| Covalent crosslinking is very important for rubbers to obtain high elasticity and high strength,but it is not conductive to repeatedly molding,healing and recycling of used rubber,resulting in resource wasting and environmental pollution.Integrating dynamic covalent bonds into crosslinked rubber could be seen as one of the effective strategies to solve the contradiction between permanent crosslinking and reforming.Besides,although the thermoplastics can be repeatedly processed,the polymeric chains may be irreversibly broken after multiple molding at high temperature,which directly leads to the deterioration of mechanical properties.Therefore,how to recover thermoplastic while maintaining the mechanical properties of the materials is a difficult target in the process of repeated processing and recycling of thermoplastics.In recent years,aromatic disulfide bonds can be exchanged at room temperature without catalysis,which can reduce the energy required for the crosslinking and molding processes,and have attracted wide attention in the fields of self-healing and recycling of polymer networks.From the time being,the mechanical properties of polymer networks containing aromatic disulfide bonds are still relatively low,and few research on their thermal stability,thermodynamic properties,surface and microphase structure is to be seen.There are few applications based on aromatic disulfide bonds.Therefore,it is becoming more and more important and urgent to apply aromatic disulfide bonds to different polymer systems to prepare a series of self-healing and recyclable polymers.In this work,we proposed a series of simple methods to prepare elastic materials with high mechanical strength,self-healing and recyclable by introducing the dynamic aromatic disulfide bonds into different polymer systems,such as polysiloxanes,polyurethane and epoxy resin.We focused on the aromatic disulfide bonds as main research object,and supramolecular elastomers based on aromatic disulfide bonds were constructed were explored.The mechanical properties,viscoelastic properties,thermal properties and applications of polymer networks were concerned,which provides systematically theoretical evidence for development supramolecular elastomers with excellent comprehensive properties.The main contents are as follows:(1)Chain-extended/crosslinked containing aromatic disulfide bonds as chain extender and crosslinking agent was constructed.Due to the presence of carbamate groups in the structure,the polysiloxane-polyurethane elastomer exhibits a high elastic state at room temperature under the synergistic effect of hydrogen bonds and dynamic aromatic disulfide bonds,and the material can be completely melted at elevated temperature.By adjusting the molecular weight and grafting density of polysiloxane,the obtained elastomer has different viscoelastic transition temperature and crosslinking density by calculation.It is proved that the chain extender/crosslinking agent(DTSA-HDI)played an important role in tuning the mechanical and thermal properties of polymer networks.Among then,polysiloxane-polyurethane elastomer showed the highest tensile strength of 2.60 MPa and outstanding reforming ability.After four times of hot-pressings,the tensile strength did not decrease obviously.As an unfilled elastomer,polysiloxane-polyurethane containing aromatic disulfide has excellent comprehensive mechanical properties.(2)As a dynamic crosslinking agent of bio-based epoxy resin,aromatic disulfide was characterized by good biodegradability,environmental friendliness and certain mechanical strength compared with other polymer systems.Purely dynamically crosslinked material was prone to stress hysteresis and stress relaxation under cyclic loadings,so permanent chemical crosslinking bonds were introduced to ensure the entropy elasticity of elastomer material,thus greatly reducing the stress hysteresis and relaxation effect.Bio-based epoxidized soybean oil(ESO)and dimer aliphatic acid(DAA)were crosslinked aromatic disulfide agent,and a biobased elastomer with self-healing properties was obtained.Adjusting the ratios of[DAA]/[DTSA] made the elastomer contain both dynamic reversible covalent crosslinking and permanent covalent crosslinking bonds.Using the elastomer as substrate,conductive carbon nanotube layer was coated by vacuum filtration,and liquid metal EGa In was used as electrode material to assemble a strain sensor set.The strain sensor can be used to monitor the change of resistance signal under different conditions,and can respond linearly to the resistance signal with strain range of 1%～40% and strain rate of 10～200 mm/min.It was observed that the resistance values decreased slightly after repeated cyclic stretching.This work provides a new approach for the recycling and degradation of epoxy resins.(3)As one of the essential components of strain sensors,elastomer substrate needs to monitor different scale of strains,considering that the strain sensors acting on various body parts are usually made of same material at present.Inspired by the unique stress strengthening effect of human soft tissue,the bottlebrush polymer was prepared by the simple reaction of polyurethane chain extension.Aromatic disulfide bonds existed in the main chain of polytetramethylene ether glycol(PTMG)to endow self-healing property to the material,and side chain was stearic acid with equal polymerization degree.The polymerization degree and bottlebrush density of the main chain were calculated by the results of gel permeation chromatography(GPC).Adjusting the feeding sequence of raw materials in the “one-pot method” to obtain bottlebrush polymer elastomer with aromatic disulfide bonds,random or block distribution of side chains and polyether.In the tensile tests,we found that the bottlebrush polymer has a stress strengthening effect similar to that of human soft tissue.When the strain is less than 100%,the stress will yield with the increase of strain.While the strain is over 100%until the sample breaks,the strain-stress curve of the material presents a quadratic relationship,and fitting results have a high correlation coefficient.In addition,the bottlebrush polymer also has excellent mechanical properties,with its tensile strength reaching 20 MPa and elongation at break exceeding 2000%.After the introduction of dynamic aromatic disulfide bonds,the bottlebrush polymer also exhibits excellent self-healing and recyclability.The substrate of strain senor not only needs good sensitivity and durability,but also should consider the different parts of action,so it has a certain degree of “adaptability”.Therefore,compared with artificial soft materials,nature soft tissues show special stress strengthening effect when subjected to different ranges of stress.Started from strain sensor,we designed the molecular structure of bottlebrush polymer which exhibited stress strengthening effect as an elastomer substrate,and explore the relationship between the structure and performance of bottlebrush polymer segment parameters,providing an idea for the network with skin “adaptability”.(4)As the base material of wearable flexible sensor,elastomer substrate needs to be in contact with human skin for a long time to monitor various motion signals.Therefore,certain antibacterial properties are required besides flexibility and strength.We successfully prepared a series of antibacterial polyurethanes elastomers with good self-healing,high mechanical and antibacterial properties by copolymerization of polyurethane prepolymer containing aromatic disulfide bonds and quaternization agents.The existence of quaternary ammonium cations greatly enhanced the tensile strength,elasticity,and toughness of the final products,with the tensile strength reaching 39.2 MPa and elongation at break reaching 2278%.The antibacterial elastomer successfully achieved 97% and 99% for Escherichia coli(E.coli)and Staphylococcus aureus(S.albus),respectively.Antibacterial elastomer has advantages in biomedical materials and other health protection and environmental hygiene products.It is also suitable for the elastic substrate sensor in direct contact with human skin. |
PDF Full Text Request