Relation of structure to properties in polyurethanes

A polyurethane bearing pendant hydroxyl groups and carbon-carbon double bonds was prepared. Curing was carried out by diisocyanates of different lengths and by an accelerated sulfur recipe. The vulcanizate properties of modulus, hardness, and resilience were found to be independent of both the length and chemical nature of the crosslink. Compression set, however, was always higher for the sulfur cures, an effect which can be attributed to the disulfide structure of this particular crosslink. Both physical and chemical evidence support this structure. Diisocyanate curing of the dolyurethane via the hydroxyl groups is a clean-cut, essentially quantitative reaction which provided an absolute method for determining the degree of crosslinking. Excellent agreement was obtained between the physical degree of crosslinking as determined by equilibrium modulus and the chemical degree of crosslinking. This agreement provides further verification of the network-statistical theory of rubber elasticity in highly swollen systems. No chain entanglement was observed.