Mechanical and Viscoelastic Properties of Polymer-Grafted Nanorod Composites from Molecular Dynamics Simulation

An understanding of the structure property relationship in polymer/nanorod (NR) nanocomposites is of fundamental importance in designing and fabricating polymer nanocomposites (PNCs) with desired properties. Here, we study the structural, mechanical, and viscoelastic properties of polymer grafted NR filled PNCs, using coarse-grained molecular dynamics simulation. The mechanical reinforcement efficiency is found to be determined by the NR/polymer interfacial properties, which are in turn modulated by the grafting density, the grafted chain length, and the graft matrix interaction strength. By systematically analyzing the evolution of the polymer-grafted NRs during mechanical deformation, we find that the NRs aligning side by side with each other do not contribute much to the mechanical reinforcement. Simulation results also indicate that the strain-dependent viscoelastic behavior (the Payne effect) originates from the failure of the local filler network and, especially, NR clusters constructed via site-to-site contacts. PNCs with low grafting density and short grafted chains are found to form NR aggregates, mostly through the site-to-site contact state, which leads to a more pronounced Payne effect as reflected in the slope of the storage modulus versus shear amplitude. Furthermore, for stronger graft matrix interactions, the NRs dispersed in the polymer matrix act as the temporary cross-linking points for a polymer shell layer-bridged NR network, accounting for the significant improvement in the mechanical property and the large increase in the Payne effect at high graft matrix interaction strengths. In general, higher grafting density, longer grafted chains, and moderate graft matrix interactions can effectively minimize the nonlinear viscoelastic behavior of PNCs.