Correlation between fractional free volume and diffusivity of gas molecules in classy polymers

Despite its oversimplifications, the free-volume approach has proven to pro vide very useful correlative and even semipredictive capabilities. This art icle is concerned with the correlation between the diffusivity, D, of gas m olecules in glassy polymers and the fractional free volume, FFV, determined by the Bondi method. The diffusivities were taken from a database, generat ed by the authors in connection with work on the new Landolt Bornstein seri es "Diffusion in Non-Metallic Solids", which encompasses a very large varie ty of glassy polymers. For a given diffusant log D is a linear function of 1/FFV as predicted by the free volume theory. However, the deviations from this relationship are significant and strongly correlated between O-2, N-2, CO2, and CH4. The systematic deviations are opposite to the predicted effe ct of the polymer jumping unit size in the free-volume concept of Vrentas a nd Duda and are interpreted in terms of an increase in the activation energ y with increasing chain stiffness. Correlation analysis further suggests th at the diffusion mechanisms of H-2 and particularly of He differ markedly f rom that of larger gas molecules. Furthermore the influence of the cohesive energy and the glass-transition temperature of the polymers is investigate d. (C) 1999 John Wiley & Sons, Inc.