Preparation And Characterization Of Vitrimers Based On The Epoxidized Diene Rubbers

Due to the excellent physical and mechanical properties,diene rubber including natural rubber,polyisoprene rubber,styrene-butadiene rubber,polybutadiene rubber,nitrile rubber,and so forth,has broad applications.The essence of entropic elasticity makes vulcanization necessary.However,the traditional vulcanization process causes serious productive pollution.And the formation of irreversible chemical cross-linked networks makes it difficult to be reprocessed in solid-state.Thermoplastic diene rubber with physical cross-linked networks,such as styrene-butadiene-styrene block copolymer(SBS),can theoretically be completely recycled,but weak intermolecular interactions lead to poor elasticity and environmental stability.Vitrimer is a class of network polymers cross-linked by dynamic covalent bonds.The dynamic covalent bonds can be reversibly broken and generated under the external stimuli,making the crosslink density constant.The introduction of vitrimer chemistry,derived from dynamic covalent chemistry,into rubber networks provides an ideal strategy for integrating the robustness of thermoset rubber and the reprocessability of thermoplastic rubber.The dynamic exchange of covalent bonds enables the network topology rearrangement,and the associative exchange mechanism of vitrimer chemistry further ensures the structural integrity of cross-linked networks.Despite the great potential of the rubber vitrimer,the design of dynamic chemistry and performance optimization of the diene rubber vitrimer remain a challenge.Due to the relatively mature industrial applications of the epoxidation modification for diene rubber,the direct transformation of the rubber vitrimer were achieved by the highly-reactive epoxy groups and the crosslinkers containing dynamic covalent bonds in this paper,with the applications of rubber vitrimers expanded to the field of selfhealing and adhesive.The main results of this paper are as follows:(1)Imine-crosslinked styrene-butadiene rubber(SBR)vitrimers with epoxidized styrene-butadiene rubber as matrix was prepared by the nucleophilic addition reaction between p-hydroxybenzaldehyde and p-aminophenol and the ring-opening reaction of epoxy and hydroxyl groups.The dynamic exchange of imine bonds provided the material with solid-phase reprocessing and chemical de-cross-linking abilities.Based on the epoxy-hydroxyl cross-linking system,acetaminophen was used as the enhancer to realize a simple one-step preparation of the double reversible cross-linking network with hydrogen bonds and imine bonds.The hydrogen bonds as sacrificial units significantly improved the tensile strength and elongation at break of the rubber vitrimer without hindering the network topology rearrangement.In addition,the hydrogen bonds also acted as physical cross-linking points to limit the creep of the rubber vitrimer at high temperatures.(2)Through the ring-opening reaction of epoxy and carboxylic groups,SBR was directly transformed into the rubber vitrimer with self-healing,solid-state plasticity,and recycling capabilities by using the diacid crosslinker containing disulfide bonds.Then the polymer with block architectures was merged with vitrimer chemistry to prepare the rubber vitrimer with nanoscale self-assembled structures.The network topology rearrangement was promoted by the microphase separation structure induced by the thermodynamic incompatible of polymer chain segments,while the restricted polymer chain movement improved the creep resistance.(3)Firstly,SBS was epoxidized,then dynamic disulfide bonds were introduced into the cross-linking network through the epoxy-carboxylic cross-linking system and the diacid crosslinker containing disulfide bonds to prepare high-performance SBS with synergistic interactions of supramolecular and vitrimer chemistry.The strong covalent cross-linking significantly enhanced the tensile strength,elasticity,solvent resistance,and heat resistance of the rubber vitrimer.And the combination of inherent physical cross-linking and additional dynamic covalent cross-linking allowed the material to exhibit shape memory and self-healing properties under thermal stimulation,while maintaining its recycling ability.SBS vitrimers also showed the potential application in the field of reusable adhesives.(4)After the epoxidation modification,polybutadiene was transformed into the rubber vitrimer with good recycling properties and solid-state plasticity by diacid crosslinkers containing disulfide bonds.The comprehensive performance of the rubber vitrimer was systematically studied by controlling the network structure parameters(the epoxy ratio,crosslinker content and crosslinker chain length),and the structureperformance relationship of the epoxidized diene rubber vitrimer was established.In addition,the balance of relaxation and creep performance in the rubber vitrimer network could be achieved by suitable cross-linking structure design.