Tobolsky has been theoretically and experimentally working for both extreme cases in which only main chains scission occurs and on the contrary only cross-linking sites scission occurs for cross-linked polymers. The studies on both scissions which occur at the same time have not been carried out theoretically or experimentally, though such cases will exist actually. From this point of view, the theoretical equations for the both scissions have been calculated and derived by the author. In deriving the theoretical equations, the ideal pyramid-type model and infinitive network structure model were discussed as the representative for the actual network structure of a polymer. The scission mechanism was assumed such that the crosslinking site scission occurs at only the cross-linking site in the both scission of main chain and cross-linking site.
The theoretical equation derived in the paper (I) was compared with the experimental results of EPM (ethylene-propylene co-polymer) as model polymer. As the results, it was found that the both scissions have expectedly occurred for this sample in air at high temperature and k=1 or the probability of both scissions is equal. These results were infered from the fact that in the chemical structure of EPM cured with the peroxide curing agents, the both chemical bonds of main chain and cross-linking site are the same as C-C bond.
Several samples of natural rubber vulcanizates were prepared. A and B samples are cured with sulfur, and C is cured with TMTD (tetramethyl thiuram disulfide) and D is natural rubber cured with peroxide. The cross-linking densities, n (0), were measured for these four samples extracted with : acetone and unextracted. The values of n (0) calculated by the Flory-Rehner swelling method were compared with those obtained by the mechanical means. The probabilities of the ratio of both scissions were indicated by kA, kB and kc for three samples except D. The degradation of the polymer structures was discussed for the values of kA, kB, kC obtained from the theoretical equation derived in the former paper.
Stress relaxation mechanisms were investigated on three types of EPT polymers in air at 109°C.These polymers differ only in the structure of the crosslinkages which are a carbon-carbon bond, a polysulfide linkage (-Sx-) and a monosulfide linkage (-S-). All the stress relaxation of a peroxide-cured EPT polymer is not due to the oxygen-induced cleavage of the main chain but to a physical flow. In the case of a sulfur-cured EPT polymer, the relaxation curve is divided into three straight lines by using the procedure X when log f (t) /f (0) is plotted against t. It was concluded that this graph could be attributed to an interchange reaction between the polysulfide linkage and an oxidative cleavage of the monosulfide linkage. On the other hand, a TT-cured EPT polymer gave a plot with a straight line.This stress relaxation could be explained by an oxidative cleavage of the monosulfide linkage. Further, authors have investigated the degree of agreement between the theoretical relation proposed by the authors and experimental results relating to TT-cured natural rubber. In this crosslinked polymer, both scissions of main chain and cross-linkage occur at the same time.