Local Structure and Dynamics in Solvent-Swollen Polymers

Abstract

We report atomistically detailed molecular dynamics simulations of benzene−polystyrene systems (0−84.2 wt % polystyrene). We have calculated solvent diffusion coefficients and have found that their composition dependence not only shows good agreement with experiment but also follows quite well the predictions by lattice models. We also show that, for the polystyrene−benzene system studied here, it is not possible to separate solvent molecules into slow ones, tightly bound to the polymer, and fast ones, not bound to the polymer. This would suggest that, in a gel, the polymer chains alone act as obstacles to solvent diffusion and not polymer decorated by a shell of solvent molecules. We have found the reorientation of benzene molecules in the gel to be nonexponential and anisotropic, the reorientation of the ring normal being slower than the in-plane reorientation. This anisotropy increases dramatically with polystyrene concentration. At the highest polymer concentration, the time scales for the two motions are separated by 3 orders of magnitude. The changes of polymer solvation and polymer dynamics with concentration are discussed. For all polymer concentrations above 50 wt %, the polymer turns out to be essentially rigid on a nanosecond time scale with only local fluctuations possible.