ABSTRACT
Much of the emphasis throughout this book, thus far, has been on microscopic mechanisms of diffusional transport in different types of materials. In metals and ionic crystals the influence of the periodic lattice and the nature of the point defects that mediate diffusional transport were highlighted. In metals, different types of defects and lattices of varying geometric structures were responsible for a diverse range of transport mechanisms that occur in these systems. In network glasses the structural disorder, coupled with the transient nonbridging sites that accommodated cationic transport, imposed certain limitations on the nature of the dynamics. Spatial correlations were imposed on the mobile species due to long-range Coulombic effects. In long-chain polymers, translational diffusion of a chain is subject to topological constraints imposed by neighboring chains (“tubes”) leading to one-dimensional motion along its own contour. Tracer diffusion and selfdiffusion were discussed in detail in order to illustrate the effect of these processes on polymers. The driving force for tracer and self-diffusion is entropic, devoid of complications associated with enthalpic interactions that would influence the magnitude of the interdiffusion coefficient, which is of particular interest in this chapter.
