A capacitance study of pseudo-Fickian diffusion in glassy polymer coatings

The permeation and sorption of water in eight coatings below the glass transition temperature was monitored using capacitance measurements. For each coating deviations from ideal-fickian diffusion were observed. The diffusion rate was found to decrease considerably. This anomalous (or non-fickian) behaviour is known in literature as pseudo-fickian diffusion or two-stage sorption. To analyse the measurements further a new method is developed. The data were transformed to a time-dependent diffusion rate. The analysis resulted in a new pseudo-fickian diffusion model, which could also be represented by a spectral distribution of diffusion rates. The results agree well with the relaxation model of Berens and Hopfenberg. It is shown that the Berens-Hopfenberg model, in particular, and pseudo-fickian behaviour, in general, can be understood as diffusion influenced by free-volume relaxation.