The microstructure of free volume in polymers is of fundamental importance in determining macroscopic behaviors of polymer materials, such as mechanical and viscoelastic behaviors. According to the free-volume theory, the diffusion of molecules in polymers within their amorphous rubbery state is primarily governed by the amount of free-volume holes. In order to highlight the properties of the free volume in polymers, much effort has been focused on macroscopic properties exhibited as a sum performance of holes, while little information from microscopic point of view are given out about the behaviors of free volume. Therefore, the relation between the atomic-scale holes and free-volume theory still remains a challenge.

The Simha-Somcynsky (SS) hole theory equation-of-state (EOS), positron annihilation lifetime spectroscopy (PALS) technique and molecular modeling can provide detailed information about vacancies in polymers from theory, experiment and simulation, respectively. The purpose of this paper is to simulate the vacancies in PVAc, and to compare the simulation results with the SS EOS, published PALS results and free-volume theory. The solvent diffusion coefficient in PVAc will also be predicted using molecular simulation. A probe molecule method has been employed to calculate the free volume. The quantitative relation between three kinds of free volume and probe radius has been determined. In addition, the infinite dilution diffusion coefficient of organic solvent in PVAc has been predicted using simulation method, and the predictions are in good agreement with reported experimental data. It is reasonable to expect this approach as a useful tool in polymer-related field.