The Effect of Temperature on the Stress-Strain Isotherms of Polyisobutylene Networks at High Elongation

Abstract

Abstract Three chemically cured, unfilled polyisobutylene networks were studied in elongation to their rupture points at several temperatures in the range 15–80°C. At the lower temperatures, the stress-strain isotherms obtained were similar to those reported for unfilled networks of natural rubber and several other polymers in that they showed an anomalous, marked increase in the modulus or reduced force at high elongations. At intermediate temperatures, this upturn in the force is significantly diminished, however, and the networks rupture at lower elongations. At the higher temperatures, there is no evidence whatever for an upturn in the force, and the elongation at rupture is further reduced. These results on polyisobutylene networks implicate strain-induced crystallization rather than limited chain extensibility as the origin of the pronounced increase in the force at high elongations. The reinforcement provided by the crystallites formed in this manner would account for the increased extensibility of the networks at the lower temperatures. Since natural rubber is very similar to polyisobutylene in its crystallizability, the anomalous stress-strain isotherms widely reported for networks of this polymer are thus probably also due largely, if not entirely, to network crystallization.