Effects of Strain-Induced Crystallization on the Segmental Dynamics of Vulcanized Natural Rubber

The effects of strain-induced crystallization on the segmental dynamics of vulcanized natural rubber (NR) have been studied by combining dielectric relaxation spectroscopy and wide-angle X-ray scattering. Segmental dynamics is clearly affected by uniaxial stretching. For low strains, λ < 3, stretching takes place without crystallization. However, a dramatic increase of the dielectric strength, Δε, is observed which has been explained by an increase of the effective dipole moment upon stretching. This effect is accompanied by an increment of the fragility parameter, m, since as free volume reduces by stretching more cooperativity is needed in order to accomplish segmental motions. For 3 < λ < 7.5, crystals develop and the inclusion of segments into the crystalline phase counteracts the increment in Δε provoked by stretching. Our results support a previous morphological model indicating that during strain-induced crystallization shorter chains are progressively incorporated into the crystals while a significant amount of longer chains remains rather coiled.