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Influence of Nanoparticles and Molecular Additives on Fragility and Collective Particle Motion in Polymer Glass-Formation
Jack DouglasNational Institute of Standards and Technology, Polymers Division, Gaithersburg, MD
We investigate the impact of the addition of molecular additives and nanoparticles (NP) on glass-formation in polymer melts by complementary molecular dynamics simulations and analytic theory. The NP cause significant changes in both fragility and the average length of string-like motion, where the magnitude and sign of the effect depends on the NP-polymer interaction and NP concentration. These dynamical changes can be interpreted via the Adam-Gibbs (AG) theory if we assume the strings represent the “cooperatively rearranging regions” of AG model, whose basic assumptions are reviewed. Our findings are consistent with fragility being a measure of the cooperativity of molecular motion and a quantitative description of the fragility changes seen in our simulations is obtained from this description.. Molecular additives are also shown to be effective at altering the fragility of glass-formation and extent of collective motion and the phenomenon of antiplasticization (stiffening in the glass state while simultaneously lowering Tg) is found to be associated with this effect. The classical entropy theory of glass-formation is considered as a complementary tool to gain analytic insights into these additive effects on polymer glass-formation.
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