Chain dynamics and strain-induced crystallization of pre- and postvulcanized natural rubber latex using proton multiple quantum NMR and uniaxial deformation by in situ synchrotron X-ray diffraction

The structural development and morphology in unvulcanized and vulcanized (both pre- and postvulcanized) natural rubber latex were studied in a relaxed state and under deformation by multiple-quantum (MQ) NMR and in situ wide-angle X-ray diffraction (WAXD), respectively. Vulcanization was carried out using both sulfur and peroxide, showing important differences on the spatial distribution of cross-links according to the source of vulcanizing agents. Sulfur prevulcanization promotes the formation of highly homogeneous networks in the dispersed rubber particles, whereas peroxide vulcanization makes broader spatial cross-link distributions. The latter is compatible with the formation of core-shell network structures. Molecular orientation and strain-induced crystallization were analyzed by both stress-strain relations and WAXD. An increase in the vulcanizing agent concentration led to an increase in modulus and crystalline fractions. For sulfur vulcanization, the additional heat treatment (postvulcanization) increased the interactions between rubber particles and unreacted vulcanizing agents. For peroxide vulcanization, the additional heat treatment led to chain scission reactions and degradation of network points.