Diffusion Barriers

Diffusion is interesting because it is slow. As a result, many studies of diffusion seek ways to make the process fast and selective. For example, most membrane separations use thin polymer films to recover particular chemicals. Nitrogen is separated from air, and water is purified by ultrafiltration. This is an active and important area of diffusion research. However, in other cases, we may seek to slow diffusion rather than to accelerate it. We may want protection from acids or oxygen. Paints and packaging are examples with this goal. In these cases we are seeking not to make diffusion fast and selective but to make it as slow as possible. The obvious way to retard diffusion is to use thicker layers of less permeable polymers. However, as the data in Figure 1 show, this may often be difficult: a polymer which is highly impermeable to water may be highly permeable to oxygen. Even when we can achieve low permeabilities of many solutes, we almost always want still thinner coatings of still less permeable materials. In some cases, we want to reduce the permeability by a factor of 10,000. This is the goal for this research: how can we reduce the permeability of any diffusion barrier by a factor of 10,000? In this paper, we discuss two possible routes for this reduction. The first is to incorporate immobile reactive groups into the diffusion barrier. These groups react with particular diffusing solutes and hence retard permeation of those solutes. Alternatively, we can use as diffusion barriers composite materials containing aligned impermeable flakes. The flakes force a tortuous path across the barrier. The first of these methods, reactive solutes, can reduce the unsteady state permeation by factor of 1000. The incorporation of aligned flakes can reduce both unsteady and steady state permeation by a factor of as much as 100. By combining these two methods, we can approach by our target of 10,000. The details of how this is accomplished are given in the remainder of this paper.