Partial Molar Volumes and Thermal Expansion Coefficients as an Explanation for Co-Solvent Effect of Penetrants in Multicomponent Polymer Mixtures

Experimental mixed‐gas sorption/dilation data and mixture densities estimated by the fitted Sanchez–Lacombe equation of state have been used to estimate the partial molar volumes (PMV) of gases and polymers in multicomponent mixtures (i.e., ternary) at conditions of industrial relevance. The method developed estimates accurately the PMV and volumetric thermal expansion coefficients of various highly soluble gases and polymers in multicomponent mixtures over a wide range of temperatures, pressures, and gas phase compositions. A comparison of solubility, volumetric thermal expansion coefficients, and PMVs of the gases involved in the studied ternary mixtures reveal that, irrespective of the polymer nature, co‐solvent effect is caused by the gas with higher solubility in the polymer phase and higher thermal expansion coefficient, which provides an explanation to the occurrence of co‐solubility effects in multicomponent gases/polymer mixtures. It has also been shown that the PMV behavior of gases in the ternary mixtures with polymers is different from their PMV behavior in the corresponding binary gas/polymer mixtures, and that the PMV of a gaseous penetrant in a multicomponent system depends on its gas phase concentration. For multicomponent gases/polymer mixtures, partial molar volume and volumetric thermal expansion coefficient of each gas dissolved in the polymer phase are estimated by using the experimental solubility or swelling data in conjunction with the Sanchez–Lacombe equation of state. The results suggest that the co‐solvent effect is provoked by the gas with higher solubility and higher volumetric thermal expansion coefficient.