Physical models of diffusion for polymer solutions, gels and solids

Diffusion in polymer solutions and gels has been studied by various techniques such as gravimetry, membrane permeation, fluorescence and radioactive labeling. These studies have led to a better knowledge on polymer morphology, transport phenomena, polymer melt and controlled release of drugs from polymer carriers. Various theoretical descriptions of the diffusion processes have been proposed. The theoretical models are based on different physical concepts such as obstruction effects, free volume effects and hydrodynamic interactions. With the availability of pulsed field gradient NMR techniques and other modern experimental methods, the study of diffusion has become much easier and data on diffusion in polymers have become more available. This review article summarizes the different physical models and theories of diffusion and their uses in describing the diffusion in polymer solutions, gels and even solids. Comparisons of the models and theories are made in an attempt to illustrate the applicability of the physical concepts. Examples in the literature are used to illustrate the application and applicability of the models in the treatment of diffusion data in various systems.