Structural Properties of Bound Layer in Polymer-Nanoparticle Composites

The modification of interfacial polymer behavior is one of the primary sources of property modifications of polymer-nanoparticle (NP) composites (PNC). For strongly favorable polymer-nanoparticle interactions, large NPs exhibit a "bound" polymer layer which has a much longer relaxation time than the surrounding polymer matrix. The difference between the relaxation time of the bound polymer and matrix decreases as the NP size decreases. Using molecular simulations, we explore the degree to which the interfacial polymer structure, and thus the bound layer properties, depends on the size of NP. We find that for larger NPs, chains orient with their longest axis parallel to the NP interface, an effect which diminishes for small NP. We also find that the strength of interfacial interaction does not have a significant effect on the structural changes, which suggests that the change in the polymer chain structure is entropically driven.