Equilibrium Swelling of Polyurethane Elastomers

Abstract

Abstract Equilibrium swelling of randomly crosslinked elastomers is in satisfactory agreement with a model that considers them to be aggregates of equivalent spheres whose diameter is proportional to Mc1/2. Solvation expands these spheres, the extent of expansion being proportional to the diameter and the strength of the solvent-polymer interaction, but is independent of the number of effective chains per volume unit of the elastomer. A potential mechanistic explanation assumes that the solvent molecules form a weak complex with the elements of the polymeric chain, which forces the segments out of their equilibrium position, thus creating a force that opposes further solvation. Both forces have a very short range and are effective only in the individual segment that has formed the polymer-solvent complex. The magnitude of the extension of the whole chain will be proportional to the number of chain segments undergoing solvation, which leads to the mathematical relationship that forms the basis of Equation (6). Since the swollen volume is many times the volume of the network rubber, it is clear that only a (comparatively) small fraction of the solvent molecules will interact directly with the polymer, many of which will just fill the space created by the expanding network. In weak solvents, or at very high levels of crosslinking, where the swelling volumes are small, the extent of swelling is likely to depend on the polymer content, and the presented mechanism does not apply.