Insights into the Vulcanization Mechanism through a Simple and Facile Approach to the Sulfur Cleavage Behavior

Abstract

The vulcanization mechanism remains unclear due to the difficulty of characterization and the complexity of the vulcanization systems. In this work, a series of epoxy model systems were used to study the sulfur cleavage behavior, from which the study process of the vulcanization mechanism was significantly simplified and a brand new sulfur-only vulcanization mechanism was revealed by the FTIR and DSC analysis. Our studies suggest that thiyl radicals derived from the homolytic cleavage of sulfur play the dominant role in the vulcanization, while the heterolytic fission of sulfur cannot occur as expected because this reaction needs 200 °C. Nevertheless, sulfur anions, which are easy to be understood derived from the heterolytic cleavage of sulfur, are actually transformed from the thiol groups that are produced through the reaction of thiyl radicals abstracting α-H atoms of allyl groups in rubber, and this maybe the reason that vulcanization mechanism remains controversial for centuries.

Catalytic inverse vulcanization

“…The starting temperature of homolytic fission for S 8 has not been agreed, with reports ranging from 140 to 181 o C 49,53–56 . That catalysts allow temperatures below this range to be used may therefore make a crucial difference to the nature of the reaction.…”

“…Despite a long history of use, the mechanism of even uncatalyzed conventional vulcanization is not fully understood, and remains complex, difficult to characterize, and controversial 42 . Conventional vulcanization has been ascribed to either radical or ionic pathways according to homolytic, or heterolytic fission of S 8 rings (supplementary figure 43) 39,42,48 , and even recently as initially radical, with ionic species generated after reaction of sulfur with organic species 49 . That said, the most widely agreed pathway for conventional vulcanization is via hydrogen abstraction of the α-position relative to the double bond, leading to a combination of crosslinking by proton substitution and addition across the double bonds, with substitutions of hydrogen for sulfur being the dominant factor (Fig.…”

Thermally-healable network solids of sulfur-crosslinked poly(4-allyloxystyrene)

“…Additionally, sulfur has a melt peak at 118.7 C. The higher sulfur content PAOS x (x ¼ 60-99) exhibit endothermic features at $106 C. The feature at $106 C in DSC analysis is attributable to an orthorhombic to monoclinic phase change (T a-b , for the a-S(s) / b-S(s) process). [45][46][47] Older literature has variably attributed this transition to occurring at 95 C, [48][49][50][51] possibly confused by the signicant variability in properties of sulfur based on its thermal history and complex phase behavior 36,49,[52][53][54][55] The peak at $115 C is attributed to the melt peak of sulfur-rich domains in the materials. The normalized integral of the sulfur melt peak decreases predictably as the wt% of sulfur is decreased (Fig.…”

Sulfur-Rich Polymers Based Cathode with Epoxy/Ally Dual-Sulfur-Fixing Mechanism for High Stability Lithium–Sulfur Battery

“…Double bonds and epoxy groups can be almost 100% converted in the epoxy/allyl compound/sulfur system during dual-curing. 50,51 Inspired by this idea, a dualsulfur-fixing mechanism of epoxy/allyl compound/sulfur system was applied in Li−S batteries to effectively reduce the irreversible loss of active materials. In the current work, we proposed for poly(sulfur-random-4-vinyl-1,2-epoxycyclohexane) (SVE) copolymers as powerful cathode materials.…”