Abstract
Mechanical properties of natural rubber vulcanized in dry and swollen states were investigated to elucidate the difference in network structures. The value of Mooney-Rivlin's C2 term for the solution-cured (SC) sample was almost zero, and was dependent on volume fraction of rubber during crosslinking. These facts suggest that the C2 term is closely related to entanglements. On the other hand, for the dry state-cured (DC) sample, an abrupt upturn appeared in the Mooney-Rivlin plot at high elongation. Such an upturn, however, was not observed for the SC sample, in which fewer entanglements exist. Furthermore, hysteresis phenomenon was not observed for the SC sample. This suggests that little or no strain-induced crystallization occurs even at high elongation for the SC sample. This was confirmed by comparing equatorial X-ray diffraction patterns and birefringences for both types of stretched samples. These experimental results for the SC sample were not due to a small amount of residual decalin; No effect of sorbed decalin was detected by stress-strain measurements in decalin atmosphere. Taking into account the entanglement effect on network structure, the relation between the network structure and mechanical properties for the two kinds of rubber samples was discussed.
