Influence of System Size in Molecular Dynamics Simulations of Gas Permeation in Glassy Polymers

Abstract

The dependence of gas permeation on system size in glassy polymers has been tested by creating several models containing different numbers of molecules of the same chain length for the ODPA-ODA homopolyimide and by subsequently determining the permeation characteristics of helium. Eight "standard size" 4150-atom, a 6225-atom, and a much more expensive computationally 56025-atom systems were generated using hybrid pivot Monte Carlo (PMC)-molecular dynamics (MD) single-chain sampling. Following the careful relaxation of the polyimide matrices, helium atoms were then inserted into these systems, and MD simulations were carried out at the same applied external conditions of constant temperature and pressure tensor. Average densities for the pure matrices all fell within 0.7% of the experimental value. Energetic and structural properties as well as solubilities and characterization of the void space were also found to be number independent, thus showing that the preparation procedure gives reproducible and reliable results. Helium diffusion was analyzed over periods up to 20 ns using different approaches, such as observation of the individual trajectories, mean-square displacements, distributions of penetrant displacements components, and van Hove correlation functions. No number dependence could be detected, whether the gas molecules were in the anomalous or in the Fickian regime.

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

“…The force-field and the parameters for the polyimide are the same as described before [10,17,18], with "bonded" anglebending, torsional and out-of-plane interactions arising from near-neighbour connections in the structure. The so-called "non-bonded" excluded-volume and electrostatic potentials are applied to all atom pairs separated by more than two bonds on the same molecule or belonging to different molecules.…”

“…1) was chosen as a test case since experimental data for both gas permeability and diffusion in this specific polyimide are available [14,15]. 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.…”

“…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].…”

Gas Permeation in Semicrystalline Polyethylene as Studied by Molecular Simulation and Elastic Model

“…Later, de Pablo et al [22] employed Gibbs ensemble Monte Carlo simulations in order to compute the solubility of short chain alkanes in polyethylene melts beyond Henry's regime. Several studies have followed, considering many different penetrant gases and more complex polymers such as polystyrene [23], poly(styrene-alt-maleic anhydride) copolymer, pols-(styrene-stat-butadiene) rubber and atactic polystyrene [24], polyimide [25], polyamide [26], polyethylene terephthalate [27,28].…”

Molecular simulation of realistic membrane models of alkylated PEEK membranes

“…The prediction of transport properties of small molecules through polymer matrices would allow for the rational selection of polymer materials used in these applications and their future optimal design. Although there has been reported an increasing number of studies on this subject over the last decades [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18][19][20], the prediction of transport properties of gas molecules through glassy polymer membranes remains a difficult target. In many of the recent studies reporting molecular simulation prediction of diffusion and solubility of small gas molecules in several membrane models of the same glassy polymer a great scatter of the predicted values is observed.…”