Structure and Dynamics of Composites of Star Polymers in a Linear Polymer Matrix

Polymer "composites" composed of star polymers dispersed in a linear polymer matrix are a limiting case of composites with polymer-tethered nanoparticles (NPs) when the core NP size is extremely small. We study this novel class of "soft-particle" nanocomposites using molecular dynamics simulations, where these composites can also exhibit attributes of polymer blends. These materials exhibit some aspects of polymer blends due to the capacity of the star polymers to swell and collapse depending on the interactions of the star polymers and polymer matrix. In particular, we find that the swelling or collapse of star polymers greatly impacts the degree of influence of the star polymer additive on the segmental dynamics of the polymer/star-polymer composite. Moreover, the swelling of the stars increases the degree of interpenetration of the polymer matrix and the star polymer segmental domains, resulting in a larger alteration of the polymer matrix dynamics. Building on this observation, we show that adding star polymers with modestly stiff arms, or simply an equivalent fraction of stiff chains, can lead to large changes in the polymer matrix dynamics as compared to the neat polymer or composite with flexible-armed stars. Apparently, stiff polymer additives are significantly more effective at altering the relaxation of the polymer matrix. As observed for ordinary "hard" nanoparticle composites, the changes in segmental dynamics and glass transition parallel the changes in the activation free energy in the high-temperature Arrhenius regime.