Kinetics of swelling of polymers and their gels.

The swelling limit or equilibrium swelling of semicrystalline or cross-linked polymers and of their gels upon immersion in liquids has been investigated extensively. Few studies, however, have dealt with the kinetics of swelling. Theoretical considerations, based on diffusion-controlled swelling, show that first-order kinetics do not apply, even though deviations during the initial and even middle stages of the swelling process may be relatively small. Extensive studies of swelling rate and equilibrium swelling of supported and unsupported gelatin films have been published. Diffusion was always fast. After it was completed, the rate of swelling was controlled by stress relaxation in the amorphous portion of the polymer network. The rate equations for this process, which also apply to regenerated cellulose, are shown to represent second-order kinetics with respect to the remaining swelling capacity. The following interpretation for the applicability of second-order kinetics to the swelling of semicrystalline polymers, such as gelatin and cellulose, is given. The rate of swelling is assumed to be directly proportional to the percent swelling capacity still available at a given time and to the total internal specific boundary area enclosing those sites capable of swelling that have not yet become hydrated and swollen at that time. The latter, in turn, is also directly proportional to the percent unrealized swelling capacity.