Modifying Fragility and Collective Motion in Polymer Melts with Nanoparticles

We investigate the impact of nanoparticles (NP) on the fragility and cooperative stringlike motion in a model glass-forming polymer melt by molecular dynamics simulation. The NP cause significant changes to both the fragility and the average length of stringlike motion, where the effect depends on the NP-polymer interaction and NP concentration. We interpret these changes via the Adam-Gibbs (AG) theory, assuming the strings can be directly identified with the abstract ``cooperatively rearranging regions'' of AG. Our findings indicate that fragility is primarily a measure of the temperature dependence of the cooperativity of molecular motion.