Determination of degree of crosslinking in natural rubber vulcanizates. Part VI. Evidence for chain scission during the crosslinking of natural rubber with organic peroxides

The possible scission of natural rubber chains during their crosslinking by organic peroxides at 140°C. has been examined: (1) by comparing the kinetics of crosslinking and of peroxide decomposition and (2) by measuring the relaxation in stress of an extended sheet of a peroxide vulcanizate subjected to further crosslinking by di‐cumyl peroxide in nitrogen. The former method is too insensitive to establish the occurrence of chain scission, but the latter method indicates that degradation of the primary network occurs during the formation of the secondary network, an effect which is ascribed to the unimolecular scission of polysoprenyl radicals competing with their bimolecular combination to from crosslinks. Under the reaction conditions prevaling in (2) it is calculated that degradation of the primary network occurs equivalent to about 10% of the new elastically effective crosslinks formed. These findings suggest that minor revision of the existing calibrations1,4 relating the physical and chemical degrees of crosslinking of a natural rubber vulcanizate is necessary, since the basis of the original chemical estimate1 was that the sole fate of polyisoprenyl radicals, derived from the reaction of natural rubber with alkyloxy‐, aralkyloxy‐ and methyl radicals, is bimolecular combination. However, the available data, coupled with the present status of stress relaxation theory, are inadequeate to permit precise corrections of the existing calibrations to be made at this stage.