Programming Exchangeable Bonds Into Diene-rubbers And Their Reinforcement And Toughening

The thermosetting polymers can be enabled with malleablility by introducing dynamic covalent bonds into crosslinked networks,which breaks the boundary between traditional thermoplastic and thermosetting polymers.Leibler pioneered the concept of vitrimer based on the thermally triggered associative exchange reactions.For rubber materials,vulcanization is prerequisite to obtain high elasticity.Currently,sulfur vulcanization and peroxide curing are the main crosslinking techniques,which yield irreversible crosslinks in the networks and make the recycling of rubber products inherently difficult.In addition,there are some deficiencies in the traditional crosslinking methods.For examples,the products vulcanized with sulfur exhibit poor aging resistance,and carcinogenic nitrosamines can be produced by certain accelerators used during sulfur vulcanization.The rubber products vulcanized by peroxides have poor scorch safety and mechanical properties.In view of the scarcity of raw rubbers,environmental pollution caused by scrap rubbers and defects of traditional vulcanization techniques,it is of great significance to develop crosslinked rubbers with both reprocessability and excellent performance starting from the new crosslinking.In this dissertation,the exchangeable bonds including?-hydroxyl ester,imine and borate ester bonds have been introduced into diene rubbers to impart the crosslinked network with malleablility.The particle and sacrificial bond strategy were adopted to strengthen and toughen the diene rubbers,and the influence mechanism of network structure on rubber properties was clarified.The main contents are as follows.?1?Aldehyde group-terminated polybutadiene rubber?APB?with different molecular weights were prepared by sequential oxidation and chain scission reactions,and subsequently crosslinked by tris?2-aminoethyl?amine through the formation of imine bond linkages.The results showed that the modulus and ultimate strength of the imine bonds crosslinked networks were consistently enhanced,while the relaxation rate and activation energy of network rearrangement were both decreased,with the decrease of the molecular weight of APB precursors and the increase in the crosslinking density of the networks.Besides,TAPB was also recycled by adding monofunctional amines to degrade the crosslinked network.This work provides a methodology to tune the mechanical and dynamic properties of vitrimer materials by altering the precursor molecular weight and network crosslinking degree.?2?Styrene-butadiene rubber?SBR?was firstly grafted with aldehyde groups by thiol-ene“click”reaction and esterification reaction,and the grafted rubber was then crosslinked with p-phenylene diamine as crosslinking agent and filled with p-phenylene diamine modified graphene as reinforcement filler.Based on the reaction between aldehyde and amino groups to yield imine linkages,the SBR/graphene composites were prepared with the formation of imine bonds in bulk and interfaces.The results showed that the incorporation of graphene led to spectacular improvements on the mechanical properties of vitrimer composites,and the mechanical properties could be further tuned through the orientation arrangement of graphene sheets induced by solid-state drawing.The crosslinked networks were able to alter their topologies via imine exchange reactions in both the bulk network and SBR-graphene interphase.And by using the photothermal effect of graphene,the remote and local reshaping of the composites could be realized.This work combines the supramechanical properties,multi-stimuli response and malleability into catalyst-free and commercially available rubber-based networks,which is of great significance to extend the realm of thermally malleable polymers.?3?The epoxy group modified silica?Esilica?was used both as crosslinking agent and reinforcing filler,and blended with carboxyl grafted SBR?CSBR?.The exchangeable?-hydroxyl ester crosslinking bonds were built at the interfaces of SBR and silica based on the reaction of epoxy and carboxyl groups,fabricating a covalently crosslinked yet reprocessable rubber vitrimer composite.The Esilica crosslinked CSBR composites exhibited high mechanical properties due to the covalent linkages in the interface and fine silica dispersion in the matrix.In addition,the interface could undergo dynamic reshuffling via transesterification reactions to alter network topology at high temperatures,conferring the resulting composites the ability to be reshaped and recycled.?4?The state of art approach to improve the mechanical properties of vitrimers is the addition of fillers,which,however,greatly restricts the chain mobility and impedes network topology rearrangement,thereby deteriorating the dynamic properties of vitrimer composites.Herein,we have demonstrated that the integration of sacrificial bonds into a vitrimeric network can remarkably enhance the overall mechanical properties while facilitating network rearrangement.Specifically,epoxidized natural rubber?ENR?were compounded and hot pressed with sebacic acid and N-acetylglycine?NAg?.Based on the chemical reaction between epoxy and carboxyl groups to yield exchangeable?-hydroxyl esters,ENR was crosslinked and meanwhile amide functionalities were introduced into the networks.The hydrogen bonds arising from amide functionalities acted in a sacrificial and reversible manner,which preferentially broke prior to the covalent framework and underwent reversible breaking and reforming events to dissipate energy under external load,which led to a rarely achieved combination of high strength,modulus and toughness.Besides,the transesterification reactions were accelerated due to the dissociation of transient hydrogen bonds and increase of the ester concentration with the grafting of NAg.?5?The elastomeric vitrimers with an integration of unparalleled mechanical properties,creep resistance and malleability were prepared by engineering Zn²⁺-imidazole complexes into the network.Specifically,SBR grafted with 2-?2-benzimidazolyl?ethanethiol was covalently crosslinked with the dithiol-containing boronic ester crosslinker.Afterwards,Zn²⁺were introduced and led to the formation of Zn²⁺-imidazole complexes into the networks.Zn²⁺-imidazole complexes could function as sacrificial units through reversible breaking and reforming events,leading to significant enhancement on the modulus,strength and toughness while maintaining the extensibility of the networks.In addition,the creep resistance at service temperature was improved,as the Zn²⁺-imidazole complexes could act as crosslinks to restrict segment mobility,whereas,the network arrangement at elevated temperatures was not affected due to the dissociation of Zn²⁺-imidazole complexes,allowing the networks to be reshaped and recycled.