Effect of network heterogeneity on tensile and tear strengths of radiation, peroxide, efficient and conventional cured polyisoprene

At any given crosslink density, the ultimate tensile and tear strengths of vulcanizates depend on the curing system and decrease in the order: conventional (CV) > efficient (EV) > peroxide > radiation cured. Measurements on three network parameters were interpreted as indicating that crosslinks were not uniformly distributed throughout the networks and that network heterogeneity decreased in parallel with a decrease in physical properties. (1) Plots of the freezing point of the gel‐solvent in a network, swollen to equilibrium, versus the volume fraction of rubber in the swollen gel, showed a straight line relationship for radiation cured networks and an increasingly large deviation from linearity on passing from radiation cured networks through peroxide and EV to CV cured networks. (2) The glass transition temperature of CV cured networks was higher than that of radiation cured networks of the same crosslink density. (3) Scanning electron micrographs of cryofracture surfaces showed a more uneven surface in CV than in radiation cured networks. The development of heterogeneity in the four networks studied is discussed in terms of the reactions that lead to crosslinking with the different curing systems.