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 investigated: (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 T-g of 5-15 degrees C were observed for all composites (2-12% w/w silica) and even pure polymer reference samples. However, after additional annealing for 72 h at 140 degrees C, all previously observed drops in T-g disappeared, and the intrinsic T-g 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 mechanical properties (i.e., storage modulus) of the fully equilibrated composites. Even though no equilibrium T-g changes were observed, the addition of silica to the PMMA matrices was observed to improve the mechanical properties of the glassy polymer host. (c) 2007 Wiley Periodicals, Inc.