Influence of sample preparation and processing on observed glass transition temperatures of polymer nanocomposites

Polymer composites composed of poly(methyl methacrylate) (PMMA) and silica (14 nm diameter) have been investigated. The influences of sample preparation and processing have been probed. Two types of sample preparation methods were inves- tigated: (i) solution mixture of PMMA and silica in methyl ethyl ketone and (ii) in situ synthesis of PMMA in the presence of silica. After removing all solvent or monomer, as confirmed using thermogravimetric analysis, and after compression molding, drops in Tg of 5-15 8C were observed for all composites (2-12% w/w silica) and even pure poly- mer reference samples. However, after additional annealing for 72 h at 140 8C, all previ- ously observed drops in Tg disappeared, and the intrinsic Tg of bulk, pure PMMA was again observed. This is indicative of nonequilibrium trapped voids being present in the as-molded samples. Field-emission scanning electron microscopy was used to show well-dispersed particles, and dynamic mechanical analysis was used to probe the me- chanical properties (i.e., storage modulus) of the fully equilibrated composites. Even though no equilibrium Tg changes were observed, the addition of silica to the PMMA matrices was observed to improve the mechanical properties of the glassy polymer host.

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