A novel poly-benzoxazinyl functionalized polyhedral oligomeric silsesquioxane and its nanocomposite with polybenzoxazine

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<ul><li><p>r N</p><p>tiom</p><p>Xu</p><p>eijing</p><p>Beisanhuan East Road No. 15, Beijing 100029, China</p><p>Received 13 August 2006; received in revised form 26 October 2006; accepted 13 November 2006</p><p>Keywords: Nanocomposites; Polyhedral oligomeric silsesquioxane; Polybenzoxazine; Cross-linking; Miscibility</p><p>tightening their network structure [6].Organic and inorganic hybrid nanocomposites</p><p>based on polyhedral oligomeric silsesquioxane</p><p>erved.</p><p>* Corresponding author.E-mail address: yuds@mail.buct.edu.cn (D. Yu).</p><p>European Polymer Journal 43</p><p>EUROPEANPOLYMER</p><p>MACROMOLECULARNANOTECHNOLOGY0014-3057/$ - see front matter 2006 Elsevier Ltd. All rights res1. Introduction</p><p>Polybenzoxazine (PBZ), as a new kind of ther-mosetting resin, is prepared from inexpensiveraw materials. By ring-opening polymerization, thePBZ can be formed without any strong catalyst,does not release low molecular weight byproductsand exhibited small shrinkage upon curing. More-over, PBZ resins possess excellent mechanical, heat</p><p>resistance and dielectric properties, low moistureabsorption and ame retardance. These advantagesof PBZ have resulted in considerable increases in thestudies of benzoxazine (BZ) monomers and theircorresponding polymers [15]. However, despitetheir high moduli and Tgs, the cross-linking densi-ties are very low in comparison to ordinary thermo-setting resins with similar properties. As a result, theproperties of PBZ may further be improved byAvailable online 10 January 2007</p><p>Abstract</p><p>The novel poly-benzoxazinyl functionalized polyhedral oligomeric silsesquioxane macromonomer (BZ-POSS), contain-ing 7.6 benzoxazine groups per molecule on average was synthesized from octaaminophenylsilsesquioxane, p-cresol andparaformaldehyde. BZ-POSS was well miscible with bisphenol A-based benzoxazine (BBZ) melt. By ring-opening copo-lymerization of BBZ and BZ-POSS under condition similar to that used for polymerizing neat BBZ, the transparentand uniform BBZ/BZ-POSS organicinorganic hybrid nanocomposites were prepared. The nano-scale dispersion of POSScores in the nanocomposite was veried by powder X-ray diraction and transmission electron microscopy studies.Dynamic mechanical analyses and thermal gravimetric analysis indicated that thermal stabilities, cross-link densitiesand the ame retardance of the nanocomposites were increased in comparison with neat PBBZ resin, although only smallamounts of inorganic POSS cores were incorporated into the systems. Structural analyses of BZ-POSS and BBZ/BZ-POSSnanocomposites are discussed herein. 2006 Elsevier Ltd. All rights reserved.Macromolecula</p><p>A novel poly-benzoxazinyl funcsilsesquioxane and its nanoco</p><p>Jun Zhang, Riwei</p><p>College of Materials Science and Engineering, Bdoi:10.1016/j.eurpolymj.2006.11.012anotechnology</p><p>nalized polyhedral oligomericposite with polybenzoxazine</p><p>, Dingsheng Yu *</p><p>University of Chemical Technology, Box 104,</p><p>(2007) 743752</p><p>www.elsevier.com/locate/europolj</p><p>JOURNAL</p></li><li><p>(POSS) have attracted considerable interest inrecent years. It is due to not only the high perfor-mance originating from the combination of advan-tages of inorganic and organic components inthese materials, but also the unique features derivedfrom their nanometer-scale architecture [7]. Thetypical POSS derivatives have the structure of acube-octameric framework covalently bonded witheight organic groups, one or more of which is reac-tive or polymerizable. The diameter of the POSScore is at nanometer-scale (body diagonal of cubicsilsesquioxane [SiO1.5]8 being 0.53 nm) [8]. If dis-persed at molecular level in a polymer matrix, thesePOSS cores can be regarded as true inorganic nano-particles. To improve the dispersion of POSS coresin organic polymers, controlling the type of organicgroups bonded to POSS cores is essential. To thisend, much eort has been made to prepare POSSmacromonomers with various functional groupssuch as amino [8,9], hydroxy [10], bromo [11], epoxy[12,13], acrylate [14,15], norbonyl [16], cinnamate[17], uorine [9] and maleimide [9] and then to copo-lymerize them with suitable polymers forming a ser-ies of organicinorganic hybrid nanocomposites.</p><p>cerned with common polymers incorporating POSSto get the improved properties, but few concernedthe modication of PBZ with POSS. Lee et al.[1820] have connected the benzoxazinyl groups toPOSS core through aliphatic chains (BZ-aliph-POSS) and copolymerized it with benzoxazinemonomer via ring-opening polymerization toform hybrid materials. But the BZ-aliph-POSSwas poorly miscible with BZ monomer and tendedto grossly aggregate forming its own domains. Thediculty lies in the fact that the inorganic silicastructure of the POSS cores does not permit theBZ-aliph-POSS macromonomer disperse well inthe organic matrix. In our previous paper, we suc-cessfully modied the PBZ resin by incorporatingoctaaminophenyl-POSS in the presence of bisoxazo-line as compatibilizer [21].</p><p>Considering the low cross-link density of thecured BZ-based resin and the poor miscibility ofPOSS cores with BZ melt, in this study, we synthe-sized and characterized a novel poly-benzoxazinylfunctionalized polyhedral oligomeric silsesquioxane(BZ-POSS) macromonomer which contained closeto octa-benzoxazinyl groups being attached to a</p><p>744 J. Zhang et al. / European Polymer Journal 43 (2007) 743752</p><p>MACROMOLECULARNANOTECHNOLOGYThe resulting materials exhibited favorable combi-nation of properties between POSS and polymers.However, most previous studies have been con-Scheme 1. Preparation of BZ-POSS aPOSS core through aromatic phenylene groups(Scheme 1). This macromonomer can be copolymer-izied with regular benzoxazine monomer as a chainnd its nanocomposite with BBZ.</p></li><li><p>J. Zhang et al. / European Polymer Journal 43 (2007) 743752 745</p><p>MACROMOLECULARNANOTECHNOLOGYcross-linking agent. The improved miscibility andinteractions between BZ-POSS macromonomerand benzoxazine matrix were expected due to theinorganic core of BZ-POSS molecule being con-cealed with eight benzoxazine groups. In addition,because the benzoxazinyl groups can react withepoxy, isocyanate and other groups [22,23], theBZ-POSS was also expected to be used further asa nano-construction site and chain cross-linker formodication of other polymer matrices. Based onthe BZ-POSS that we obtained, the novel hybridnanocomposites of bisphenol A-based BZ (BBZ)with BZ-POSS were prepared by thermally inducedring-opening reaction. The structure, morphologyand properties of the BBZ/BZ-POSS nanocompos-ites were studied.</p><p>2. Experimental section</p><p>2.1. Materials</p><p>Bisphenyl A-based benzoxazine (BBZ) monomerwas prepared from aniline, paraformaldehyde, andbisphenol A by using a solvent method describedin our previous paper [21]. Octaphenylsilsesquiox-ane (OPS) was obtained from Aldrich Co., Ltd.Paraformaldehyde, p-cresol, and solvents such astetrahydrofuran (THF), acetonitrile, and ethylace-tate were of analytically pure grade and purchasedfrom Beijing Reagent Co., China. THF was driedover molecular sieves and distilled over CaH2. Allother reagents were used as received without furtherpurication.</p><p>2.2. Synthesis of OAPS</p><p>The POSS macromonomer used to prepare BZ-POSS was octaaminophenyl polyhedral oligomericsilsesquioxane (OAPS), synthesized from OPS byfollowing literature method [8,9]. FT-IR (KBr):3369, 3220 (NH), 1119 cm1 (SiOSi); 1H NMR(DMSO-d6), (d, ppm): 7.86.0 (b, 2.0H), 5.33.7(b, 1.0H); GPC: Mn 1147, Mw 1330, Mw/Mn 1.16.</p><p>2.3. Synthesis of BZ-POSS</p><p>To a ask equipped with a mechanical stirrer,20 g (0.19 mol) of melted p-cresol and 2 g of OAPS(1.73 mmol) were charged. The mixture was stirredat 40 C until the solid dissolved completely. Thesolution was cooled to 5 C, and then 2.4 g of para-</p><p>formaldehyde (80 mmol) was added. The solutionwas stirred for an additional 5 min, then graduallyincreased in temperature to 80 C and stirred for6 h. The resulting transparent, light brown solutionwas combined with 30 mL of ethylacetate and pre-cipitated in 1 L hexane. The obtained solid waswashed with acetonitrile, redissolved in 20 mL ofethylacetate and reprecipitated into 500 mL ofhexane. The faintly yellow powder product wasdried under vacuum. Yield 2.9 g (68%). FT-IR(KBr): 1228 cm1 (COC), 948 cm1 (COC),1501 cm1 (C6H3), 1119 cm</p><p>1 (SiOSi); 1HNMR (DMSO-d6) (d, ppm): 6.37.8 (b, 7.0H),3.85.5 (b, 3.8H), 3.6 (s, 0.20H); GPC: Mn 2162,Mw 2737, Mw/Mn 1.27.</p><p>2.4. Preparation of BBZ/BZ-POSS nanocomposite</p><p>BBZ monomer (0.95 g) and BZ-POSS (0.05 g)were dissolved in THF (10 mL) and stirred at roomtemperature for 1 h. The THF was then removed bydistillation under reduced pressure. The blend ofBBZ and BZ-POSS was a light yellow solid. Themixture was cast and cured in a mold at 100 Cfor 1 h, 130 C for 1 h, 160 C for 1 h, 180 C and200 C for 3 h each and then post-cured at 220 Cfor 1 h. The procedure was repeated to prepareother hybrid materials by varying the amount ofBZ-POSS (0, 10, and 15 wt%). The cured sampleswith dimensions of 25 5 1 mm3 were transparentand had a red-wine color.</p><p>2.5. Measurements and techniques</p><p>The FT-IR measurements were conducted on aNicolet-60SXB spectrophotometer at room temper-ature (25 C). All samples were prepared as pelletsusing spectroscopic grade KBr.</p><p>The 1H NMR measurement was carried out on aBruker AV600 MHz spectrometer at 25 C. Thesamples were dissolved with dimethyl sulfoxide-d6and the solution was measured with tetramethyl-silane (TMS) as the internal reference.</p><p>Molecular weight polydispersities were deter-mined by a Waters 5152410 Gel-permeation Chro-matography (GPC), H3+H5+H6E l-styragelcolumns. The system was calibrated with standardpolystyrene, using THF as the eluent.</p><p>Powder X-ray diraction (XRD) patterns wererecorded with a D/Max 2500 VB2+/PC based ana-lytical diractometer. The work voltage and currentwere 40 kV and 50 mA, respectively. CuKa</p><p>(k = 1.54 A) radiation with a Ni lter was used.</p></li><li><p>OAPS and BZ-POSS powder were mounted andpressed on the glass holder and scanned from 2to 40. BBZ/BZ-POSS samples were mounted onan aluminum holder and scanned following thesame method for BZ-POSS.</p><p>Transmission electron microscopy (TEM) wasperformed on Hitachi H-800 TEM operated at 200KV. The samples were trimmed using a microtomeand the specimen sections (ca. 70 nm in thickness)were placed in 400 mesh copper grids for analysis.</p><p>Dynamic mechanical analyses (DMA) were con-ducted on a Rheometrics ScienticTM DMTA V at1 Hz at a heating rate of 3 C min1 over the tem-perature range of 50300 C.</p><p>Thermogravimetric analyses (TGA) were per-formed with a Toshiba Netzsch 209 C thermogravi-metric analyzer under nitrogen from ambienttemperature up to 800 C at a heating rate of10 C min1 in all cases.</p><p>746 J. Zhang et al. / European Polymer Journal 43 (2007) 743752</p><p>MACROMOLECULARNANOTECHNOLOGYFig. 1. FT-IR spectra of (a) OAPS, (b) BBZ, (c) BZ-POSS, (d)PBBZ and (e) BBZ/BZ-POSS nanocomposite with a BZ-POSS3. Results and discussion</p><p>3.1. Synthesis and characterization of BZ-POSS</p><p>The OAPS was prepared by following litera-ture methods [9]. In the FT-IR spectrum ofOAPS (Fig. 1, curve a), two broad peaks at 3369and 3220 cm1, which are assigned to mNH, wereobserved. The SiOSi bonds in silsesquioxanecages of OAPS could be characterized by thecontent: 15 wt%.stretching bands at 1119 cm1 in FT-IR spectrum.For 1H NMR data of OAPS, the ratio of integra-tion intensity for protons of amine groups and aro-matic groups equals to 1:2. These results indicatethat every benzyl ring has one amino group inOAPS, corresponding to the OAPS structure. Inaddition, the narrow polydispersity of OAPS (Mw/Mn = 1.16) also indicated the absence of oligomericmaterials resulting from POSS cage decomposition.</p><p>Various functional groups can be theoreticallyderived from the eight corner amino groups ofOAPS. Tamaki et al. [9] prepared N-Fluorene-POSSfrom OAPS. But the product contains only 5 u-orene groups per silica cube on average accordingto 1H NMR analysis. They explained that the di-culty for the complete substitution is due to the ste-ric hindrance in the POSS macromonomer. In thispaper we prepared the novel BZ-POSS by reactionof OAPS, paraformaldehyde and cresol in yield of68%. In order to avoid any p-position substitutionof phenol, the p-cresol was used instead of phenolwhich is conventionally used for BZ preparation.In addition, during the BZ ring-closure process,we have encountered the formation of insolubleby-products. This phenomenon may arise from theadditional Mannich reaction of carbene-iminiumwith the aromatic groups of dierent POSS mole-cule forming some cross-linking or branched struc-tures. Thus we used the bulk p-cresol as bothreactant and solvent to prevent the side-reactions.</p><p>The structure of resulting BZ-POSS was con-rmed by spectroscopic and XRD methods. FT-IR spectra (Fig. 1, curve c) shows the asymmetricCOC stretching at 1228 cm1 and the COCcyclic acetal vibration at 948 cm1 as in the oxazinering. The stretching of tri-substituted benzene ringat 1501 cm1 is also clearly noted. The disappear-ance of the characteristic absorption bands at 3369and 3220 cm1, attributed to the asymmetric andsymmetric vibration modes of the amino groups ofOAPS (Fig. 1, curve a) is also detected, indicatingthe complete conversion of amino groups into BZgroups. The SiOSi bonds in silsesquioxane cageof BZ-POSS are conrmed by a stretching vibrationat 1119 cm1 [17,21,24].</p><p>The structure of BZ-POSS was also characterizedby XRD study (Fig. 2). Both the patterns for OAPSand BZ-POSS show a single crystalline peak andamorphous halo. The crystalline peak is related tosome long-range order in the macromonomer. Theamorphous halos in OAPS and BZ-POSS curves</p><p>arise from the presence of amino and BZ group</p></li><li><p>osites</p><p>J. Zhang et al. / European Polymer Journal 43 (2007) 743752 747</p><p>OLECULARNANOTECHNOLOGYsubstitutional isomers [9,17], respectively, haphaz-ardly packing at the molecular level. It was interest-ing to note that the crystalline peak in the patternfor OAPS at 2h = 8 (d = 1.1 nm, by Braggs equa-tion) corresponds roughly to the diameter of octa-phenylsilsesquioxane cage as pointed out by Choiet al. [8]. After BZ functionalization in the patternfor BZ-POSS the crystalline peak shifts to2h = 5.3 (d = 1.7 nm). The greater d spacing couldbe related to the diameter of sti BZ-POSS. Thus wecalculated the diameter of the BZ-POSS moleculeaccording to the bond lengths and angles (Scheme 2).We used 11.1 nm for the extension of the length of</p><p>Fig. 2. XRD patterns of (a) OAPS and BBZ/BZ-POSS nanocomp10 wt%, (e) 5 wt%, (f) 0 wt%.octaphenylsilsesquioxane cage [8]. The lengths of thesingle bonds NCph, NC and NO are 0.143 nm[25], 0.147 nm and 0.141 nm [26], respectively. Thus,the diameter for the BZ-POSS molecule was calcu-lated to be (11.1) + 2 [0.143 + 0.147 + 0.141 cos(180 109)] = 1.651.75 nm, in accordance</p><p>Scheme 2. Schematic illustration for calculating the length ofBZ-POSS. (a) 11.1 nm; (b) 0.143 nm; (c) 0.147 nm and (d)0.141 nm cos(180 109).with the crystalline peak at 2h = 5.3 determinedby XRD.</p><p>The 1H NMR spectrum of BZ-POSS is shown inFig. 3. There is no peak disappearance or obviouschange in integration...</p></li></ul>


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