. Sci. A Polym.
Chem., 2013, 51, 1359.
87
22 A. Takahashi, Y. Rho, T. Higashihara, B. Ahn, M. Ree and M. Ueda,
Macromolecules, 2010, 43, 4843.
23 S. Wu, S. Peng, N. Hameed, Q. Guo and Y.-W. Mai, Soft Matter, 2012, 8,
688.
24 X. Lu, W. P. Steckle, Jr. and R. A. Weiss, Macromolecules, 1993, 26, 6525.
25 X. Lu, W. P. Steckle and R. A. Weiss, Macromolecules, 1993, 26, 5876.
26 S. Wu, Q. Guo, S. Peng, N. Hameed, M. Kraska, B. Stühn and Y.-W. Mai,
Macromolecules, 2012, 45, 3829.
27 S. Wu, Q. Guo, T. Zhang and Y.-W. Mai, Soft Matter, 2013, 9, 2662.
28 R. Chollakup, W. Smitthipong, C. D. Eisenbach and M. Tirrell,
Macromolecules, 2010, 43, 2518.
29 J. N. Hunt, K. E. Feldman, N. A. Lynd, J. Deek, L. M. Campos, J. M. Spruell,
B. M. Hernandez, E. J. Kramer and C. J. Hawker, Adv. Mater., 2011, 23,
2327.
88
Chapter Five
Hybrid HIPE Organogels with Oil Separation Properties
5.1 Abstract Hybrid HIPE organogels were prepared from pre-formed hybrid organogels
which were formed from a triblock ionomer SSEBS and –NH2 modified Fe3O4
nanoparticles via charge-driven assembly. Magnetic macroporous materials can
be obtained from these hybrid HIPE organogels simply by removal of solvents,
and these porous materials have been confirmed to be excellent candidates for
the separation of oil from water.
This chapter is reproduced from the article: T. Zhang, Y. Wu, Z. Xu and Q. Guo. Chem.
Commun. 2014, 50, 13821. Reprinted with permission from RSC, copy right 2014.
AUTHORSHIP STATEMENT
1. Details of publication and executive author
Title of Publication Publication details
Hybrid high internal phase emulsion (HIPE) organogels with oilseparation properties
Chemical Communications2014, 50, 13824 13827.
Name of executive author School/Institute/Division ifbased at Deakin; Organisationand address if non Deakin
Email or phone
Qipeng Guo Institute for Frontier Materials [email protected]
2. Inclusion of publication in a thesis
Is it intended to include this publication in a higherdegree by research (HDR) thesis?
Yes If Yes, please complete Section 3If No, go straight to Section 4.
3. HDR thesis author’s declaration
Name of HDR thesis author ifdifferent from above. (If thesame, write “as above”)
School/Institute/Division if basedat Deakin
Thesis title
Tao Zhang Institute for Frontier Materials Polymer organogelsstabilized HIPE organogelsand applications in oil fields
If there are multiple authors, give a full description of HDR thesis author’s contribution to thepublication (for example, how much did you contribute to the conception of the project, the design ofmethodology or experimental protocol, data collection, analysis, drafting the manuscript, revising itcritically for important intellectual content, etc.)I contributed about 70 % to the conception of the project. I also designed the methodology andexperimental protocol, collected most of the data, analysed the data and drafted the manuscript.
I declare that the above is an accurate description ofmy contribution to this paper, and the contributionsof other authors are as described below.
Signatureand date
10/12/2015
4. Description of all author contributions
Name and affiliation of author Contribution(s) (for example, conception of the project, designof methodology or experimental protocol, data collection,analysis, drafting the manuscript, revising it critically forimportant intellectual content, etc.)
Tao Zhang at Institute for FrontierMaterials
Contributed about 70 % to the conception of the project.Designed the methodology and experimental protocol,collected most of the data, analysed the data and drafted themanuscript.
Dr. Yuanpeng Wu at Institute forFrontier Materials, DeakinUniversity, and at School ofMaterials Science and Engineering,Southwest Petroleum University,Chengdu, China
Contributed about 10 % to the conception of the project andhelped the preparation of magnetic particles.
Dr. Zhiguang Xu at Institute forFrontier Materials
Contributed 5 % to the conception of the project and helpedwater contact angle measurements.
Prof. Qipeng Guo at Institute forFrontier Materials
He is my supervisor. He contributed about 15 % to theconception of the project and revised the paper critically.
5. Author DeclarationsI agree to be named as one of the authors of this work, and confirm:i. that I have met the authorship criteria set out in the Deakin University Research
Conduct Policy,ii. that there are no other authors according to these criteria,iii. that the description in Section 4 of my contribution(s) to this publication is
accurate,iv. that the data on which these findings are based are stored as set out in Section 7
below.
If this work is to form part of an HDR thesis as described in Sections 2 and 3, I furtherv. consent to the incorporation of the publication into the candidate’s HDR thesis
submitted to Deakin University and, if the higher degree is awarded, thesubsequent publication of the thesis by the university (subject to relevantCopyright provisions).
Name of author Signature* Date
Tao Zhang 10/12/2015
Dr. Yuanpeng Wu 10/12/2015
Dr. Zhiguang Xu 10/12/2015
Prof. Qipeng Guo 10/12/2015
6. Other contributor declarations
I agree to be named as a non author contributor to this work.
Name and affiliation of contributor Contribution Signature* anddate
Dr. Yuanpeng Wu at Institute forFrontier Materials, Deakin University,and at School of Materials Science andEngineering, Southwest PetroleumUniversity, Chengdu, China
Contributed about 10 % to theconception of the project andhelped the preparation of magneticparticles.
10/12/2015
Dr. Zhiguang Xu at Institute for FrontierMaterials
Contributed 5 % to the conceptionof the project and helped watercontact angle measurements. 10/12/2015
Prof. Qipeng Guo at Institute forFrontier Materials
Contributed about 15 % to theconception of the project andrevised the paper critically. 10/12/2015
* If an author or contributor is unavailable or otherwise unable to sign the statement ofauthorship, the Head of Academic Unit may sign on their behalf, noting the reason for their
unavailability, provided there is no evidence to suggest that the person would object to beingnamed as author
7. Data storage
The original data for this project are stored in the following locations. (The locations must bewithin an appropriate institutional setting. If the executive author is a Deakin staff memberand data are stored outside Deakin University, permission for this must be given by the Headof Academic Unit within which the executive author is based.)
Data format Storage Location Date lodged Name of custodianif other than theexecutive author
Electronic raw data, analysed data andpaper
Computers ofrelated labs ofInstitute forFrontier Materialsat DeakinUniversity
2014 Institute forFrontier Materials
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If the publication is to be included as part of an HDR thesis, a copy of this form must beincluded in the thesis with the publication.
89
5.2 Introduction HIPEs consist of a continuous phase and an internal/dispersed phase, with
the total volume fraction of the internal phase exceeding 74%.1 HIPEs are
usually highly viscous, and they are also known as gel emulsion.2 HIPEs can be
solidified to prepare various materials with low density and porous structures
through polymerization of the continuous phase, and they are also called
polyHIPEs.3, 4 The polyHIPEs exhibit a variety of applications such as use in
tissue engineering,5 sensor,6 drug delivery7 and hydrogen storage.8 The route to
prepare polyHIPEs usually includes three steps.9 First, a stable HIPE is prepared
with a polymerizable continuous phase, and then the continuous phase is
polymerized to solidify the microstructures of emulsion. Finally, removal of the
residue of the continuous phase and the dispersed phase to obtain porous
materials with low density. To prepare HIPE, surfactants and solid particles are
most commonly used as stabilizer. However, surfactants show low efficiency,
where as much as 5-50% (w/v) of the continuous phase is required to stabilize
HIPEs.10, 11 Solid particle-stabilized HIPEs are known as Pickering emulsions.12,
13 Phase inversion may occur in Pickering emulsions, although some inorganic
particles have been successfully used as stabilizer. As a result, preparation of
hybrid materials from polyHIPEs can be complicated.
Recently, solidification of HIPEs has been achieved directly by applying
gels as stabilizer,14-19 and they are also called HIPE organogels or hydrogels
according to the kinds of the continuous phase. Ngai and co-workers first
reported HIPE hydrogels and organogels stabilized by ionisable polymer
microgel particles, and porous materials have been obtained from the HIPE
organogels.14, 15 By introducing inorganic particles into these gel particle-
stabilized HIPE gels, hybrid HIPE gels can be obtained, and these gels can be
used as template to prepare various porous inorganic materials.20, 21 However,
there has been no report on the preparation of hybrid materials from these hybrid
HIPE gels, which may result from the lack of strong interaction between the
microgel particles and inorganic particles.
Herein we describe a new strategy to prepare hybrid HIPE organogels from
hybrid organogels, which were formed from a triblock ionomer SSEBS and an
90
inorganic nanoparticle, (3-aminopropyl)triethoxysilane modified Fe3O4
nanoparticles, by charge-induced assembly. Fe3O4 was chosen as inorganic
component because of its magnetic responsiveness, which endows some
advantages for separation by applying a magnetic field. In our previous study,
we pioneered the fabrication of HIPE organogels formed from polymer
organogels, which were from SSEBS and a series of hydrophilic PPI
dendrimers.19 By tuning the hydrophilicity and functional groups on Fe3O4
nanoparticles, strong interactions may occur between SSEBS and the inorganic
nanoparticles. The sulfonated polystyrene blocks of SSEBS and modified Fe3O4
nanoparticles work together to stabilize water droplets in organic solvents. The
middle EB blocks may connect different droplets to form three dimensional
networks. When the volume fraction of water droplets in the networks exceeds
74%, hybrid HIPE organogels are formed. After removal of solvents, these gels
transform into a type of magnetic materials with low density, which are excellent
candidates for oil absorption from water.
5.3. Experimental section 5.3.1 Materials and preparation of samples
Materials. Triblock ionomer SSEBS was prepared by sulfonation of SEBS
according to the method described in our previous study.22 The SEBS contains
28 % of polystyrene and after sulfonation, the SSEBS has 15.0 mol % of
polystyrene on SEBS grafted with -SO3H groups by titration. Fe3O4
nanoparticles were prepared via co-precipitation method according to reference 23. Deionized water was used in all experiments.
Preparation of –NH2 modified Fe3O4 nanoparticles. –NH2 modified Fe3O4
nanoparticles were prepared by surface silanization of Fe3O4 nanoparticles. In
details, 0.95 g of the Fe3O4 nanoparticles were dispersed into 20 mL of water,
and 0.5 mL of (3-Aminopropyl)triethoxysilane in 2 mL of ethanol was added
into the Fe3O4 nanoparticles solution at 25 . After 24 hours, the surface
modi ed Fe3O4 nanoparticles were separated and washed with deionized water
for several times with the help of a magnet. Subsequently, –NH2 modified Fe3O4
nanoparticles were re-dispersed into 20 mL of THF for further use. The synthesis
of the Fe3O4 nanoparticles was characterized by transmission electron
91
microscopy (TEM), and from the result in Figure 5.1, it is observed that the sizes
of the nanoparticles vary from several to about 20 nm.
Figure 5.1 TEM imagine of –NH2 modified Fe3O4 nanoparticles.
Preparation of pre-formed hybrid organogels. Hybrid organogels were
obtained by mixing SSEBS solution and –NH2 modified Fe3O4 nanoparticles. 5 %
(w/v) SSEBS solution was obtained by dissolving SSEBS into mixed solvents
of toluene and methanol (98/2, v/v). Hybrid organogels formed when SSEBS
solution and dispersed Fe3O4 nanoparticles mixed at 10:1 (v/v).
Preparation of hybrid HIPE organogels. HIPE organogels were prepared
by shearing a mixture of aqueous salt solutions (4, 5, 6 or 7 mL) with different
NaCl concentrations (0.25, 0.5 or 1 mol L-1), THF (1 mL), pre-formed hybrid
organogels (1 mL) for about 2 minutes with a Vortex mixer at 3,400 rpm. The
fraction of water phase in HIPE organogels was determined by control
experiment described in our previous paper.19 The volume fractions of water
phase are 75%, 80%, 84% and 88% in corresponding to the addition of 4, 5, 6
and 7 mL of aqueous salt solution, respectively. THF is necessary in preparation
of hybrid HIPE organogels from SSEBS/Fe3O4 organogels although it is
believed the ionic domains are hydrophilic. Control experiment showed that a
precipitate formed without addition of THF.
92
Removal of solvents from the hybrid HIPE organogels. The hybrid HIPE
organogels were poured into diethyl ether under mechanical stirring, and after
collected from diethyl ether, the hybrid organogels were washed twice with
water. Subsequently, the hybrid HIPE organogels were freeze drying for 48
hours.
5.3.2 Rheological experiments
Rheological experiments were carried out on a TA DHR 3 rheometer with
a cone-plate geometry at 25 °C. A cone with a tilt angle of 2o and a diameter of
40 mm was used, and gap width was set to be 200 um. A solvent trap was used
to minimize the effect of evaporation. Frequency sweeps with an angular
frequency from 0.5 to 600 rad s-1 were performed at a strain of 1 %. For accuracy,
the data from 0.5 to 200 rad s-1 were used.
5.3.3 Fourier-transform infrared (FTIR) spectroscopy
FTIR spectra were obtained with a Bruker Vertex 70 FTIR spectrometer.
To prepare samples for FTIR, SSEBS solution, Fe3O4 dispersed solution and the
newly formed mixtures of SSEBS solution and –NH2 modified Fe3O4 dispersed
solution were was dropped onto the KBr disk, respectively. The solvents were
evaporated in fume hood first and then dried in vacuum oven at 40 oC for 2 hours
prior to measurements. The spectra were recorded in the range of 600 – 4000
cm-1 at a resolution of 4 cm-1.
5.3.4 Conductivity measurement
The conductivity of the hybrid HIPE organogels was determined on a
SevenEasy conductivity meter (Mettler-Toledo GmbH, Switzerland) at 25
degrees. The conductivities of water and organic solvents were also studied for
comparison, where water and organic solvents used were from the control
experiment described above.
5.3.5 Confocal microscopy
Confocal imaging was taken on a laser scanning confocal microscope (Leica
SP5, Leica Microsystems CMS GmbH, Germany). Magnetic HIPE organogel
93
with 80 % of the dispersed phase was transferred onto a glass slide and observed
immediately. A laser with wavelength of 405 nm was used to excited pyrene in
the organic phase.
5.3.6 Optical microscopy
Optical imaging was taken on an optical microscope (Olympus BX51).
Magnetic HIPE organogels with 80 %, 84 % and 88% of the dispersed aqueous
phase were transferred onto a glass slide and observed at 100 times immediately.
Porous material from HIPE organogel (88% of dispersed phase, SSEBS/Fe3O4
nanoparticles = 10/1, using 0.5 mol/L aqueous NaCl solution) was cut into
pieces, and then put onto a glass slice for observation.
5.3.7 Contact angle measurements
Contact angle measurements of the porous materials were carried out in a
KSV CAM 101 contact angle instrument. Distilled water was used for the
analyses on the samples, and the measurements were taken at room temperature.
For accuracy, the water contact angle was calculated from 5 measurements
which were obtained from different parts of the same sample.
5.3.8 Small-angle X-ray scattering (SAXS)
SAXS experiments were carried out at the Australian Synchrotron on the
small/wide-angle X-ray scattering beamline, and the camera length used is 7
meters. The samples were put into a multiwall gel plate, and the background
correction was done with the same solvent used.
5.3.9 Scan electron microscopy (SEM)
Surface morphologies were imaged with a NEOSCOPE JCM-5000 SEM.
The samples used are same with these for optical microscopy, and the surfaces
were coated with a thin gold layer. The images were taken under an accelerating
voltage of 10 kV.
5.3.10 Oil absorption evaluation
94
Oil absorption capacity experiments were carried out as follows. Typically,
20 mL of oil was dropped into 50 flasks with 20 mL of water in it, and then
about 0.2 g of hybrid material was put into the flasks. It was observed the hybrid
material absorbed oil very quickly, and after enough time to reach equilibrium,
the hybrid materials were picked up with a tweezer or magnetic bar. They were
weighed on a balance after laying on a filter paper to remove the excess oil. Oil
absorption capacity was calculated as follows: absorption capacity = ,
where and stand for the weight of hybrid materials before and after oil
absorption.
5.4 Results and discussion 5.4.1 The formation of charge-driven assembled organogels
Hybrid organogels were obtained by mixing SSEBS solution and -NH2
modified Fe3O4 nanoparticles dispersed in THF, and the formation of the
organogels was in 10-30 seconds, which was confirmed by tube-inversion
method.24 The SSEBS prepared from SEBS containing 28%, 42% and 68% of
polystyrene are labelled as SSEBS1, SSEBS2 and SSEBS3, respectively.
100 101 102
102
103
1 rad s-1
10 rad s-1
Dyn
amic
Mod
ulus
(Pa
s)
Strain (%)
(a)
G' G"
100 101 10210-2
10-1
100
101
102
103
SSEBS1/Fe3O4
SSEBS2/Fe3O4
SSEBS3/Fe3O4
Dyn
amic
Mod
ulus
(Pa
s)
Frequency (rad s-1)
(b)
G' G"
Figure 5.2 Dynamic moduli G’ and G” for (a) SSEBS1/Fe3O4 organogel with 1
and 10 rad s-1 as a function of strain (b) SSEBS1, SSEBS2 and SSEBS3/Fe3O4
organogels as a function of oscillatory shear frequency.
95
The formation of organogels was verified by rheological measurements, and
these results are shown in Figure 5.2. Figure 5.2(a) reveals the typical results of
storage moduli (G’) and loss moduli (G”) of SSEBS1/Fe3O4 organogels at 1 and
10 rad s-1 as a function of strain and from the results, it can be observed that the
linear viscoelastic regime ranges to almost 100%, indicative of the flexibility of
EB blocks of SSEBS1 in the toluene/methanol solution. The results are similar
to those of SSEBS1/PS-b-P2VP organogels. Figure 5.2 (b) G’ and G” of SSEBS1,
SSEBS2 and SSEBS3/Fe3O4 organogels as a function of frequency
measurements, and it can be seen that the fractions of EB blocks in the SEBS
affect the strengths of SSEBS/Fe3O4 organogels greatly. For SSEBS1/ Fe3O4
organogels, the G’ are larger than the corresponding the G”, indicative of
formation of three-dimensional networks in organic solvents. With the increase
of polystyrene content to 42%, the SSEBS2/ Fe3O4 showed the formation of gels
at low frequency and properties of fluids at high frequency. However, further
increasing the polystyrene contents in the SEBS to 68%, no three-dimensional
networks was formed, which was revealed by the facts that G’ are lower than the
corresponding G”.
FTIR spectroscopy was applied to study the ionic interaction between
SSEBS1 and -NH2 modified Fe3O4 nanoparticles after evaporation of solvents.
FTIR spectra in Figure 5.3 shows that with the increase of -NH2 modified Fe3O4
nanoparticles in the mixtures, characteristic peaks of the protonated amine
moieties can be observed at 1541 and 1560 cm-1.25 Moreover, the characteristic
peak of –SO3H at 1538 cm-1 transfers to 1533 cm-1 which is the characteristic
absorption peak of –SO3- groups.26 This is in accordance with the ionic
interactions observed between sulfonic acid groups and amine moieties in our
previous studies on blends of triblock ionomer SSEBS.22, 27
Small angle X-ray scattering (SAXS) measurement was applied to elicit the
morphology of SSEBS1/Fe3O4 hybrid organogels, and their SAXS profiles are
shown in Figure 5.4. Our previous results have shown that there is no
microphase separation in SSEBS solutions.22 However, obvious peaks occurred
in SAXS profiles for SSEBS1/Fe3O4 hybrid organogels, implying the
appearance of microphase separation in these organogels. Combine the results
96
from FTIR and SAXS, it is safe to conclude that these hybrid organogels are
formed from charge-driven assembly.
1500 1550 1600 1650
10/1
Abs
orba
nce
(a.u
.)
Wavenumber (cm-1)
1560
1542
1533
1538
1/0
30/1
SSEBS/Fe3O4
(a) (b)
Figure 5.3 (a) a typical photo of magnetic organogels and (b) FT-IR spectra of
SSEBS1, mixtures with SSEBS1/-NH2 modified Fe3O4 nanoparticles = 30/1
and 10/1 (v/v) in the range of 1500–1650 cm-1.
23.3 nmSSEBS/Fe3O4
20/322.9 nm
0.0 0.2 0.4 0.6 0.8 1.0
q (nm-1)
Inte
nsity
(a.u
.)
22.9 nm
20/1
20/2
Figure 5.4 SAXS profiles for SSEBS1/Fe3O4 hybrid organogels with
SSEBS/Fe3O4 = 20/3, 20/2 and 20/1.
97
5.4.2 The formation of HIPE organogels
A grey hybrid gel was obtained by mixing the pre-formed hybrid organogel,
THF and an aqueous salt solution, and the formation of the new gels was again
confirmed by tube-inversion method.24 Control experiments indicate that the
ionic interaction between SSEBS1 and -NH2 modified Fe3O4 nanoparticles
induces the formation of the new gels, since SSEBS1, -NH2 modified Fe3O4,
SSEBS/non-modified Fe3O4 nanoparticles or SEBS/-NH2 modified Fe3O4
nanoparticles cannot drive the formation of the gels under the same condition.
Please note, the SSEBS2/Fe3O4 and SSEBS3/Fe3O4 showed similar properties
to these of SSEBS1/Fe3O4 organogels in the formation of HIPE organogels, and
thus without specific state, the following HIPE organogels are prepared and
studied from SSEBS1/Fe3O4 organogels.
The structures of the newly-formed gels were studied by measuring their
conductivity, and the conductivity of the gels was about 0 S cm-1, which
indicates that the two phase system was formed with organic solvent as the
continuous phase.19 So they are called organogels. The volume fraction of the
internal phase was over 74%, and thus hybrid high internal phase organogels
were formed.
(a) (b)
Figure 5.5 (a) a typical photo of HIPE organogels and (b) confocal image of
hybrid HIPE organogels, excited by laser with a wavelength of 405 nm for the
98
pyrene labelled organic phase, scale bar: 50 m.
To verify the formation of high internal phase organogels, confocal
microscopy was applied to these gels. The confocal image of the gel shown in
Figure 5.5 confirmed the formation of water-in-oil organogels, with toluene
(pyrene labelled) as the continuous organogel phase with entrapped water
droplets. The size of water droplets varied from several to tens of micrometres,
which was in the size range of emulsions, and thus hybrid HIPE organogels were
formed.
Figure 5.6 Optical micrographs (Mag. × 100) of the hybrid HIPE organogels
with (a) 80%, (b) 84% and (c) 88% of the internal phase.
The morphologies of the hybrid HIPE organogels were further observed by
optical microscopy, and the photos are shown in Figure 5.6. It can be seen that
the sizes of the dispersed phase ranges from a few to tens of micrometres, and
this result is similar to that obtained by confocal microscopy. Few water droplets
deformed in the hybrid HIPE organogels with 80% of the internal phase, and the
deformed droplets increased gradually with an increase of internal phase. Most
of the spheres get deformed into polyhedron structures when the internal phase
is up to 88%. With the increase of the internal phases from 80% to 88%, the sizes
of water droplets also increase. This is quite reasonable considering the
emulsifying efficiency of hybrid organogels. The emulsifying efficiency can be
studied by calculating the total dispersed phase/continuous phase interfacial area
(S) of these HIPE organogels from the following formulate:
(1)
where R is the average dispersed droplet radius, and VT is the volume of the
99
dispersed phase included in the HIPE organogels. It seems that some deformed
water droplets emerged into a bigger one in the organogel with 88% of the
internal phase, which is well known as coalescence. The coalescence can be
induced by the low emulsifying efficiency of gel particles.
In the preparation of the hybrid HIPE organogels, aqueous NaCl solution
was added to the mixture step by step. It was noticed that with the increase of
water, the mixtures transformed from fluid to gels. The gels flowed very slowly
in an inversed vial with a 75% internal phase, and no flow behaviour was
observed for a few hours with increasing the internal phase to 88 %. The effects
of water content on the strength of the hybrid HIPE organogels were studied
quantitatively by rheological measurements. From the results of dynamic
frequency measurements shown in Figure 5.7, It can be seen that the elastic
modulus G’ is higher than the corresponding viscous modulus G”, indicating the
formation of three-dimensional networks. Both G’ and G” increase with the
increase of the dispersed water phase in the hybrid HIPE organogels, which is a
typical property of HIPEs.
Figure 5.7 G’ and G” of SSEBS1/Fe3O4 HIPE organogels with volume fractions
of the internal phase at 75, 80, 84 and 88 % as a function of oscillatory shear
frequency.
The effect of the amount of Fe3O4 on the mechanical properties of HIPE
100 101 102100
101
102
75 % 80 % 84 % 88 %
Dyn
amic
Mod
ulus
(Pa)
Frequency (rad s-1)
G' G"internal phase
100
organogels was also studied, since it determines the magnetism of the organogels.
It can be seen from Figure 5.8(a) that the strengths of these organogels are not
affected by increasing the amount of Fe3O4 nanoparticles in the gels. These result
indicates that a large amount of Fe3O4 nanoparticle can be loaded onto the hybrid
HIPE organogels, increasing the magnetism without decreasing the mechanical
properties of these organogels.
Figure 5.8 G’ and G” of SSEBS1/Fe3O4 HIPE organogels with (a) different
Fe3O4 nanoparticles at and (b) different salt concentrations.
The hybrid HIPE organogels are induced by ionic interaction between
SSEBS and inorganic nanoparticles, and it is expected that electrolytes will
affect the formation or strength of the hybrid HIPE organogels. Control
experiments showed that the hybrid HIPE organogels cannot be obtained by
using water instead of NaCl aqueous solution, perhaps because water cannot
enter the microdomains formed from sulfonated polystyrene blocks of SSEBS
and surface modified Fe3O4. Other types of salt such as KCl and K2SO4 were
also tried, and it was found that they could be used in the situation.
The effect of salt concentration on the strength of these organogels was
studied quantitatively by rheological measurements, and the results for dynamic
frequency measurements are shown in Figure 5.8(b). From the results, it can be
observed that the strength of these organogels increases with the salt
100 101 102100
101
102
103
10:1 10:2 10:3
Dyn
amic
Mod
ulus
(Pa)
Frequency (rad s-1)
G' G"
(a)
100 101 102100
101
102
0.25 mol/L0.5 mol/L1 mol/L
Dyn
amic
Mod
ulus
(Pa)
Frequency (rad s-1)
G' G"
(b)
101
concentration, which will be a benefit for the application of the hybrid HIPE
organogels in oil spill recovery at sea.
The stability of emulsions can be classified into droplet stability and
dispersion stability. In HIPEs, the interaction between droplets hinders the
flocculation and as a result, they usually showed high dispersion stability. Here,
the hybrid HIPE organogels are quite stable, and the stability has been confirmed
by an aging experiment. The experiment was conducted in closed vials. It was
observed about 5% (v/v) of water was separated from the hybrid HIPE
organogels in the first week, which indicates that these organogels are not always
in the thermodynamic equilibrium state. Upon further observation of these
organogels for two months, no further change was observed, indicating no
obvious coarsen, coalescence and flocculation occurred in these hybrid HIPEs.
5.4.3 HIPE xerogels
Figure 5.9 SEM micrographs of macroporous materials formed from HIPE
organogel with an 88% internal phase at different magnifications.
Porous materials with a low density (also named as HIPE xerogels) have
been obtained after removal of the solvent from the hybrid HIPE organogels by
freezing drying. The porous structures were observed by optical microscopy and
scan electron microscopy (SEM). The SEM micrographs of the HIPE xerogels
with different magnifications are shown in Figure 5.9, and it can be seen that the
sizes of pores range from tens of micrometres to about 100 micrometres, which
shows the pores from the dispersed phase is well reserved. From the photos at a
high magnification, it can be seen the presence of pore throats (windows) on the
walls of large pores, which could be ascribed to the aggregation of SSEBS
102
occurred during the removal of solvents although most micro-structures have
been reserved. As the pores are large, they can be observed under optical
microscopy, and a typical imagine is shown in Figure 5.10 (a). The xerogels have
a low density of 0.04 g cm-3, which is consistent with the volume shrinkage
during the removal of solvents. The hybrid materials show magnetic responsive
properties, which has been confirmed by applying a magnetic field using a
magnet as shown in Figure 5.10 (b). As the HIPE xerogels are magnetic
responsive, it is expected that they can be collected under the help of magnetic
field even though they are quite weak.
Figure 5.10 (a) optical micrograph (Mag. ×500) of macroporous materials
formed from HIPE organogel with an 88% internal phase and (b) the optical
photo of the HIPE xerogels with a magnet.
These HIPE xerogels were prepared from hybrid HIPE organogels which
were formed from charge-driven assembled polymer organogels, it is expected
that microphase separation is preserved in the macroporous materials. The
morphology of the macroporous materials was studied with SAXS, and from the
SAXS profiles listed in Figure 5.11, obvious peaks can be observed for these
macroporous materials prepared from SSEBS1, SSEBS2 and SSEBS3/Fe3O4
organogels, demonstrating that microphase separation still exists in these porous
103
materials although the average distances between the neighbouring
microdomains have changed.
Inte
nsity
(a.u
.)
SSEBS3/Fe3O4
SSEBS2/Fe3O4
0.1 1
SSEBS1/Fe3O4
q (nm-1)
Figure 5.11 SAXS profiles of SSEBS1, SSEBS2 and SSEBS3/Fe3O4 HIPE
xerogels.
Figure 5.12 Photograph of a water droplet on the surface of porous materials
taken during contact measurements.
The surface properties was examined by contact angle measurements, and a
typical photo of a water droplet on the surface of an HIPE xerogels is shown in
Figure 5.12. The modified Fe3O4 nanoparticles and the sulfonated polystyrene
blocks of SSEBS can be hydrophilic, but the middle EB block of SSEBS is
104
hydrophobic. From the results of the contact angle measurements, it can be
calculated that the average water contact angle of the porous material is 136°,
indicating that the material is hydrophobic. Thus the xerogels should exhibit
high absorption of hydrophobic solvents and oils but a low absorption of water.
They should be able to apply in the oil spill recovery, and thus the oil water
separation properties was studied here.
5.4.4 Oil water separation performance
The magnetic HIPE xerogels were studied for the separation of oil and water,
and results showed these xerogels absorbed oil from an oil-water mixture in a
few seconds, which is quickest amongst the reported absorbers used for the
separation of oil and water. The magnetic xerogels absorbed a variety of organic
solvents and oils including THF, hexane, toluene, xylene, chloroform, petroleum
spirit, dichloroethane, vegetable oil, gasoline and engineer oil. The HIPE
xerogels exhibited various absorption capacity to different kinds of oil from
water, and the results are shown in Figure 5.13. It can be seen that the absorption
rates range from 15 to 27 times of the original weight of HIPE xerogels, and the
absorption capacities are quite large in comparison with those porous materials
reported.
Figure 5.13 Oil absorption capacity of hybrid materials formed from hybrid
HIPE organogels.
After the absorption of solvents or oil from water surface, these xerogels
0
5
10
15
20
25
30
Abs
orpt
ion
Cap
acity
(tim
es)
Petroleum spirit
Engine oilDiesel
Gasoline
Vegetable oil
Chloroform
Dichloroethane
TolueneXylene
Hexane
105
still floated on the water surface, which facilitates the collection of xerogels after
absorption. As the newly formed xerogels still contains magnetic nanoparticles,
they can be collected from water by applying a magnetic field. As a result, the
strengths of the HIPE xerogels are not so important for practical applications, as
they can still be easily collected with an outer magnetic field even they break up.
These HIPE xerogels were formed from commercial available polymers with
simple modification, and thus they are inexpensive in comparison with low
molecular weight gelators.28 Moreover, unlike organogels formed from low
molecular-weight gelators,29, 30 no co-solvent is used in the separation of oil and
water, which can be cheap and environmentally friendly. Thus, we believe that
the magnetic materials from hybrid HIPE xerogels can be excellent candidates
for collection of oil from water.
5.5 Conclusions Hybrid HIPE organogels have been prepared from hybrid organogels, which
are formed from a triblock ionomer SSEBS and -NH2 functionalized Fe3O4
nanoparticles by charge-induced assembly. After removal of solvents, the hybrid
HIPE organogels transformed into a magnetic porous material, and the porous
materials have a low density of 0.04g cm-3. These porous materials absorbed a
variety of organic solvents and oils, and showed high absorption capacity,
absorption rate and re-usability from oil water mixture. The porous materials
can be collected with a magnet and absorbed oil can be recovered by simple
squeezing out. Therefore, these porous materials can be an excellent candidate
for the separation of oil and water. The sulfonation degrees and block lengths
can be easily tuned, and many kind of inorganic nanoparticles can be modified
with -NH2 groups, we believe that this method can be a versatile route to
prepared hybrid HIPE organogels and materials.
5.6 References 1 K. J. Lissant, B. W.Peace, S. H. Wu and K. G.Mayhan, J. Colloid
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2 A. Menner, R. Powell and A. Bismarck, Macromolecules, 2006, 39, 2034.
3 H. Zhang and A. I. Cooper, Soft Matter, 2005, 1, 107.
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4 M. Claire Hermant, B. Klumperman and C. E. Koning, Chem. Commun.,
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5 H. Ma, J. Hu and P. X. Ma, Adv. Funct. Mater., 2010, 20, 2833.
6 C. Zhao, E. Danish, N. R. Cameron and R. Kataky, J. Mater. Chem., 2007,
17, 2446.
7 J. Andersson, J. Rosenholm, S. Areva and M. Lindén, Chem. Mat., 2004,
16, 4160.
8 F. Su, C. L. Bray, B. Tan and A. I. Cooper, Adv. Mater., 2008, 20, 2663.
9 Z. Li, M. Xiao, J. Wang and T. Ngai, Macromol. Rapid. Commun., 2013,
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10 A. Barbetta and N. R. Cameron, Macromolecules, 2004, 37, 3188.
11 J. M. Williams, Langmuir, 1991, 7, 1370.
12 V. O. Ikem, A. Menner and A. Bismarck, Angew. Chem. Int. Ed. Engl.,
2008, 47, 8277.
13 A. Menner, V. Ikem, M. Salgueiro, M. S. P. Shaffer and A. Bismarck,
Chem. Commun., 2007, 4274.
14 Z. Li, T. Ming, J. Wang and T. Ngai, Angew. Chem., Int. Ed., 2009, 48,
8490.
15 G. Sun, Z. Li and T. Ngai, Angew. Chem., Int. Ed., 2010, 49, 2163.
16 Y. Chen, N. Ballard, F. Gayet and S. A. F. Bon, Chem. Commun., 2012,
48, 1117.
17 Y. Chen, N. Ballard and S. A. F. Bon, Chem. Commun., 2013, 49, 1524.
18 X. Chen, K. Liu, P. He, H. Zhang and Y. Fang, Langmuir, 2012, 28, 9275.
19 T. Zhang and Q. Guo, Chem. Commun., 2013, 49, 11803.
20 Z. Li, X. Wei, T. Ming, J. Wang and T. Ngai, Chem. Commun., 2010, 46,
8767.
21 X. Li, G. Sun, Y. Li, J. C. Yu, J. Wu, G. H. Ma and T. Ngai, Langmuir,
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22 T. Zhang and Q. Guo, Chem. Commun., 2013, 49, 5076.
23 Y. Wu, T. Zhang, Z. Zheng, X. Ding and Y. Peng, Mater. Res. Bull., 2010,
45, 513.
24 A. R. Hirst and D. K. Smith, Langumir, 2004, 20, 10851.
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25 A. Takahashi, Y. Rho, T. Higashihara, B. Ahn, M. Ree and M. Ueda,
Macromolecules, 2010, 43, 4843.
26 J. Wang, W. H. de Jeu, P. Müller, M. Möller and A. Mourran,
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27 S. Wu, S. Peng, N. Hameed, Q. Guo and Y.-W. Mai, Soft Matter, 2012, 8,
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28 M. Suzuki and K. Hanabusa, Chem. Soc. Rev., 2010, 39, 455.
29 S. R. Jadhav, P. K. Vemula, R. Kumar, S. R. Raghavan and G. John,
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108
Chapter Six
Reactive Polymer Organogels Stabilized HIPE Organogels
with Oil Separation Properties
6.1 Abstract Reactive charge-driven assembled polymer organogels were investigated as
HIPE organogel stabilizer and after reaction, these HIPE organogels obtained
were strong enough to be dried at room temperature. The reactive polymer
organogels were formed from a block ionomer SSEBS and a benzoxazine
monomer. The formation of reactive polymer organogels was confirmed by
tube-inversion method, rheological measurements and small-angle X-ray
scattering. The formation of HIPE organogels was studied by rheological
measurements and confocal microscopy. After reaction of benzoxazine in HIPE
organogels, three-dimensional macroporous polymers were obtained by direct
removal of the dispersed aqueous phase at room temperature. The macroporous
structures were confirmed by SEM and the macroporous polymers showed
excellent properties for oil water separation, including high absorption rate and
relatively large absorption capacity to a variety of organic solvents and oils,
simple recovery method by squeezing and re-usability. Therefore, reactive
polymer organogels as HIPE organogels stabilizer provide a new route to
prepare macroporous materials from HIPE organogels at room temperature.
109
6.2 Introduction HIPEs are emulsions having the total fraction of the internal/dispersed
phase exceeding 74% by volume.1 HIPEs are commonly highly viscous
due to their high fraction of the dispersed phase, and thus they are also
called gel emulsions.2 HIPEs can be solidified to produce macroporous
materials with a low density via the polymerization of the continuous
phase, and the obtained macroporous materials are also called polyHIPEs.1
PolyHIPEs have a wide range of application in sensor,3 tissue
engineering,4 hydrogen storage5 and oil water separation.6 Commonly,
there are three steps to prepare polyHIPEs.7 First, a stable HIPE is
prepared with a polymerizable continuous phase, and then the continuous
phase is polymerized to solidify the structures of HIPEs. Finally, remove
the continuous phase and the residue of the dispersed phase to obtain
macroporous materials with a low density.
To obtain polyHIPEs, stable HIPEs are required and they are
traditionally stabilized by surfactants and solid particles. For surfactants,
careful selection is required to avoid phase inversion at a high dispersed
phase ratio according to Bancroft rule,8 and usually a high amount of
surfactants (5 – 50 w/v % of the continuous phase) is needed.9 Particles
stabilized emulsions are known as Pickering emulsions, and it is challenge
to prepare Pickering HIPEs, as phase inversion tends to occur though some
particles have been successfully used to stabilize HIPEs.10-14
With the development of HIPE stabilizers, macroporous materials can
be obtained by direct solidification of the HIPE stabilizers (gel particles).15
These HIPEs show a flow-free behaviour,15 demonstrating that HIPE
hydrogels or organogels were formed according to their type of the
continuous phase.16, 17 These HIPE stabilizers are usually cross-linked
with a high molecular-weight, and thus macroporous materials can be
obtained by directly drying.15 The macroporous materials are commonly
weak as there is no chemical interaction between these stabilizers. The
development of novel HIPE stabilizers is a promising route to prepare
macroporous materials directly from HIPE stabilizers.18-21 Dual-stabilizer
110
method has been developed to prepare strong macroporous materials from
HIPE gels,18, 19 where one stabilizer was sacrificed and another was
subsequently cross-linked by chemical reaction. Recently, we have
reported HIPE organogels stabilized by charge-driven assembled polymer
organogels which were formed from a triblock ionomer SSEBS and PPI
dendrimers.22 Macroporous materials were obtained from the HIPE
organogels and they were studied for oil spill recovery.21 The materials are
weak and thus limit their collection. The development of magnetic HIPE
stabilizer endows magnetism to macroporous materials, which does not
enhance the strength though facilitates the collection.20
Herein, we report a reactive polymer organogels as HIPE organogel
stabilizer. The reactive polymer organogels were formed from a triblock
ionomer SSEBS and a benzoxazine monomer via charge-driven assembly.
The benzoxazine monomer is reactive and can react with themselves
without addition of any initiator in both pre-formed organogels and the
corresponding HIPE organogels. After reaction, the newly formed
polybenzoxazine is cross-linked, which bridges the SSEBS chains,
enhancing the strength of the macroporous materials. As a result, the
dispersed aqueous phase could be removed from HIPE organogels at room
temperature. Thus, reactive polymer organogels stabilized HIPE
organogels provide a new approach to prepare macroporous materials at
room temperature.
6.3. Experimental section 6.3.1 Materials and preparation of samples
Materials. Triblock ionomer SSEBS was prepared by sulfonation of SEBS
according to literatures.23, 24 The sulfonation degree was determined to be 15.0%
based on titration method. Aniline, paraformaldehyde and bisphenol-A are
purchased from Sigma-Aldrich Co. The other reagents and solvents were
analytical grade and used directly.
Synthesis of benzoxazine. Benzoxazine monomer was synthesized via a
solventless method according to references.25, 26 In short, 3.72 g of aniline, 2.4 g
of paraformaldehyde and 4.48 g of bisphenol-A (2:4:1) were added to a 100 mL
111
round-bottom flask, and then the flask was connected to the Schlenk line system
with 3 pump-thaw cycles to remove oxygen. The reaction was conducted at
110 °C for 30 minutes with nitrogen protection. After reaction, the product was
dissolved into dichloromethane, and washed twice with Na2CO3 solution and
then three times with brine solution. After dry with Na2SO4, solvent was
removed and benzoxazine was obtained. The structure was confirmed by 1H
NMR, and the spectrum of benzoxazine is shown in Figure 6.1. The
characteristic peaks assignable to methylene (O-CH2-N) and methylene (Ar-
CH2-N) of oxazine ring were observed at 5.32 and 4.57 ppm, respectively. The
methyl proton of bisphenol-A showed peaks at 1.57 ppm, and the proton on the
Ar ring were shown from 6.71 to 7.09 ppm. The theoretical ratio of the
methylene proton of benzoxazine to the methyl proton on bisphenol is 4 to 6,
and the result was found to be 3.4 to 6. Thus the ratio of the ring-closed
benzoxazine structure was calculated to be 85%.
Figure 6.1 1H-NMR spectrum of the synthesized benzoxazine in CDCl3.
Preparation of reactive polymer organogels. Reactive polymer organogels
were prepared by mixing SSEBS solution with benzoxazine solution. SSEBS
was dissolved into a mixture of toluene/methanol (98:2, v/v) to obtain 5% (w/v)
112
of SSEBS solution. 10% (w/v) of benzoxazine solution was obtained by
dissolving the as-obtained benzoxazine monomer into THF.
SSEBS/benzoxazine organogels with [-SO3H]/[N] = 1/4, 1/8 and 1/16 were
obtained by mixing 1 mL of SSEBS solution with required amount of
benzoxazine solution, and they were denoted as 1OG, 2OG and 3OG,
respectively.
Preparation of high internal phase emulsion (HIPE) organogels. HIPE
organogels were prepared by shearing a mixture of 1 mL of the pre-formed
organogels, 1 mL of THF and 4.5 mL of aqueous NaCl solution (0.5M). These
HIPE organogels were named as 1HIPEOG, 2HIPEOG and 3HIPEOG
corresponding the pre-formed 1OG, 2OG and 3OG, respectively.
Polymerisation of polymer organogels and HIPE organogels.
SSEBS/benzoxazine organogels and the corresponding HIPE organogels were
polymerised at 75 °C in sealed vials for 2 and 4 hours.
Preparation of macroporous materials. Macroporous materials were
obtained by evaporation of HIPE organogels after aging for 4 weeks at 40 °C.
These macroporous materials were denoted as 1HIPEXG, 2HIPEXG and
3HIPEXG corresponding to 1HIPEOG, 2HIPEOG and 3HIPEOG,
respectively.
6.3.2 Rheological measurements
Rheological experiments were carried out on a TA DHR 3 rheometer with
a cone-plate geometry at 25 °C. A cone with a diameter of 40 mm and a tilt angle
of 2o was used, and gap width was set to be 52 m and 500 m for organogels
and HIPE organogels, respectively. A solvent trap was used to minimize the
effect of evaporation. Dynamic frequency sweeps were performed at a strain of
1 %, and stress relaxation measurements were carries out at a strain of 10%.
6.3.3 Conductivity measurement
The conductivity of the SSEBS/benzoxazine HIPE organogels was
measured on a SevenEasy conductivity meter (Mettler-Toledo GmbH,
Switzerland) at room temperature. The conductivity of each sample was the
average value of 5 results from the same sample. The conductivities of water
113
phase and organic phase were also studied for comparison, where water and
organic phases were from the control experiment described in our previous paper.
6.3.4 Small-angle X-ray scattering (SAXS)
SAXS experiments were carried out at the Australian Synchrotron on the
small/wide-angle X-ray scattering beamline, and the camera length used is 3
meters. SSEBS solution was injected into a capillary tubes and
SSEBS/benzoxazine organogels were placed in to a gel plate for data collection.
The background correction was carried out with the same solvent used.
6.3.5 Confocal microscopy
Confocal image was taken on a laser scanning confocal microscope (Leica
SP5, Germany). 1HIPEOG, 2HIPEOG or 3HIPEOG was transferred onto a
glass slide for observation. Light at wavelength of 405 nm was used to excited
pyrene dissolved in the organic phase.
6.3.6 Scan electron microscopy (SEM)
Surface morphologies of 1HIPEXG, 2HIPEXG and 3HIPEXG were
conducted on a Zeiss Supra 55VP at an acceleration voltage of 5.0 kV. The
surfaces were coated with a thin gold layer prior to observation.
6.3.7 Contact angle measurements
Contact angle measurements of 1HIPEXG, 2HIPEXG and 3HIPEXG
were conducted on a KSV CAM 101 contact angle instrument. Distilled water
was used for the analysis of the samples and the measurements were taken at
room temperature.
6.3.8 Oil absorption capacity
Oil absorption capacity measurements were carried out according to our
previous methods.20 In brief, 15 mL of oil was mixed with 20 mL of water in 50
ml beaker, and then about 0.3 g of macroporous material was put into the beaker.
After the oil absorption of these macroporous materials reached equilibrium,
these materials were picked up with a tweezer. They were weighed after removal
114
of surface oil with a filter paper. Oil absorption capacity was calculated as
follows: absorption capacity = , where and represent the weights of
these macroporous materials before and after oil absorption.
6.3.9 Absorption rate
The absorption rate of these macroporous materials was investigated with
toluene and engine oil. Briefly, a certain weight of macroporous materials (m0)
was put into toluene or engine oil, and then monitored weights of these
macroporous materials (mt) with time. The weight gained of these macroporous
materials at time t was defined as (mt-m0).
6.3.10 Recovery rate
These macroporous materials can be reused by simple mechanical
squeezing out the absorbed organic solvent or oil. The recovery rate was
calculated as , where is the weight of the recovered oil or
organic liquid, the weight of HIPE xerogels at the saturated state and
m0 stands for the initial weight of the xerogels.
6.4 Results and discussion 6.4.1 SSEBS/benzoxazine organogels
Reactive SSEBE/benzoxazine organogels were formed in a few seconds
upon mixing the individual SSEBS solution and benzoxazine solution, and the
formation of organogels was confirmed by tube-inversion method.27 All the
organogels were yellowish without visible heterogeneity, showing that no
macrophase separation occurred. Control experiment showed that no
organogels were formed by mixing SSEBS solution with the same amount
of THF without benzoxazine monomer. Therefore, the formation of
organogels is ascribed to the interaction between SSEBS and benzoxazine
monomers. Considering the properties of –SO3H groups of SSEBS and
benzoxazine monomers, it is believed that the formation of organogels is
induced by ionic interaction, which can be inferred from the fast formation. The
mechanical strengths of organogels decreased with the increase of amount of
115
benzoxazine in these organogels, as the concentration of benzoxazine in the
organogels is much higher than that based on stoichiometry.
100 101 102101
102
103
1OG 2OG 3OG
Dyn
amic
Mod
ulus
(Pa)
Frequency (rad s-1)
G' G"
Figure 6.2 Dynamic moduli G’ (filled) and G” (hollow) of 1OG, 2OG and 3OG
as a function of oscillatory shear frequency.
The formation of organogels was verified by rheological measurement.
Dynamic frequency measurements of organogels were conducted after
mixing the two individual solutions in 20 – 30 seconds, and the results are
shown in Figure 6.2. From the results, it can be seen that the elastic moduli
(G’) are high than the corresponding viscous moduli (G”), indicative of
the formation of three-dimensional networks. Moreover, the strengths of
these organogels increase with the increase of [-SO3H]/[N] in the
organogels, probably because a smaller fraction of benzoxazine monomers
could react with –SO3H groups with the increase of benzoxazine
monomers.
Benzoxazine monomers can be transform into polybenzoxazines by heating
or acid-catalysed polymerization.26, 28 Here it is believed that all the –SO3H
groups are neutralized in 1OG, 2OG and 3OG, and thus these organogels were
heated for polymerization, where a relative low temperature at 75 °C was chose
considering the solvents in these organogels. The polymerization of benzoxazine
116
monomer in the 1OG, 2OG and 3OG was confirmed by the change in
mechanical strength of organogels. A typical result from dynamic frequency
measurements of 1OG is presented in Figure 6.3(a), and it can be seen that the
G’ are higher than the corresponding G” for control organogel, organogels
heated for 2 hours and 4 hours, showing that three-dimensional networks were
formed in all the organogels. The mechanical strengths of 1OG heated for 2 and
4 hours are stronger than the control 1OG, indicting the polymerization of
benzoxazine monomers in these organogels. The change in mechanical strengths
can also be verified by the results from the stress relaxation measurements in
Figure 6.3(b).
100 101 102100
101
102
103
Control2 hours at 75 oC4 hours at 75 oC
Dyn
amic
Mod
ulus
(Pa)
Frequency (rad s-1)
G' G"
1: 0.1(a)
10-1 100 101 10210-3
10-2
10-1
100
101
102
103
Control 2 hours at 75 oC 4 hours at 75 oC
G(t)
(Pa)
Time (s)
(b)
Figure 6.3 (a) Dynamic moduli G’ (filled) and G” (hollow) as a function of
oscillatory shear frequency and (b) stress relaxation for control 1OG,
polymerized at 75 °C for 2 and 4 hours 1OG.
The polymerization of benzoxazine monomer in SSEBS/benzoxazine
organogels can also be inferred from the morphology change, which was
revealed by SAXS. It can be observed from SAXS profiles in Figure 6.4(a)
that there is no peak for SSEBS solution, implying no microphase
separation in the solution, which is agreeable with those results obtained
before.29 However, obvious peaks appear in 1OG, 2OG and 3OG,
117
indicative of the existence of microphase separation in these organogels.
Based on the q value, it can be calculated the average distances between
the neighbouring domains to be 27.8 nm. Although these first peaks are
quite narrow, it is still impossible to describe the exact morphologies of
the organogels as the resolution of higher order peaks are poorer with the
increase of ionic interaction of SSEBS organogels.30 According to the
results reported from similar composition, it is assumed that the charged
sulfonated polystyrene blocks of SSEBS and charged benzoxazine
monomer form the solvophobic cores, which are connected by the
solvophilic mid blocks.29 The extra benzoxazine monomer may dissolve
in the toluene outside of these solvophobic core, which can be inferred
from the fact that the location and shape of these first peaks do not change
with the amount of benzoxazine monomer in these organogels. The
formation of these organogels and the occurrence of microphase
separation are formed from the ionic interaction between SSEBS and
benzoxazine monomer, and thus it is believed that the formation of these
organogels is driven by charge-induced assembly.
Figure 6.4(b) shows the change in morphology of 2OG with heating, and
the location of the first peak does not change after heating at 75°C for 2 hours,
implying the reaction of benzoxazine monomer does not change the average
distances between the neighbouring domains. It is noted that the first peak
becomes broad after polymerisation for 2 hours, indicating the distribution of
cores becomes less ordered. Further increase the heating time to 4 hours, the first
peak shift to 0.363 nm-1, corresponding the average distances between the
neighbouring domains to be 17.3 nm. Meanwhile, the peak is much broad,
indicative of less ordered structures in the polymerised organogels.
118
Inte
nsity
(a.u
.)SSEBS solution27.8 nm
1OG
2OG
(a)
0.0 0.5 1.0 1.5
q (nm-1)
3OG
0 hour
27.8 nm
2 hours
0.0 0.5 1.0 1.5
4 hours
Inte
nsity
(a.u
.)
q (nm-1)
17.3 nm
(b)
Figure 6.4 SAXS profiles of (a) SSEBS solution, 1OG, 2OG and 3OG; (b)
2OG under 75 °C for 0, 2 and 4 hours.
6.4.2 SSEBS/benzoxazine HIPE organogels
A white gel was formed by shearing the pre-formed 1OG, 2OG or
3OG, aqueous NaCl solution and THF, and the formation of gels was also
confirmed by tube-inversion method.27 Control experiment showed that no
gels could be formed from polymerised 1OG, 2OG or 3OG under the
same conditions, and this result can be explained by the facts that
hydrophilic amines are required to form HIPE organogels,22 indicating
polybenzoxazines are more hydrophobic than the corresponding
benzoxazine monomers. This show the hydrophilicity of benzoxazine
monomer is critical to the formation of the white gels. Control experiments
also showed that no white gels can be formed when aqueous salt solution
is replaced by deionized water.
Based on the fact that the volume fraction of water phase in the gels is
over 74% from composition and the properties of the gels, it is assumed
that they are organogels with the organic solvents as the continuous phase
and water as the dispersed phase. The assumption was confirmed by
conductivity measurements, and the conductivity of the gels is about 0 S
119
cm-1, which is almost same as that of organic solvents. These white gels
have high ratio of the internal phase and organic component as the
continuous phase, and thus they are high internal phase organogels.
Figure 6.5 A typical confocal image of HIPE organogels, excited by laser at a
wave length of 405 nm for the pyrene labelled organic phase, scale bar: 50 m.
The formation of high internal phase organogels was verified by
confocal microscopy, and a typical confocal image is shown in Figure 6.5.
From the image, it can be seen that the organogels were formed with
toluene (pyrene labelled) as the continuous phase and water as the disperse
phase. The sizes of water drops vary from several to tens of micrometres,
which is in the size range of emulsions, and thus HIPE organogels were
formed.
The benzoxazine monomers in 1HIPEOG, 2HIPEOG and
3HIPEOG are still reactive, which are confirmed by rheological
measurement. A typical result of dynamic frequency measurements for
1HIPEOG is shown in Figure 6.6. According to Figure 6.6(a), the G’ are
higher than the corresponding G” for the controlled 1HIPEOG,
confirming the results from tube-inversion experiments. G’ of 1HIPEOG
increased after heating for 2 and 4 hours at 75 °C, indicating the
polymerization of benzoxazine monomers occurred in HIPE organogels.
120
The results are also agreeable to these from stress relaxation measurements
in Figure 6.6(b).
10-2 10-1 100101
102
103
G" Fresh 2 hour at 75 oC 4 hour at 75 oC
Dyn
amic
Mod
ulus
(Pa)
Frequency (rad s-1)
G'
(a)
10-1 100 101 102100
101
102
103
Fresh 2 hours at 75 oC 4 hours at 75 oC
G(t)
(Pa)
Times (s)
(b)
Figure 6.6 (a) Dynamic moduli G’ (filled) and G” (hollow) as a function of
oscillatory frequency and (b) stress relaxation for fresh prepared, polymerized
at 75 °C for 2 and 4 hours 1HIPEOG.
The stability of these HIPE organogels was checked by aging
experiment in closed vials at 0 °C. It showed that about 2% (v/v) of water
was separated from the HIPE organogels in the first few days and no
further separation was observed in the following six months. This results
showed that the fresh prepared HIPE organogels were not in the
thermodynamic equilibrium state.
6.4.3 SSEBS/benzoxazine HIPE xerogels
The HIPE organogels become tough with deep colour after 2 weeks at room
temperature, which may be explained by transformation of benzoxazine
monomer to polybenzoxazine in the HIPE organogels. These
SSEBS/polybenzoxazine HIPE organogels became strong enough to support
their three-dimensional networks during and after the evaporation of solvents at
room temperature. As a result, the solvents in HIPE organogels can be
evaporated in room temperature directly to form macroporous materials. It is
121
noted that freezing-drying technology is always required to prepare
macroporous materials from dual-stabilizer and unreactive polymer organogels
stabilized HIPE gels,18-21 and that only single-layered materials were obtained at
room temperature.15 Here, the dispersed phase can be removed from reactive
polymer organogel stabilized HIPE organogels, demonstrating the high strength
of the reacted HIPE organogels. These materials have a low density of 0.078,
0.065 and 0.080 g cm-3 for 1HIPEXG, 2HIPEXG and 3HIPEXG, respectively.
These values are higher than that obtained from hybrid HIPE organogels,20
probably because great shrinkage occurred during the process of solvent
evaporation at room temperature.
The pore morphology of 1HIPEXG, 2HIPEXG and 3HIPEXG was
observed by SEM and the results are shown in Figure 6.7. It can be seen that the
size range of porous materials vary from several to tens of micrometres, which
are smaller than the water droplets in HIPE organogels. This could be explained
by the volume shrinkage during the polymerization of benzoxazine and the
evaporation of solvents. 1HIPEXG may induce the shrinkage of materials
during evaporation of solvents. In the 2HIPEXG and 3HIPEXG, the structures
are mainly dependent on the process of benzoxazine polymerization, as the
polybenzoxazine is strong enough to support the structure of porous structures
during the solvent evaporation. In the materials 3HIPEXG, the benzoxazine
monomers polymerised on the walls of pores to form thick walls.
Figure 6.7 SEM micrographs of 1HIPEXG, 2HIPEXG and 3HIPEXG.
122
The surface property was examined by contact angle measurements, and
typical photos of water droplets on the surfaces of 1HIPEXG, 2HIPEXG and
3HIPEXG are shown in Figure 6.8. It can be seen that the average water contact
angles of these macroporous materials are 122°, 126° and 129°, respectively,
indicating that all these materials are hydrophobic. Moreover, contact angles
increase with the increase of benzoxazine in these macroporous materials. This
can be ascribed to the fact that the middle EB block of SSEBS and
polybenzoxazine are hydrophobic though sulfonated polystyrene blocks of
SSEBS and charged benzoxazine can be hydrophilic.
Figure 6.8 Photographs of water droplets on the surfaces of (a) 1HIPEXG, (b)
2HIPEXG and (c) 3HIPEXG taken during contact measurements.
Figure 6.9 shows the absorption rates of 1HIPEXG, 2HIPEXG and
3HIPEXG to toluene and engine oil, and these absorption rates are quite high
in comparison with other porous materials reported for oil water separation.31 It
can be seen that 2HIPEXG has the highest absorption rate, which reached its
saturated state in a few minutes. While 3HIPEXG showed lowest absorption
rate, and it took about 50 minutes to reach the saturated state. 1HIPEXG has
moderate absorption rate though it has the highest absorption capacity. These
results show that the absorption rate is highly dependent on the internal porous
structures.
123
0 10 20 30 40 50 600
5
10
15
20
25
Abso
rptio
n (g
/g)
Time (minutes)
1HIPEXG 2HIPEXG 3HIPEXG
Toluene Engine oil
Figure 6.9 Absorption rates of toluene and engine oil for 1HIPEXG, 2HIPEXG
and 3HIPEXG.
As shown in Figure 6.10, these HIPE xerogels could absorb a variety of
organic solvents and oils, including hexane, toluene, xylene, dichloroethane,
chloroform, vegetable oil, gasoline, diesel and engine oil. The absorption
capacity is relative high though it varies with organic solvents or oils as well as
HIPE xerogels. On the whole, 1HIPEXG has the highest absorption capacity
while 3HIPEXG is lowest. On the whole, the absorption capacity decreases with
the increase of benzoxazine fraction in these HIPE xerogels. It is reasonable as
polybenzoxazine formed three-dimensional networks, which decrease the
swelling ability of these HIPE xerogels. The absorption capacity is quite high in
comparison with xerogels and HIPE monoliths from low molecular weight
gelators.31, 32 It is noted that the absorption capacity of these
SSEBS/benzoxazine HIPE xerogels is lower than the previous reported hybrid
HIPE xerogels and highly porous materials, but they are less expensive.
124
HexaneToluene
Xylene
Dichloroethane
Choroform
Vegatable oil
GasolineDiesel
Engine oil0
5
10
15
20
25
Abs
orpt
ion
Cap
acity
(tim
es)
1HIPEXG 2HIPEXG 3HIPEXG
Figure 6.10 Oil absorption capacity of 1HIPEXG, 2HIPEXG and 3HIPEXG.
0 5 10 15 20 25 300
5
10
15
20
25
30
Saturated Squeezed
Abs
orpt
ion
Cap
acity
(tim
es)
Number of cycles
Rec
over
y
0.0
0.2
0.4
0.6
0.8
1.0
Recovery rate
Figure 6.11 Recovery rate of 1HIPEXG for engine oil.
Just as the SSEBS/Fe3O4 xerogels we reported before, the absorbed oil
could be recovered easily by mechanical squeeze, which should be more
economical than solvent wash.32 The recovery rate of 1HIPEXG for toluene is
presented in Figure 6.11, and from the result, it can be seen that the recovery
rate is at about 0.8, and the HIPE xerogels can be re-usable for 30 times without
obvious change in the oil absorption capacity and recovery rate. Although
2HIPEXG and 3HIPEXG can be used in the same way and have almost same
recovery rate, they broke into small pieces after 15 and 7 times squeezes.
125
6.5 Conclusions
We presented reactive polymer organogels stabilized HIPE
organogels which were used to prepare macroporous materials by direct
drying at room temperature. The reactive polymer organogels were formed
from a triblock ionomer SSEBS and a benzoxazine monomer via charge-
driven assembly. The benzoxazine monomer is reactive both in pre-
formed organogels and HIPE organogels with heating. Macroporous
materials were obtained by directing drying these HIPE organogels at
room temperature. These macroporous materials obtained are confirmed
to be an excellent candidate for oil water separation.
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127
Chapter Seven
HIPE Xerogels for Enhanced Oil Spill Recovery
7.1 Abstract Oil spills cause serious damage to the aquatic ecosystem and require quick
cleanup. Herein we report HIPE xerogels for the first time as oil absorbents for
enhanced oil spill recovery. The HIPE xerogels absorb diesel from the water–oil
mixture in 20–30 seconds. The absorption capacity of the HIPE xerogels ranges
from 20 to 32 times for different kinds of oils, and the oils can be recovered
simply by being squeezed out, with a recovery rate around 80%. They can be
reused at least 40 times without obvious deterioration in oil separation properties
from 0 to 45 °C. These novel xerogels are suitable for practical use in oil spill
reclamation and wastewater treatment.
This chapter is reproduced from the article: Y. Wu, T. Zhang Z. Xu and Q. Guo. J. Mater.
Chem. A 2014, 4, 35055. Reprinted with permission from RSC, copy right 2014.
AUTHORSHIP STATEMENT
1. Details of publication and executive author
Title of Publication Publication details
High internal phase emulsion (HIPE) xerogels for enhanced oil spillrecovery
Journal of Materials ChemistryA 2015, 3, 1906 1909.
Name of executive author School/Institute/Division if based atDeakin; Organisation and address ifnon Deakin
Email or phone
Qipeng Guo Institute for Frontier Materials [email protected]
2. Inclusion of publication in a thesis
Is it intended to include this publication in a higherdegree by research (HDR) thesis?
Yes If Yes, please complete Section 3If No, go straight to Section 4.
3. HDR thesis author’s declaration
Name of HDR thesis author ifdifferent from above. (If thesame, write “as above”)
School/Institute/Division if basedat Deakin
Thesis title
Tao Zhang Institute for Frontier Materials Polymer organogelsstabilized HIPE organogelsand applications in oil fields
If there are multiple authors, give a full description of HDR thesis author’s contribution to thepublication (for example, how much did you contribute to the conception of the project, the design ofmethodology or experimental protocol, data collection, analysis, drafting the manuscript, revising itcritically for important intellectual content, etc.)I contributed about 70 % to the conception of the project. I designed about 65% of experimentalprotocol, analysed most of the data and rewrite about one third of the manuscript.
I declare that the above is an accurate description ofmy contribution to this paper, and the contributionsof other authors are as described below.
Signatureand date
10/12/2015
4. Description of all author contributions
Name and affiliation of author Contribution(s) (for example, conception of the project, designof methodology or experimental protocol, data collection,analysis, drafting the manuscript, revising it critically forimportant intellectual content, etc.)
Tao Zhang at Institute for FrontierMaterials
I contributed about 70 % to the conception of the project. I alsodesigned about 65% of experimental protocol, analysed thedata and rewrite about one third of the manuscript.
Dr. Yuanpeng Wu at Institute forFrontier Materials, DeakinUniversity, and at School ofMaterials Science and Engineering,Southwest Petroleum University,Chengdu, China
He contributed about 15 % to the conception of the project. Hecollected the data, analysed the data partially and draftedmanuscript.
Dr. Zhiguang Xu at Institute forFrontier Materials
He contributed 5 % to the conception of the project and helpedwater contact angle measurements.
Prof. Qipeng Guo at Institute forFrontier Materials
He is my supervisor. He contributed about 10 % to theconception of the project and revised the paper critically.
5. Author DeclarationsI agree to be named as one of the authors of this work, and confirm:i. that I have met the authorship criteria set out in the Deakin University Research
Conduct Policy,ii. that there are no other authors according to these criteria,iii. that the description in Section 4 of my contribution(s) to this publication is
accurate,iv. that the data on which these findings are based are stored as set out in Section 7
below.
If this work is to form part of an HDR thesis as described in Sections 2 and 3, I furtherv. consent to the incorporation of the publication into the candidate’s HDR thesis
submitted to Deakin University and, if the higher degree is awarded, thesubsequent publication of the thesis by the university (subject to relevantCopyright provisions).
Name of author Signature* Date
Tao Zhang 10/12/2015
Dr. Yuanpeng Wu 10/12/2015
Dr. Zhiguang Xu 10/12/2015
Prof. Qipeng Guo 10/12/2015
6. Other contributor declarations
I agree to be named as a non author contributor to this work.
Name and affiliation of contributor Contribution Signature* anddate
Dr. Yuanpeng Wu at Institute forFrontier Materials, Deakin University,and at School of Materials Science andEngineering, Southwest PetroleumUniversity, Chengdu, China
Contributed about 15 % to theconception of the project. Collectedthe data, analysed the data partiallyand drafted manuscript. 10/12/2015
Dr. Zhiguang Xu at Institute for FrontierMaterials
Contributed 5 % to the conceptionof the project and helped watercontact angle measurements. 10/12/2015
Prof. Qipeng Guo at Institute forFrontier Materials
Contributed about 10 % to theconception of the project andrevised the paper critically. 10/12/2015
* If an author or contributor is unavailable or otherwise unable to sign the statement ofauthorship, the Head of Academic Unit may sign on their behalf, noting the reason for theirunavailability, provided there is no evidence to suggest that the person would object to beingnamed as author
7. Data storage
The original data for this project are stored in the following locations. (The locations must bewithin an appropriate institutional setting. If the executive author is a Deakin staff memberand data are stored outside Deakin University, permission for this must be given by the Headof Academic Unit within which the executive author is based.)
Data format Storage Location Date lodged Name of custodianif other than theexecutive author
Electronic raw data, analysed data andpaper
Computers ofrelated labs ofInstitute forFrontier Materialsat DeakinUniversity
2014 Institute forFrontier Materials
This form must be retained by the executive author, within the school or institute in whichthey are based.
If the publication is to be included as part of an HDR thesis, a copy of this form must beincluded in the thesis with the publication.
128
7.2 Introduction
Oil spills have become one of the major environmental issues.1 Oil spills
cause serious and irrecoverable damage to the marine environment and the
ecosystem, and finally affect the human health. Therefore, these petroleum-
based pollutants must be removed from water surfaces speedily and efficiently.2
Common methods to clean the oil spills include combustion, mechanical
treatment using oil sorbent materials, chemical treatment by dispersants, and
bioremediation with microorganisms.3 Among these methods, the use of
sorbents is one of the most desirable options for the treatment of oil spills owing
to the economy and efficiency.
Many kinds of sorbent materials have been developed for oil spill cleanup.4-
6 However, the currently used sorbents have some drawbacks: slow oil
absorption, low absorption capacity, production of toxic wastes, high cost and
low recovery efficiency.7 In recent years, phase-selective organogels have been
used as sorbents to selectively remove oil from water due to their unique
efficiency for large-scale oil cleanup.8, 9 Phaseselective organogels based on
sugar-derivatives,9 mannitol,10 aromatic amino11 or carbohydrates12 have been
prepared and investigated for oil spill recovery. However, these organogels still
have limitations: for practical applications, a heating–cooling cycle is usually
applied to initiate gelation13 or a watermiscible carrier solvent (ethanol,
tetrahydrofuran and dioxane) is needed for the introduction of gelators into oil
spills.11 The heating–cooling approach is costly and complicated while the
water-miscible solvent route consumes a huge amount of solvent and leaves
toxic chemicals in the sea which has deleterious effects on marine life.10 Lee et
al. prepared xerogels from phase-selective organogels to absorb spilled oil from
water.14 Though these xerogels are of low-cost and are effective, their absorption
capacity is very low (ca. 4 g g-1). HIPE xerogels are another important series of
xerogels and are developed nowadays for treatment of oil spill. Porous
polystyrene15 and poly(tertiary-butylmethacrylate) xerogels16 were prepared by
using the HIPE gel as the template and these polymer xerogels exhibited oil
absorption properties. However, low-molecular mass gelators (cholesteryl
derivatives) which are costly are necessary in these procedures.
Recently, we reported the preparation of HIPE organogels from charge-
129
driven assembled polymer organogels,17-19 and it is interesting to find that these
HIPE organogels exhibit oil separation properties.19 Herein we report HIPE
xerogels as oil absorbents for the cleanup of oil spills. The HIPE xerogels with
high porosity and low density were obtained by removing solvents from HIPE
organogels by freeze drying, and HIPE organogels were prepared from PPI
dendrimers and a block ionomer sulfonated SSEBS. The HIPE xerogels obtained
were examined as sorbents for oil spill recovery, and they showed high oil
absorption capacity, absorption rate and reusability. To the best of our knowledge,
this is the first polymer xerogel based on charge-driven assembled polymer
HIPE organogels. And the polymer xerogels exhibit excellent oil absorption and
reusable properties.
7.3 Experimental section
7.3.1 Materials and preparation of samples Materials. Three kinds of SEBS with 28, 42 and 68 mol % of polystyrene
blocks were bought from Sigma-Aldrich. Three kinds of SSEBS was prepared
from these SEBSs respectively according to the procedure in previous reports.17,
20 The sulfonation degree of SSEBS was tested by titration method which was
described detailed in our previous work.17 And the titration results showed that
the sulfonation degrees of SSEBS samples prepared from SEBS with 28, 42 and
68 mol % polystyrene blocks were 15, 13.3 and 11.6%, respectively. PPI
dendrimers of generation 2 were supplied by Sigma-Aldrich and used as
received. Other chemicals such as toluene, methanol, THF, NaCl, diethyl ether
were of reagent grade. Distilled water was used throughout the experiment.
Preparation of HIPE organogels. HIPE organogels were prepared
according to the route reported in our previous work.18 In typical, 2 % (w/v) of
PPI dendrimers in toluene was mixed with 5 % (w/v) of SSEBS solution in
toluene and methanol (v/v, 98:2) and then the mixture was stirred acutely to form
organogels. Subsequently, tetrahydrofuran and 1M aqueous NaCl solutions were
added to the pre-formed organogels, and after shearing the mixture for 5 minutes
with a Vortex mixer at 3,400rpm, HIPE organogels were obtained.
Preparation of HIPE xerogels. The as-prepared HIPE organogels were
dispersed into diethyl ether under stirring. Then the mixture was poured into
130
huge volume of water under vigorous stirring and filtered after stirring for 2
hours. The HIPE xerogels were obtained by freeze drying for 24 hours.
7.3.2 Characterization of HIPE xerogels
Confocal imaging of HIPE organogels was performed on a laser scanning
confocal microscope (Leica SP5, Leica Microsystems CMS GmbH, Germany).
The porous structure of HIPE xerogels was observed using a Neoscope JCM-
5000 SEM (JEOL, Japan). Contact angle was tested on a KSV CAM 101 contact
angle instrument.
7.3.3 Characterization of oil absorption performance
The oil absorption performance of HIPE xerogels can be characterized by
oil absorption capacity, absorption rates and reused property of the oil
absorbents.2
The mass absorption capacity can be defined as (ms-m0)/m0, where m0 and
ms represent the weights of the xerogels before and after oil-absorption,
respectively. A known weight of HIPE xerogels (m0) were put into a beaker
containing oil or organic liquid and left quiescent for overnight. Then the
xerogels were lifted and dried to remove residual surface oil/organic liquid
before recording the weight of ms. The oil absorption capacity can be calculated
from the above equation.21
The absorption rate of HIPE xerogels was investigated by putting a certain
weight of HIPE xerogels (m0) into the oil or organic liquid, then monitoring the
weight of xerogels (mt) with time. The weight gained of HIPE xerogels at time
t was defined as ((mt-m0)/m0).22
The reused property of HIPE xerogels was carried out by simple mechanical
squeeze, and the absorbed oil or organic liquid was recovered from the xerogels.
The recovery was calculated as mre/(ms-m0), where mre is the weight of the
recovered oil or organic liquid, ms is the swollen xerogels weight (g) at the
saturated absorption state and m0 is the initial weight of the xerogels. 23
The water absorption capacity was tested according to the approach for the
determination of the oil absorption capacity. The initial weight of HIPE xerogels
was measured as m0. Then, the xerogels were put on the surface of distilled water,
131
0.5M NaCl aqueous solution and sea water, and after absorption for 2 hours, the
weights of the xerogels were recorded as mw. The water absorption capacity can
be deduced through (mw-m0)/m0. 2
7.4 Results and discussion 7.4.1 Formation of SSEBS/DAB-8 HIPE xerogels
The formation of HIPE organogels was confirmed by tube-inversion
method, conductivity measurements and rheological measurements. The
morphology of these HIPE organogels was observed by confocal microscopy,
and microphase separation was revealed by SAXS. These results can be found
in Chapter 4. A typical confocal micrograph is shown in Figure 7.1(a), and it can
be observed that the dispersed domains are water phase and that the sizes of the
water droplets vary from several to tens of micrometres. It is possible to obtain
porous HIPE xerogels with low density after the removal of solvents due to the
high volume fraction of the dispersed phase in these HIPE organogels.18
Figure 7.1 (a) Confocal image of HIPE organogels; (b) SEM image of HIPE
xerogels.
The HIPE xerogels were obtained after the removal of water and solvents
by freeze drying. The morphology of the HIPE xerogels was observed by SEM.
It can be seen from Figure 7.1(b) that the HIPE xerogels exhibit a porous
structure. However, the porous network structure was retained after freeze-
drying as shown in Figure 7.1(b). With the fraction of the water phase being over
74% by volume in HIPE organogels, the as-prepared HIPE xerogels are highly
132
porous and lightweight. The densities are 0.108, 0.069 and 0.103 g cm-3 for the
as-prepared HIPE xerogels from 28%, 42% and 68% of polystyrene in SSEBS,
respectively. The density difference of the HIPE xerogels may be explained by
the well-define structures of pores. Due to the existence of high porosity and
oleophilicity, the HIPE xerogels are expected to have a high absorption capacity
and absorption rate for oils.
7.4.2 Absorption capacity
Figure 7.2 Separation of diesel from water by HIPE xerogels. The diesel was
dyed with oil red o.
To examine HIPE xerogels as effective oil absorption agents, a variety of
oils and organic solvents, including diesel, gasoline, engine oil and a number of
organic solvents, were employed to mimic the spilled oils. It was found that the
HIPE xerogels absorbed oil from water–oil mixtures quickly, perhaps because
the existence of pores with large sizes. HIPE xerogels were observed floating on
the surface of water after the absorption, as both the densities of oils and porous
materials are lower than that of water.
Figure 7.2 illustrates a typical separation process of oil from water by HIPE
xerogels. Diesel can be absorbed completely by the HIPE xerogels in one minute
and then the resultant oil absorbed xerogels remain floating on the water layer.
The oil absorbed in the xerogels can be recovered by simply squeezing out with
a recovery rate of about 81.6% (Figure 7.4a). This process has demonstrated that
the spilled oil can be successfully separated and reclaimed from water with HIPE
133
xerogels as sorbents. The other organic liquids can also be recovered by the
xerogels in a similar way to diesel. In comparison with other methods such as
vaporization24 and washing with solvents25 or burning,26 the squeezing-out
method for reusing oil sorbents is relatively simple and inexpensive. As the
HIPE xerogels can absorb oil/organic solvents from the surface of water, it is
expected that this process can be scaled-up, promising for practical applications.
The absorption capacity of different oils and organic solvents with the HIPE
xerogels was investigated. Saturated absorption was reached after these xerogels
were immersed into the oils or organic solvents overnight. The absorption
capacity of the xerogels is calculated from the weight of oils or organic solvents
in the HIPE xerogels relative to the dry weight of the xerogels, and the results
are shown in Figure 7.3a. These xerogels exhibit high mass absorption capacities
for a variety of solvents: 19.8 g g-1 (hexane), 22.3 g g-1 (toluene), 24.9 g g-1
(xylene), 22.6 g g-1 (dichloroethane), 32.2 g g-1 (chloroform), 24.2 g g-1
(vegetable oil), 21.2 g g-1 (gasoline), 23.8 g g-1 (diesel), 31.5 g g-1 (engine oil)
and 15.1 g g-1 (crude oil).
Figure 7.3 Absorption capacities of the HIPE xerogels for various organic
solvents and oils.
The results show that the absorption capacities of the HIPE xerogels are
higher than those of many xerogels14, 27 and are close to or higher than those of
oil absorbents or organogels reported (shown in Table 7.1).28, 29 Such high
absorption performance can be attributed to their porous nature and oleophilic
134
properties of the alkyl and aromatic moieties in the xerogels, which leads to a
large capillary action for wetting by oils or organic solvents.30
Table 7.1. Comparison of absorption capacities of several absorbents
Absorbents Oils/solvents and absorption capacities (g/g)
Ref.
Phase-selective xerogels gasoline (5.83), diesel (4.59) 21
Lignin-based xerogels gasoline (c.a. 2.1), toluene(c.a. 2.5) 27 Polymeric sponges diesel (c.a. 40) 29 Polymethylsilsesquioxane xerogels
kerosene (c.a. 7.8), toluene (c.a. 8.5) 28
PolyHIPEs gasoline (16.49), n-hexane (4.45), mineral ether (5.02), kerosene (6.19), benzene (18.71), dichloromethane (20.21), salad oil (2.75), machine oil (2.51)
15
Poly (tertiary-butylmethacrylate) HIPE xerogels
kerosene (8.17), benzene (15.37), dichloromethane (17.33), used oil (4.98)
16
Polymer SEBS HIPE xerogels
gasoline (21.2), diesel (23.8), dichloroethane (22.6), toluene (22.3)
This work
7.4.3 Absorption rate and recovery rate
The oil absorption rate of HIPE xerogels was tested using toluene and diesel
as model organic solvents and oils; the results are presented in Figure 7.4. It is
noted that toluene and diesel were absorbed into the xerogels quickly in the first
few minutes, and the absorption equilibrium was reached within 50 minutes for
toluene and within 60 minutes for diesel. The absorption rate is much higher
than that of xerogels prepared from phase-selective organogels,12 oleophilic
polyelectrolyte gels31 and PAN/DMSO gels32 which may need a few days to
reach saturated absorption.
135
Figure 7.4 weight gain by the HIPE xerogels in toluene and diesel versus time.
Figure 7.5 Absorption capacities of HIPE xerogels based on SEBS with 28 mol %
of polystyrene blocks for organic solvents and oils.
Two other HIPE xerogels prepared from SEBS samples with 28 and 68 mol%
of polystyrene blocks also exhibit high oil absorption capacities, and their results
are shown in Figure 7.5 and 7.6. It was expected that HIPE xerogels from 28%
of polystyrene blocks have the highest absorption capacity and from 68% exhibit
the lowest capacity, because higher middle block ratio means more oils can be
trapped. However, the experimental results are quite different from the
assumption, perhaps because the absorption capacity is determined by not only
the middle block contents but also the void fraction in the porous materials. As
136
shown by SEM, both pores and windows exist in these HIPE xerogels, which
makes the exact investigation of pore fraction difficult.
Figure 7.6 Absorption capacities of HIPE xerogels based on SEBS with 68
mol % of polystyrene blocks for organic solvents and oils.
The reusability of HIPE xerogels for oil absorption – recovery was studied
as one of the key factors to recover the oil spill effectively, reduce the cost and
reuse the oil sorbents.33 The HIPE xerogels can be reused for oil spill recovery
by a repeatable absorbing–squeezing out–separation process. Figure 7.7
demonstrates such an absorption–recovery process of oil from the water–oil
mixture with HIPE xerogels, and that the xerogels can be used repeatedly.
Figure 7.7 Absorption-squeeze process of HIPE xerogels separating diesel from
water.
The reusability of HIPE xerogels for absorption and recovery of diesel was
examined for 40 cycles. It can be seen from Figure 7.8 that the absorption and
recovery capacity of the HIPE xerogels do not change obviously even after 40
cycles. The diesel absorbed by the HIPE xerogels can be recovered with a
recovery rate of 75–82%.
137
The environmental temperature, such as the temperature of sea water,
changes with the climate. The effects of temperature were attended, and the oil
absorption of HIPE xerogels was also examined at 0 and 45 °C. As shown in
Figure 7.8b and c, the oil recovery remains almost unchanged although the
absorption capacity is only slightly affected when the temperature is down to
0 °C or increased to 45 °C. The HIPE xerogels developed in this study are the
first polymer xerogels with such a high reusability in a broad range of
temperatures. The excellent reusability of these HIPE xerogels within a broad
range of temperature can reduce the cost of the process.
138
Figure 7.8 Absorption capacities of HIPE xerogels and spilled oil recovery with
diesel absorption–squeezing-out cycle number at (a) 20 °C, (b) 0 °C and (c)
45 °C.
Figure 7.9 Optical images of a droplet of (a) distilled water, (b) 0.5 M NaCl
aqueous solution and (c) seawater on the surface of HIPE xerogels.
The surface property of HIPE xerogels was studied with contact angle
measurements, and the results are shown in Figure 7.9. In the contact angle
measurements, three types of liquids including distilled water, 0.5 M NaCl
aqueous solution and seawater were used, and the contact angle of HIPE
xerogels are 144, 137 and 138°, respectively. These results demonstrated that the
xerogels are hydrophobic for all the three liquids, and as a result, HIPE xerogels
should have a highly selective absorption for oil from oil water mixture.
The assumption has been confirmed by the water absorption experiment.
The water absorption capacity of these HIPE xerogels was examined with
different aqueous media such as distilled water, 0.5 M aqueous NaCl solution
and sea water. From the results shown in Figure 7.10, it can be seen that the mass
139
gained after 2 hours was only 0.82 g g-1 in distilled water, 0.95 g g-1 in 0.5MNaCl
aqueous solution, and 0.89 g g-1 in seawater. The low absorption capacity to
water is insignificant in comparison with the high absorption capacity to oils and
organic solvents.
Figure 7.10 Water absorption of HIPE xerogels in distilled water, 0.5M NaCl
solution and sea water.
7.5 Conclusions HIPE xerogels prepared in this work exhibited high absorption capacity and
easy recovery for a wide range of oils and organic solvents from oil–water
mixtures. The absorption capacity of the HIPE xerogels ranges from 20–32 times
to different kinds of oils, and the oils can be recovered simply by being squeezed
out, with a recovery rate as high as 80%. The HIPE xerogels absorb oil from the
water–oil mixture in 20–30 seconds. The HIPE xerogels can be reused more than
40 times without obvious deterioration in oil separation properties in a broad of
temperature range from 0 to 45 °C. These HIPE xerogels are suitable for
practical use in oil spill reclamation and wastewater treatment.
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24 H. Bi, X. Xie, K. Yin, Y. Zhou, S. Wan, L. He, F. Xu, F. Banhart, L. Sun
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Chapter Eight
Conclusions and Perspective for Future Work
8.1 General conclusions …………………………………… This research has developed a novel route to stabilize HIPE organogels by
polymer organogels which were formed from a triblock ionomer by charge-
driven assembly. The triblock ionomers were obtained by sulfonation of a
commercially available triblock copolymer SEBS, which were designed for the
purposes to prepare polymer organogels, HIPE organogels all the way to HIPE
xerogels for the applications in oil fields. This work presents a series of original
methods to fabricate polymer organogels and HIPE organogels, where ionic
interaction between –SO3H groups of SPS and oppositely charged components
forms the cores which are connected by the middle EB blocks of SSEBS to form
three-dimensional networks. Several factors such as sulfonation degree, the
composition of SSEBS, the type of counterion on the structures and mechanical
properties were studied. The general conclusions of this research are described
below.
a) Development of novel multiresponsive polymer organogels from block
ionomer SSEBS and oppositely charged components including block
copolymer, PPI dendrimers, benzoxazine molecules and surface modified
nanoparticles via charge induced assembly. The solvophobic ionic
complexes aggregate to form cores which are connected and stabilized by
the middle EB blocks of SSEBS to form three dimensional networks. These
polymer organogels are responsive to acid, amine and salt as these ionic
interactions can be affected by those chemicals.
b) HIPE organogels could be prepared from charge-driven assembled polymer
organogels which were formed from SSEBS and oppositely charged
component such as PPI dendrimer, surface modified nanoparticles and
benzoxazine. Water may enter the hydrophilic ionic microdomains, and
weakens the ionic interactions, leading to the dentangement of polymer
143
chains. The morphology of the pre-formed polymer organogels are
determined by the type of PPI dendrimers, sulfonation degrees and
polystyrene contents of SSEBS, but the morphologies of HIPE organogels
are mainly dominated by polystyrene contents of SSEBS. Salt concentration,
SSEBS and the ratios of the internal phase could affect the strengths of HIPE
organogels.
c) Porous materials (HIPE xerogels) could be obtained from HIPE organogels
by simple removal of solvents, and these structures of pores were reserved.
These HIPE xerogels are hydrophobic, and selectively absorb oil from water
surface. They showed excellent performances in oil spill recovery and oil
water separation considering their high absorption capacity, rate, simple
recovery and reusability.
d) Incorporation of functional components into HIPE organogels may endow
new functionality to these porous materials, facilitating their preparation or
applications. Magnetic responsive HIPE xerogels were obtained from
hybrid HIPE organogels by incorporating surface modified Fe3O4
nanoparticles, and the hybrid xerogels facilitate the collections after
absorption with a help of magnetic field. With the incorporation of reactive
benzoxazine to these charge-driven assembled polymer organogels and
corresponding HIPE organogels, the HIPE xerogels could be obtained at
room temperature, which facilitating the preparation of porous materials.
In conclusion, HIPE organogels were successfully stabilized by charge-driven
assembled polymer organogels which were prepared from SSEBS and
oppositely charged components. Porous materials were also prepared from these
HIPE organogels and all of them showed excellent performance for oil spill
recovery and oil water separation.
8.2 Perspective for future work …………………………. Charge-driven assembled polymer organogels have been investigated to
stabilize HIPE organogels in this work. These HIPE organogels are also studied
for their applications in petroleum fields including oil water separation and oil
spill recovery. As can be seen, more results on HIPE organogels have been
published but not included in this thesis, as a concise and topic-focused thesis is
144
expected. In addition to these results, following works can be carried out in
future to continue this research in this area:
a) Further identify the formation mechanism of HIPE organogels from charge-
driven assembled polymer organogels, establish a phase diagram of ionomer
structures to form HIPE organogels, and monitor the morphological
transition during the transformation of charge-driven assembled polymer
organogels to HIPE organogels.
b) Develop polyHIPEs from polymer organogels stabilized HIPE organogels,
and study the components of polymer organogels and HIPE organogels on
the morphology and mechanical strengths of polyHIPEs.
c) Investigate the pore size and distribution of HIPE xerogels on the
performances of oil absorption capacity, rate, re-usability and recovery rate.
d) Study these HIPE xerogels for the real-life application of HIPE xerogels for
oil spill recovery including absorption capacity, rate, recovery and re-
usability.