Particle Dynamics and Rheology of Polymer-Grafted Nanoparticle Composites with Chemical Heterogeneities

We investigate the effect of chemical heterogeneities and architectures of interfacial chains on mechanical properties of polymer nanocomposites. Magnetite (Fe₃O₄) nanoparticles adsorbed with flexible poly(methyl methacrylate) (PMMA) chains in poly(methyl acrylate) (PMA) matrices relax at longer times than those of particles with the rigid adsorbed polymer. This confirms our previous results on the reinforcement of highly entangled, packed adsorbed layers around nanoparticles. In the case of grafted Fe₃O₄ nanoparticle system, particles with short grafts surprisingly exhibit a longer relaxation time than the longer chains, implying a stronger interphase formation in grafted systems. A series of polymer-grafted nanoparticles with different chain lengths at the same graft density are synthesized and their composites with chemically different matrix polymers are prepared to explore the dynamic heterogeneity effect on the mechanical performance of composites. Small-amplitude oscillatory shear (SAOS), temperature sweep, and deformation at large shear experiments are carried out to understand the effect of graft chain conformation and molecular weight effect on the linear viscoelastic behavior and inter-diffusion of the graft-matrix interfacial regions where graft chains become glassy and liquid with temperature modulations around the T_g of graft chains.