The Effect of Swelling on the Elasticity of Polyisobutylene Networks at High Elongation

Abstract

Sulfur-cured, unfilled networks of butyl rubber (essentially polyisobutylene, PIB) were swollen to varying degrees with a nonvolatile diluent. A study in elongation of their rupture points was carried out at sufficiently low temperature to permit strain-induced crystallization in the unswollen state. The stress—strain isotherms thus obtained differed somewhat from those similarly obtained for crystallizable networks of cis-1, 4-polybutadiene, in that for example the strain-induced crystallization never caused an abrupt decrease in the modulus. The marked increases in modulus which did occur at high elongations in the case of the unswollen PIB networks were found to diminish, and eventually to be suppressed entirely, with increased swelling. Both the ultimate strength and maximum extensibility decreased substantially with an increase in swelling. The fractional decreases in both quantities were much larger than for typical noncrystallizable networks of poly(dimethylsiloxane), but the ultimate strength was found to be much more sensitive to the reinforcing effect of the strain-induced crystallization.