Two-cycle strain-induced crystallization behavior of peroxide cross-linked solid guayule natural rubber

Authors: JUNKONG,, JUNKONG - Mahidol University; OHASHI,, T - Kyoto University; PHAKKEEREE,, T - Kyoto University; MIYAJI,, K - Kyoto University; IWASAKI,, S - Kyoto University; Cornish, Katrina; IKEDA,, YUKO - Kyoto University

Submitted to: Macromolecular Materials and Engineering
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/6/2025
Publication Date: 2/26/2025
Citation: Junkong,, J., Ohashi,, T., Phakkeeree,, T., Miyaji,, K., Iwasaki,, S., Cornish, K., Ikeda,, Y. 2025. Two-cycle strain-induced crystallization behavior of peroxide cross-linked solid guayule natural rubber. Macromolecular Materials and Engineering. 310(6). Article 2400349. https://doi.org/10.1002/mame.202400439.
DOI: https://doi.org/10.1002/mame.202400439

Interpretive Summary: Polymer crosslinking is essential to the mechanical performance of natural rubber products. Natural rubber can crosslinked in various ways including sulfur linkages and carbon to carbon bonds. In this study, guayule was crosslinked using peroxide - which makes carbon-carbon linkages. Natural rubber has superior performance to synthetic polymers because it is able to form crystallites under stress and strain. Thus, as a natural rubber product is stretched or compressed it becomes stronger. The formation and dissolution of crystallites can be examined in real time using synchrotron wide-angle X-ray diffraction measurements. We have shown that reproducible cyclic stress strain curves and crystallite formation are maintained by the thermodynamically stable peroxide cross-linking network. These findings provide valuable insights for manufacturing both conventional and high-quality products.

Technical Abstract: Peroxide cross-linked guayule natural rubbers (P-gGR) were prepared from solid green guayule natural rubber (gGR). Tensile and dynamic mechanical properties of P-gGR samples of three different network chain-densities are characterized. All P-gGR samples demonstrate highly reproducible tensile behaviors under two repeated cycles. For the first time, strain-induced crystallization (SIC) behaviors of P-gGR were measured with synchrotron wide-angle X-ray diffraction measurements under two repeated cycles by exposing the incident beam for 70 ms every 3 s. Highly reproducible SIC phenomena were observed, and apparent crystallite sizes, average volumes, indexes of average number and orientation fluctuations of crystallites were confirmed to not change significantly during cyclic deformation. Non-rubber components such as phospholipids, glycolipids, fatty acids, esters, sterols, and terpenes in gGR barely inhibit the second SIC of the P-gGR and do not cause deterioration of its mechanical properties. The reproducibility of cyclic stress-strain curves and cyclic SIC behavior of the P-gGR sample is ascribed to the thermodynamically stable peroxide cross-linking network. As a viable alternative to Hevea NR in the rubber industry, these findings on gGR provide valuable insights for manufacturing both conventional and high-quality products.