Poly(methylmethacrylate)-silica nanocomposites films from surface-functionalized silica nanoparticles
Introduction
Polymer-silica nanocomposite materials [1], [2] received wide attention of researches for their attractive properties to be potentially applied in protective coatings, high refractive index films, optical waveguide materials, and flame retardants [3], [4], [5], [6]. Among the nanocomposite materials, one of the most widely studied is poly(methyl methacrylate)-silica nanocomposite materials [7], [8], [9], [10], [11], [12], [13]. The sol–gel process with using alkoxysilane compounds as precursors was commonly utilized in preparing PMMA-silica nanocomposite materials. However, owing to the poor thermal stability of PMMA, the curing temperature of the nanocomposite materials was limited, so as to result in an incomplete condensation of Si–OH groups. The residual Si–OH groups in the nanocomposite materials obtained showed significant harms to the optical and thermal properties of the PMMA-silica nanocomposite materials. To overcome this problem, preformed nanosized colloidal silica was used to prepare PMMA-silica nanocomposite materials [12], [13], [14]. Kashiwagi et al. [13] disclosed the preparation of PMMA-silica nanocomposites from in situ polymerization of MMA together with silica nanoparticles. Homogeneous dispersion of the silica particles in the PMMA matrix was found. However, there were not covalent linkages between the organic and inorganic domains to reduce the glass transition temperature of the nanocomposites. To introduce chemical bonding between the PMMA chains and silica particles, Yu et al. [14] used 3-(trimethoxysilyl)propyl methacrylate to modify the silica surface, and then copolymerized the modified silica particles with MMA. A post curing process was applied to promote the conversion of the silanol dehydration reactions. In Yu's work covalent linkages were introduced to promote the miscibility between organic and inorganic domains. However, sol–gel reaction was still involved in preparing nanocomposite materials to reduce the thermal stability of the resulted products.
Here, allylglycidylether (AGE) was utilized as a surface modifier for silica nanoparticles [15]. The AGE-modified silica particles were then copolymerized with MMA to form PMMA-silica nanocomosite films. This approach introduced covalent bonds between PMMA and silica particles and did not employ alkoxysilane compounds and sol–gel process in the preparation process, consequently to overcome the drawbacks found in the reported literatures. Transparent PMMA films having very high silica contents, superior thermal stability, low surface roughness, and high hardness were obtained.
Section snippets
Materials
Nanoscale silica particles (CS) were purchased from Nissan Chemical Co., Japan. The commercial product of MIBK-ST, in which 30–31 wt% of silica (particle size: 10–20 nm) was dispersed in methylisobutylketone (MIBK), was used. Allylglycidylether (AGE) and methylmethacrylate (MMA) from Aldrich Co. were used as received. The AGE modified silica particles (AGE-CS) were prepared through reacting AGE with silica particles according to the reported method [15]. The reaction composition in weight ratio
Results and discussion
The polymerization of MMA with AGE-CS was successfully performed through the conventional free-radical polymerization process (Fig. 1). After the reaction, an increase in viscosity of the reaction solution was observed. All of the reaction solutions were transparency and precipitation and gel phenomena did not occur during the polymerization. The PMMA-silica nanocomposite films were obtained from spinning-coating the solution on a glass plate. The chemical compositions of the nanocomposite
Conclusion
It was concluded that the covalent bond linkages between the inorganic silica and organic PMMA polymer provided extreme high compatibility of these two domains to result in the high silica contents (over 70 wt%) of the nanocomposite materials. The prepared nanocomposite films exhibited extremely high hardness, surface planarity, and good thermal stability. On the other hand, incorporation of the silica particles did not altered the thermal degradation behaviors of PMMA.
Acknowledgements
Financial support on this work from the Ministry of Economic Affairs of Taiwan is highly appreciated. The authors also thank Prof. R.-J. Jeng at National Chung Hsing University (Taichung, Taiwan) for his kind help in AFM measurements.
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