High density polyethylene was biaxially oriented by high-pressure forging and subsequently low-pressure thermal forming. The permeability of O2, CO2, and CH4 was reduced slightly by biaxial orientation, the diffusivity of the gases was found to increase with the degree of orientation. The solubility decreased markedly with orientation. Density measurements indicated that crystallinity increases with the extent of biaxial deformation. Based on the concept of particle flow in viscous media, the variation of gas diffusivity with orientation is viewed as a frictional resistance effect. The diffusivity is inversely proportional to a phenomenological friction coefficient; which can be related to orientation by a shape factor s. The diffusivity for the oriented state, D, is then related to that of the nonoriented state, D0, as D = D0/(1 + sϕ). The variation of diffusivity selectivity with orientation is also formulated based on this consideration. Results on both biaxially and uniaxially oriented HDPE are examined in light of this model.
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