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 (Vw), occupied (Vocc), and accessible (Vacc) volumes of polymers, using polyimides as examples. In this approach, Vocc and Vacc values are obtained as characteristics of polymer–gas systems. Correlations between Vocc and the gas permeability (P) and diffusion (D) coefficients are demonstrated. Accessible volume, Vacc, determined experimental densities, and Vocc calculated using a Monte Carlo technique, as well as the fractional accessible volume (FAV) (Vacc/Vsp), where Vsp 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.imageDefinition of occupied volume.

A new model for characterizing the free volume of a glassy polymer { gas systems is proposed. An improved method for the calculation of occupied volume per monomer unit was developed within the limits of this model. The model assumptions, error estimates and algorithm e¯ciencies are described. Using the example of polyvinyltrimethylsilane, it is shown that linear dependences of logarithms of the di¬usion and the permeability coe¯cients on speci c accessible volume for inert gases exist.