Self-Healing and Recyclable Vulcanized Polyisoprene Based on a Sulfur-Rich Copolymer Cross-Linking Agent Derived from Inverse Vulcanization

Abstract

The sulfur vulcanization process has been widely used in the rubber industry, but it is difficult to obtain self-healing and recyclable vulcanized rubber. The latter can be obtained by introducing a dynamic bond network, but it is difficult to achieve good self-healing and mechanical strength simultaneously. In this contribution, based on an inverse vulcanization reaction, a new type of sulfur-rich copolymer was synthesized by bulk copolymerization of thioctic acid and sulfur at elevated temperatures. During the vulcanization process, the carboxyl groups in the copolymer react with zinc oxide to form zinc carboxylate, which interacts with N-methylimidazole to vulcanize the polyisoprene rubber. The resulting vulcanized rubber exhibits exceptional mechanical strength and elastic properties, comparable to those of commercial polyisoprene, with a tensile strength of up to 24.2 MPa and an elastic recovery rate of 96.1%. Small molecule model reactions demonstrate that N-methylimidazole accelerates the disproportionation reaction of disulfide bonds and polysulfide bonds in the system, leading to the self-healing and recyclable properties of the novel vulcanized rubber. In the presence of nucleophilic catalysts, excellent self-healing and recycling performances of the obtained vulcanized polyisoprene can be realized. After the selfhealing process, the tensile strength and fracture strain of vulcanized polyisoprene recovered by 95.0 and 83.3%, respectively. After thermal compression repair at 120 °C for 6 h, the sample's tensile strength and elongation at break reached 95.0 and 83.3%, respectively, of the original sample. After the first recycling, the sample's tensile strength and elongation at break were restored to 69.8 and 84.8%, respectively, of the original sample. In the polyisoprene/carbon black composite, the sulfur-rich copolymer serves not only as a cross-linking agent but also generates sulfur radicals during vulcanization, which are captured by carbon black, thereby grafting onto the surface of carbon black. This greatly improves the dispersion of carbon black in the rubber matrix and enhances the interfacial adhesion of the composite. Thanks to the strong radical capture ability of carbon black, the amounts of N-methylimidazole and zinc oxide can be reduced (to 0.5 parts and 2 parts, respectively), avoiding stress concentration in the rubber network caused by excessive cross-linking and thus affecting the mechanical properties. In addition, the N-methylimidazole-catalyzed S−S bond disproportionation reaction also endows the polyisoprene/carbon black composite with dynamic properties.