SWELLING KINETICS OF POLYMERS

The equilibrium swelling of semicrystalline or crosslinked polymers and of their gels upon immersion in liquids has been investigated extensively. On the other hand, few studies have dealt with the kinetics of swelling. Swelling kinetics are important for designing controlled-release devices for drugs and agricultural pesticides based on swellable polymer matrices, and for predicting the release rates of the active ingredients. Theoretical considerations, based on diffusion-controlled swelling, show that first-order kinetics does 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. The rate is controlled by stress relaxation in the swelling polymer network. The rate equations of these studies, which have also been reported to apply to cellulose, are shown to represent second-order kinetics with respect to the remaining swelling capacity. An interpretation for the applicability of second-order kinetics to the swelling of semicrystalline polymers such as gelatin and cellulose is given.