| Abstract: | Concentration dependent diffusion coefficients explain Case II type diffusion. Solutions to the diffusion equation yield straight lines for weight gain vs time when the exponential variation of the diffusion coefficient is included. Boundary layer resistance together with concentration dependent diffusion coefficients explain Super Case II type diffusion. The increase in the rate of weight gain after a prolonged period is predictable. The surface concentration rises slowly to its equilibrium value in this case. Boundary layer resistance appears to be quite common, particularly where the penetrant has low affinity (not a true solvent) for the polymer, such as water in most polymers or hexane in polystyrene. Boundary layer resistance is quite significant for coatings (where film thicknesses are usually measured in microns), accounting for the majority of the resistance in some cases. A method to separate diffusional resistance from the surface resistance is suggested, but studies at different film thicknesses appear mandatory. |
