Glass transition behavior of alumina/polymethylmethacrylate nanocomposites

Alumina/polymethylmethacrylate (PMMA) nanocomposites were synthesized using 39-nm nanoparticles and in situ free-radical polymerization. At filler concentrations greater than 0.5 wt.%, the glass transition temperature, T g, was observed to decrease precipitously by 25 °C compared to the neat polymer. At smaller weight fractions, there were no changes in the composite T g. The abrupt changes seem to indicate a threshold at which a significant volume fraction of polymer has higher mobility that brings about the decrease in T g. Consistent with this behavior, the T g depression was suppressed by coating the nanoparticles to make them compatible with the matrix.