Permeability of High-polymer-films to Gases and Vapors

Abstract

IN RECENT years a number of investigators (1, 6, 7, 11), employing different experimental techniques, have reported data on the transmission of gases through polymer films. The present investigation was undertaken to supplement existing data, particularly with respect to the "permanent gases" nitrogen, oxygen, and carbon dioxide, and to study the effect of variables such as temperature and film thickness on the rate of permeation. In addition, the permeability of a number of polymer films to the two organic vapors methyl bromide and ethylene oxide has been studied at a number of temperatures and pressures. The experimental arrangement used to measure the fate of per-meation was an adaptation of the high vacuum technique used by Barrer (8) and van Amerongen (2). The permeation of gases and vapors through polymer films appears to be primarily diffusion-controlled. Thus, when the stationary state of flow is attained, the flux, q-i.e., the amount of gas passing through the polymer film per unit area and time-satisfies Fick's first law in that. _-Pi) _ P(Pi-Pi) m 9-l ~ l where pi and p2 are the pressures of the gas on both sides of the barrier, l is the thickness of the film, D is the diffusion constant, S is the solubility coefficient, and P is the permeability constant, which is equal to DS. For the permanent gases, D and S are essentially constant for all pressures; this is in contradistinction to the case of organic vapors, where D and S often increase with increasing pressure (, 9). The temperature dependence of D and S is given by the usual exponential function; hence, the temperature dependence of the permeability constant is given also by the Arrhenius equation P = Pc exp (-Ep/BT) (2) where EP, the permeation activation energy, is the sum of the activation energy for diffusion E¿ and the heat of solution, AH, of the gas in the polymer. EXPERIMENTAL PROCEDURE The experimental method used to measure the rate of gas transmission is essentially similar to that used by Barrer (8, 4)· The apparatus consists of a stainless steel permeability cell connected by means of Kovar seals to a high vacuum system. The perme-ability cell and the equipment and method of operation have been described in detail (8). The permeability constants are given in units of cubic centimeters of gas at standard temperature and pressure per second per square centimeter of area, 1 mm. in thickness, at a pressure difference of 1 cm. of mercury across the film. The gases used in this study, obtained from the Matheson. Co., were of the following purities: nitrogen, 99.9%; oxygen, 99.6%; carbon dioxide, 99.9%; methyl bromide, 99.4%; and ethylene oxide, 99.5%. The polymer films used were donated by a number of companies and are described in Table I. To ensure accurate permeability values, several samóles were cut from various parts of the polymer films and the permeability was determined for each of them. In general, good agreement was obtained between the various samples, as shown in Table II. Measurements made at different temperatures were always concluded by returning to the original temperature and repeating the determination. In this way, the effects of any irreversible changes in the film, such as crystallization or degradation, could be detected. Literature values, when available, were either similar to or greater than the measured values. These considerations led to the belief that the values reported in this paper are genuine characteristics of the polymer film concerned and that errors due to pinholes and other imperfections have been avoided.