Molecular orientation and stress relaxation during strain-induced crystallization of vulcanized natural rubber

Birefringence and tensile stress of vulcanized natural rubber samples were measured simultaneously during fast deformation and subsequent holding at a prefixed extension ratio. The deviation from the stress-optical law due to finite chain extensibility could be clearly detected, which became pronounced with an increase in true stress. The increase in birefringence during the holding process was related to the different degrees of molecular-chain orientation to be incorporated into strain-induced crystals according to the network-chain density and the strain ratio of holding. During the holding process, normalized stress decreased linearly with the logarithm of elapsed time in a manner that is almost independent of network-chain density. These experimental results could be successfully explained by assuming a certain fraction of network chains that behave in a manner similar to that of a fluid.