Sorption and diffusion in multicomponent polymers

The solution, diffusion, and permeation of low molecular weight substances in polymeric materials are governed by the relative chemical compositions and physical structures of the penetrant molecule and the polymer. Those factors determine the chain segmental mobility, defect structures, and interactions which control the sorption magnitude and penetrant molecular mobility within the polymer. The transport of relatively noninteracting penetrant molecules in a polymer almost always follows the classical behavior predicted by Fick's law relationships with a constant (or nearly so) diffusion coefficient. An increase in generalized interactions (van der Waals, etc.) leads to increased sorption of the penetrant, with consequent increase in plasticization of the polymer, such that the diffusion process often becomes concentration-dependent. Then, depending upon the relative rates of polymer relaxation processes concurrent with the sorption-diffusion process, the overall transport process may exhibit Fickian behavior with a simple concentration-dependent diffusion coefficient or it may deviate significantly from that behavior due to complicating relaxation effects. Systems in which there may be more specific interactions (hydrogen-bonding, polar, ionic) often show a pronounced dependence of transport behavior on compositional, temporal, and spatial parameters which relate to changes in sorption modes. In these present investigations, we have used copolymer systems, prepared under controlled conditions, to gain some further insight into the dependence of polymer structure on variations in composition and how these variations affect or are reflected by transport and relaxation properties.