The efficiency of the oxidative chain scission
in natural rubber vulcanizes has been measured for a simple MBTS vulcanizate with and without antioxidants. This was accomplished by making concurrent measurements of the chain scission
velocity and oxidation rate. The chain scission
velocity was obtained from stress
relaxation measurements which were corrected for the relaxation not due to oxygen by means of relaxation in vacuo. The reciprocal scission efficiency, defined as the number of molecules of O2 per chain scission, was found to be 1.5 as a mean for six vulcanizates both with and without antioxidant. The decrease in network chains was found to be first order. From the theoretical analysis by Bueche such behavior has been interpreted to indicate a nonrandom scission process with the scission occurring at the crosslinked site. The form of the dependence of the reciprocal scission efficiency on time, in vulcanizates with and without antioxidant, is consistent with nonrandom scission. Experiments on Butyl vulcanizates and the known effect of crosslink density or combined sulfur on oxidation rate further strengthen the concept of scission at the crosslinked site.
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