Correlation of Gas Permeability and Diffusivity with Selectivity: Orientations of the Clouds of the Data Points and the Effects of Temperature

For an extended data set for amorphous glassy polymers, the pair correlations P-i-P-j fulfill with very good accuracy (R-2 about 0.9 or higher) where P-j and P-j are the permeability coefficients of the gases M-i and M-j in the same polymers. This allowed us to determine the orientation (medians) of the clouds of the data points on the correlations log P-i versus alpha(ij) = P-i/P-j. It was shown that for various gas pairs the slopes of these medians are negative and increase by absolute value when the ratio of kinetic diameters of the penetrants increases. For the first time a similar analysis was carried out for pair correlations of the diffusion coefficients D-i and D-j. On the diagrams log D-i versus alpha(Dij) = D-i/D-j, the medians show the same trend as in the diagrams for permeability coefficients. The effects of temperature on the orientation of the clouds of the data points and upper bounds (limiting lines) of the diagrams log P-i versus alpha(ij) = P-i/P-j were investigated using the available values of the activation energy of permeation. It was shown that for all the gas pairs the medians tend to shift down along the selectivity axis when temperature increases. Depending on a selected gas pair the slopes of the medians and upper bounds get either steeper or weaker. It was discussed that the positions and the slopes of the medians and limiting lines are sensitive to the Arrhenius parameters of gas permeation and temperature.