Mechanism of gas permeation through polymer membranes. Progress report, September 1, 1978-May 31, 1979

Abstract

A new, more exact derivation of the free-volume model of permeation is presented in this report. The free-volume parameters A/sub d/, B/sub d/, and ..gamma.. have been determined for CO/sub 2/ in polyethylene from diffusivity measurements at 20 and 35/sup 0/C and at pressures of up to 23 atm. The polyethylene was in the form of semicrystalline rods, and the measurements were made with a new type of high-pressure diffusivity apparatus operating under both isobaric and isothermal conditions. The above parameters were used in conjunction with the free-volume model to predict permeability coefficients for the transport of CO/sub 2/ through polyethylene membranes as a function of pressure. The values of the predicted permeability coefficients were found to agree to within 35% or better (on the average) with those obtained directly from permeability measurements with thin polyethylene membranes. The agreement is within the expected limits, considering the assumptions of the free-volume model and the uncertainty in the diffusivity and permeability measurements. A method of determining free-volume parameters directly from permeability measurements was developed for dilute penetrant-polymer systems. This method was used to obtain values of A/sub d/, B/sub d/, and ..gamma.. for CO/sub 2/, CH/sub 4/, C/sub 2/H/sub 4/, C/sub 2/H/sub 6/, and C/sub 3/H/sub 8/ in polyethylene membranes. The values of B/sub d/ and ..gamma.. are in about the same range, respectively, as reported for other penetrant-polymer systems. No data appear to have been published previously for A/sub d/. According to the theory, B/sub d/ is thought to be related to the minimum hole size required for the displacement of a penetrant molecule in the polymer, and should depend on both the size and shape of the molecule. This was confirmed by a linear correlation of B/sub d/ with the effective molecular diameters of the above gases; these diameters make allowance for possible orientation of anisometric molecules during diffusion.