A molecular dynamics simulation study of the influence of free surfaces on the morphology of self-associating polymers

Molecular dynamics simulations of thin films and bulk melts of model self-associating polymers have been performed in order to gain understanding of the influence of free surfaces on the morphology of these polymers. The self-associating polymers were represented by a simple bead-necklace model with attractive groups (stickers) at the chain ends (end-functionalized polymer) and in the chain interior (interior-functionalized polymer). The functionalized groups were found to form clusters in the melt whose size is representative of that found experimentally in many ionomer melts. While the size distribution and shape of the clusters in the thin films were found to be relatively unperturbed compared to their corresponding bulk melts, the morphology of the self-associating melts was found to be significantly perturbed by the free surfaces. Specifically, a strong depletion of stickers near the interface and the emergence of clearly defined layers of stickers parallel to the surface was observed. Increased bridging of clusters by the functionalized polymers was also observed near the free surface. We conclude that these effects can be associated with a high free energy for stickers in the low-density interfacial regime: stickers prefer to be in the higher-density interior of the film where relatively unperturbed sticker clusters can form.