Wetting and Chain Packing across Interfacial Zones Affect Distribution of Relaxations in Polymer and Polymer-Grafted Nanocomposites

Abstract

Polymers exhibit deviations from their bulk physical properties in the vicinity of solid interfaces due to changes in configurations, entanglements, and relaxation dynamics at the interfacial regions. By comparing grafted and nongrafted polymer nanocomposite systems based on poly(methyl methacrylate) and silica, we show that the distribution of relaxation times exhibits both commonly reported slower mobility and faster modes that depend on the nature of the interfacial zone, matrix molecular weight, and loading level of nanomaterials. These findings are derived from studies using broadband dielectric spectroscopy (BDS) and differential scanning calorimetry (DSC) to probe molecular and interfacial dynamics. By systematically examining nanocomposites based on nonfunctionalized “bare” Si nanoparticles (NPs) dispersed in PMMA matrices and on PMMA-grafted Si NPs (PMMA-g-NPs) in PMMA matrices, we probe the effects of interfacial interactions and confinement in each of these cases on the glass transition temperature, Tg, the mean time scales, and spectral shapes of the dielectric relaxation. The faster relaxation modes are attributed to the increasing importance of chain wetting and packing in the interfacial zones around nanofillers, especially in the polymer-grafted system. These insights are used to generate a unifying molecular framework that explains the enhancement in numerous macroscopic physical properties of polymer and polymer-grafted nanocomposites, which suits them for myriad applications.