Activated Transport In Polymer Grafted Nanoparticle Melts

We measure the activation energy for the local segmental dynamics of polymer chains densely grafted to nanoparticles (NPs) using quasielastic neutron scattering. We aim to understand the underpinning physics of the experimentally measured enhanced gas transport in polymer grafted nanoparticle-based membranes relative to the neat polymer (without NPs), especially the permeability maximum, which occurs at intermediate chain lengths. We find that the activation energy goes through a minimum as a function of chain length, while the elementary jump size goes through a maximum around the same chain length. These results, likely, are the dynamic consequence of a structural transition of the grafted polymer brush from "extended" to "interpenetrated" with increasing chain length at fixed grafting density. Evidently, the regimes of different graft chain lengths near this structural transition are associated with lower activation energy, likely due to fluctuation effects, which also lead to enhanced gas transport.