A. Thran et al., Correlation between fractional free volume and diffusivity of gas molecules in classy polymers, J POL SC PP, 37(23), 1999, pp. 3344-3358
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.