Effects of Monomer Size on Polymer Mass Transport at an Interface

Polymer mass transport in the bulk has been a topic widely studied in the literature with the main phenomelogical aspects, presently, well understood. These studies are, in general, of great importance to the understanding of the phenomenology of, for example, polymers solutions and polymer melts in the bulk. However, polymer diffusion at an interface, as for example, a crystalline substrate has been a much less researched topic [Desai et al., Phys. Rev. Lett. 98, 218301 (2007)]. Present studies have focused on smooth surfaces defined by a potential field, which are unrealistic for actual substrates as their structure is not taken into account. In fact, we expect monomer diffusion to take place by thermally activated processes over a corrugated energy landscape, instead of moves over a smooth potential. Here, we study the effect of the substrate structure on the diffusion constant of the polymer, by taking into account its crystalline structure, an FCC (100) substrate orientation. Specifically, we vary the monomer radius relatively to that of the substrate particles and study how the diffusion constant varies. We show that the influence of the substrate is important and leads to a non-monotonic diffusion constant with the ratio of the monomer radius relatively to that of substrate particles. This behavior is primarily identified with the total energy, consisting of the polymer energy and the substrate-monomer interaction. The major contribution, leading to the non-monotonic behavior of the diffusion constant, is associated with the substrate-monomer interaction, but there is also, a monotonic dependence, given by the polymer energy.
Concomitantly, we also study the Arrhenius behavior of the diffusion
process at various temperatures. Finally, we also study the size dependence of the diffusion constant and discuss the implications of a finite lifetime of polymers of different sizes on a substrate.