Theory of solubility of gases in polymers

Abstract

Lattice–hole theory in the formulation of Simha and Somcynsky (SS) is employed to compute the free energy of mixing and the chemical potential of the dissolved gas. The corresponding quantity in the vapour phase is obtained from tabulated values of the second and third virial coefficients. This makes it possible to proceed beyond the limit of Henry’s law. Three gases, argon, methane and carbon dioxide, and two polymers, polyethylene and cis-polybutadiene, are considered. Previous equation-of-state analyses in terms of SS theory provide requisite structural parameters and, where required, are modified or extended. The pressure- and temperature-dependent solubilities derived are reasonable in the light of literature values for similar polymers at atmospheric pressure and 298K. We compare our results with empirical correlations for natural rubber between solubility and gas critical temperature on one hand and heats of solution on the other. The outcome is gratifying. The literature contains the characteristic parameters of the theory for many polymers. Thus applications of the theory to further polymer–gas combinations are indicated. We note an inversion, i.e. positive temperature coefficients of nitrogen solubility in polystyrene and polybutadiene. ©1997 SCI