Theory of coupled activated relaxation in dense polymer-particle mixtures: effects of size ratio, particle loading and interfacial attraction

Dense polymer-particle mixtures represent a wide range of systems including polymer nanocomposites, polymerized ionic liquids, and bio-related materials. Understanding and predicting the dynamical and mechanical properties of such hybrid systems is of practical importance and high theoretical interest. Here we study the dynamics of polymer nanocomposites using the Elastically Collective Nonlinear Langevin Equation (ECNLE) theory with structural correlations obtained from Polymer Reference Interaction Site Model (PRISM) approach. The latter captures the depletion, steric stabilization, and tight bridging states of structural organization. We focus on the effects of particle-segment size ratio, interfacial (cross) attraction strength, and particle loading on the segmental relaxation time, glass or gel like mechanical properties, and kinetic arrest. Cooperative motions of segments and particles is treated in a dynamically self-consistent manner, and compared to the limiting case of pinned particles. We find a rich dynamical behavior with both plasticization and anti-plasticization regimes, and a nanocomposite shear modulus which can be either softened or reinforced. The key physics relates to the interplay between geometric packing and physical bonding effects.