ABSTRACT
Predicting whether or not a given chemical will attack a given polymer is important. Hansen solubility parameters (HSP) have been used for this purpose as discussed elsewhere in this book. Consideration of the absorption and diffusion of the chemical in the polymer is often required in addition to HSP in order to make reliable predictions, however. This has been discussed in particular in Chapter 12 through Chapter 14, where chemical resistance, barrier properties, and environmental stress cracking are treated in detail. Chemicals with smaller and more linear molecules absorb and diffuse more readily than those with larger and more bulky structures. Surface resistance to absorption is sometimes so dominating that absorption does not occur in some cases, even though this might be expected based on simple HSP considerations. This chapter examines surface resistances in connection with absorption and diffusion in polymers in order to help improve understanding of these factors and to emphasize the necessity of simultaneous consideration of surface resistance when absorption rates and diffusion within the bulk of the polymer itself are of interest. Methods to measure surface resistance and concentration-dependent diffusion coefficients are discussed. Solving the diffusion equation with simultaneous consideration of surface resistance and with a concentration dependent diffusion correctly models absorption, desorption, film formation by solvent evaporation, and various forms of so-called anomalous diffusion such as “time-dependent,” Case II, and Super Case II. Surface phenomena such as surface resistance to absorption deserve far more attention than has been given in the past.
