Unexpected diffusion behavior of gas molecules in crystalline poly(4‐methyl‐1‐pentene)

We have studied the behavior of carbon dioxide and methane in crystalline poly(4‐methyl‐1‐pentene) (PMP) by means of molecular simulation. Gas diffusion coefficients in the direction of the crystal axes have been determined by equilibrium and nonequilibrium molecular dynamics techniques. We find that both gases diffuse along the channels of free volume between the PMP helices as well as perpendicular to these channels. The diffusion of CH₄ is more isotropic than that of CO₂: It diffuses perpendicular to the helices with almost the same rate as parallel. This is attributed to the fact that CH₄ dissolves better in the outer perimeters of the PMP helices which are not dissimilar to fluid alkanes. A map of the local relative free energy of sorption within the unit cell is calculated both by test‐particle insertion and from the residence periods of gas molecules at certain positions. It also shows that the barriers to perpendicular diffusion are lower for CH₄ than for CO₂.