Occupied and Accessible Volumes in Glassy Polymers and Their Relationship with Gas Permeation Parameters

Abstract In order to relate the microstructure of polymer chains to the gas permeation properties, a novel method is proposed for the calculation of the van der Waals ( V w ), occupied ( V occ ), and accessible ( V acc ) volumes of polymers, using polyimides as examples. In this approach, V occ and V acc values are obtained as characteristics of polymer–gas systems. Correlations between V occ and the gas permeability ( P ) and diffusion ( D ) coefficients are demonstrated. Accessible volume, V acc , determined experimental densities, and V occ calculated using a Monte Carlo technique, as well as the fractional accessible volume ( FAV ) ( V acc / V sp ), where V sp is the specific volume of polymers, also correlate well with the P and D values. The slopes of these correlations increase with increasing molecular size of the diffusing molecules. In addition, the parameters of the equations for D ( FAV ) and P ( FAV ) strongly depend on conformation rigidity (Kuhn segment) of the polyimides. An interpretation of the results obtained is presented. Definition of occupied volume. image Definition of occupied volume.