Influence of morphology on the transport properties of crystalline polymers
The sorption and diffusion of penetrant of low molecular weight proceeds almost exclusively through the amorphous component of the semicrystalline polymer consisting of chain folds, tie molecules and free chain ends. Hence they depend on its transport properties and geometrical distribution which are both modified by mechanical and thermal treatment. The morphological changes induced by such treatment and the ensuing changes of transport properties cannot be described in a satisfactory manner by crystallinity and orientation. That is particularly true for drawn material with fibrous structure exhibiting a high anisotropy of diffusion and a drastic decrease of sorption and diffusion. Both effects are the consequence of the microfibrillar morphology with a large fraction of highly aligned taut tie molecules which by their closer packing eliminate many sorption sites, enormously reduce the diffusivity, and increase its concentration dependence and activation energy. The axial alternation of crystal blocks and amorphous layers making the microfibrils almost impermeable in the longitudinal direction produces the high anisotropy of diffusivity which may be reduced during plastic deformation of the fibrous structure by the increased number of interfibrillar tie molecules. Annealing relaxes the amorphous component by reducing the fraction of taut tie molecules, thus restoring the transport properties to values as observed in undrawn material. Another example of the overwhelming influence of morphology are the hard elastomers where the formation of a great many submicroscopical holes yields permeability increased by many orders of magnitude with no change in crystallinity and only modest increase of crystal orientation.