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.

A molecular dynamics simulation study of surface effects on gas permeation in free-standing polyimide membranes

“…We have recently carried out a comparative study of gas permeation in two fully atomistic models of a glassy polyimide using molecular dynamics (MD) simulations [10] in order to test for possible "skin-effects" [3,[11][12][13] related to the presence of interfaces with specific features. The 40-mer ODPA-ODA homopolyimide ( Fig.…”

“…In addition, the preparation procedure for atomistic models of ODPA-ODA in the pure bulk had been earlier optimized and validated with respect to other experimental properties [16][17][18]. The first system studied [10] was an isotropic 27-chain 56025-atom bulk model of the ODPA-ODA amorphous phase [10,18], which was periodic in three dimensions. The second system was a 68-chain 141100-atom model of an actual membrane, which was created using an original procedure based on the experimental solvent casting process [10].…”

“…The first system studied [10] was an isotropic 27-chain 56025-atom bulk model of the ODPA-ODA amorphous phase [10,18], which was periodic in three dimensions. The second system was a 68-chain 141100-atom model of an actual membrane, which was created using an original procedure based on the experimental solvent casting process [10]. The membrane had a width of 13 nm, i.e.…”

“…In the former case [10], calculations were speeded-up by a factor of 70 using a soft-repulsive Weeks-Chandler-Andersen (WCA) potential for the non-bonded interactions. While this allowed for diffusive equilibrium and good statistics to be attained in both bulk and membrane models, the polymer membrane had to remain constrained between both walls in order to compensate for the absence of attractive forces, and thus only the presence of a high-density interface was tested [10].…”

“…While this allowed for diffusive equilibrium and good statistics to be attained in both bulk and membrane models, the polymer membrane had to remain constrained between both walls in order to compensate for the absence of attractive forces, and thus only the presence of a high-density interface was tested [10]. In the present work, we include more realistic Lennard-Jones (LJ) and Coulombic interactions for non-bonded interactions.…”