We employ both traditional molecular graphics techniques to show atomic motion and surface rendering methods to display the redistribution of penetrant-accessible volume in the polymer. The penetrants are quasi-stationary for longer periods of time while they are located in cavities that exist in the polymer. During these residence times they explore the cavity by oscillation and by rotational or librational motions, but do not leave it. Hence, no net penetrant flow is achieved in these periods. Exactly how long the quasi-stationary periods last depends strongly on the size of the penetrants, the details of their interactions with the polymer, the structure and flexibility of the polymer matrix, and so on.
Diffusion is caused by quick leaps of penetrant molecules from one cavity to another.
The time for such a jump event is much shorter than the quasi-stationary periods, typically in the order of a few picoseconds. They seem to confirm unanimously the hopping pattern described above. We display the free volume by a surface-rendering technique borrowed from biomolecular simulations. They consist of a single PIB chain of 80 monomers interacting with its periodic images due to the cubic periodic boundary conditions. The equations of motion are integrated by the leap-frog algorithm with a time step of 2 fs.