Transport Mechanisms of Gases in Annealed Linear Low Density Polyethylene Films

The effect of the pressure difference on the permeability of carbon dioxide, oxygen, and nitrogen through annealed LLDPE films is studied. The isotherms showing the dependence of the permeability coefficient on the pressure of the upstream chamber, p o, exhibit an anomalous decrease in the low-pressure region whose location is shifted to lower values of p o as the temperature of the isotherms decreases. The diffusion coefficient increases with p o, the increase being larger the higher is the temperature. The curves showing the dependence of the solubility coefficient on the pressure of the upstream chamber exhibits the same pattern as that of the permeability coefficient. The strong dependence of the diffusive characteristics of gases on p o in the low-pressure region was interpreted in terms of the dual mode theory. The values of the apparent diffusion coefficients for the molecules in the dissolved and the trapped modes are estimated. In the same way, the Henry's law solubility coefficient, k D, and the Langmuir sorption capacity, C ‘ H, are obtained. The values of k D and C ‘ H for different gases are somewhat lower than those reported for glassy membranes. A detailed study on the effect of p o on the activation energies associated with both the permeability and the diffusion coefficients is reported.