The permeation of carbon dioxide through polyethylene membranes has been studied at pressures up to 54.4 atm. and a t temperatures above and below the critical temperature of the gas (31.0"C.l. The permeability coefficient is independent of pressure a t the highest experimental temperature (61 .O"C.), but becomes increasingly pressure-dependent as the temperature is lowered. The principle of corresponding states can be used to correlate the solubility of both gases and vapors in polyethylene over a wide range of temperatures. This principle can also be invoked to obtain an upper limit for the penetrant pressure above which the permeability coefficient becomes pressure-dependent. The effect of pressure on the permeability, solubility, and diffusivity of gases and vapors in polyethylene is discussed in some detail.Although the mechanism of gas permeation through plastic membranes has been studied by many investigators, surprisingly little information is available on the effect of gas pressure on the rate of permeation. In general, the literature suggests that the permeation of gases with low critical temperatures, such as helium, hydrogen, oxygen, and nitrogen, can be described under steady state conditions by a simple form of Fick's law, and that the permeability coefficients for these gases are independent of pressure (1 ) . Carbon dioxide and hydrogen sulfide have been reported to exhibit a similar behavior (1 to 3 ) .On the other hand, the permeability coefficients for many organic vapors, which are characterized by high critical temperatures, were found to be strongly pressure dependent (1, 4, 5).

An improved cell which permits the measurement of permeabilities of membranes to gases over a wide range of temperatures and gas pressures is described. The measurements are made by the variable volume method, under constant pressure differential across the membrane. The cell lends itself particularly well to routine tests, because it does not require calibration or the use of vacuum techniques. The performance of the cell is discussed, and typical experimental results are presented. A modified permeability cell of the same type for high‐pressure studies is also described. Measurements with this apparatus show that the rate of gas permeation obeys, in some cases, a single from of Fick's law, even under pressure differentials across the membrane as high as 800 psi (54 atm.). The paper also compares permeability data obtained by the variable volume and the variable pressure methods. The permeability of 0.002 in.‐thick Alathon 15 polyethylene to oxygen and nitrogen was determined between 0 and 50°C. by the two methods, using the same sample of membrane in situ, and the measurements were found to agree within experimental error. Permeabilites of 0.010 in.‐thick samples of Alathon 15 polyethylene to nitrogen, oxygen, helium, and carbon dioxide obtained in the same temperature range by the variable volume method were 15–30% higher than the corresponding data determined by the variable pressure method. This discrepancy could be due to the fact that the variable pressure measurements with the thicker membrances may not have been made under true steady‐state conditions, although permeabilities were determined from apparently linear sections of permeated gas pressure vs. time curves. A critical re‐examination of the methods used to determine permeability constants is suggested.

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