INTERFACIAL STRUCTURE AND DYNAMICS OF MACROMOLECULAR LIQUIDS - A MONTE-CARLO SIMULATION APPROACH

Dynamic Monte Carlo simulations are performed on a dense liquid consisting of freely jointed, 20-bead long chain molecules in a cubic lattice in the vicinity of solid walls. The objective is to elucidate the equilibrium structure and dynamic behavior of polymer melts at interfaces. The microscopic structure of the polymeric liquid is significantly affected by the presence of solid walls. Segment density is enhanced near the strongly attractive surfaces and depleted near the weakly attractive surfaces. The distribution of chain centers of mass is peaked at a distance slightly less than one radius of gyration from the solid. Chain shapes are pronouncedly flattened adjacent to a solid wall and gradually assume unperturbed bulk characteristics as one moves away from the wall. The self-diffusion coefficient of chains, evaluated by monitoring their mean-squared center of mass displacement as a function of simulation time, is dramatically reduced near strongly adsorbing walls; on the contrary, chain mobility is enhanced near weakly adsorbing walls. The spatial dependence of self-diffusivity is highly correlated with, but not exclusively governed by, the behavior of local segment density in the interfacial region. The diffusive motion of chains is inherently anisotropic near a surface. Potential barriers between strongly attractive surface sites have only a minor effect on the self-diffusivity. The longest relaxation time of chains is determined as a function of their center of mass position by analyzing the long-time behavior of the end-to-end distance vector autocorrelation function. Strong segment-wall attraction prolongs relaxation times in the surface region appreciably, in comparison to the bulk. Moreover, potential barriers to the lateral motion of adsorbed segments exert a profound influence on the rate of molecular relaxation. Adding such barriers to a strongly attractive surface can cause the relaxation time of chains located near the surface to rise by as much as 80%.