Difference in the stability of network between natural rubber and polyisoprene vulcanizates

Stability of network against the mechanical stress during stretching was evaluated from the in-situ ESR measurements during stretching for vulcanized natural rubber (NR) and polyisoprene (IR). Large amounts of unpaired electrons (free radicals) were produced during stretching of vulcanized NR. However, the amounts were less in vulcanized IR. That is, the stability of network against the mechanical stress was lower for NR than for IR. Combined these results with the data for deproteinized natural rubber, the low stability of network structure in NR was explained by the inhomogeneous network which was formed by hydrogen bond at terminal groups of NR and by sulfur linkages, in addition to the inhomogeneous vulcanization induced by protein in NR. The explanation was supported by the results of permanent set for the vulcanized samples.