Thermomechanical and Surface Properties of NovelPoly(ether urethane)/Polyhedral OligomericSilsesquioxane Nanohybrid Elastomers
Juanjuan Tan,1,2 Zhiyuan Jia,1,2 Dekun Sheng,1,2 Xin Wen,1,2 Yuming Yang1
1 Key Laboratory of Polymer Ecomaterials, Changchun Institute of Applied Chemistry,Chinese Academy of Sciences, Changchun 130022, China
2 Graduate School of the Chinese Academy of Sciences, Beijing 10080, China
A series of linear polyurethane (PU) elastomerscontaining various amounts (ca. 2–10 wt%) of a diol-functionalized polyhedral oligomeric silsesquioxane(diolPOSS) were synthesized from organic solventdispersions. Fourier transform infrared spectroscopywas used to characterize the chemical structure of thediolPOSS-reinforced PU. Transmission electron micros-copy (TEM) indicated that the diolPOSS aggregationwas formed in the polymer matrix at the scale of 20–50nm. The introduction of diolPOSS into such system ledto high glass transition temperature, enhanced storagemodulus, and improved stability compared with thepristine PU according to the differential scanning calo-rimetry, dynamic mechanical analysis, and thermalgravimetric analysis, respectively. Moreover, contactangle measurements indicated a significant enhance-ment in surface hydrophobicity as well as a reductionin surface free energy after introducing diolPOSSinto the synthesized PU. The improvement of surfaceproperties could be ascribed to the enrichment ofthe diolPOSS moiety on the surface of the hybrids.POLYM. ENG. SCI., 51:795–803, 2011. ª 2011 Society ofPlastics Engineers
INTRODUCTION
In recent years, organic–inorganic hybrid composites
have undergone a significant evolution because of the
outstanding properties of these materials compared with
pristine polymers [1]. Polyhedral oligomeric silses-
quioxane (POSS)-based polymers are emerging as one
of the most promising high-performance materials for
fundamental studies as well as industrial applications.
POSS, first reported in 1946 [2], is a cube-octameric
molecule with an inner inorganic Si��O��Si framework
that is externally covered with reactive or nonreactive
organic substituents, so it possesses organic–inorganic
hybrid structure at molecular level. POSS moieties can
be incorporated into polymer systems through either
simple melt/solution blending [3, 4] or polymerization
via functional substituents at the vertexes of POSS
cages [5, 6]. The resulting POSS–polymer hybrid sys-
tems displayed various significantly enhanced proper-
ties, including increased toughness, decreased flamma-
bility, ultraviolet stability, and oxidation resistance [7,
8]. In most cases, the enhanced properties are achieved
by incorporation of only small amounts (\10 wt%) of
POSS that are far below the levels needed for tradi-
tional fillers.
Polyurethane (PU), a well-known versatile engineering
material, can be tailored to meet the highly diversified
demands of modern technologies, such as coatings, adhe-
sives, reaction injection molding, fibers, foams, rubbers,
thermoplastics elastomers, and composites [9, 10]. With
the increasing demand in a wide range of application,
considerable academic and industrial efforts have been
devoted to preparing novel PU materials with improved
properties. During the past years, extensive work has been
reported in the literature on the modification of PUs via
POSS. The PU/POSS hybrids are normally obtained from
waterborne dispersions or nondispersions. In previous
studies, two typical kinds of POSS with different func-
tional groups have been synthesized into PU from homo-
geneous aqueous solution [11–14]. The first one is 2,3-
propanediol propoxy-heptaisobutyl-POSS (diol-POSS),
which significantly reduced the surface free energy and
thus increased surface hydrophobicity of PUs [11, 12].
The second one is 3-(2-aminoethylamino) propyl-heptai-
sobutyl-POSS (diamino-POSS), which was incorporated
into aqueous PU dispersions [13, 14] to enhance the ther-
momechanical and surface properties of PU. As for the
Correspondence to: Yuming Yang; e-mail: [email protected]
Contract grant sponsor: Chinese Academy of Sciences.
Contract grant sponsor: Commission of Science Technology and Industry
for National Defense.
DOI 10.1002/pen.21877
Published online in Wiley Online Library (wileyonlinelibrary.com).
VVC 2011 Society of Plastics Engineers
POLYMER ENGINEERING AND SCIENCE—-2011
PU/POSS hybrids obtained from nondispersions, Neu-
mann et al. [15] reported the synthesis of an octa(isocya-
nate)-functionalized POSS macromer, which can subse-
quently be used to prepare organic–inorganic hybrid PUs.
Zheng and coworkers [16, 17] used octa-functional POSS
as cross-linking agents to react with PU prepolymer to
form octa-armed star-like PU prepolymer with silses-
quioxane core, and then the chain was extended into PU
networks. Fu et al. [18, 19] prepared PU elastomers con-
taining octasiloxane–POSS used for a chain extender
through one of its corner groups grafting onto the macro-
molecular backbone of PU chains. Janowski and Pieli-
chowski [20] investigated the thermo(oxidative) stability
of PU/POSS nanohybrid elastomers using thermal gravi-
metric analysis (TGA). Oaten and Choudhury [21] synthe-
sized transparent PU hybrid containing an open-cage
POSS for thin film application.
In this work, a new type of diolPOSS was chosen
as precursor for PU synthesis. The whole synthetic pro-
cess was performed in two mixed organic solvents to
improve the dispersion of the monomers and facilitate
the polymerization reaction. The purpose of this article
is to prepare a novel poly(ether urethane)/POSS nano-
hybrid elastomer and investigate the effect of POSS on
the thermomechanical and surface properties of PU
elastomers in detail.
EXPERIMENT
Materials
4,40-Diphenylmethane diisocyanate (MDI), poly(pro-
pylene glycol) (PPG, Mw ¼ 2000), and dibutyltin dilau-
rate (DBTDL) catalyst were purchased from Aldrich.
MDI and DBTDL were used as received, and PPG was
dehydrated under vacuum at 1108C for 2 hr before use.
Trans-CyclohexaneDiolIsobutyl POSS (C36H78O14Si8,
diolPOSS), a white powder, was obtained from Hybrid
Plastics (USA). The molecular structure is given in
Scheme 1. 1,4-Butanediol (BDO), toluene, and n-hexanewere purchased from Shanghai Chemical Reagent Co.
(Shanghai, China). BDO was distilled under reduced pres-
sure. Toluene and n-hexane were dried over 4 A molecu-
lar sieves and then distilled from sodium benzophenone
ketyl for further purification.
Preparation of Samples
In a dry 250-mL three-necked round-bottom flask
equipped with a mechanical stirrer, a nitrogen inlet, and a
condenser, MDI and diolPOSS were first dissolved into
mixed solvents of toluene and n-hexane under ultrasonic
SCHEME 1. Synthesis of the PU–diolPOSS hybrids.
796 POLYMER ENGINEERING AND SCIENCE—-2011 DOI 10.1002/pen
dispersion and then allowed to react with each other for 30
min with the existence of DBTDL at room temperature.
Later, calculated PPG (NCO Index ¼ 2:1, where [NCO]
and [OH] are the concentration of NCO groups of MDI and
total OH groups of diolPOSS and PPG, respectively) was
charged into the flask while stirring, and the reaction flask
was immersed in an oil bath at 608C. The isocyanate
(NCO) content was monitored during the reaction using the
standard dibutylamine back-titration method [22]. About 1
hr later, the NCO content reached the theoretical NCO
value (the content of left NCO was nearly 50% of that in
the original state). Then, the prepolymer was chain
extended with BDO, and the reaction continued for another
3 hr at 608C to complete polymerization. Finally, the mix-
tures were precipitated with n-hexane after some additional
toluene added and dried under vacuum at 608C for 1 week.
The pristine PU was prepared in the same way without
addition of diolPOSS and n-hexane.The films of the pristine PU and PU hybrids with dif-
ferent concentrations of diolPOSS were hot pressed at
1608C and 10 MPa with a hold time of 3 min, followed
by quenching to room temperature between two thick
metal blocks.
Measurements
Fourier Transform Infrared Spectroscopy. Fourier
transform infrared spectroscopy (FTIR) measurements
were conducted on a FTIR-6100 spectrometer (JASCO,
Tokyo, Japan) equipped with an IMV-4000 multichannel
infrared microscope (JASCO, Tokyo, Japan) and a MCT
detector in the transmission mode at room temperature.
The sample films were prepared by dissolving the poly-
mers with tetrahydrofuran (10 wt%) and then the solu-
tions casting onto KBr windows. The residual solvent was
removed in a vacuum oven at 608C for 2 hr. In all cases,
the spectra was recorded 32 scans at a resolution of 2/cm
in the wavenumber range of 4000–500 cm21.
Wide-Angle X-ray Diffraction. Wide-angle x-ray dif-
fraction (WAXD) experiments were performed by the use of
Rigaku D/Max-II B X-ray diffractometer with a Cu anode
(CuKa1 ¼ 1.5406 A). DiolPOSS powder and 1-mm thick
PU hybrid films were analyzed by WAXD at room condi-
tions. All of the measurements were operated at 40 kV and
200 mA from 4 to 408 at a 2y scan rate of 28/min.
Transmission Electron Microscopy. The morphology
of PU/POSS hybrids was observed using a transmission
electron microscope (TEM) (JEOL JEM-1011). The speci-
men for TEM observations was about 50–60-nm in thick-
ness, which was prepared by ultramicrotoming under
cryogenic conditions using a leica ultramictome (Ultracut
R with FCS).
Differential Scanning Calorimetry. The calorimetric
measurements were performed on a TA Instrument differ-
ential scanning calorimetry (DSC) Q20 with a Universal
Analysis 2000 under a continuous dry nitrogen purge (50
mL/min). Indium was used for temperature and enthalpy
calibration. All samples were sealed in aluminum pans
with mass in the range of 5–10 mg. All measurements
were conducted at a heating rate of 108C/min following
heat–cool–heat procedure from 280 to 1208C. Glass tran-sition temperatures (Tg) were determined by the midpoint
of enthalpy change during the second heating.
Dynamic Mechanical Analysis. The dynamic mechani-
cal tests were performed on DMA/SDTA861e (Mettler
Toledo, Switzerland) in the shear-sandwich mode. The
samples were cut on the 1-mm thick PU films with a di-
ameter of 10 mm and measured under a nitrogen atmos-
phere from 2100 to 1308C at a heating rate of 38C/min,
a frequency of 1 Hz, and a deformation �0.3%.
Thermal Gravimetric Analysis. A Mettler Toledo
TGA STAR gravimetric analyzer was used to investigate
the thermal stability of all PU samples. The specimens
(about 10 mg) were heated under a nitrogen atmosphere
from ambient temperature up to 6008C at a heating rate
of 208C/min in all cases. The temperature at which the
rate of mass loss (Tmax) is up to maximum was evaluated
from the differential thermogravimetry curves.
Contact Angle Measurements. The static contact angle
measurements with ultrapure water and diiodomethane
were performed on a Kruss DSA100 (Germany) apparatus.
A droplet of water (or diiodomethane) was dropped onto
the surface of PU films about 5–10 lm. The evolution of
the droplet shape was recorded by a CCD video camera and
analyzed to determine the contact angle. About 10 inde-
pendent measurements were performed and the average
contact angle was reported. The specimens were dried in a
vacuum oven for 12 hr before measurement.
X-ray Photoelectron Spectroscopy. X-ray photoelec-
tron spectroscopy (XPS) experiments of PU hybrid films
were performed on a ThermoElectron ESCALAB 250
spectrometer at room temperature by using an Al and K
X-ray source (1486.6 eV). The spectra were recorded at
908 takeoff angle with 100 eV pass energy. The binding
energies were calibrated by using the containment carbon
(C 1s ¼ 284.6 eV).
RESULTS AND DISCUSSION
Preparation of the Inorganic–Organic Hybrids
The synthesis process of PU–diolPOSS hybrids is illus-
trated in Scheme 1. The contents of diolPOSS in the
hybrids were set at 2, 5, 8, and 10 wt%, and the corre-
sponding samples were marked as PU2, PU5, PU8, and
PU10, respectively. The compositions of the pristine PU
DOI 10.1002/pen POLYMER ENGINEERING AND SCIENCE—-2011 797
and PU–diolPOSS hybrids are summarized in Table 1. To
compensate for the inclusion of diolPOSS monomers, the
content of PPG was reduced from 77 to 65 wt% of the
whole hybrids as the diolPOSS content varied from 0 to
10 wt%. All samples can dissolve completely into polar
solvents such as N,N-dimethylformamide and tetrahydro-
furan, which proved that there was no covalent cross-link-
ing in their molecular structure.
The FTIR spectra of the diolPOSS, the pristine PU,
and PU–diolPOSS hybrids are shown in Fig. 1. For the
pristine PU, the stretching vibrations of the N��H groups
occurred at 3300 cm21, together with the carbonyl bands
at 1732 cm21 were indicative of the presence of urethane
moieties. For the diolPOSS, the broad band above 3000
cm21 was typical of the stretching vibrations of the
hydroxyl groups, and the band at 1106 cm21 was ascribed
to the stretching vibration of Si��O��Si groups in the sil-
sesquioxane cages. For the PU–diolPOSS hybrids, this
band at 1106 cm21 unfortunately overlapped with that of
the aliphatic ether C��O��C. It is worth noticing that the
disappearance of the bands at 2250–2275 cm21, which
are characteristic of isocyanate, indicated the completed
reaction between the hydroxyl and isocyanate groups.
WAXD
Figure 2 shows the WAXD patterns of the diolPOSS
monomer and all synthesized samples. The pure diolPOSS
monomer presented characteristic diffraction peaks at 2yof 8.18, 10.88, 12.08, and 18.98, corresponding to the crys-
tallization of POSS, which are similar to the previous
reports [12, 13]. In particular, the most intense and sharp-
est diffraction peak at 2y of 8.18 is attributed to the 101
reflection of POSS rhombohedral unit cell [23]. In con-
trast, the pristine PU (PU0) showed only one broad amor-
phous halo centered at 2y of 19.58. With the incorporation
of 2 wt% POSS, the pattern of PU2 resembled that of
PU0; with further increase of diolPOSS content, the dif-
fraction for PU0 still existed, and the peak shifted to a
slightly lower angle, indicating that the introduction of
diolPOSS expanded the intermolecular main chain spac-
ing. When the content of diolPOSS was [2 wt%, another
broad peak appeared at 2y of 9.18, which should be
ascribed to the aggregation of diolPOSS, indicating the
partial crystallization of diolPOSS in the hybrids [13, 24].
But the deviation from the diffraction of pure diolPOSS
indicated that the presence of diolPOSS in hybrids was
different from the 3D crystals of pure diolPOSS mono-
mer. It is apparent that the crystallization behavior and
crystal morphology of diolPOSS were influenced by the
pendent state in the long polymer chains.
Morphology of the PU–diolPOSS Hybrids
For further confirming the diolPOSS distribution in the
hybrids, the morphology of the diolPOSS-containing PU
hybrids was observed by TEM. Figure 3 representatively
shows the TEM micrograph of the hybrids containing 5
wt% (PU5) and 10 wt% (PU10) of diolPOSS. According
to the difference in transmitted electronic density between
organic PU polymer and inorganic POSS component, the
TABLE 1. Compositions of PU–diolPOSS hybrids.
Samples
DiolPOSS
% (w/w)
MDI %
(w/w)
PPG %
(w/w)
BDO %
(w/w) Molar ratio of reactantsa
PU0 0.0 19.3 77.2 3.5 2.000:0.000:1.000:1.000
PU2 2.0 19.7 74.7 3.6 2.000:0.053:0.946:1.000
PU5 5.0 20.3 71.0 3.7 2.000:0.128:0.872:1.000
PU8 8.0 21.1 67.1 3.8 2.000:0.202:0.798:1.000
PU10 10.0 21.4 64.7 3.9 2.000:0.244:0.756:1.000
a The reactants are MDI, diolPOSS, PPG, and BDO.
FIG. 1. FTIR spectra of the diolPOSS, the pristine PU, and PU hybrids
containing diolPOSS.
FIG. 2. WAXD patterns of the diolPOSS monomer, the pristine PU,
and PU–diolPOSS hybrids.
798 POLYMER ENGINEERING AND SCIENCE—-2011 DOI 10.1002/pen
dark area is assigned to the POSS domains, whereas the
white region to PU matrix. As can be seen, at relatively
low diolPOSS content such as 5 wt%, only small amounts
of diolPOSS-aggregated spherical particles were found
(Fig. 3a); However, at relatively high diolPOSS concen-
trations such as 10 wt%, significant aggregation was
found, and the particle diameter was about 20–50 nm
(Fig. 3b). The results are in good agreement with those of
WAXD, further suggesting that diolPOSS aggregations
were formed, and the aggregation extent enhanced with
the diolPOSS content increasing.
Thermal Behavior
DSC analysis of samples detected only the glass transi-
tion of the polyether phase (the soft phase), without evi-
dence of other higher temperature thermal transitions (the
hard phase) [12]. The pristine PU and diolPOSS-contain-
ing PU hybrids were subjected to thermal analysis and the
DSC curves are presented in Fig. 4. It can be seen that
the pristine PU displayed a glass transition at 2428C,much lower than room temperature, indicating that this
material is a typical elastomer in the ambient condition.
For the diolPOSS-containing hybrids, all of the DSC ther-
mograms displayed single glass transition temperatures
(Tgs) in the experimental temperature range (280 to
1208C). With an increasing fraction of diolPOSS in the
polymer, the Tg is slightly shifting to a higher tempera-
ture. This result was in good agreement with many other
kinds of POSS-containing materials, such as POSS-rein-
forced epoxy resins [25, 26], POSS/PMMA composites
[27], styrene/styryl-POSS [28], and poly(norbornene-co-norbornene POSS) [29] copolymers. In the reports of Xu
et al. [30], it was interpreted that the Tg of hybrid poly-
mer depended on three factors: (1) inert diluent role of
POSS to reduce the self-association dipole–dipole interac-
tion of parent polymer molecules (negative contribution),
(2) the dipole–dipole interaction between POSS siloxane
and the polar carbonyl of organic polymer species, and
(3) the POSS–POSS inter-and/or intermolecular interac-
tion. In our case, the POSS–PU and POSS–POSS interac-
tion played a dominant role and resulted in an increase of
Tg. It is worth noticing that although the Tgs of the PU
hybrids were higher than that of the pristine PU, they did
not vary significantly with the content of diolPOSS
increasing, and the Tg of the hybrid with 10 wt% dio-
lPOSS only increased to 2358C. In Lee’s opinion [25],
the presence of POSS cages, because of their nanoscopic
size and their chemical incorporation directly into the
polymer chains, effectively hindered the motion of macro-
molecular chains, thus the Tg of the matrix increased. In
the present case, the rigid diolPOSS cages were tethered
to the hard segments of PU macromolecular chains, so
the confinement effect on molecular chain motion of soft
segments was not evident. This speculation will be con-
firmed in the following dynamic mechanical analysis
(DMA) results. Furthermore, it is noted that the width of
FIG. 3. TEM images for the PU containing 5 wt% (a) and 10 wt% (b) diolPOSS hybrids.
FIG. 4. The DSC curves of the pristine PU and PU–diolPOSS hybrids
at a heating rate of 108C/min.
DOI 10.1002/pen POLYMER ENGINEERING AND SCIENCE—-2011 799
the endothermic transition was not affected with the addi-
tion of diolPOSS. This fact suggested that these diolPOSS
cages were completely incorporated within the chains and
did not form macroscopic phase-separated domains.
Dynamic Mechanical Properties
DMA was measured to characterize the dependence of
viscoelastic properties on temperature for the pristine PU
and PU–diolPOSS hybrids. The shear storage modulus
(G0) and loss tangent (tand) for all samples are shown in
Fig. 5a and b, revealing the dependences on diolPOSS
content. As shown in Fig. 5a, it is obvious that the stor-
age moduli of the glassy state for all diolPOSS-containing
hybrids are close to that of the pristine PU. However, the
introduction of diolPOSS brought the enhancement of the
storage modulus in the rubbery state, and the increase
trend was dependent on the diolPOSS loading. The dio-
lPOSS cages were chemically bonded to the macromolec-
ular chains during polymerization, indicating that the
incorporation of diolPOSS into the PU interrupted the
movements of polymer chains because of the reinforcing
effect of diolPOSS [31]. In the glassy state, the molecular
chain segments were frozen, so the rigid cages of dio-
lPOSS had little contribution to the modulus; along with
temperature rising, the molecular chain segments went
into motion, whereas the rigid diolPOSS cages would
hold back.
The loss factor tand, calculated as the ratio of the loss
modulus to storage modulus, is sensitive to structural
transformation of the material. As shown in Fig. 5b, there
were two peaks in the tand � temperature curves for the
pristine PU because the sensitivity of DMA is much
higher than that of DSC [32]. One was located at 2308C,a well-defined relaxation peak, which was attributed to
the glass transition of the soft segments in the matrix.
With the incorporation of diolPOSS, this relaxation peak
shifted to higher temperature regularly accompanied with
significantly broadened width and declined height. But it
was still lower than 2208C when the diolPOSS content
reached 10 wt%. The other, centered at around 358C, wasascribed to the glass transition of the hard segment. It
became weaker and weaker and eventually vanished with
the increase of diolPOSS content. From the synthesis
route in Scheme 1, it should be noted that the diolPOSS
cages were grafted onto the hard segments of the macro-
molecular chains, so they played a greater negative role
in the motion of hard segmental chain than that of the
soft segmental chain.
Thermal Stability
TGA was used to evaluate the thermal stability of the
pristine PU and PU hybrids containing diolPOSS. As
shown in Fig. 6a, all samples displayed similar degrada-
tion profiles, suggesting that the existence of diolPOSS
component did not significantly alter the degradation
mechanism of the polymer matrix. The decomposition
process of all samples in inert atmosphere ran mainly in
two steps, and this result can be observed more obviously
in the derivatives of the TGA curves as a function of tem-
perature (shown in Fig. 6b). The temperature for the first
maximum rate of weight loss, which can be connected
with the degradation of the hard segments of PU [20],
was located at ca. 3508C for the pristine PU, whereas this
temperature for PU hybrids slightly decreased. The sec-
ond maximum weight loss, attributed to the degradation
of soft segments of PU, was situated at ca. 4008C, and it
shifted to higher temperature on the addition of diolPOSS.
With increasing diolPOSS loading, the inorganic ingredi-
ent enhanced, resulting in a higher char yields as
expected. For example, at 6008C, there was about 2.7
wt% residue for the pristine PU and about 4.7 wt% resi-
due for the PU hybrids containing 10 wt% diolPOSS. The
increase in char yields implied that there were fewer vola-
tiles being released from the hybrids at high temperature.
During the decomposition process of PU hybrids, once
the hard segments started to decompose, the bulk materi-
FIG. 5. The dynamic mechanical spectra of the pristine PU and PU–
diolPOSS hybrids, (a) storage modulus (G0) vs. temperature and (b) loss
tangent (tand) vs. temperature.
800 POLYMER ENGINEERING AND SCIENCE—-2011 DOI 10.1002/pen
als were covered with ceramic residue composed of silica,
resulting in a drop of the rate of volatiles’ release from
the materials and leading to an improvement of the ther-
mal stability at high temperature as well. On the basis of
the above results, it is estimated that the thermal stability
of hybrids was not significantly enhanced with the inor-
ganic component tethered into the PU macromolecular
chains [16, 17, 33].
Surface Properties
It is widely known that organosilicon compounds are
generally of low surface energy. In the present case,
the diolPOSS cages were covalently connected with PU
chains. The surface properties of the PU hybrids were
investigated in terms of the measurements of static con-
tact angles against water and diiodomethane. The results
are summarized in Table 2. The contact angle of the
pristine PU was approximately 978 measured with
water. On diolPOSS added to the system, the contact
angle increased with the percentage of diolPOSS rising
and reached up to 114.58 for the PU hybrid containing
10 wt% diolPOSS. This observation indicated that the
hydrophobicity of the materials was significantly
enhanced; that is, the surface free energy of the materi-
als was reduced. The surface free energies of the
hybrids containing various percentages of diolPOSS
were calculated according to the geometric mean model
[34, 35]:
gL 1þ cos yð Þ ¼ 2 gDS gDL
� �12
h iþ 2 gPSg
PL
� �12
h i(1)
gS ¼ gDS þ gPS (2)
where y is contact angle, and gL is the liquid surface
tension; gPL and gDL are the polar and dispersive compo-
nents of gL, respectively. The calculated surface free
energies of the hybrids were also presented in Table 2.
It should be noted that the total surface free energies
of the hybrids were reduced from 36.5 to 18.5 mN�malong with the increasing of diolPOSS content. To
minimize the polymer–air surface tension, the material
with a lower surface energy is located at the surface
[36, 37]. Therefore, there may exist a thermodynamic
driving force for the migration of diolPOSS to the free
surface.
To verify this possibility, the surface elemental compo-
sitions of the PU hybrids were investigated by means of
FIG. 6. TGA measurements for the pristine PU and PU–diolPOSS
hybrids at 208C/min heating rate under a nitrogen atmosphere, (a) TGA
curves and (b) derivative TGA curves.
TABLE 2. Static contact angles and surface free energies of the
pristine PU and diolPOSS-containing PU hybrids.
Samples hH2Oo � rð Þ hCH2I2
o � rð Þ cDS
mN�m�1
cPS
mN�m�1
cS
mN�m�1
PU0 97.0 6 2.3 47.0 6 1.8 36.20 0.29 36.49
PU2 104.3 6 0.6 58.2 6 2.6 30.46 0.05 30.51
PU5 109.4 6 2.7 72.4 6 1.8 21.85 0.12 21.97
PU8 113.5 6 2.1 77.1 6 1.0 19.57 0.03 19.60
PU10 114.5 6 1.6 79.1 6 2.1 18.49 0.03 18.52
FIG. 7. XPS survey spectrum of the PU hybrid containing 5 wt%
diolPOSS.
DOI 10.1002/pen POLYMER ENGINEERING AND SCIENCE—-2011 801
XPS. Figure 7 shows a representative XPS spectrum of
the thin hybrid film containing 5 wt% diolPOSS. The C
1s peak centered at 284.6 eV is responsible for the contri-
bution of several types of carbon atom from the PU and
diolPOSS components. The oxygen atoms present in the
urethane, which were chemically bonded to carbon atoms,
are detected at the peak of a higher binding energy. The
O 1s peak at approximately 532.0 eV is mainly ascribed
to the C��O, C¼¼O of the urethane and a bit to the oxy-
gen present in the diolPOSS cages. The Si atoms from
the silsesquioxane moiety of diolPOSS contributed the Si
2s and Si 2p signals at 153.2 and 103.6 eV, respectively.
The N 1s peak occurred at 400.5 eV corresponding to the
amide group of the urethane. The relative concentrations
of C, O, N, and Si on the surfaces of the PU hybrids, cal-
culated from the corresponding photoelectron peak areas,
are listed in Table 3. It is obvious that carbon and oxygen
were dominant on the detected surfaces of all samples.
The concentrations of elemental Si increased with the dio-
lPOSS content going up in the PU hybrids, and it is espe-
cially noteworthy that its experimental concentrations on
the surface of the hybrids were significantly higher than
its theoretical vales calculated in terms of the feed ratio.
For instance, the surface content of Si is 5.08 mol% when
the concentration of diolPOSS is 5 wt%. This value is
much higher than the theoretical value 0.42 mol%. This
observation suggests that there existed the enrichment of
a Si-containing moiety on the surface of the hybrids [38,
39]. This result helps us to understand the enhancement
of the surface hydrophobicity for the POSS-modified
polymer composites.
CONCLUSIONS
A series of linear poly(ether urethane)–diolPOSS
hybrid elastomers were synthesized using two organic sol-
vents as dispersing agent for the reactant monomers. DSC
results showed that the soft segment chains of the hybrids
displayed increased glass transition temperature in com-
parison with the pristine PU, resulting from the strong
POSS–PU and POSS–POSS interaction. According to
DMA results, it can be seen that the addition of diolPOSS
enhanced the storage modulus of the rubbery state and
caused various degrees of confinement to the soft and
hard segmental chain motions. In terms of TGA, the ther-
mal stability of the hybrids had a limited enhancement,
as evidenced by the rate of volatiles’ release from the
materials and the increased char yields. Contact angle
measurements indicated that the hydrophobicity of the
hybrids was significantly enhanced while the surface
free energy reduced, with regard to the enrichment of the
diolPOSS moiety on the surface of the hybrids, which
was evidenced by XPS.
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TABLE 3. The elemental compositions of the surfaces of diolPOSS-
containing PU hybrids determined by means of XPS.
Samples
Experimental composition
(mol%)
Theoretical composition
(mol%)
C O Si N C O Si N
PU2 72.36 21.19 4.90 1.54 77.94 20.32 0.17 1.56
PU5 72.71 20.49 5.08 1.73 77.65 20.28 0.42 1.64
PU8 73.81 19.01 5.29 1.90 77.36 20.23 0.68 1.73
PU10 71.89 20.56 5.65 1.90 77.14 20.22 0.86 1.78
802 POLYMER ENGINEERING AND SCIENCE—-2011 DOI 10.1002/pen
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DOI 10.1002/pen POLYMER ENGINEERING AND SCIENCE—-2011 803
