Diffusion of small molecules in semicrystalline polymers: Water in polyethylene

摘要

AbstractPublished data on the solubility properties and diffusion coefficients for water in polyethylene are critically examined and related to the permeability coefficients analyzed in a previous paper. The data examined in the present article are even more variable than the permeability data, the measured diffusion coefficients varying over a fourteenfold range. Solubilities vary less from specimen to specimen but do so in an order opposite to that of the diffusion coefficients. Accordingly the permeability coefficients vary much less than do the diffusion coefficients. In an examination of the permeation process, as it must occur through semicrystalline polymer films, it is demonstrated that a systematic error is introduced if permeation through such films is described by means of equations developed for homogeneous materials. The error has been evaluated by means of an electrical analog and equations describing diffusion in semicrystalline polymers are derived which complement those already in existence for diffusion in homogeneous polymers. By means of these equations it is shown that the variations in permeability and diffusion coefficients cannot be explained on the basis of reasonable variations in the amount of crystallinity between otherwise comparable polymers or by reasonable variations in the way in which the crystalline regions occur in the amorphous matrix. The conclusion previously reached to the effect that variations in the structure of the amorphous phase from specimen to specimen are responsible for irreproducible transport properties still appears valid.