  The diffusion of small molecular penetrants through polymeric materials
represents an important fundamental problem, relevant to the design of
materials for applications such as coatings and membranes. Polymer networks
hold promise in these applications, because dramatic differences in molecular
diffusion can result from subtle changes in the network structure. In this
paper, we use molecular simulation to understand the role that crosslinked
network polymers have in governing the molecular motion of penetrants. By
considering the local, activated alpha relaxation time of the penetrant and its
long-time diffusive dynamics, we can determine the relative importance of
activated glassy dynamics on penetrants at the segmental scale versus entropic
mesh confinement on penetrant diffusion. We vary several parameters, such as
the crosslinking density, temperature, and penetrant size, to show that
crosslinks primarily affect molecular diffusion through modification of the
matrix glass transition, with local penetrant hopping at least partially
coupled to the segmental relaxation of the polymer network. This coupling is
very sensitive to the local activated segmental dynamics of the surrounding
matrix, and we also show that penetrant transport is affected by dynamic
heterogeneity at low temperatures. To contrast, only at high temperatures and
for large penetrants or when the dynamic heterogeneity effect is weak does the
effect of mesh confinement become significant, even though penetrant diffusion
more broadly empirically follows similar trends as established models of mesh
confinement-based transport.
