Inorganic–organic nanocomposites of polybenzoxazine with octa(propylglycidyl ether) polyhedral oligomeric silsesquioxane

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<ul><li><p>InorganicOrganic Nanocomposites of Polybenzoxazinewith Octa(propylglycidyl ether) Polyhedral OligomericSilsesquioxane</p><p>YONGHONG LIU, SIXUN ZHENG</p><p>Department of Polymer Science and Engineering, Shanghai Jiao Tong University, 800 Dongchuan Road,Shanghai 200240, Peoples Republic of China</p><p>Received 25 August 2005; accepted 31 October 2005DOI: 10.1002/pola.21231Published online in Wiley InterScience (www.interscience.wiley.com).</p><p>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 organicinorganic 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: 11681181,2006</p><p>Keywords: nanocomposites; polybenzoxazine; polyhedral oligomeric silsesquioxane;thermal properties; thermsets</p><p>INTRODUCTION</p><p>The concept of incorporating inorganic (or organo-metallic) blocks into organic polymers has beenwidely accepted to obtain materials with some newand improved properties.16 Polyhedral oligomericsilsesquioxane (POSS) reagents, monomers, andpolymers are emerging as new chemical feedstock</p><p>for the preparation of organicinorganic nanocom-posites;718 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.</p><p>The polymers reinforced with well-denednanosized POSS molecules represent a class ofimportant polymer nanocomposites. Typical POSSmonomers possess the structure of cubeocta-meric frameworks with eight organic vertexgroups, one or more of which is reactive or poly-merizable. Depending on the functionalities of</p><p>Correspondence to: S. Zheng (E-mail: szheng@sjtu.edu.cn)</p><p>Journal of Polymer Science: Part A: Polymer Chemistry, Vol. 44, 11681181 (2006)VVC 2005 Wiley Periodicals, Inc.</p><p>1168</p></li><li><p>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</p><p>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 efciently.11,14,21,22 The POSSmonomers with higher functionalities can beemployed to prepare POSS-containing thermoset-ting nanocomposites.2328</p><p>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 exibility.2931 The potential applica-tions of PBA-a have motivated the preparation ofthermosetting polymers with improved proper-ties.3234 Recently, PBA-a-based inorganicor-ganic nanocomposites have been prepared via theintercalation or exfoliation of layered silicates orthe solgel process.35,36 However, the modica-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 themodication 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 fullled via phenolichydroxyls of PBA-a and epoxide groups of Ope-POSS. The goal of this work is to demonstrate theefciency of this approach through the investiga-tion of the properties of the resulting materials.This approach is different from previous reportson the modication of thermosets (e.g., epoxyresin) by octafunctionalized POSS, in whichoctafunctionalized POSS molecules solely actas nanocrosslinking sites and the crosslinking</p><p>densities of thermosets remain virtually invari-ant.25,3745</p><p>In this work, we rst 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).</p><p>EXPERIMENTAL</p><p>Materials</p><p>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 puried 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.2933</p><p>Synthesis of Octahydrosilsesquioxane (H8Si8O12)</p><p>H8Si8O12 was synthesized according to a literaturemethod47 with some modication (Scheme 1).Typically, FeCl3 (140 g) was dissolved in 200 mLof methanol, and the solution was charged to a5000-mL, three-necked, round-bottom ask 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 ask, andanhydrous K2CO3 (70 g) and CaCl2 (50 g) werecharged to the ask with continuous stirring for</p><p>INORGANICORGANIC NANOCOMPOSITES 1169</p></li><li><p>another 12 h. The solution was concentrated to 50mL via rotary evaporation after the solids were l-tered out. The evaporation of the solvent affordedwhite crystals (10.3 g) with a yield of 19.4%.</p><p>Fourier transform infrared (FTIR; cm1, KBrwindow): 2275 (SiH), 1121 (SiOSi,), 860(SiH). 1H NMR (chloroform-d, ppm): 4.23.29Si cross polarization/magic-angle-spinning solidNMR (ppm): 85.5.</p><p>Synthesis of OpePOSS</p><p>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-bottomask equipped with a magnetic bar, which wasdried with a repeated exhaustingrelling pro-cess with highly pure nitrogen. Anhydrous toluene(10 mL) and AGE (3 mL) were charged, and vedrops 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 94%).</p><p>FTIR (cm1, KBr window): 3056, 2995, 2934,2873 cm1 (alky CH), 1255 (COC of epox-ide), 1103 (SiOSi), 906 (epoxide). 1H NMR(chloroform-d, ppm): 3.723.42 [m, CH2O(CH2)3Si)] 3.503.35 (m, SiCH2CH2CH2O; all the res-</p><p>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).</p><p>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).</p><p>29Si NMR (CDCl3, ppm): 65.2 (aaddition), 67.6 (b addition, small). Matrix-assisted laser desorption/ionization time-of-ightmass spectroscopy (product Na): 1359.1 Da.</p><p>Reaction of the Model Compounds</p><p>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 90%).</p><p>FTIR (cm1, KBr window): 3425 (OH), 3064,3040, 2935, 2873 (CH), 1110 (SiOSi), 915cm1 (epoxide). 1H NMR (chloroform-d, ppm): 7.27,6.95 (protons of aromatic rings, 4.7H), 4.14 (OCH2CHOH, 1.0H), 3.98 (OCH2CHOH, 2.3H),3.56 (SiCH2CH2CH2O, 2.4H), 3.45 (CH2OPh,</p><p>Scheme 1. Synthesis of H8Si8O12 and OpePOSS.</p><p>1170 LIU AND ZHENG</p></li><li><p>2.2H), 1.97 (OH, 1.1H), 1.69 (SiCH2CH2CH2O,2.4H), 0.64 (SiCH2CH2CH2O, 2.0H).</p><p>Preparation of the POSS-Containing PBA-aNanocomposites</p><p>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.</p><p>Measurements and Techniques</p><p>FTIR Spectroscopy</p><p>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 smallakes for the measurements. All the specimenswere sufciently thin to be within a range inwhich the BeerLambert law was obeyed. In allcases, 64 scans at a resolution of 2 cm1 wereused to record the spectra.</p><p>NMR Spectroscopy</p><p>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.</p><p>Matrix-Assisted Ultraviolet Laser Desorption/Ionization Time-of-Flight Mass Spectroscopy</p><p>Gentisic acid (2,5-dihydroxybenzoic acid) was usedas the matrix with THF as the solvent. Thematrix-assisted laser desorption/ionization time-of-ight mass spectroscopy experiment was carried</p><p>out on an IonSpec HiRes matrix-assisted laserdesorption/ionization mass spectrometer equippedwith a pulsed nitrogen laser (k 337 nm; pulsewith 3ns). This instrument was operated at anaccelerating potential of 20 kV in a reector mode.Sodium was used as the cationizing agent, and allthe data shown are for positive ions.</p><p>Scanning Electron Microscopy (SEM)</p><p>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.</p><p>Atomic Force Microscopy (AFM)</p><p>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 0.31 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.</p><p>Transmission Electron Microscopy (TEM)</p><p>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.</p><p>INORGANICORGANIC NANOCOMPOSITES 1171</p></li><li><p>DSC</p><p>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 (Tgs) were taken as the midpoint ofthe capacity change.</p><p>DMA</p><p>The dynamic mechanical tests were carried outon a dynamic mechanical thermal analyzer(MKIII, Rheometric Scientic, Ltd. Co., UnitedKingdom) with a temperature range of 150 to280 8C. The frequency w...</p></li></ul>