Strain-induced crystallization in elongated polyisobutylene elastomers

Elastomeric networks were prepared from a sample of polyisobutylene having 2 mol% isoprene units as unsaturated sites for free-radical crosslinking. Four samples having different degrees of crosslinking were obtained by peroxide thermolysis, and were then studied with regard to their thermoelastic properties in elongation. Each network strip was elongated at 338 K to ensure that it was completely amorphous, and then held at this constant length. The temperature was then slowly decreased at intervals, and the equilibrium value of the elastic force recorded for each temperature. The temperatures at which crystallization was observed from changes in the stress-temperature dependence were taken to be approximate melting points. The temperature range over which crystallization occurred was found to narrow considerably with increase in elongation. It was found that at lower elongations, the nominal melting points increased gradually with increase in elongation, while at higher elongations, their dependence on elongation became linear and more pronounced. The dependence of the melting points on the molecular weights between crosslinks and on strain were documented, and then compared with relevant theory. For each degree of crosslinking, estimated degrees of crystallinity were found to increase substantially and linearly with increase in elongation.