Revisit of crystal orientation in a vulcanizate of natural rubber under planar elongation

Abstract

The orientation of the crystalline lattice formed as a consequence of strain-induced crystallization (SIC) was revisited for a natural rubber (NR) sheet undergoing planar elongation by being stretched in one direction while keeping the other dimension unchanged. For this purpose, two-dimensional wide-angle X-ray diffraction patterns were measured by changing the inclination angle of the incident X-ray beam to the specimen sheet. The intensity of the reflection from the (120) plane of the orthorhombic crystal increased with increasing inclination angle from the normal direction of the specimen sheet; this result indicated that the orientation of the (120) planes was parallel to the surface of the specimen sheet. Moreover, the intensity of the (200) plane reflection reached a maximum at a slightly inclined angle from the normal of the specimen surface. WAXD measurements were also conducted using different X-ray wavelengths (0.06, 0.10, and 0.15 nm) to evaluate the inclined angle of the ac plane with respect to the surface of the specimen. By changing the X-ray wavelength, the distribution of reciprocal lattice points was altered such that the inclined angle of the incident X-ray beam to the specimen sheet needed to be changed to meet the diffraction condition. The inclined angle at the maximum intensity of the (200) reflection increased as a function of the X-ray wavelength. Based on this result, the preferential orientation of the ac planes was determined, whereby the ac plane was not completely parallel to the surface of the specimen sheet but was slightly inclined by 6.4°. Thus, a dual orientational state was deduced. Therefore, the preferential orientation of the ac plane was nearly parallel to the surface of the specimen sheet, with the inclined angle ranging from 6.4° to 19.6° within the volume of the specimen irradiated by the incident X-ray beam. Natural rubber exhibits the strain-induced crystallization (SIC). By using WAXD, the orientation of NR crystal formed by SIC under planar elongation was revisited. We found that the orientational state of the crystal lattice possesses a continuous margin of the orientation angle between 6.4 and 19.6° for the ac plane with respect to the surface of the specimen sheet in the real NR specimen. This orientational state could be accomplished as a result of balancing the preferential parallel orientation of (120) planes (the slip planes) and C = C planes with respect to the surface of the specimen sheet.