A critical investigation of polyethylene gas permeability

The effect of polymer density on the gas permeability of polyethylene film has been studied. The experimental data for permeability P are described by the equation P = K (1 — d ) n , where K is a constant characteristic of each gas and d is the sample density. This equation is similar to previous analytical descriptions of the permeability—density relation. Values of K and n ( n ≈ 2) and confidence limits for P are given for N 2 , O 2 , and CO 2 . Permeability—density values are reported for approximately 100 film samples for each of the three gases over a density range from 0.899 to 0.975 g./cc. The density was measured for each sample inserted in the permeability apparatus. The above mathematical model chosen to describe the experimental data is interpreted to mean that the effect of the crystalline phase is to decrease both the gas solubility and diffusion. Gas solubility and diffusion are linear functions of the volume fraction of amorphous material. Permeability, the product of diffusion and solubility, is consequently proportional to the square of the volume fraction of amorphous material. Permeability is related to density rather than volume fraction of amorphous material to avoid choosing a single value for the density of the amorphous fraction. The experimental results suggest additional lowering of the permeability due to restraints on the segmental motion of the amorphous chains.