Morphology of strain-induced crystallization of natural rubber. Part II. X-Ray studies on cross-linked vulcanizates

Abstract

The results of wide- and small-angle X-ray studies on bulk vulcanizates of natural rubber are described. At room temperature no discrete X-ray peaks occur at small angles, even for highly strained (700%) specimens for which a significant degree of crystallinity is present at room temperature. Below the isotropic melting point of the network, discrete small-angle X-ray scattering develops for both unstrained and strained specimens. The Bragg periodicity corresponds closely to the longitudinal lamellar spacing from electron micrographs of thin films under corresponding conditions. The X-ray long period exhibits significant thermal reversibility. Upon heating, following initial crystallization at −25°C, the long period continuously increases from about 140 Å at −25°C to about 250 Å at 17°C. After heating, progressive cooling from room temperature results in continuous decreases in long period and at least 80% of the initial increase on heating is recovered. These results in conjunction with electron microscopy data (Part I) strongly suggest that thermally reversible changes in physical lamellar periodicity occur for natural rubber and are discussed in detail in light of current concepts concerning long period reversibility in bulk polymers.