Molecular-dynamics simulation of diffusion of small penetrants in polymers

Abstract

Molecular dynamics simulations of diffusion of a small-molecule penetrant in an amorphous polymer matrix have been carried out for the examples of methane in polyethylene (PE) and methane in polyisobutylene (PIB). Particular attention waa paid to ensuring that the nonbonded potentiale representing the polymerpolymer bead interactions in the matrix were adequately calibrated. Accurate representation of equation of state (P-V-2') behavior for the polymers was used aa the criterion for this. The simulatione were carried out over a wide range of temperatures. The results are in accord with available experimental data. The temperature variation of the diffusion coefficients for PE was non-Arrhenius in character and of the WLF type. Analysis of the diffusion trajectories in PE indicates that the diffusion mechanism changes significantly over the temperature range studied. At lower temperatures, the diffusant motion is characterized by relatively long periods of quiescence interspersed with fairly large jumps. However, at higher temperatures, a broad spectrum of frequent jumps obtains. The diffusion rates are obviously very sensitive to the free volume and hence to the molecular packing. However, the polymer chain mobility is found to play a role aa well. The diffusion activation energy at higher temperature ia similar to that for the decay of the torsional angle autocorrelation function for the polymer. It ie also found that increaaing the torsional barrier reduces the diffusion rate. In PIB where the free volume ie significantly lees than that in PE, the diffusion rates are much lower than those in the latter. The mechanism over the range studied resembles that of the lower temperature regime in PE.

Molecular modelling of oxygen and water permeation in polyethylene

“…Pant and Boyd [16] used a simpler conversion for the diffusion coefficient which was derived from an expression for the dielectric tensor [64]. With the assumption that diffusion in the crystalline regions is negligible, the diffusion coefficient is:…”

“…This could either be due to the oxygen jumping between voids in the polymer matrix, or the motion of the polymer chains annihilating and creating voids in the matrix, or a combination of these processes. defined as the distance between two adjacent intervals [16]. The distributions in Fig.…”

“…These techniques can predict properties at both the macroscopic and molecular (microscopic) levels. At the macroscopic level it is possible to calculate diffusion [14][15][16] and solubility coefficients [14,17,18], and at the microscopic level it is possible to analyse properties such as clustering of the penetrant molecules [19], free volume [17] and rotations of polymer chains [20]. The work presented here illustrates the fact that a single model can provide properties at both the macroscopic and microscopic levels.…”

Transport Properdines of Gases in Polymers: Bibliographic Review

“…The recent development of techniques of microstructure numerical simulation makes possible the modelling of transport phenomena in polymers, especially by application of Monte-Carlo simulation techniques, molecular dynamics (MD), Brownian motions. In recent studies, MD simulations allowed to estimate rather precisely the diffusion coefficients in rubbery polymers, for example, for CH 4 in PE (Pant and Boyd, 1992), for He and CH 4 in PDMS (Sok and Berendsen, 1992). The transport of diffusing molecules in glassy polymers is much more complex because such polymers are not in a state of true thermodynamic equilibrium considering the scale of diffusion measurements.…”

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.…”