Strain-included crystallization of un-vulcanized natural rubber measured by synchrotron X-ray diffraction

Strain-induced crystallization of un-vulcanized natural rubber (NR) was investigated by tensile test and synchrotron wideangle X-ray diffraction (WAXD) measurement with respect to naturally occurring network. NR was purified by deproteinization, acetone-extraction and transesterification to remove proteins, free fatty acids and linked fatty acids, respectively, which resulted, in turn, in decomposition of the naturally occurring network. The stress at break of un-vulcanized NR, green strength, was similar to that of deproteinized NR (DPNR), whereas it decreased after acetone extraction followed by transesterification. The transesterified DPNR did not show an abrupt increase in stress versus strain and its green strength was comparable to that of synthetic cis-1,4,- polyisoprene. The abrupt increase in stress versus strain was associated with the strain-induced crystallization, detected by WAXD measurement. The effect of strain-induced crystallization was proved with soluble and gel fractions of NR to be due to the naturally occurring network. The effect of fatty acids on green strength and strain-induced crystallization was also investigated.