The effect of atmospheric humidity on the ozone degradation of chloroprene rubber (CR) was investigated. CR vulcanizates were exposed to 50 pphm ozone at 40 °C for 168 h under relative humidity (RH) conditions of 80%, 50%, and 25%. After ozone exposure, ozone cracks were observed on all samples, with the largest and deepest cracks occurring most frequently at 50%RH. Nevertheless, the tensile properties exhibited the most significant decrease at 80%RH. To clarify the ozone degradation mechanism under high humidity conditions, the chemical and structural changes of CR were examined. The results showed that, in addition to the typical crosslinking reactions associated with ozone degradation, molecular chain scission and dehydrochlorination reactions were accelerated at high humidity. These reactions produced deliquescent and hygroscopic degradation products, increased the internal moisture content of the rubber, and reduced crystallinity. As a result, CR exposed to ozone under high humidity conditions showed a marked decrease in tensile properties despite having relatively small and few ozone cracks. These results suggest that visual observation alone cannot adequately evaluate the ozone resistance of rubber, and that incorporating mechanical property testing is effective.

The tensile deformation behavior of semicrystalline polymer solids is discussed from a structural viewpoint, with emphasis on yielding, necking, and strain hardening. Molecular orientation scarcely develops from the onset of deformation to the yield point, whereas pronounced orientation appears during post-yield deformation and necking. Yielding triggers fragmentation of lamellar assemblies and collapse of the spherulitic structure, leading to cooperative rearrangement of lamellar cluster units. Subsequent strain hardening in the ultra-drawing regime is governed by the progressive formation of taut tie molecules extracted from the crystalline phase.