Diffusion of small molecules in semicrystalline polymers. II. The permeability of unoriented polymer films

Abstract

AbstractIn a previous publication, the diffusion of small molecules through semicrystalline polymers was compared with the flow of electrical current through a heterogeneous conductor. In this way it was possible to solve the diffusion equations numerically, and to apply the results successfully to the description of the permeability of some polyethylene films as a function of their density. In the present paper, the electrical model has been investigated further and in particular, the effect of blind alleys in the diffusion path has been described. The theory has then been applied to the permeability data published recently by Lasoski and Cobbs for three series of unoriented polymer films differing primarily in crystalline content. Although the experimental data do not conform perfectly to the theory, a large area of agreement has been established, and in view of the complexity of the problem this agreement appears to be satisfactory. The theory developed here requires that the activation energy for permeation be substantially independent of the degree of crystallinity. This has been shown to be in harmony with activation energy data determined on polyethylene specimens of different densities, both by permeability measurements and by nuclear magnetic resonance measurements. The abrupt increase in the apparent activation energy for the permeation of natural rubber films by small molecules below the temperature of crystallization has been shown to be due, most probably, to the reduction of the amorphous volume fraction as the temperature is decreased.