Effect of Interfacial Structure on Permeation Properties of Glassy Polymers

Abstract

Gas permeation has been studied in two fully atomistic molecular models of a glassy polyimide, which differ by their chain configurations and packing. The first polyimide system is an isotropic 56 025-atom bulk model of the amorphous phase while the second is a 141 100-atom model of an actual membrane. The preparation procedure of the membrane was based loosely on the experimental solvent-casting process. The membrane model exhibits density oscillations at the interfaces with chains being aligned and flattened parallel to it. The structuration persists throughout the membrane, and this leads to the gas diffusing in a slightly anisotropic way in the center of the membrane model. However, the diffusion coefficient obtained using either approximate analytical solutions or numerical solutions to the one-dimensional diffusion equation as well as a time-lag approach was found to be very similar to that in the bulk, which was evaluated from mean-square displacements, probability density distributions of displacement vector components, and the van Hove self-correlation function. Solubilities obtained from Widom's insertion technique and from the equilibrium density of gas within the matrix were in very good agreement for the membrane model. Despite a small drop in solubility in the region corresponding to the density peak of the polymer at the interface, the solubility coefficient also remained similar to that of the bulk. The changes in configurations and the high-density interface have thus little effect on the permeability of the gas used in this case.