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

AY Liu and H Emamy and JF Douglas and FW Starr, MACROMOLECULES, 54, 3041-3051 (2021).

DOI: 10.1021/acs.macromol.0c02500

We use molecular dynamics simulations to study how the chain length affects the structure and segmental dynamics of polymer-nanoparticle (NP) composites at semidilute NP concentrations. For NPs having relatively strong interactions with the polymer, we can approximate the system as an ideal NP dispersion, which isolates the effect of direct interactions among the NPs. By varying both the chain length N and NP concentration, we examine regimes where the chain size (i.e., chain radius of gyration R-g) is small compared to the NP separation d (d/R-g > 1), as well as cases where d/R-g < 1, so that chains readily "bridge" between the NPs. We find that the fraction of such bridging chains in our simulations can be expressed as a universal function of d/R-g. Structurally, the polymers slightly elongate near the NP interface and the chains tend to align their longest axis with the NP interface. We show that the effect of NPs on the chain structure is nearly chain length-independent, whereas the effect on chain alignment extends farther from the NP surface as the chain length increases. Chains that bridge between NPs must significantly elongate when the NP separation is large compared to the chain dimensions (d/R-g > 1). Although these bridging chains have a longer relaxation time than nonbridging chains, they do not make a substantial contribution to the overall nanocomposite segmental relaxation time for the conditions studied because the bridging chains represent only a small fraction of the system. Note that relaxation at the scale of chains may differ for bridging chains. When NP separation is comparable or smaller than the chain size (d/R-g < 1), bridging chains are more prevalent, but their properties are more similar to nonbridging chains than when d/R-g > 1. Accordingly, the variation of the computational glass transition temperature Tg with chain length essentially mirrors the trend for the reference pure polymer melt, where Tg increases with chain length and roughly saturates at large polymer mass. Overall, for the conditions studied, bridging chains are found to have a small impact on the segmental dynamics of nanocomposites.

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