Free volume analysis of organic vapor diffusion in polyurethane membranes

Mutual diffusion coefficients of penetrant-polymer systems comprising two o rganic compounds (benzene and tetrachloroethylene) and four polyurethane me mbranes (spandex from E.I. du Pont de Nemours & Company, Inc., and three Es tane(R) polymers from B.F. Goodrich Speciality Chemicals) were measured as a function of temperature (30-60 degrees C) and organic vapor activity by p erforming transient desorption experiments with a McBain quartz-spring micr obalance apparatus. Diffusion coefficients increased with temperature and d ecreased with polymer hard segment content. Fujita's free volume theory, co rrected for the presence of hard segment domains in the polyurethanes, desc ribed well the dependence of organic penetrant diffusion on concentration a nd temperature. Equations correlating the free volume parameters to the pol ymer morphology, as quantified by the hydrogen bonding index (HBI), were ge nerated. These equations, in combination with the Flory-Rhener equation for benzene and tetrachloroethylene vapor sorption and polymer morphology corr elations for the Flory-Rhener parameters, were able to predict to within 12 % the mutual diffusion coefficients as a function of concentration, tempera ture, and hard segment content. (C) 2000 Elsevier Science B.V. All rights r eserved.