Effects of Chain Length on the Structure and Dynamics of Semi-Dilute Nanoparticle-Polymer Composites

We use molecular dynamics simulations to study how chain length affects the structure and dynamics of polymer-nanoparticle (NP) composites at a semi-dilute NP concentration with an ideal dispersion of NPs. By varying both chain length and NP concentration, we examine regimes where chain size (i.e. chain radius of gyration R_g) is small compared to the NP separation d (R_g/d < 1), as well as cases where R_g/d > 1, so that chains can “bridge” between the NPs. Generally, we find that polymers slightly elongate near the NP interface and the chains tend to align their longest axis with the NP interface. Chains that bridge between NPs must significantly elongate when the NP separation is large compared to the chain dimensions. These bridging chains have a longer relaxation time than non-bridging chains, but surprisingly they do not make a substantial contribution to the overall composite relaxation for the conditions studied. Accordingly, the variation of glass transition temperature T_g with chain length essentially mirrors the trend for the reference pure polymer melt, where T_g increases with chain length and roughly saturates at large mass. Overall, for the conditions studied, bridging chains are found to have a surprisingly small impact on the composite properties.