  We investigate the impact of nanoparticles (NP) on the fragility and
cooperative string-like 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 string-like motion, where the effect
depends on the NP-polymer interaction and the NP concentration. We interpret
these changes via the Adam-Gibbs (AG) theory, assuming the strings can be
identified with the "cooperatively rearranging regions" of AG. Our findings
indicate fragility is primarily a measure of the temperature dependence of the
cooperativity of molecular motion.
