Case II Sorption

Abstract

Since the advent of synthetic high polymers there have been many observations of their ability to absorb substantial quantities of penetrating liquids. In some cases the equilibrium swelling is sufficient to double the original volume or more, and materials which are originally hard and perhaps glassy become rubbers or gels. The early development of interest in sorption phenomena was reflected in a general discussion of the Faraday Society on Swelling and Shrinking in 1946.¹ At this meeting there were several reports of moving boundaries in swelling polymer systems and linear, as opposed to Fickian, sorption kinetics. A growing recognition of the reality and significance of linear kinetics was marked some 20 years later by Professor Alfrey (a participant at the 1946 meeting) who christened such behaviour ‘Case II’.² Once named, the process has appeared to attract increasing interest. The essential attributes, namely sharp step-like penetrant profiles moving inwards from the surfaces with constant velocity which give, for sheet specimens, weight gain plots which are also linear with time, have now been frequently observed and widely recorded (e.g. Fig. 1). Also, further detailed features of the process, such as the induction period and front acceleration in the latter stages, have recently been noted for some conditions.