Mechanical Behavior of Polymer Nanocomposites via Atomistic Simulations: Conformational Heterogeneity and the Role of Strain Rate

Polymer nanohybrids with a high fraction of nanofillers have been found to exhibit improved mechanical properties compared to the neat polymer homogeneous systems. Since polymer-based materials are characterized by a broad range of relaxation times, it is expected that their response under external load would depend on the actual rate of the applied deformation. In this work, we investigate the heterogeneous mechanical behavior in glassy poly(ethylene oxide)/silica nanoparticles (PEO/SiO2) nanocomposites via detailed atomistic molecular dynamics simulations. Our goal is to unravel the effect of strain rate on the mechanism of polymer nanocomposite reinforcement, within the glassy state, by directly probing the mechanical properties at the molecular level. By applying tensile deformations with various strain rates we clearly demonstrate that the value of the applied strain rate strongly affects the mechanical properties of the PEO/SiO2 model systems, inducing a transition from a rubber-like behavior, at low strain rate, to a more brittle one, at high strain rates. Then, we further investigate the mechanical heterogeneity in glassy PEO/SiO2 systems by probing directly the stress and strain fields for various conformations of adsorbed (trains, tails, loops, and bridges), and free polymer chains. Our data emphasize the importance of both train and bridge conformations on the mechanical reinforcement of the polymer nanocomposites. Last, we also probe the mobility of various chain conformations, under different applied strain rates, and their contribution to the overall mechanical behavior of the nanocomposite, during the deformation process.