Inorganic–Organic Nanocomposites of Polybenzoxazinewith Octa(propylglycidyl ether) Polyhedral OligomericSilsesquioxane

YONGHONG LIU, SIXUN ZHENG

Department of Polymer Science and Engineering, Shanghai Jiao Tong University, 800 Dongchuan Road,Shanghai 200240, People’s Republic of China

Received 25 August 2005; accepted 31 October 2005DOI: 10.1002/pola.21231Published online in Wiley InterScience (www.interscience.wiley.com).

ABSTRACT: Octa(propylglycidyl ether) polyhedral oligomeric silsesquioxane (OpePOSS)was used to prepare the polybenzoxazine (PBA-a) nanocomposites containing polyhe-dral oligomeric silsesquioxane (POSS). The crosslinking reactions involved with the for-mation of the organic–inorganic networks can be divided into the two types: (1) thering-opening polymerization of benzoxazine and (2) the subsequent reaction betweenthe in situ formed phenolic hydroxyls of PBA-a and the epoxide groups of OpePOSS.The morphology of the nanocomposites was investigated by means of scanning electronmicroscopy, transmission electron microscopy, and atomic force microscopy. Differentialscanning calorimetry and dynamic mechanical analysis showed that the nanocompo-sites displayed higher glass-transition temperatures than the control PBA-a. In theglassy state, the nanocomposites containing less than 30 wt % POSS displayed anenhanced storage modulus, whereas the storage moduli of the nanocomposites contain-ing more than 30 wt % POSS were lower than that of the control PBA-a. The dynamicmechanical analysis results showed that all the nanocomposites exhibited enhancedstorage moduli in the rubbery states, which was ascribed to the two major factors,that is, the nanoreinforcement effect of POSS cages and the additional crosslinkingdegree resulting from the intercomponent reactions between PBA-a and OpePOSS.Thermogravimetric analysis indicated that the nanocomposites displayed improvedthermal stability. VVC 2005 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 44: 1168–1181,

2006

Keywords: nanocomposites; polybenzoxazine; polyhedral oligomeric silsesquioxane;thermal properties; thermsets

INTRODUCTION

The concept of incorporating inorganic (or organo-metallic) blocks into organic polymers has beenwidely accepted to obtain materials with some newand improved properties.1–6 Polyhedral oligomericsilsesquioxane (POSS) reagents, monomers, andpolymers are emerging as new chemical feedstock

for the preparation of organic–inorganic nanocom-posites;7–18 the polymers containing POSS arebecoming the focus of many studies because of thesimplicity in processing and the excellent compre-hensive properties of this class of hybrid materials.

The polymers reinforced with well-definednanosized POSS molecules represent a class ofimportant polymer nanocomposites. Typical POSSmonomers possess the structure of cube–octa-meric frameworks with eight organic vertexgroups, one or more of which is reactive or poly-merizable. Depending on the functionalities of

Correspondence to: S. Zheng (E-mail: szheng@sjtu.edu.cn)

Journal of Polymer Science: Part A: Polymer Chemistry, Vol. 44, 1168–1181 (2006)VVC 2005 Wiley Periodicals, Inc.

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POSS molecules, various approaches can be usedto incorporate POSS molecules into polymers. Forinstance, monofunctional POSS monomers candirectly be grafted onto macromolecular chainsby copolymerization or reactive blending,19,20

whereas bifunctional POSS monomers will allowthe incorporation of the silsesquioxane buildingblocks into macromolecular backbones, althoughit is still a challenge to synthesize scaled quanti-ties of monomers efficiently.11,14,21,22 The POSSmonomers with higher functionalities can beemployed to prepare POSS-containing thermoset-ting nanocomposites.23–28

Polybenzoxazines (PBA-a) are a class of attrac-tive phenolic resins that can be used as matricesof high-performance composites because of theirsuperior mechanical properties and high-temper-ature stability. PBA-a possesses excellent pro-cessability through a very wide range of molecu-lar design flexibility.29–31 The potential applica-tions of PBA-a have motivated the preparation ofthermosetting polymers with improved proper-ties.32–34 Recently, PBA-a-based inorganic–or-ganic nanocomposites have been prepared via theintercalation or exfoliation of layered silicates orthe sol–gel process.35,36 However, the modifica-tion of PBA-a via POSS was less involved. Changet al.28 reported the synthesis of a monofunc-tional POSS benzoxazine (BA-a) monomer, whichwas then used to reinforce PBA-a. In their work,the POSS molecules were grafted onto PBA-a net-works. Nonetheless, the interest still exists tointroduce octafunctionalized POSS into PBA-a toobtain nanocomposites with improved thermome-chanical properties. In this strategy, octafunction-alized POSS molecules could act as nanocross-linking sites, and thus the thermomechanicalproperties of the materials could be furtherimproved. To this end, we present studies on themodification of PBA-a with octa(propylglycidylether) polyhedral oligomeric silsesquioxane (Ope-POSS). In this system, the additional nanocros-slinking can be formed apart from the crosslink-ing reaction from the ring-opening polymeriza-tion of BA-a, which can be fulfilled via phenolichydroxyls of PBA-a and epoxide groups of Ope-POSS. The goal of this work is to demonstrate theefficiency of this approach through the investiga-tion of the properties of the resulting materials.This approach is different from previous reportson the modification of thermosets (e.g., epoxyresin) by octafunctionalized POSS, in whichoctafunctionalized POSS molecules solely actas nanocrosslinking sites and the crosslinking

densities of thermosets remain virtually invari-ant.25,37–45

In this work, we first synthesized OpePOSS,and then the POSS molecules were incorporatedinto PBA-a to obtain the POSS-containing nano-composites. The intercomponent reaction be-tween PBA-a and OpePOSS was investigated viaa model compound method. The thermomechani-cal properties were addressed on the basis of dif-ferential scanning calorimetry (DSC), dynamicmechanical analysis (DMA), and thermogravi-metric analysis (TGA).

EXPERIMENTAL

Materials

Trichlorosilane (HSiCl3; 98%) was kindly sup-plied by Shanghai Lingguang Chemical Co.(China) and was used as received. The otherreagents, such as ferric chloride (FeCl3), anhy-drous NaHCO3, concentrated hydrochloric acid,anhydrous K2CO3, CaCl2, allyl glycidyl ether(AGE), phenol, 4,40-isoproylidenediphenol, ani-line, and paraformaldehyde, were of chemicallypure grade and were purchased from ShanghaiReagent Co. (China). The solvents, such as tol-uene, dichloromethane, 1,4-dioxane, and metha-nol, were obtained from commercial sources andwere further purified in the general way beforeuse. The platinum-containing Karstedt catalystwas prepared with chloroplatinic acid hexahy-drate (H2PtCl6.6H20) and 1,3-divinyltetramethyl-siloxane.46 BA-a was synthesized with the proce-dures reported by Ishida and coworkers.29–33

Synthesis of Octahydrosilsesquioxane (H8Si8O12)

H8Si8O12 was synthesized according to a literaturemethod47 with some modification (Scheme 1).Typically, FeCl3 (140 g) was dissolved in 200 mLof methanol, and the solution was charged to a5000-mL, three-necked, round-bottom flask eq-uipped with a mechanical stirrer. ConcentratedHCl (100 mL), petroleum ether (1750 mL), and tol-uene (250 mL) were added to the system in succes-sion. With vigorous stirring, a mixture of HSiCl3(100 mL) with petroleum ether (750 mL) wasadded dropwise within 9 h. With vigorous stirringfor an additional 30 min, the upper petroleumether layer was transferred to another flask, andanhydrous K2CO3 (70 g) and CaCl2 (50 g) werecharged to the flask with continuous stirring for

INORGANIC–ORGANIC NANOCOMPOSITES 1169

another 12 h. The solution was concentrated to 50mL via rotary evaporation after the solids were fil-tered out. The evaporation of the solvent affordedwhite crystals (10.3 g) with a yield of 19.4%.

Fourier transform infrared (FTIR; cm�1, KBrwindow): 2275 (Si��H), 1121 (Si��O��Si,), 860(Si��H). 1H NMR (chloroform-d, ppm): 4.23.29Si cross polarization/magic-angle-spinning solidNMR (ppm): �85.5.

Synthesis of OpePOSS

OpePOSS was synthesized via the hydrosilyla-tion between H8Si8O12 and AGE as depicted inScheme 1. In a typical experiment, 1.0 g ofH8Si8O12 was added to a 25-mL, round-bottomflask equipped with a magnetic bar, which wasdried with a repeated exhausting–refilling pro-cess with highly pure nitrogen. Anhydrous toluene(10 mL) and AGE (3 mL) were charged, and fivedrops of the Karstedt catalyst were added at roomtemperature. After vigorous stirring for 30 min,the reactive system was heated to 95 8C to per-form the reaction for 36 h to ensure the hydrosily-lation to completion. The solvent and excessiveAGE were removed under decreased pressure toafford a viscous liquid (2.95 g, yield ¼ 94%).

FTIR (cm�1, KBr window): 3056, 2995, 2934,2873 cm�1 (alky C��H), 1255 (C��O��C of epox-ide), 1103 (Si��O��Si), 906 (epoxide). 1H NMR(chloroform-d, ppm): 3.72–3.42 [m, CH2O(CH2)3Si��)] 3.50–3.35 (m, SiCH2CH2CH2O; all the res-

onance between 3.72 and 3.35 were integrated tobe 4.3H, 3.16 (OCH2CH, epoxide, 1.0H), 2.79,2.60 (CH2 epoxide, 2.1H), 1.64 (SiCH2CH2CH2O,2.5H), 0.62 (SiCH2CH2CH2O, 2.0H). 13C NMR(chloroform-d, ppm): 73.6 (SiCH2CH2CH2O), 71.6[CH2O(CH2)3Si], 51.0 [OCH2CH(epoxide)], 44.4[CH2 (epoxide)], 23.1 (SiCH2CH2CH2O), 8.2 (SiCH2CH2CH2O). 29Si NMR (CDCl3, ppm): �65.2 (aaddition), �67.6 (b addition, small). Matrix-assisted laser desorption/ionization time-of-flightmass spectroscopy (product þ Naþ): 1359.1 Da.

Reaction of the Model Compounds

To verify the additional crosslinking reactionbetween PBA-a and OpePOSS, phenol wasselected as the model compound of PBA-a to reactwith OpePOSS. Phenol (1.13 g, 0.012 mol) andOpePOSS (1.30 g, 0.00125 mol with respect to theepoxide groups) were mixed at room temperature.The reaction was carried out at 180 8C for 4 h in anitrogen atmosphere. The excessive phenol waswashed out with hot deionized water, and theproduct was dried in vacuo at 60 8C to afford aviscous liquid (yield ¼ 90%).

FTIR (cm�1, KBr window): 3425 (O��H), 3064,3040, 2935, 2873 (C��H), 1110 (Si��O��Si), 915cm�1 (epoxide). 1H NMR (chloroform-d, ppm): 7.27,6.95 (protons of aromatic rings, 4.7H), 4.14 (OCH2CH��OH, 1.0H), 3.98 (OCH2CH��OH, 2.3H),3.56 (SiCH2CH2CH2O, 2.4H), 3.45 (CH2��O��Ph,

Scheme 1. Synthesis of H8Si8O12 and OpePOSS.

1170 LIU AND ZHENG

2.2H), 1.97 (OH, 1.1H), 1.69 (SiCH2CH2CH2O,2.4H), 0.64 (SiCH2CH2CH2O, 2.0H).

Preparation of the POSS-Containing PBA-aNanocomposites

The desired amounts of BA-a and OpePOSS weremixed, and the smallest amount of dichlorome-thane was used to facilitate the mixing. The major-ity of the solvent was evaporated at room tempera-ture, and the residual solvent was removed viadrying in vacuo at 60 8C for 2 h to afford the homo-geneous and transparent mixtures. The mixtureswere poured into aluminum foil and sealed. Thecuring was carried out at 180 8C for 4 h. PBA-ahybrid materials with OpePOSS concentrations upto 40 wt % were obtained.

Measurements and Techniques

FTIR Spectroscopy

The FTIR measurements were conducted on aPerkinElmer Paragon 1000 Fourier transformspectrometer at room temperature (25 8C). Thespecimen for OpePOSS was prepared by tetrahy-drofuran (THF) solution casting onto KBr win-dows, the majority of the solvent was evaporatedat room temperature, and the residual solventwas removed via drying in vacuo at 60 8C for 2 h.The samples of the composites were granulatedand mixed with KBr powder to press into smallflakes for the measurements. All the specimenswere sufficiently thin to be within a range inwhich the Beer–Lambert law was obeyed. In allcases, 64 scans at a resolution of 2 cm�1 wereused to record the spectra.

NMR Spectroscopy

The NMR measurements were carried out on aVarian Mercury Plus 400-MHz NMR spectrome-ter at 27 8C. The samples were dissolved withdeuteronated chloroform. The 1H, 13C, and 29Sispectra were obtained with tetramethylsilane asthe internal reference.

Matrix-Assisted Ultraviolet Laser Desorption/Ionization Time-of-Flight Mass Spectroscopy

Gentisic acid (2,5-dihydroxybenzoic acid) was usedas the matrix with THF as the solvent. Thematrix-assisted laser desorption/ionization time-of-flight mass spectroscopy experiment was carried

out on an IonSpec HiRes matrix-assisted laserdesorption/ionization mass spectrometer equippedwith a pulsed nitrogen laser (k ¼ 337 nm; pulsewith ¼ 3ns). This instrument was operated at anaccelerating potential of 20 kV in a reflector mode.Sodium was used as the cationizing agent, and allthe data shown are for positive ions.

Scanning Electron Microscopy (SEM)

To investigate the morphology of the POSS-con-taining hybrids, the samples were fracturedunder cryogenic conditions with liquid nitrogen.The fractured surfaces so obtained were im-mersed in dichloromethane at room temperaturefor 30 min. If the POSS did not participate in thecrosslinking reaction, it could be preferentiallyetched by the solvent, whereas the PBA-a matrixphase remained unaffected. The etched speci-mens were dried to remove the solvents. All speci-mens were examined with a Hitachi S-210 scan-ning electron microscope at an activation voltageof 15 kV. The fracture surfaces were coated withthin layers of gold of about 100 A.

Atomic Force Microscopy (AFM)

AFM experiments were carried out with a Nano-scope III scanning probe microscope. The phaseimages were obtained by the operation of theinstrument in the tapping mode at room tempera-ture. Images were taken at the fundamental reso-nance frequency of the Si cantilevers, which wastypically around 300 kHz. A typical scan speedduring recording was 0.3–1 line/s with scan headswith a maximum range of 16 � 16 lm2. The phaseimages represent the variation of the relativephase shifts and are thus able to distinguishphases in terms of their material properties. Toinvestigate the phase structure of POSS-contain-ing PBA-a hybrids, the samples were fracturedunder cryogenic conditions with liquid nitrogen,and the smooth fractured surfaces so obtainedwere used for morphological observations.

Transmission Electron Microscopy (TEM)

TEM was performed on a JEM 2010 high-resolu-tion transmission electron microscope at an accel-erating voltage of 200 kV. The samples weretrimmed with a microtome, and the specimen sec-tions (ca. 70 nm in thickness) were placed in 200-mesh copper grids for observation.

INORGANIC–ORGANIC NANOCOMPOSITES 1171

DSC

The calorimetric measurements were performedon a PerkinElmer Pyris-1 differential scanningcalorimeter in a dry nitrogen atmosphere. Theinstrument was calibrated with standard indium.All the samples (ca. 10 mg) were heated from 20to 250 8C, and the DSC curves were recorded at aheating rate of 20 8C/min. The glass-transitiontemperatures (Tg’s) were taken as the midpoint ofthe capacity change.

DMA

The dynamic mechanical tests were carried outon a dynamic mechanical thermal analyzer(MKIII, Rheometric Scientific, Ltd. Co., UnitedKingdom) with a temperature range of �150 to280 8C. The frequency was 1.0 Hz, and the heat-ing rate was 5.0 8C/min. The specimen dimen-sions were 1.2 � 2 � 0.1 cm3. The experimentswere carried out from the ambient temperatureuntil the samples became too soft to be tested.

TGA

A PerkinElmer thermal gravimetric analyzer(TGA-7) was used to investigate the thermalstability of the hybrids. The samples (ca. 10 mg)were heated in an air atmosphere from the ambi-ent temperature to 800 8C at a heating rate of20 8C/min in all cases. The thermal degradation

temperature was taken as the onset temperatureat which 5 wt % weight loss occurred.

RESULTS AND DISCUSSION

Synthesis and Characterization of thePOSS Monomer

The synthesis route of octa(propylglycidyl ether)POSS is shown in Scheme 1. As the starting mate-rial, H8Si8O12 was first prepared via the hydrolyticcondensation of HSiCl3 in a methanol solution cat-alyzed by FeCl3.

47 Under this condition, we ob-tained a yield of�20%, which was higher than that(12%) of Bassindale and Gental48 and comparableto that (23%) reported by Crivello et al.49 Thehydrosilylation between H8Si8O12 and AGE wasused to synthesize the octafunctional silsesquiox-ane (i.e., OpePOSS). Figure 1 shows the FTIR spec-trum of the product of hydrosilylation. In the FTIRspectrum of OpePOSS, the band at 906 cm�1 isattributed to the stretching vibration of epoxidegroup. A feeble remnant stretching vibration bandof Si��H is still discernible at 2275 cm�1, indicat-ing that the hydrosilylation did not exactly occur tocompletion. Nonetheless, the fact that the signalsindicating the presence of Si��H bond were not bedetected in the 1H (d ¼ 4.23 ppm) and 29Si (d¼ �85.5 ppm) NMR spectra of the product [seeFig. 2(A,C)] suggests that a quite high conversa-tion of Si��H bonds was obtained under this reac-tion condition, as reported by Laine et al.23 Hydro-silylation often produces mixtures of productsresulting from both a addition and b addition tothe olefin unless b addition can be efficiently sup-pressed. In this work, the reaction was catalyzedby the Karstedt catalyst (a platinum complex with1,3-divinyltetramethyldisilane), and the 1H and13C NMR spectra of OpePOSS [Fig. 2(A,B)] showthat the signal of methyl was too weak to detect,implying that the portion of b-addition productswas very small. In the 29Si NMR spectrum [Fig.2(C)], there are two resonances at �65.2 and �67.6ppm. The major resonance is assigned to a-additionproducts, whereas the small one at �67.6 ppm isassigned to the corresponding b-addition products.The low intensity of the resonance for b additionindicates that the a addition dominates in thehydrosilylation under this condition. The resultsare comparable to those of H8Si8O12 addition withother olefin-containing compounds catalyzed byH2PtCl6.

50,51 The FTIR and NMR results indicatethat OpePOSS was obtained.

Figure 1. FTIR spectrum of OpePOSS.

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Inorganic–Organic Nanocompositesof PBA-a with POSS

Formation of the Hybrid Composites

Before curing, all the mixtures composed of BA-aand OpePOSS were homogeneous and transpar-ent, and this suggests that the two componentsare completely miscible in the composition rangeinvestigated. The mixtures of BA-a and OpePOSSwere cured at an elevated temperature, and thePOSS-containing PBA-a hybrids were formed asdepicted in Scheme 2. The crosslinking network-forming reactions can be divided into the follow-ing two principal steps: (1) the ring-opening poly-merization of BA-a at the elevated temperatureand (2) the subsequent reaction between phenolichydroxyls of PBA-a and epoxide groups of Ope-POSS. The occurrence of the first reaction willafford a great number of phenolic hydroxyls inthe macromolecular backbone of PBA-a (Scheme2), whereas the second reaction will create addi-tional crosslinking between PBA-a networks andOpePOSS. FTIR spectroscopy was employed toexamine the degree of the curing reaction afterthe POSS was introduced to the systems. Shownin Figure 3 are the FTIR spectra of BA-a, PBA-a,and hybrid composites containing 5, 10, 20, 30,and 40 wt % POSS. The pure BA-a is character-ized by the stretching vibration of oxazine ringsat 945 cm�1 (see curve A). Under this curing con-dition, the curing reaction was performed to com-pletion, as evidenced by the disappearance of theoxazine band (see curves B–G). On the otherhand, all the epoxide bands (at 915 cm�1) also vir-tually vanished for the POSS-containing hybridcomposites, and this indicated that OpePOSSparticipated in the crosslinking reaction. It is pro-posed that with the reaction between PBA-a andOpePOSS, the phenolic hydroxyls are convertedinto the secondary alcohol hydroxyls in hydroxyether structural units (��O��CH2��CH(OH)��CH2��O��) and the number of secondary hy-droxyl groups increases with an increasing con-centration of OpePOSS. This was evidenced bythe observation that the stretching bands ofhydroxyl groups significantly shifted to higherfrequencies with an increasing concentration ofOpePOSS.

Reaction of the Model Compounds

To verify the reaction between the phenolic hy-droxyl groups of PBA-a and epoxide groups ofOpePOSS, the reaction of the model compounds

Figure 2. NMR spectra of OpePOSS: (A) 1H, (B) 13C,and (C) 29Si.

INORGANIC–ORGANIC NANOCOMPOSITES 1173

was carried out under conditions identical tothose for the preparation of the hybrid compo-sites. Phenol was selected as the model compoundof the phenolic moiety of PBA-a to react with Ope-

POSS. Figure 4 presents the FTIR spectra of phe-nol, OpePOSS and the product of the reactionfrom their stoichiometric mixture. Phenol wascharacterized by the stretching vibration band of

Scheme 2. Preparation of PBA-a nanocomposites containing POSS.

Figure 3. FTIR spectra: (A) BA-a, (B) the control PBA-a, (C) the nanocomposite con-taining 5 wt % OpePOSS, (D) the nanocomposite containing 10 wt % OpePOSS, (E)the nanocomposite containing 20 wt % OpePOSS, (F) the nanocomposite containing30 wt % OpePOSS, and (G) the nanocomposite containing 40 wt % OpePOSS.

1174 LIU AND ZHENG

phenolic hydroxyl groups at 3334 cm�1 (curve A).In curve B, the stretching vibration bands at 1103and 910 cm�1 were assigned to the Si��O��Simoiety and epoxide groups of OpePOSS, respec-tively. Under this condition, the band of Ope-POSS at 910 cm�1 was virtually depleted, andthis indicated that the intercomponent reactionbetween phenol and OpePOSS was performed tocompletion. After the reaction, the hydroxylstretching vibration band shifted to a higher fre-quency (at 3456 cm�1) in comparison with phenol(at 3334 cm�1). This result was ascribed to theoccurrence between phenolic hydroxyls and epox-ide groups of OpePOSS because this reaction con-verted the phenolic hydroxyls into secondaryalcohol hydroxyls. This observation is in a goodagreement with the comparison of the FTIR spec-tra in the range of 3000–4000 cm�1 between thecontrol PBA-a and the hybrid composites. Shownin Figure 5 is the 1H NMR spectrum of the prod-uct of the reaction for the model compoundstogether with the assignment of the spectrum.From the integration intensity of the correspond-ing resonance, it is judged that the stoichiometricreaction between OpePOSS and phenol occurredunder identical reaction conditions. The reactionof the model compounds (i.e., phenol and Ope-

POSS) further verified that the reaction betweenphenolic hydroxyls and epoxide groups of POSSreadily occurred.

Morphology of the Nanocomposites

All the cured hybrids were homogeneous andtransparent, and this implied that no phase sepa-ration occurred at least on the scale greater thanthe wavelength of visible light. The morphologyof the POSS-containing hybrids was investigatedwith SEM. Shown in Figure 6 is the SEM micro-graph of the fractured surface of a hybrid, con-taining 40 wt % POSS, frozen under cryogenicconditions with liquid nitrogen. The sampleexhibited a featureless morphology, and no local-ized domains were detected; this indicated thatOpePOSS participated in the formation of cross-linked PBA-a networks.

The morphology of the hybrids was furtherinvestigated with AFM and TEM. A typical AFMheight image of a hybrid containing 40 wt %POSS is presented in Figure 7 (left). The surfaceappears to have been free of visible defects andquite smooth. The phase-contrast image (Fig. 7,right) shows a homogeneous surface, indicatingthat the surface of the fractured ends was compo-

Figure 4. FTIR spectra of phenol and its stoichiometric mixture with OpePOSS ther-mally treated at 180 8C for 4 h in a nitrogen atmosphere: (A) phenol, (B) OpePOSS, and(C) the reacted product.

INORGANIC–ORGANIC NANOCOMPOSITES 1175

sitionally homogeneous; that is, no localizedareas of POSS aggregates were observed at thescale of several nanometers. Figure 8 representa-tively shows a TEM micrograph of a sectionedhybrid containing 40 wt % POSS. For compari-son, the TEM image was taken at the edge of thesectioned sample and the background. The darkarea (the portion of the hybrid) was quite homo-geneous, and no localized domains were detectedat this scale; this suggested that the POSS com-ponent was homogeneously dispersed in the con-tinuous matrix at the nanoscale. Both AFM andTEM substantiated that the nanocomposites weresuccessfully obtained.

Thermomechanical Properties

Glass-Transition Behavior

The POSS-containing PBA-a nanocomposites weresubject to calorimetric measurements. The DSC

curves of the plain PBA-a and the nanocompo-sites are presented in Figure 9. The control PBA-a displayed a glass transition at 163 8C. For thePOSS-containing nanocomposites, all the DSCcurves exhibited single Tg’s in the experimentaltemperature range of 20–260 8C. In comparisonwith the control PBA-a, the nanocompositesshowed enhanced Tg’s, which increased withincreasing POSS content. The glass-transitionbehavior was further confirmed by the results ofDMA. Shown in Figure 10 are plots of tan d asfunctions of temperature for the control PBA-aand its nanocomposites containing up to 40 wt %POSS. In the DMA spectra, PBA-a exhibited awell-defined a-relaxation transition centered at183 8C, which was attributed to the glass transi-tion of the polymer. The PBA-a nanocompositescontaining POSS also clearly displayed single atransitions in the temperature range of �150 to280 8C. The Tg’s of the nanocomposites were sig-

Figure 5. NMR spectrum of a stoichiometric mixture of phenol and OpePOSS ther-mally treated at 180 8C for 4 h in a nitrogen atmosphere.

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nificantly higher than that of the control PBA-aand increased with increasing the concentrationof POSS. In addition, the magnitude of the a tran-sitions dramatically decreased with an increasingconcentration of POSS. This phenomenon couldbe interpreted in terms of the restriction (or con-finement) effect of POSS cages on the segmentalmotion of macromolecular chains near the glasstransition. The increased crosslinking densitiesresulting from the intercomponent reaction be-tween PBA-a and OpePOSS also contributed tothe decrease in the magnitude of the a transition.In the DMA spectra, the nanocomposites pos-

sessed lower temperature relaxation peaks (i.e.,b transitions) at approximately �80 8C, andthe magnitude of the transition increased withan increasing content of POSS (see Fig. 10).The b transition could be ascribed to themotion of the hydroxyl ether structural unit[��O��CH2��CH(OH)��CH2��], which is gener-ally observed in amine-crosslinking epoxy res-ins.52–54 In this case, the reactions between phe-nolic hydroxyls of PBA-a and epoxide groups ofOpePOSS afforded similar structural units, theamount of which increased with an increasingconcentration of OpePOSS. The appearance ofthe b transition in the DMA spectra of the nano-composites was in good agreement with theresults of FTIR and NMR, indicating that addi-tional crosslinking occurred between PBA-a andOpePOSS.

In POSS-containing polymer nanocomposites,the increase in Tg’s is generally interpreted onthe basis of the nanoreinforcement of POSS cageson polymer matrices, which could restrict themotions of macromolecular chains. In this case,the Tg enhancement is not only ascribed to thenanoreinforcement of POSS cages but also attrib-uted to the additional crosslinking of the nano-composites resulting from the intercomponentreaction between OpePOSS and PBA-a. The Tg’sof the POSS-containing nanocomposites did notmonotonously increase with an increase in thePOSS contents, and there was a maximum of Tg

(ca. 199 8C) for the nanocomposites containing 30wt % POSS; the nanocomposite containing 40 wt

Figure 7. AFM images of the nanocomposites containing 40 wt % POSS: heightimage (left) and phase-contrast image (right).

Figure 6. SEM micrograph of a PBA-a nanocompo-site containing 40 wt % POSS (the fracture end wasetched with chloroform for 30 min).

INORGANIC–ORGANIC NANOCOMPOSITES 1177

% POSS displayed a lower Tg (�186 8C, DSCresult). The glass-transition behavior cannot beexplained solely on the basis of the so-callednanoreinforcement effect. In fact, the inclusion ofthe bulky POSS cages could give rise to increasedfree volume of materials apart from the nanorein-forcement of POSS cages on the polymer matrix.The increased free volume will result in a depres-sion of Tg.

55,56 Similar results have been observedin other POSS-containing nanocomposites.

Dynamic Storage Modulus Analysis

Shown in Figure 11 are plots of the storage mod-uli as functions of temperature for the controlPBA-a and the nanocomposites with OpePOSSconcentrations up to 40 wt %. In the glass state(�50 to 100 8C), the dynamic storage moduli ofthe nanocomposites with POSS concentrationslower than 30 wt % were significantly higherthan that of pure PBA-a. The modulus increasedwith an increasing concentration of OpePOSS. Inthe hybrid nanocomposites, the cubic POSS cageswere homogeneously dispersed in the PBA-amatrix on the nanoscale, and thus the increasedmodulus could be attributed to the nanoreinforce-ment effect of POSS cages on the polymer matrix.Nonetheless, the nanocomposites with a higherPOSS loading (>20 wt %) gave a storage moduluslower than that of the control PBA-a. The explan-ation for this observation could be based on the

two opposing effects of POSS cages on the matri-ces of the materials. On the one hand, the nano-reinforcement effect of POSS on the polymermatrix will result in an increased modulus of thematerials. On the other hand, the inclusion ofPOSS in the system could give rise to decreaseddensities of the nanocomposites, which will giverise to a decrease in the moduli of the nanocompo-sites. The decreased densities could be due to theincrease in the porosity of the nanocomposites incomparison with the control PBA-a.28,55,56 Theincrease in the free volume of the nanocompositeshas been evidenced by the depression in Tg’s ofthe nanocomposites. The second portion of poros-ity can be attributed to the nanoporosity of thePOSS core with a diameter of 0.54 nm in a POSScage.

Moreover, the storage moduli of the rubberyplateau for all the POSS-containing nanocompo-sites were significantly higher than that of thecontrol PBA-a and increased with an increasingconcentration of POSS. The modulus of the rub-ber plateau for polymer networks is generallyrelated to the crosslinking densities of the materi-als. Under these experimental conditions, thecontrol PBA-a did not display a stable rubberyplateau; this was in marked contrast to the casesof the nanocomposites containing POSS. The factthat the storage moduli of the nanocompositeswere significantly higher than that of the control

Figure 8. TEM micrograph of a nanocomposites con-taining 40 wt % POSS.

Figure 9. DSC curves of the control PBA-a and itsnanocomposites with POSS.

1178 LIU AND ZHENG

PBA-a suggested a significant nanoreinforcementof POSS cages, which were covalently bondedwith the crosslinking networks of PBA-a. In addi-tion, the additional crosslinking resulting fromthe intercomponent reaction between PBA-a andOpePOSS also significantly contributed to theincrease in the storage moduli of the rubbery pla-teau for the nanocomposites. This case was inmarked contrast to the case of monofunctional-ized POSS-modified PBA-a.

Thermal Stability

TGAwas applied to evaluate the thermal stabilityof the POSS-containing nanocomposites. Shownin Figure 12 are the TGA curves of the controlPBA-a and the nanocomposites, recorded in anair atmosphere at 20 8C/min. Within the experi-mental temperature range, all the samples dis-played similar degradation curves, and this sug-gested that the existence of POSS did not signifi-cantly alter the degradation mechanism of thematrix polymers. For the control PBA-a, the ini-tial decomposition occurred at approximately310 8C, and no ceramic yield was obtained in airas expected. However, the nanocomposites dis-played a significantly enhanced initial decomposi-tion temperature (Td). The Td’s increased with anincreasing concentration of POSS in the compo-

sites. For the nanocomposite containing 40 wt %POSS, Td even reached 390 8C. In addition to theenhanced Td, the rates of mass loss from segmen-tal decomposition were significantly decreased forthe nanocomposites. In terms of the Td’s and ce-ramic yields, the thermal stability of the hybridswas significantly enhanced with the inclusion ofthe inorganic component.

It has been proposed that the tether structure iscrucial to the improvement of the thermal stabil-ities of POSS-containing nanocomposites.23,24 Inthis case, POSS cages participated in the forma-tion of the crosslinked network; that is, the POSScages were tethered onto the polymer matrix. Inaddition, the nanoscaled dispersion of POSS cagesin PBA-a matrices is an important factor contribu-ting to enhanced thermal stability. It is plausibleto propose that mass loss from segmental decom-position via gaseous fragments could be sup-pressed by well-dispersed POSS cubes at themolecular level. Similar results were also found infully exfoliated polymer–clay nanocomposites.57,58

Therefore, the improved thermal stability of thenanocomposites could be a result of the combinedeffects of the formation of an aromatic tether struc-ture between the PBA-a matrix and POSS cagesand the nanoscaled dispersion of POSS cages inthe PBA-a matrix.

Figure 11. Plots of the dynamic storage modulus (E0)as a function of temperature for the control PBA-a andits nanocomposites containing POSS.

Figure 10. Plots of tan d as a function of temperaturefor PBA-a and its nanocomposites with POSS.

INORGANIC–ORGANIC NANOCOMPOSITES 1179

CONCLUSIONS

OpePOSS was incorporated into PBA-a to pre-pare PBA-a nanocomposites with POSS concen-trations up to 40 wt %. The curing reactions werestarted from an initially homogeneous binarymixture of BA-a and OpePOSS. It was found thatthere was an intercomponent reaction betweenthe phenolic hydroxyls of PBA-a and the epoxidegroups of OpePOSS, which was confirmed withspectroscopic (i.e., FTIR and NMR) and modelcompound methods. The intercomponent reactionsignificantly enhanced the crosslinking degree ofthe nanocomposites in comparison with the con-trol PBA-a. DSC and DMA showed that that thenanocomposites displayed higher Tg’s in compari-son with the control PBA-a. The nanocompositescontaining less than 30 wt % POSS displayedhigher storage moduli in the glassy state thanthe control PBA-a. Nonetheless, in the glassystate, the storage moduli of the nanocompositescontaining more than 30 wt % POSS were lowerthan that of the control PBA-a, and this could beresponsible for the porosity of the nanocompo-sites. The moduli of all the nanocomposites inrubbery states were significantly higher thanthose of the control PBA-a, and this was ascribedto the two major factors, that is, the nanorein-forcement effect of POSS cages and the increasedcrosslinking degree resulting from the intercom-

ponent reaction between PBA-a and OpePOSS.TGA indicated that the thermal stability of thepolymer matrix was improved by the introductionof POSS, and the properties of oxidation resis-tance of the materials were significantly en-hanced. The improved thermal stability could beascribed to the nanoscaled dispersion of POSScages and the formation of a tether structure ofPOSS cages with the PBA-a matrix.

This financial support of the Shanghai Science andTechnology Commission (China) under a key project(no. 02DJ14048) is acknowledged. The partial supportof the Natural Science Foundation of China is alsoacknowledged (project nos. 20474038 and 50390090).The author also thank the Shanghai Education Devel-opment Foundation for its award to ‘‘Shuguang Schol-ars’’ (2004-SG-18).

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