Understanding competing mechanisms for glass transition changes in filled elastomers

In polymeric nanocomposites, shifts in the glass transition temperature (T_g) that increase monotonically with particle loading have been attributed to the interphase, in ideally dispersed, attractive systems. However, in elastomeric composites a trend has emerged that shows T_g shifts first towards higher and then towards lower temperatures with increasing filler volume fraction, when measured via mechanical methods (DMA). At high filler loadings (>10 vol%), glass transition temperatures have been recorded below that of the base polymer, even for systems with attractive interactions between polymer and filler. One-dimensional analytical models and three-dimensional finite elements models were used to investigate the source of a mechanically-induced negative T_g shift in highly filled systems. The results attribute the origin of the shift towards higher temperatures as an effect of the interphase, while the subsequent shift to lower temperatures as an apparent relaxation time shift that arises solely due to the addition of stiff elastic particles. These replicated shifts explain a consistent trend across the literature and provide some considerations for those designing elastomeric composites with high filler loading.