Investigation of Glass-Rubber Relaxation Characteristics in Polymer Nanoparticle Composites

The glass transition and sub-glass relaxation properties of glassy polymer nanocomposites have been investigated by dynamic mechanical and dielectric techniques. Two model polymer nanocomposite series were examined: polyetherimide [PEI] + SiO₂, and polymethylmethacrylate [PMMA] + SiO₂. Both native and surface-modified commercial silica nanoparticles were employed. For nanocomposite formulations with favorable polymer-particle interactions, the inclusion of moderate to high levels of nanoparticle loading led to the emergence of a dual-Tg response that encompassed a bulk Tg event (matching Tg for the unfilled polymer), and a second elevated Tg corresponding to relaxations involving chain segments constrained in the vicinity of the polymer-particle interface. The characteristics of this dual-Tg response were evaluated relative to particle loading levels, morphology and the quality of the interface. The combination of dynamic mechanical and dielectric methods provides a comprehensive view as to the influence of the particles both with respect to local sub-glass motions, as well as the larger-scale processes inherent to the glass transition.