synthesis, micelle formation, and bulk properties of poly(ethylene...

Synthesis, Micelle Formation, and Bulk Properties of Poly(ethylene glycol)-

b-Poly(pentafluorostyrene)-g-polyhedral Oligomeric Silsesquioxane

Amphiphilic Hybrid Copolymers

H. HUSSAIN,1 B. H. TAN,1 K. Y. MYA,1 Y. LIU,1 C. B. HE,1 THOMAS P. DAVIS1,2

1Institute of Materials Research and Engineering (IMRE), A*STAR (Agency for Science, Technology and Research),

3, Research Link, Singapore 117602

2Centre for Advanced Macromolecular Design (CAMD) School of Chemical Sciences and Engineering, UNSW,

Sydney, New South Wales 2052, Australia

Received 25 August 2009; accepted 3 October 2009

DOI: 10.1002/pola.23773

Published online in Wiley InterScience (www.interscience.wiley.com).

ABSTRACT: The synthesis, micelle formation, and bulk properties

of semifluorinated amphiphilic poly(ethylene glycol)-b-poly-

(pentafluorostyrene)-g-cubic polyhedral oligomeric silsesquioxane

(PEG-b-PPFS-g-POSS) hybrid copolymers is reported. The syn-

thesis of amphiphilic PEG-b-PPFS block copolymers are

achieved using atom transfer radical polymerization (ATRP) at

100 �C in trifluorotoluene using modified poly(ethylene glycol)

as a macroinitiator. Subsequently, a proportion of the reactive

para-F functionality on the pentafluorostyrene units was

replaced with aminopropylisobutyl POSS through aromatic

nucleophilic substitution reactions. The products were fully char-

acterized by 1H-NMR and GPC. The products, PEG-b-PPFS and

PEG-b-PPFS-g-POSS, were subsequently self-assembled in

aqueous solutions to form micellar structures. The critical mi-

celle concentrations (cmc) were estimated using two different

techniques: fluorescence spectroscopy and dynamic light scat-

tering (DLS). The cmc was found to decrease concomitantly with

the number of POSS particles grafted per copolymer chain. The

hydrodynamic particle sizes (Rh) of the micelles, calculated from

DLS data, increase as the number of POSS molecules grafted per

copolymer chain increases. For example, Rh increased from

�60 nm for PEG-b-PPFS to �80 nm for PEG-b-PPFS-g-POSS25 (25

is the average number of POSSparticles grafted copolymer chain).

Static light scattering (SLS) data confirm that the formation of

larger micelles by higher POSS containing copolymers results

from higher aggregation numbers (Nagg), caused by increased

hydrophobicity. The Rg/Rh values, where Rg is the radius of gyra-

tion calculated fromSLS data, are consistent with a spherical parti-

cle model having a core-shell structure. Thermal characterization

by differential scanning calorimetry (DSC) reveals that the grafted

POSS acts as a plasticizer; the glass transition temperature (Tg) of

the PPFS block in the copolymer decreases significantly with

increasing POSS content. Finally, the rhombohedral crystal

structure of POSS in PEG-b-PPFS-g-POSS was verified by wide

angle X-ray diffraction measurements. VC 2009 Wiley Periodicals,

Inc. J PolymSci Part A: PolymChem48: 152–163, 2010

KEYWORDS: amphiphilic block copolymers; atom transfer radical

polymerization; fluoropolymers; micelle; polyhedral oligomeric sil-

sesquioxane; self-assembly; semifluorinated

INTRODUCTION Fluoropolymers have attracted a great dealof attention both from academia and industry as they canimpart chemical resistance, thermal stability, low surfaceenergy, low dielectric constant, and low refractive index tomaterials.1,2 Recent advances in polymerization have enabledthe synthesis of well-defined semifluorinated copolymerswith defined architectures and compositions, precisely con-trolled chain lengths and narrow molecular weight distribu-tions. These predesigned polymers have been exploited in arange of applications including low surface energy coat-ings,3–5 stimuli responsive brushes,6–8 surface modifiedmembranes for water treatment,9,10 and fouling release coat-ings.11 Well-defined semifluorinated copolymers have beensynthesized using living polymerization techniques, such as

anionic,12 cationic,13 and living radical.14–21 Hansen et al.22

and Hirao et al.23 have recently published excellent reviewson the design of well-defined fluorinated homo- and copoly-mers by living radical polymerization techniques, such asnitroxide mediated radical polymerization (NMRP),24,25 tran-sition-metal-catalyzed atom transfer radical polymerization(ATRP)26,27 and reversible addition fragmentation chaintransfer (RAFT)/macromolecular design through the inter-change of xanthates (MADIX).28–32 ATRP, in particular, hasbeen widely exploited for the synthesis of well-defined semi-fluorinated amphiphilic block copolymers.4,17,19,33–35 Livingradical polymerization works by establishing a dynamic equi-librium between a low concentration of active propagatingspecies and a dormant species unable to propagate or

Additional Supporting Information may be found in the online version of this article. Correspondence to: Thomas P. Davis (E-mail: t.davis@unsw.

edu.au), H. Hussain (E-mail: [email protected]) or C. B. He (E-mail: [email protected])

Journal of Polymer Science: Part A: Polymer Chemistry, Vol. 48, 152–163 (2010) VC 2009 Wiley Periodicals, Inc.

152 INTERSCIENCE.WILEY.COM/JOURNAL/JPOLA

terminate.36 The equilibrium can be established by eitherthe persistent radical effect37,38 or by addition-fragmentationchemistry.39–42

Polyhedral oligomeric silsesquioxane (POSS) has a well-defined, nanometer scale, cage-like cubic structure.43 POSSnanocages can be functionalized easily with organic func-tional groups (reactive or inert) that facilitate miscibilityand/or covalent incorporation into organic polymers, leadingto nanohybrids with improved bulk properties such as glasstransition temperature, mechanical strength, thermal andchemical resistance, and low dielectric constant.44–52 Severalrecent studies have reported a decrease in glass transitiontemperature on the incorporation of POSS into polymers toform nanocomposites.53,54 We recently reported fluorinatedhybrid star polymers having POSS nanocages as the coreswith fluorinated arms synthesized by ATRP.55 In this work,we report the synthesis, self-assembly, and bulk propertiesof POSS containing amphiphilic semifluorinated poly(ethyl-ene glycol)-b-poly(pentafluorostyrene)-g-polyhedral oligo-meric silsesquioxane (PEG-b-PPFS-g-POSS) hybrid blockcopolymers, where the pendant POSS molecules are statisti-cally distributed along the PPFS block. The influence of thePOSS functionality on the material and solution properties ofthe block copolymer is investigated.

EXPERIMENTAL

MaterialsTrisilanolisobutyl POSS was purchased from Hybrid Plastics(product no.: SO1450). Aminopropyltriethoxysilane (APTEOS)and triethylamine (TEA) were purchased from Aldrich. Tri-ethylamine was distilled over calcium hydride (CaH2) undernitrogen before use. Tetrahydrofuran (THF) was purified bydistillation over Na/benzophenone under a nitrogen atmos-phere immediately before use. 2,3,4,5,6-pentafluorostyrene(Aldrich, 99%) was purified by an alumina column. Anhy-drous trifluorotoluene (Aldrich, 99%), 2-bromoisobutyrylbromide (Aldrich, 98%) CuBr (Aldrich, 99.99%,), 2,2-bipyri-dine (Strem Chemicals, 99þ%), and n-ethyldiisopropylamine(Fluka, 98%) were used as received. PEG (Mn ¼ 5000 g/mol) was purchased from Fluka and dried under vacuumbefore use.

Synthesis of Aminopropylisobutyl POSSTrisilanolisobutyl POSS (5.0 g, 6.32 mmol) was dissolved in an-hydrous THF (50 mL) under an argon atmosphere. Triethyl-amine (2.6 mL, 18.96 mmol) was then added, and the mixturewas stirred at room temperature for 10 min. Aminopropyltrie-thoxysilane (1.5 mL, 6.32 mmol) was added dropwise underargon and stirring continued at room temperature for 1 day.The viscous liquid was obtained after completion of the reac-tion. The crude product was dissolved in a small amount of dryTHF and purified by precipitation in an excess amount ofmethanol. The final product was obtained after filtrationand drying in a vacuum oven at 40 �C for 2 days (yield:>92%). 1H-NMR (d-chloroform): 0.58–0.60 (SiCH2); 0.94–0.96 (SiCH2CH(CH3)2); 1.82–1.88 (SiCH2CH(CH3)2); and2.68–2.70 (Si CH2CH2CH2NH2). The structure of the prod-uct was further verified by Matrix-Assisted Laser Desorp-

tion/Ionization-Time of-Flight Mass Spectrometry (MALDI-TOF-MS). In addition, a monomodal GPC profile (Mn ¼1122, Mw ¼ 1128 g/mol, and PDI ¼ 1.01) confirmed thatno higher molecular weight byproducts were formed dur-ing the reaction, confirming the successful synthesis ofaminopropylisobutyl POSS.

Synthesis of PEG-b-PPFS Diblock CopolymerMPEG (Mn ¼ 5000 g/mol) was transformed into an ATRPmacroinitiator by reaction with 2-bromoisobutyryl bromidein THF at room temperature.56 MPEG macroinitiator (0.14mmol), 2,3,4,5,6-pentafluorostyrene (22.1 mmol), and anhy-drous trifluorotoluene (1.4 mL) were mixed under argon.The mixture was deoxygenated by four freeze-pump-thawcycles, followed by the addition of CuBr (0.14 mmol) and2,2-bipyridine (0.42 mmol) under argon. The contents weresubjected to two more freeze-pump-thaw cycles to ensurecomplete oxygen removal. The mixture was maintained at100 �C for �3 h before air exposure and dilution with THF.The product solution was purified through an aluminacolumn, precipitated in hexane, and dried under vacuum at50 �C overnight.

Grafting of Aminopropylisobutyl POSS to PEG-b-PPFSA typical reaction is described as follows: PEG-b-PPFS(0.3 g), aminopropylisobutyl POSS (0.19 g), and n-ethyldiiso-propylamine (80 lL) were dissolved in dried THF (2.5 mL)and refluxed at 80 �C for �21 h. Aminopropylisobutyl POSSis not soluble in methanol and ethylacetate. Therefore, toremove the unreacted POSS materials, the product was dis-persed in methanol or ethylacetate and centrifuged repeat-edly. The upper transparent layer of solution containing thedesired product was recovered. The final product was recov-ered by solvent evaporation using a rotary evaporator andsubsequent drying under in vacuum at 40 �C overnight. Thepurity of the product was verified by GPC, where traces dueto unreacted POSS species disappeared after purification.The synthesis of block copolymers and subsequent graftingof POSS is outlined in Scheme 1.

Characterization1H-NMR spectra were recorded on a Bruker 400 MHz spec-trometer in d-chloroform. GPC analyses were carried out inTHF at 35 �C with a flow rate of 1 mL/min using a Waters2690 system fitted with an evaporative light scattering de-tector (Waters 2420) and a UV detector (Waters 996, photo-diode array). The GPC was calibrated with both PS andPMMA standards. TEM images were obtained using a JEOL2100 transmission electron microscope operating at anacceleration voltage of 200 kV. TEM samples were preparedby dip coating carbon coated copper grids with block copoly-mer solutions. Samples for surface characterization were pre-pared by spin coating the copolymer solutions in chloroform(10 mg/mL) on cleaned silicon chips. The static and advanc-ing and receding contact angles of water on copolymer surfa-ces were measured using a Rame-Hart goniometer. DSC char-acterization was performed using a TA Instruments DSCQ100 at a heating rate of 10 �C min�1. Wide angle X-ray

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PROPERTIES OF PEG-b-PPFS-g-POSS HYBRID COPOLYMERS, HUSSAIN ET AL. 153

diffraction (WAXD) studies were conducted on a BrukerGADDS XRD system.

Dynamic Light ScatteringRoom-temperature light scattering measurements were madewith a Brookhaven BI-200SM goniometer equipped with aBI9000AT digital correlator. An inverse Laplace transformationof REPES in the GENDIST software package was used to obtain

decay time distribution functions with the probability to-rejectset at 0.5. The copolymer concentration in aqueous solution was0.2 mg/mL while the scattering angle was set at 90�. From theexpression C ¼ Dappq

2, the apparent translational diffusion coef-ficients, Dapp, were determined. C is the decay rate, which is theinverse of the relaxation time, s; q is the scattering vectordefined as q ¼ (4pn sin(h/2)/k) (where n is the refractive indexof the solution, h is the scattering angle, and k is the wavelength

SCHEME 1 Synthesis of amphiphilic PEG-b-PPFS-g-POSS hybrid copolymer.

JOURNAL OF POLYMER SCIENCE: PART A: POLYMER CHEMISTRY DOI 10.1002/POLA


of the incident laser light in vacuum).57–59 The particle interac-tions in dilute solution can be assessed using a virial expansion,describing the concentration; c, dependence of the apparent dif-fusion coefficient, Dapp:

58

Dapp cð Þ ¼ D0 1þ kcDc� �

(1)

where kcD is the diffusion constant.

After extrapolation to infinite dilution, the diffusion coeffi-cient at infinite dilution, D0 and hence the hydrodynamicradius (Rh) can be determined by the Stoke-Einstein relation-ship:57,59

Rh ¼ kBT6pgD0

(2)

where kB, g, and T are the Boltzmann constant, viscosity ofsolvent, and the absolute temperature, respectively.

The dynamic light scattering (DLS) method provides a simplebut accurate method to obtain the size distribution of themicelles using the cumulants method where the field autocor-relation function, g1(t) is fitted to a second order polynomial,which yields the first cumulant, �C and the second cumulant,l2.

60 The first cumulant, �C describes the average decay rateof the distribution, whereas the second cumulant, l2 is ameasure of the width of the distribution w(C) and hence theeffective polydispersity of the sample. For moderately poly-disperse spherical particles in solution, the following relationhas been derived:57

l2=�C2 � ðMw=Mn � 1Þ=4 (3)

where Mn is the number average molecular weight, Mw theweight average molecular weight, and the ratioMw/Mn is knownas the polydispersity index. The method of cumulants is verysensitive to the distribution of particle sizes, thus providing aquantitative measure of the dispersity of particle sizes in thesample. Information on the polydispersity of the micellar aggre-gates in aqueous solutions obtained using eq 3 is included inTable 2.

Static Light ScatteringStatic light scattering (SLS) was used to measure and analyzethe time–average scattered intensities where the weight–av-erage molecular weight ( �Mw) and the second virial coeffi-cient (A2) of the block copolymer unimers and micelles canbe determined.61,62 The refractive index increment of the co-polymer solution, (dn/dc) was measured using a BI-DNDCdifferential refractometer at a wavelength of 620 nm todetermine the refractive index increment (dn/dc) of each so-lution. The instrument was calibrated primarily with potas-sium chloride (KCl) in aqueous solution.

Fluorescence SpectrometryThe excitation spectra (300–360 nm) of the block copoly-mers in aqueous solutions were recorded with an emissionwavelength of 395 nm and the excitation and emission band-widths being set at 3 nm. The ratios of the peak intensities

at 338 nm and 333 nm (I338/I333) of the excitation spectrawere analyzed as a function of polymer concentration. Thecmc values were taken from the intersection of the tangentto the curve at the inflection with the horizontal tangentthrough the point at the low concentrations.

Micelle Formation of PEG-b-PPFS in Aqueous SolutionThe PEG-b-PPFS and PEG-b-PPFS-g-POSS hybrid copolymersproved to be insoluble in water. Consequently, the copoly-mers were first dissolved in a small volume of THF, followedby the slow addition (�2 mL/min) of a known volume ofwater. In a typical process, 0.4 mg of PEG-b-PPFS was dis-solved in 0.2 mL of THF, and the solution was diluted with2 mL of deionized water. To ascertain the complete evapora-tion of THF from the solution, the 2.2 mL and 2 mL volume,which represents the volume of solution with and withoutTHF, respectively, was marked on the sample vial. The massof the initial 2.2 mL polymer solution was also measured.Next, the solution was left to evaporate at room temperatureuntil the volume drops to the 2 mL level, and the mass wasobserved to decrease until it remains constant where the du-ration taken is �24 h. The drop in the volume and mass ofthe solution is mainly due to the complete evaporation ofTHF (0.2 mL THF was added to dissolve the polymer) asvery minimal amount of water evaporates at roomtemperature.

RESULTS AND DISCUSSION

Amphiphilic PEG-b-PPFS diblock copolymer was synthesizedby ATRP using a modified PEG (5000 g/mol) as the macroi-nitiator. The ATRP of pentafluorostyrene was carried out at100 �C in anhydrous trifluorotoluene as the solvent usingCuBr/2,2-bipyridine as the catalyst system. 1H-NMR

FIGURE 1 GPC traces of PEG-macroinitiator, PEG-b-PPFS, and

PEG-b-PPFS-g-POSS25.

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spectroscopy and GPC analyses were used to characterizethe obtained PEG-b-PPFS. The GPC chromatograms of thePEG macroinitiator and the respective PEG-b-PPFS diblockcopolymer are shown in Figure 1. A successful chain exten-sion was confirmed by a significant shift in the GPC chromat-ogram of the macroinitiator (to a higher molecular weight)after the reaction. A 1H-NMR spectrum of the amphiphilic

PEG-b-PPFS copolymer in d-chloroform is shown in Figure 2,where signals a at �3.6 ppm and signals b in the region�1.8 to 2.9 ppm can be assigned to PEG (ACH2ACH2AOA)Aand the PPFS block (ACH2ACHA)A, respectively. The exis-tence of signals from both PEG and PPFS in the 1H-NMRspectrum in combination with the GPC data indicates thesuccessful synthesis of well-defined amphiphilic PEG-b-PPFS

FIGURE 2 1H-NMR spectrum of PEG-b-PPFS and PEG-b-PPFS-g-POSS6.



TABLE 1 Characteristic Data of PEG-ini, PEG-b-PPFS, and PEG-b-PPFS-g-POSS

Sample Mna Mn

b Mwc Mw/Mn

PEG-macroinitiator 5,600 8,400 n.d. 1.08

PEG-b-PPFS 12,600 20,900 21,450 6 207 1.10

PEG-b-PPFS-g-POSS6 12,700 21,300 26,700 6 315 1.10

PEG-b-PPFS-g-POSS19 14,200 23,700 38,100 6 422 1.14

PEG-b-PPFS-g-POSS25 15,100 25,400 43,400 6 385 1.14

Mn values (g/mol) were evaluated by GPC against

n.d., not determined.a PS and bPMMA standards, ccalculated from Zimm plots obtained from SLS measurements of the respective

copolymer solutions in THF.

FIGURE 3 (a) Average scattering light intensity (kcps), (b) intensity ratio (I338/I333), obtained from DLS and fluorescence measure-

ments, respectively, as function of PEG-b-PPFS-g-POSS6 copolymer concentration (mg/mL) in aqueous solution, and (c) cmc of

PEG-b-PPFS-g-POSS copolymers, obtained from fluorescence and DLS measurements, as function of average number of POSS

molecules grafted per copolymer chain. The line drawn in (c) has no significance or theoretical basis.

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diblock copolymers. By comparing the integration values in1H-NMR spectrum corresponding to PPFS and PEG block, thecomposition of the block copolymer was estimated asPEG113-b-PPFS111. However, for simplicity, we will representit as PEG-b-PPFS in the subsequent discussion.

Aminopropylisobutyl POSS was synthesized by the reactionof trisilanolisobutyl POSS with aminopropyl triethoxy silanein THF at room temperature using triethylamine as acatalyst. The structure of aminopropylisobutyl POSS was con-firmed using 1H-NMR, MALDI-TOF, and GPC. The aminopro-pylisobutyl POSS was then grafted to the PPFS block of PEG-b-PPFS block copolymer through nucleophilic aromaticsubstitution reaction at the para-F of the pentafluorostyrenearomatic rings.63,64 The grafting of POSS to PPFS block wasdone under THF reflux using ethyldiisopropylamine as a cat-alyst. The 1H-NMR spectrum of PEG-b-PPFS-g-POSS shown inFigure 2 confirms POSS functionalization; signals c � 0.58–0.60 ppm (SiCH2); d � 0.94–0.96 ppm (SiCH2CH(CH3)2); ande � 1.82–1.88 ppm (SiCH2CH(CH3)2) in addition to the pres-ence of PEG and PPFS blocks. Wide angle X-ray diffraction(WAXRD) investigations (vide infra), also confirmed a POSScrystal structure in the POSS grafted copolymers. However,the overall efficiency of the substitution reaction appears tobe lower as some aminopropylisobutyl POSS remainunreacted even after very long reaction times; �20 h insome cases, and hence, extensive purification was required.GPC analysis of the copolymer chains, post-POSS-functionali-zation also demonstrates a clear change in the hydrodynamicvolume while maintaining a narrow polydispersity (seeagain, Fig. 1). Interpretation of the GPC data fails to establishany correlation between overall grafting efficiency and mo-lecular weight. This analytical insensitivity can be ascribedto competing effects of increasing molecular weight (higherhydrodynamic volume) and increasing graft density (lowerhydrodynamic volume). The GPC data is, therefore, compro-mised and can only be seen as a rough estimate of molecularweight.65 SLS was used to evaluate the absolute molecularweights of PEG-b-PPFS-g-POSS with the data reported inTable 1. The molecular weights obtained using SLS are sig-nificantly higher than those estimated by GPC, particularlyfor higher POSS containing samples. The average number ofPOSS molecules grafted per block copolymer chain was esti-mated by comparing the absolute molecular weights of PEG-b-PPFS and the respective PEG-b-PPFS-g-POSS. The synthe-sized samples are abbreviated as PEG-b-PPFS-g-POSSX,where X represents the average number of POSS moleculesgrafted per copolymer chain.

Micelle FormationMicelle formation by PEG-b-PPFS and PEG-b-PPFS-g-POSScopolymers in aqueous solution was investigated by fluores-cence and dynamic/SLS measurements. The critical micelleconcentration (cmc) was evaluated by measuring the scatter-ing light intensity and fluorescence (using pyrene as probe)as a function of copolymer concentration in aqueous solu-tions at room temperature. As an example, the data reportedin Figure 3(a,b) show the light scattering intensity (fromDLS measurements) and intensity ratio of I338/I333 (from

fluorescence measurements), respectively, as a function ofPEG-b-PPFS-g-POSS6 (concentration (mg/mL)) in aqueoussolution. The sharp inflection point, where the scattering in-tensity [Fig. 3(a)] and I338/I333 [Fig. 3(b)] increases sharply,was defined as the cmc of the copolymers. Thus, the cmc ofPEG-b-PPFS-g-POSS6 was calculated to be �0.01 mg/mLfrom DLS and fluorescence measurements. The influence ofPOSS content on the cmc of copolymer was also investigatedas shown in Figure 3(c). The data in Figure 3(c) (obtainedfrom both DLS and fluorescence measurements) show adecrease in cmc with increasing grafted POSS molecules percopolymer chain. This behavior can be attributed to increas-ing hydrophobicity of the copolymers with increasing POSScontent.

Intensity autocorrelation functions (icfs), at scattering angleof 90�, for micelles formed from PEG-b-PPFS and PEG-b-PPFS-g-POSS with different number of POSS moleculesgrafted per copolymer chain are shown in a log–log plot inFigure 4 for the copolymer concentration of 0.1 mg/mL. Theexperimental data reveal just one relaxation process for allthe investigated samples. Analysis of the initial decay of theicfs, i.e., applying the cumulant method, reveals rather lowpolydispersity values obtained using eq 3. The measuredPDIs of the micelles are summarized in Table 2. The narrowmicelle size distributions are confirmed by the unimodalrelaxation times, s distribution functions obtained frominverse Laplace transformations. Figure 5(a) shows the dis-tribution functions, at a scattering angle of 90�, for PEG-b-PPFS and PEG-b-PPFS-g-POSS with different number of POSSmolecules grafted per copolymer chain at polymer concentra-tions of 0.1 mg/mL. The peak maxima shift toward higher relax-ation times with increasing POSS molecules grafted per copoly-mer chain, corresponding to the formation of larger micelles. Toconfirm that real particles are detected, an angle dependentmeasurement of decay rates C (the reciprocal of relaxation time,s) was performed for all the samples. Figure 5(b) shows the

FIGURE 4 Intensity autocorrelation functions obtained at scat-

tering angle of 90� and polymer concentration of 0.1 mg/mL

for (*) PEG-b-PPFS; (h) PEG-b-PPFS-g-POSS6; (~) PEG-b-

PPFS-g-POSS19; and (^) PEG-b-PPFS-g-POSS25.



dependence of the decay rate C on q2 at a scattering angles rang-ing from 60� to 120� and copolymer concentration of 0.1 mg/mL according to C ¼ Dappq

2, where C exhibits good linearrelationship with q2, confirming that the observed peaks in Fig-ure 5(a) originating from the translational diffusion of the copoly-mer micelles.57,59

The apparent diffusion coefficients Dapp for micelles with dif-ferent number of POSS molecules grafted per copolymerchain is obtained from the plot of C versus q2 and are plot-ted as a function of copolymer concentration, as shown inFigure 6. The micelle sizes (Rh) of all the samples show nosignificant dependence on the copolymer concentration inrange of 0.1–0.5 mg/mL as confirmed by the linearity of theapparent diffusion coefficient Dapp in this concentrationrange. This result is consistent with measurements made inthe dilute concentration regime, indicating little micelle–mi-celle interaction. The results also indicate that the micellesare formed according to a closed association mechanism.57,59

In this mechanism, the equilibrium concentration of theunimer in the copolymer micellar system is very close to thevalue of cmc, and the number unimers per micelle is con-stant thus monodisperse micelles are formed. After extrapo-lation to infinite dilution, the diffusion coefficient at infinitedilution, D0 can be obtained and the hydrodynamic radius

(Rh) of micelles with different number of POSS moleculesgrafted per copolymer chain was determined by the Stoke-Einstein relationship (eq 2). The results of the DLS measure-ments are summarized in Table 2, revealing an increase inmicelle size from (Rh) of �60 nm for PEG-b-PPFS to �80 nmfor PEG-b-PPFS-g-POSS25 with increasing POSS moleculesgrafted per copolymer chain.

Further experiments were performed to evaluate the influ-ence of POSS on the block copolymer micellization. SLS stud-ies were conducted on PEG-b-PPFS and PEG-b-PPFS-g-POSScopolymers at 25 �C in THF and aqueous solutions at scat-tering angles ranging from 30� to 120� at 10� intervals. Therespective weight–average molecular weights, Mw,single (singlecopolymer chains, measured in THF) and Mw,agg (micellaraggregates, measured in aqueous solution), were estimatedfrom their respective Zimm plots by extrapolating c and h tozero. The calculated values of Mw,single are given in Table 1.The calculated micellar parameters from SLS measurementsare summarized in Table 2 revealing a significant increase inweight–average molecular weight of micelle, Mw,agg from�995 � 103 g/mol for PEG-b-PPFS to �6530 � 103 g/molfor PEG-b-PPFS-g-POSS25. The corresponding aggregationnumbers Nagg calculated from the ratio of the molecularweight of aggregates (Mw,agg) to that of single copolymer

FIGURE 5 (a) Relaxation time, s distribution functions of micelles of PEG-b-PPFS and PEG-b-PPFS-g-POSS with different number

of POSS molecules grafted per copolymer chain at scattering angle of 90� and copolymer concentration of 0.1 mg/mL. (b) Depend-

ence of decay rate C on q2 of (*) PEG-b-PPFS; (h) PEG-b-PPFS-g-POSS6; (~) PEG-b-PPFS-g-POSS19; and (^) PEG-b-PPFS-g-

POSS25 at 0.1 mg/mL copolymer concentration.

TABLE 2 Characteristic Parameters of Micelles Formed by PEG-b-PPFS and PEG-b-PPFS-g-POSS

in Aqueous Solutions at 25 8C

Sample Rh (nm) Rg (nm) Rg/Rh PDI

Mw,agg � 10�3

(g/mol) Nagg

A2 � 10�5

(cm3 mol g�2)

PEG-b-PPFS 60.2 6 1.1 37.5 6 0.7 0.63 1.15 955 6 8.7 45 12 6 0.15

PEG-b-PPFS-g-POSS6 65.0 6 1.5 42.4 6 0.9 0.65 1.19 2,000 6 18.4 75 9.5 6 0.11

PEG-b-PPFS-g-POSS19 73.5 6 1.4 51.2 6 1.0 0.70 1.16 5,500 6 35.1 144 4.6 6 0.05

PEG-b-PPFS-g-POSS25 80.2 6 1.7 56.6 6 1.0 0.70 1.17 6,530 6 53.2 150 4.8 6 0.06

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chain (Mw,single), i.e., Nagg ¼ Mw,agg/Mw,single, thereforeincreased from 45 to 150 for this particular example. Alsogiven in Table 2, the second virial coefficient, A2, signifyingthe polymer–solvent interactions, decreased with increasingPOSS content in the block copolymer, consistent with adecrease in copolymer solubility with increasing POSS con-tent in aqueous media.

The ratio, Rg/Rh, is a characteristic parameter that providesuseful information on the geometry of colloidal particles,66

for example, for a uniform sphere (same density throughout),Rg/Rh � 0.775 and for anisotropic geometry, resulting fromrod- or disk like structures >0.775. Rg/Rh was calculated, forthe block copolymer samples, from the radius of gyration, Rg(Table 2, obtained from SLS measurements) and Rh ofmicelles. The data reported in Table 2 show that the Rg/Rhvalues are <0.775 for all the block copolymers, indicating a

spherical micelle geometry with a higher density at the cen-ter, i.e., a core-shell structure,67 where PEG would constitutethe shell and PPFS-g-POSS the core of micelle. Previously,our group and others have observed similarly lower Rg/Rhvalues for block copolymer micelles.67–70 The data in Table 2also reveal that the Rg/Rh increases from �0.63 for PEG-b-PPFS to �0.7 for PEG-b-PPFS-g-POSS25, a value much closerto the hard sphere model value, suggesting the formation oflarger and compact micelle cores as a result of increasedPOSS content and higher aggregation number (Nagg).

Micelle formation and structure can also be altered by exter-nal parameters, such as changes in temperature/solvent orthe presence of additives, such as electrolytes.71–73 Subse-quently, we extended our investigations to probe micelle for-mation by PEG-b-PPFS in a polar organic solvent, viz., metha-nol (a selective solvent for PEG block). DLS data revealed theformation of micelles in methanol (smaller than those foundin aqueous solution). The hydrodynamic radius, Rh, was cal-culated as �24.5 nm for PEG-b-PPFS (less than half the sizein aqueous solution). Furthermore, the hydrodynamic sizesof micelles formed in methanol are independent of the blockcopolymer concentration in the range 1.0–5.0 mg/mL (datanot shown), i.e., the micellization follows a close associationmechanism. TEM micrographs of the micelles formed byPEG-b-PPFS in methanol solution are shown in Figure 7(a,b)for different polymer concentrations. The micelles appear tohave coalesced into a beaded structure as the solvent evapo-rated. The reason for the formation of these structures isunknown; however, similar behavior has been reported pre-viously for micelles formed by amphiphilic block copolymersof PEG and fluorinated methacrylate.74 The solvent depend-ent particle sizes can be rationalized by solvent quality/polarity, i.e., water is a more polar solvent than methanoland hence the interfacial tension between the solvophobic/hydrophobic moiety; PPFS and solvent will be higher inaqueous solution favoring larger micelles. In contrast, metha-nol induces a lower interfacial tension between solvophobic/hydrophobic blocks and the solvent can favor the stabiliza-tion of smaller micelles. In other words, the higher

FIGURE 7 TEMmicrographs of micelles formed by PEG-b-PPFS in methanol, obtained after transferring (a) 3 mg/mL and (b) 6 mg/mL

block copolymer solution inmethanol.

FIGURE 6 Apparent translational diffusion coefficients as a

function of copolymer concentration for (*) PEG-b-PPFS; (h)

PEG-b-PPFS-g-POSS6; (~) PEG-b-PPFS-g-POSS19; and (^)

PEG-b-PPFS-g-POSS25. The solid line represents the linear

extrapolation to zero concentration to obtain the diffusion coef-

ficient at infinite dilution, D0.



solvophobic interactions of PPFS block with water whencompared with methanol drive the formation of largermicelles with higher aggregation number in aqueous solutionas compared in methanol. We recently observed similarbehavior for micelle formation by poly(styrene)-b-poly(n-vinylpyrrolidone) in aqueous and methanol solutions.70

Surface PropertiesContact angles of water droplets on copolymer surfaces weremeasured to evaluate the surface activity of the copolymers.The copolymer surfaces were prepared by spin coating THFsolutions of the copolymers on cleaned silicon wafers andcuring at 120 �C, under nitrogen for �6 h. Each experimentwas performed three times, and the average values of staticand advancing and receding contact angles are presented inTable 3. The data reveal a substantial increase in contactangle with increasing POSS content in the block copolymers,for example, the static contact angle increased from 82.5� 60.7� on PEG-b-PPFS to 98� 6 0.6� on the surface of PEG-b-PPFS-g-POSS25, which can be ascribed to the increasedhydrophobicity of amphiphilic block copolymer with higherPOSS content. The data reveal a significant hysteresis (hadv �hrec) effect for all the copolymer surfaces, becoming morepronounced with increasing POSS content in the copolymers.

Furthermore, the data in Table 3 also reveal that surfacerearrangements occur during the curing process as evi-denced by an increase in the contact angle on a PEG-b-PPFS-g-POSS25 surface from hstatic ¼ 92.3� 6 0.7� for a freshlyprepared surface to 98.0� 6 0.6� on a cured surface.

Bulk (Condensed State) PropertiesDSC CharacterizationDSC thermograms (heating cycles) of PEG-b-PPFS and PEG-b-PPFS-g-POSS samples are given in Figure 8. A significantendothermic peak associated with the melting of PEG crys-tallites can be seen at Tm � 48 �C in Figure 8(a) for PEG-b-PPFS block copolymer, which becomes less significant,and shifts toward lower melting temperature in Figure 8(b)(Tm� 43 �C) and (c) (Tm� 41 �C), suggesting a significant influ-ence of POSS on PEG crystallization, leading to the formation ofless ordered, smaller PEG crystallites with increased pendantPOSS groups. The POSS nanocages appear to plasticize.

PPFS as revealed by a decrease in glass transition tempera-ture, Tg, of PPFS block in copolymer, from �95 �C in PEG-b-PPFS [Fig. 8(a)] to �60 �C for PEG-b-PPFS-g-POSS25[Fig. 8(c)]. A similar decrease in Tg caused by POSS has also

been observed for semitelechelic POSS polymethacrylates53

and perfluorocyclobutyl aryl ether copolymers having pend-ant POSS nanocages.54

Wide Angle X-Ray Diffraction (WAXD)Figure 9 shows wide angle X-ray diffraction profiles of (a)PEG5k initiator and (b) PEG-b-PPFS-g-POSS19. The strongpeaks, highlighted in WAXD pattern of PEG5k-ini [Fig. 9(a)],at 2h � 19� and �23� (d-spacing � 4.7 Å and 3.9 Å, respec-tively), originating from the monoclinic crystal structure ofPEG. In Fig. 9(b), the assigned diffractions at 2h � 8.1�,�10.9�, �12.1�, and �9.1�, corresponding to d-spacing; 10.9,8.1, 7.3, and 4.6 Å, respectively, are associated with a rhom-bohedral crystal structure of POSS molecules. The diffractionprofiles of POSS molecules measured herein are similar tothose reported previously for closely related POSS mole-cules.44,50,51,75,76 No diffraction from PEG in PEG-b-PPFS-g-POSS19 copolymers was observed [Fig. 9(b)]. Recently, Kimand Mather76 reported the influence of POSS end caps on

FIGURE 8 DSC thermograms (heating cycle) of (a) PEG-b-PPFS,

(b) PEG-b-PPFS-g-POSS6, and (c) PEG-b-PPFS-g-POSS25. Heat-

ing rate was 10 �C/min. Each thermogram is scaled differently

for clarity of the data.

TABLE 3 Contact Angles of Water on PEG-b-PPFS and PEG-b-PPFS-g-POSS Surfaces

Samples

Static Contact

Angle (hstatic)Advancing Contact

Angle (hadv)Receding Contact

Angle (hrec) hadv � hrec

PEG-b-PPFS 82.5� 6 0.7� 89.4� 6 0.5� 80.1� 6 0.6� 9.3�

PEG-b-PPFS-g-POSS6 91.2� 6 0.6� 95.2� 6 0.7� 84.9� 6 0.7� 10.3�

PEG-b-PPFS-g-POSS19 94.3� 6 0.5� 100.9� 6 0.6� 89.1� 6 0.5� 11.8�

PEG-b-PPFS-g-POSS25 98.0� 6 0.6� 105.2� 6 0.4� 92.4� 6 0.5� 12.8�

PEG-b-PPFS-g-POSS25 (as prepared) 92.3� 6 0.7� 101.1� 6 0.5� 87.9� 6 0.4� 13.2�

ARTICLE


the crystallization of PEG (various chain lengths) showingthat diffraction (WAXD) patterns weakened with decreasingPEG chain length. This was attributed to a confining effect ofadjacent POSS crystalline phases on crystallizing PEG phases.In the present system, however, the disturbance to PEO crys-tallization could also be attributed to PPFS amorphousblocks chemically linked to PEG in addition to the influenceof POSS molecules. It must be mentioned here that the pend-ant POSS molecules causes significant hindrance to PEG crys-tallization but does not eliminate it. This can be also sup-ported by DSC data which still show a small melting peakfor PEG. The absence of PEG signals from the WAXS profileshown in Figure 9(b) could only be due to overlap bythe broad halo in the region from �13� to �32�, which canbe ascribed to the disordered amorphous PPFS block in thecopolymer.

SUMMARY

Well-defined amphiphilic diblock copolymers of PEG andPPFS have been synthesized by atom transfer radical poly-merization using modified PEG as a macroinitiator. Differentamounts of aminopropylisobutyl POSS was then grafted tothe PPFS block of the PEG-b-PPFS through aromatic nucleo-philic substitution reactions at the para F position of the aro-matic rings. The resultant amphiphilic PEG-b-PPFS-g-POSShybrid copolymers formed micelles in aqueous mediaand the micelle size increased with increasing POSS contentin the block copolymers. The increased hydrodynamic sizesof the micelles were attributed to higher aggregation num-bers of micelles because of enhanced hydrophobicity afterthe incorporation of POSS nanocages. This was verified bySLS measurements. Furthermore, contact angles of water onthe spin coated copolymer surfaces increased with increasinghydrophobic POSS content in the block copolymer. Thegrafted POSS nanocages appear to act as plasticizer asrevealed by a significant decrease in glass transition temper-ature of the PPFS block in the copolymer. Wide angle X-ray

diffraction data reported for PEG-b-PPFS-g-POSS copolymersconfirmed characteristic signals corresponding to a POSSrhombohedral crystal structure.

The authors gratefully acknowledge financial support from theInstitute of Materials Research and Engineering under theAgency for Science, Technology, and Research (ASTAR). T. P.Davis acknowledges the Australian Research Council for theaward of a Federation Fellowship.

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