Thermal and Mechanical Properties of Poly(methyl methacrylate) Nanocomposites Containing Polyhedral Oligomeric Silsesquioxane

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<ul><li><p> Thermal and mechanical properties of poly(methyl methacrylate) nanocomposites containing polyhedral oligomeric silsesquioxane </p><p>Chunbao Zhao1,2,a, Xin Wang2, Xujie Yang2, Wei Zhao1 1 Faculty of Microelectric Engineering, Nanjing College of Information Technology, Nanjing, 210046, </p><p>China </p><p>2 Key Laboratory for Soft Chemistry and Functional Materials of Ministry of Education, Nanjing University of Science and Technology, Nanjing, 210094, China </p><p> </p><p>Keywords: poly(methyl methacrylate), polyhedral oligomeric silsesquioxane, nanocomposites, thermal stability. </p><p>Abstract. A series of poly(methyl methacrylate) (PMMA) composites containing polyhedral </p><p>oligomeric silsesquioxane (POSS) were produced by bulk polymerization. The morphology, thermal </p><p>and mechanical properties of the composites were characterized by X-ray diffraction (XRD), </p><p>transmission electron microscopy (TEM), thermogravimetric analyses (TGA) and dynamic </p><p>mechanical analyses (DMA). Results show that the octa(3-chloropropyl)-POSS (ocp-POSS) and </p><p>trisilanolphenyl-POSS (triol-POSS) have high compatibility with PMMA and can be uniformly </p><p>dispersed into PMMA matrix. The separate incorporation of these two types of POSS contributes to </p><p>the improvement of thermal stability of PMMA composites. When the content of POSS was 7.5 wt%, </p><p>the thermal decomposition temperatures (5% mass loss) of PMMA composites with ocp-POSS and </p><p>triol-POSS were increased by about 104 C and 130 C, respectively. The increase of triol-POSS </p><p>content in the PMMA matrix gave slight enhanced storage modulus before glass transition. </p><p>Introduction </p><p>Poly(methyl methacrylate) (PMMA) is a type of very useful thermoplastic polymer with many </p><p>excellent properties such as good exibility, extraordinary optical clarity, high strength and desirable </p><p>weatherability. However, its lower thermal stability restrains it from applications in higher </p><p>temperature region. To improve the thermal stability of PMMA, many inorganic fillers such as silica, </p><p>clay and carbon nanotubes were introduced into the PMMA matrices to form corresponding </p><p>composites[1-3].Nevertheless, it is noted that the incorporation of the inorganic fillers usually lead to </p><p>the decrease of transparency, which may limit the applications of PMMA materials in some fields. </p><p> During the passed a few years, polyhedral oligomeric silsesquioxanes (POSS) have attracted </p><p>considerable attention as molecular silica for polymer nanocomposites formation. POSS molecule </p><p>has special organic-inorganic hybrid cage structure and can be easily incorporated into polymer </p><p>systems through blending, grafting or copolymerization[4,5] to obtain high performance composites. </p><p>Recent studies on POSS-containing polymer nanocomposites involving PMMA system have been </p><p>reported[6-10]. Nanocomposites of PMMA containing POSS have exhibited enhancement in thermal </p><p>and mechanical properties[8,10].However, the previous research mainly focused on the POSS </p><p>molecules which can form covalent bonds with PMMA backbone. Relatively speaking, much less is </p><p>known about the influence of POSS cages without covalent attachment on the thermal, mechanical </p><p>and morphological properties of the resulting PMMA composites. </p><p>In the present study, two different types of POSS, octa(3-chloropropyl)-POSS (ocp-POSS) and </p><p>trisilanolphenyl-POSS (triol-POSS), were used to prepare transparent PMMA nanocomposites by </p><p>bulk polymerization. The morphology, thermal and mechanical properties of the composites were </p><p>characterized by XRD, TEM, TGA and DMA. Differences of morphology and properties between the </p><p>composites with two different types of POSS were investigated and discussed. </p><p>Advanced Materials Research Vols. 557-559 (2012) pp 304-308Online available since 2012/Jul/26 at (2012) Trans Tech Publications, Switzerlanddoi:10.4028/</p><p>All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of TTP, (ID:, Purdue University Libraries, West Lafayette, United States of America-09/09/13,11:28:53)</p></li><li><p> Experimental section </p><p>Materials. Methyl methacrylate (MMA), azobis-(isobutyronitrile) (AIBN) were of analytical grade, </p><p>obtained from Shanghai Chemical Reagents Co., China. MMA monomer was distilled from calcium </p><p>hydride under reduced pressure. AIBN was refined in heated ethanol and kept in a dried box. </p><p>Ocp-POSS was prepared according to Ref.[11]. Triol-POSS was supplied by Hybrid Plastics </p><p>Company and used as received. All other solvents were used as received. </p><p>PMMA/POSS samples preparation. The PMMA/POSS nanocomposites were prepared by bulk </p><p>polymerization. A desired amount of POSS and AIBN were added in MMA monomer and </p><p>prepolymerized for about 0.5h under nitrogen atmosphere at 75 C. Then the solution was cast into a </p><p>glass mould and polymerized at 50 C for 48 h. The transparent castings were thermally treated at </p><p>100C for 1 h. The POSS contents of samples were 2.5, 5.0, 7.5 wt% respectively. A reference </p><p>sample of pure PMMA was prepared using the same polymerization method. </p><p>Characterization. Dynamic mechanical analysis (DMA) was conducted using a DMA Q800 (TA) </p><p>instrument with bending mode at an oscillatory frequency of 1 Hz. The temperature scan was </p><p>performed at 3C/min heating rate in the range from room temperature to around 200C. </p><p>Thermogravimetric analysis (TGA) was performed with a Mettler-Toledo TGA/SDTA 851e </p><p>apparatus from 50 to 600 C at a heating rate of 20 C/min under nitrogen. X-ray diffraction (XRD) </p><p>was recorded on a Bruker D8 X-ray diffractometer using Cu K radiation (=0.1542 nm) with the </p><p>range of the diffraction angle of 2=2~60 at 40 kV and 30 mA. The TEM micrographs were taken </p><p>with a JEOL JEM-2100 transmission electron microscopy at an accelerating voltage of 200 kV. </p><p>Results and discussion </p><p>Dynamic mechanical analysis. The dynamic mechanical spectra of PMMA composites containing </p><p>ocp-POSS and triol-POSS are shown in Fig. 1 and Fig. 2, respectively. The spectra of storage </p><p>modulus E and loss factor tan display only one relaxation process corresponding to the glass </p><p>transition of the composites with different POSS loading, quite similar to the behavior of the pure </p><p>PMMA. The storage modulus E values of PMMA/ocp-POSS composites (Fig. 1) slightly decrease </p><p>with the increasing of the ocp-POSS content. While, the storage modulus values of the </p><p>PMMA/triol-POSS composites before the glass transition are higher than that of the pure PMMA </p><p>(Fig. 2). The difference of the storage modulus between two types of composites is mainly attributed </p><p>to the structures of the filled POSS. Compared with triol-POSS, the ocp-POSS molecule is covered </p><p>with chloropropyl substituents, which can lead to a relatively stronger plasticization effect on PMMA </p><p>matrix. </p><p>From the Fig. 1 and Fig. 2, the glass transition temperature (Tg) values for these PMMA/POSS </p><p>composites are slightly lower than that of the pure PMMA. The factors leading to the variation of Tg </p><p>of POSS-containing composites include POSS species, loading and the composites structures[4]. In </p><p>our previous work[8], the PMMA composites containing octavinyl-POSS displayed a remarkable </p><p>improvement of Tg, which was ascribed to the severe hindering effect of POSS. In this study, </p><p>however, the hindering effect of used POSS is much weaker than octavinyl-POSS, because there is no </p><p>strong chemical covalent bond linkage between the POSS and PMMA matrix. </p><p>Advanced Materials Research Vols. 557-559 305</p></li><li><p>Fig.1 Dynamic mechanical spectra of pure </p><p>PMMA and PMMA/ocp-POSS composites. </p><p>(The inset is the zoom-in plot of E data in the </p><p>temperature range of 50~100 C) </p><p>Fig.2 Dynamic mechanical spectra of pure </p><p>PMMA and PMMA/triol-POSS composites. </p><p>(The inset is the zoom-in plot of E data in the </p><p>temperature range of 50~100 C) </p><p>Thermal analyses. Figure 3 shows the weight-loss curves of the PMMA composites with varying </p><p>concentrations of POSS. It can be seen that the thermal decomposition temperatures of the PMMA </p><p>composites containing POSS are higher than that of the pure PMMA, and enhance markedly with </p><p>increasing POSS content in two different systems. Compared with pure PMMA, the thermal </p><p>decomposition temperatures (5% mass loss) of PMMA composites with 7.5wt% of ocp-POSS and </p><p>triol-POSS were increased by about 104C and 130C, respectively. It is noted that the PMMA </p><p>composites filled with triol-POSS exhibit better thermal stability for a given POSS loading. We </p><p>proposed two reasons for this difference. One is the rigidity of triol-POSS molecules with benzene </p><p>rings. The other is the silicon hydroxyl of triol-POSS, which can form hydrogen bonds with PMMA </p><p>chains. In addition, the residual char yields of the PMMA composites increase with increasing POSS </p><p>content as expected, which may be favorable to improve the flame resistance of PMMA composites. </p><p>Fig. 3 TGA curves of pure PMMA and PMMA/POSS composites </p><p>Morphology (XRD and TEM). X-ray diffraction was used to characterize the dispersion of the </p><p>PMMA/POSS composites. Diffraction patterns for the ocp-POSS and triol-POSS systems are shown </p><p>in Fig. 4(a) and (b), respectively. The characteristics of the XRD patterns for the two composite </p><p>systems are similar at comparable loadings of POSS. In the figure, a broad amorphous peak at ~14.3o </p><p>was attributed to the amorphous PMMA matrix. In all the diffraction patterns of PMMA/POSS </p><p>composites, the peaks corresponding to ocp-POSS and triol-POSS are not observed, which is similar </p><p>to the result reported by Zhang W et al.[12] for PMMA/octacyclopentyl-POSS composites. This </p><p>observation clearly indicates that the ocp-POSS and triol-POSS are well dispersed into the PMMA </p><p>matrix leading to the suppression of crystallization of pure POSS[13]. Further more, it is noted that </p><p>the amorphous peaks intensity of PMMA matrix shows a decreasing trend with the increasing of </p><p>POSS concentration in the two types of PMMA composites. This is possibly due to that the addition </p><p>of ocp-POSS and triol-POSS has an influence on the arrangement of PMMA chains. </p><p>306 Advanced Materials and Processes II</p></li><li><p>Fig.4 The XRD patterns of POSS and PMMA/POSS composites </p><p>The above results were also conrmed by the TEM images of the composites. Figure 5(a) and (b) </p><p>show the TEM images of the PMMA composites with 7.5 wt% of ocp-POSS and triol-POSS, </p><p>respectively. To contrast with the background, the TEM images were taken at the edge of the </p><p>sectioned composites. It is seen that the dark areas (the portion of the PMMA composites) are quite </p><p>homogenous and no localized domains were detected at this scale, implying that the two types of </p><p>POSS components were homogenously dispersed in the continuous PMMA matrix at the nanoscale. </p><p>The result is also consistent with the observed phenomena that the ocp-POSS and triol-POSS were </p><p>completely resolved in PMMA prepolymer and formed transparent and viscous liquid during the </p><p>course of composites preparation. </p><p>Fig. 5 The TEM images of PMMA composites with 7.5 wt% of ocp-POSS (a) and triol-POSS (b) </p><p>Conclusions </p><p>In this paper, the results of XRD and TEM showed that the ocp-POSS and triol-POSS have high </p><p>compatibility with PMMA matrix. All of these two types of POSS, when incorporated separately, can </p><p>improve the thermal stability of PMMA significantly. The thermal decomposition temperatures </p><p>(Tdec,5% mass loss) of PMMA composites with 7.5wt% of ocp-POSS and triol-POSS were increased </p><p>by about 104 C and 130 C, respectively. The PMMA composites filled with triol-POSS exhibit </p><p>better thermal stability for a given POSS loading. The PMMA composites containing ocp-POSS </p><p>showed slightly reduced storage modulus values due to plasticization effects, while the incorporation </p><p>of triol-POSS slightly improved storage modulus values of PMMA composites before glass </p><p>transition. </p><p>Acknowledgment </p><p>We acknowledge the nancial support of Jiangsu Planed Projects for Postdoctoral Research Funds </p><p>(1001015B), Nature Science Foundation of Jiangsu Province of China (BK2011838) and Qinglan </p><p>Project of Jiangsu Province. </p><p>Advanced Materials Research Vols. 557-559 307</p></li><li><p> References </p><p>[1] H. Wang, P. Xu, W. Zhong, L. Shen and Q. Du: Polym. Degrad. Stabil. 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