  While recent efforts have shown how local structure plays an essential role
in the dynamic heterogeneity of homogeneous glass-forming materials, systems
containing interfaces such as thin films or composite materials remain poorly
understood. It is known that interfaces perturb the molecular packing nearby,
however, numerous studies show the dynamics are modified over a much larger
range. Here, we examine the dynamics in polymer nanocomposites (PNCs) using a
combination of simulations and experiments and quantitatively separate the role
of polymer packing from other effects on the dynamics, as a function of
distance from the nanoparticle surfaces. After showing good qualitative
agreement between the simulations and experiments in glassy structure and creep
compliance, we use a recently developed machine learning technique to decompose
polymer dynamics in our simulated PNCs into structure-dependent and
structure-independent processes. With this decomposition, the free energy
barrier for polymer rearrangement can be described as a combination of
packing-dependent and packing-independent barriers. We find both barriers are
higher near nanoparticles and decrease with applied stress, quantitatively
demonstrating that the slow interfacial dynamics is not solely due to polymer
packing differences, but also the change of structure-dynamics relationships.
Finally, we present how this decomposition can be used to accurately predict
strain-time creep curves for PNCs from their static configuration, providing
additional insights into the effects of polymer-nanoparticle interfaces on
creep suppression in PNCs.
