Room Temperature Self-healing Polyurethane Elastomer Based On Aromatic Disulfide Metathesis

Self-healing polymer is currently becoming the subject of increasing interest as an emerging class of smart materials.In comparison to the extrinsic self-healable category with sophisticated synthesis process and limited healing cycles,the intrinsic self-healable category is drawing growing concerns due to its literally infinite reversibility.Since polymeric materials are widely and heavily used in engineering field,it is of great significance to prepare robust polymeric materials equipped with fast healing ability under moderate conditions.Generally peaking,intrinsic self-healing polymers based on reversible covalent bonds have greater bonding energy therefore exhibiting superior mechanical performances than that based on reversible non-covalent bonds.Nevertheless,the former usually needs more strict conditions such as elevated temperature to proceed the healing process because of its confined chain mobility.Therefore,the current researches put great emphasis on how to resolve the dilemma and realize self-healing under readily available or mild conditions.In that case,this paper focused on the preparation,characterization and self-healing mechanism exploration of thermoset as well as thermoplastic elastomers with excellent mechanical performances and room temperature self-healing ability on the basis of high structural versitility of polyurethane and aromatic disulfide metathesis that can undergo at room temperature.For thermoset polyurethane elastomers,the presence of the cross-linking points is responsible for the slow chain movement that hinders the self-healing process.Accordingly,this paper adopt the six-membered ring of tri-functional homopolymer of hexamethylene diisocyanates(tri-HDI)to serve as the crosslinking skeleton,providing the network with a tough skeleton and thus considerable strength.Meanwhile,the chain extender,4,4'-dithiodianiline(DTDA),was applied to introduce disulfide bonds that are well-distributed around the cross-linking point,connecting the tough skeleton with soft main chains.The resulting polyurethane-urea exhibited an ultra tensile strength as high as 7.7MPa and a corresponding self-healing efficiency of 74% under ambient temperature,suggestingthat the ongoing disulfide metathesis placed in cross-linkages can promote the achievement of balanced status in crosslink/un-crosslink and bring about higher segment mobility and healing efficiency.Both healed at 60 ? for 24 h,the healing efficiency of disulfide-embedded polyurethane-urea was 97.4% while that of disulfide-free polyurethane-urea stood at 58.0%,indicating an disulfide metathesis contribution of 67.9% on top of H-bonding interactions.For thermoplastic polyurethane elastomers,the aggregated status affected by both soft/hard segment crystallinity and the micro-phase separation is the key to strike the balance between inter-molecular interactions and segment/chain mobility.Disulfide embedded benzoic acid has been innovatively taken as the chain extender in this paper to prepare amide-based polyurethane.The resulting elastomer behaves like classic thermosets at ambient conditions for its tensile stress–strain curve exhibits yielding points with a Young's modulus as high as 49.5MPa,a ultra tensile strength of 12.5MPa and a elongation at break of 668%.In the meantime,however,manual compression for 10-20 s on its fractured surfaces at 25? can recover 98%,53.3%,and 18% of its pristine value in Young's modulus,UTS and elongation at break,respectively,indicating its fast self-healing ability at room temperature.Besides,the full recovery can be achieved by healing for 12 h at 50?.Variations in microscopic structure and characterization on macroscopic performance suggest that,along with a proper molecular weight of soft segment(PBA2000),the coexistence of isophorone diisocyanate(IPDI)and amide bonds could form an ideal aggregated structure status that responsible for its balanced performance between self-healing ability and mechanical property.On the one hand,favoured by a moderate degree of micro-phase separation and tensile stretch-extension crystallization,the resulting poly(urethane-amide)exhibits superior toughness and excellent strength at room temperature.On the other hand,the low crystallinity imparts elastomer with relatively fast segment movements despite its slow chain diffusion dynamics,thus facilitating rapid recombination of H-bonding as well as disulfide metathesis between fractured surfaces.In comparison to the existing self-healing materials,this thermoplastic polyurethane-amide system possesses twin advantages of facile synthesis process and superior robustness favoured by tensile stretch-extension crystallization,making it a promising matrix in variousindustries.