Gas sorption and permeation of glassy polymers with microvoids

Gas sorption and permeation of physically treated glassy polymeric membranes is studied from the viewpoint of the transport mechanism and the control of microvoids in the membrane. Physical treatments, such as sub-T-g annealing, thermal quenching, and CO2 pressure conditioning were found to be effective for the control of microvoids in the glassy state, as well as gas sorption and permeation. The Langmuir saturation constant C-H' of the dual-mode sorption model decreased with a sub-T-g annealing period for the copolymer (vinylidene cyanide-alt-vinyl acetate), copoly(VDCN-VAc). On the other hand, Henry's solubility coefficient k(D) and Langmuir affinity constant b did not change markedly with sub-T-g annealing. The permeability coefficient of copoly(VDCN-VAc) decreased with the sub-T-g annealing period. Furthermore, the dual-mode parameter C-H' and the CO2 permeability coefficient of polyamic acid, polyimide, poly(2,6-dimethyl-1,4-phenylene oxide), and polycarbonate increased markedly upon thermal quenching and CO2 pressure conditioning from the liquid state. The gas sorption isotherm and pressure dependence of the permeability coefficients for the polymers studied here are explained in terms of the dual-mode sorption model and partial immobilization model characteristic to a glassy polymer, respectively. It was found that sub-T-g annealing, thermal quenching, and CO2 pressure conditioning control gas sorption and permeation, as well as the occurrence of microvoids in glassy polymeric membranes. (C) 2003 Elsevier Ltd. All rights reserved.