(142c) Influence of Nanoparticles on the Local Mechanical Properties of Polymeric Materials

Nanocomposite materials are encountered in a number of high-performance applications, but the source of the improved structural and mechanical attributes of nanocomposites has remained elusive. In particular, little is known about the ageing of nanocomposite glasses, or the change of their mechanical attributes over extended periods of time. Fundamental studies of nanocomposite polymeric materials have often been challenged by the proclivity of nanoparticles to phase separate or aggregate, raising questions about the equilibrium nature (or lack thereof) of existing reports. Development of improved nanocomposite materials would benefit considerably from a molecular-level understanding of the structure and dynamics of nanocomposite polymeric glasses as a function of nanoparticle shape, size, and interaction parameters. In this work, we have performed molecular simulations of an entangled polymer and an entangled polymer nanocomposite, and we have studied the effects of ageing and deformation on the mechanical properties of each system. Recent simulation studies have shown that the mechanical properties of amorphous solids are heterogeneous, and such materials can even exhibit local domains with a negative modulus. We find that, as both the pure polymer and the nanocomposite age in the glassy state, they become more mechanically homogeneous; that is, their respective distributions of local mechanical properties becomes more narrow, and the nanocomposite is shown to age more quickly. Upon deformation, the mechanical properties become more heterogeneous, analogous to the way in which mechanical deformation can "rejuvenate" a glassy material. Additionally, we determine the effect of the nanoparticles on the plateau modulus of the polymer by reducing our polymeric systems to their entanglement network. We find that nanoparticles increase the density of entanglements near their surface, and the nanoparticles themselves behave as entanglement traps. As the material is deformed, it is found that the primitive paths of the polymer begin to condense onto the nanoparticles, further deforming the entanglement network. Finally, we discuss how heterogeneity in the shape of the nanoparticles affects the local mechanical properties and the entanglement network.