Distribution of Relaxation Dynamics in Polymer Nanocomposites: Wetting and Chain Packing Effects*

Broadband dielectric spectroscopy and differential scanning calorimetry are used to study dynamics and glass transition temperature of poly(methyl methacrylate)/silica nanoparticle (NP) composites. By systematically examining nanocomposites based on non-functionalized SiO₂ NPs dispersed in PMMA matrices and on PMMA-grafted SiO₂ NPs in PMMA matrices, we probe the effects of interfacial interactions and confinement in each of these cases on segmental dynamics and T_g. In addition to slower mobility assigned to interfacial relaxations, faster modes – dependent in NP loading level and the molecular weight of the matrix – also arise due to confinement effects. These faster modes are more pronounced in nanocomposites with polymer-grafted nanoparticles. We explain these observations in a unifying way by invoking the concepts of chain wetting and packing in interfacial zones around nanofillers. As a result, these insights provide a possible molecular understanding of well-known macroscopic phenomena in polymer nanocomposites, which may inform efforts to strengthen materials or tailor the nanocomposites for specific applications.