Study On Preparation And Sensing Performance Of Self-healing Conductive Elastomers Based On Reversible Dynamic Bonds

Tactile perception is one of the main functions of human perception of external environment.In recent years,flexible sensors based on this natural human perception capability have received extensive attention from researchers,providing critical information in biomedical science and human health monitoring in applications such as biocompatible electronic platforms,implantable sensors,epidermal electronic devices,wearable Point of care(POC)diagnostics,and gaining rapid development.Compared with the traditional rigid sensor,the flexible sensor has obvious stretchability,arbitrary deformability and good biocompatibility.These characteristics provide great application value for detecting biological signals and soft robots.The realization of the sensor depends on the internal electrical signal transmission mode,according to the transmission principle can be roughly divided into two types.The first is a flexible electronic sensor,which mainly relies on the existence of electrons and holes in the system.These two electron-hole pairs can carry internal electrical signals.The second is the flexible ion sensor,which works by using the free positive and negative ions in the system.The positive and negative ions are transmitted in the channel of the system to generate electrical signals.The composition of flexible sensor cannot be separated from the selection of stretchable conductor.According to the fundamental properties of the matrix,the tensile conductors can be roughly divided into three categories,namely hydrogels,ionic gels and elastomers.Hydrogels and ionic gels are usually composed of physical(chain entangling)or chemical bond cross-linked polymer network and a certain amount of solvent(water or ionic liquid and other organic solvents).Although this composition can provide a soft and comfortable humanmachine interaction experience for the base material,the solvent in the system is easy to evaporate naturally,and when it is squeezed by mechanical external force and deformed,the solvent in the system will often leak out from the polymer network,which seriously affects its practical application.As a result,the safety,reliability and stability of the prepared sensor parts are greatly reduced.Elastomers have become a research hotspot in the field of flexible sensing due to their solventfree,excellent environmental tolerance and fatigue resistance.Using elastomer as stretchable conductor can greatly improve the service life and safety of the sensor.However,elastomers inevitably encounter destructive problems such as mechanical damage in the practical use process,which will greatly reduce the applicability and life of elastomers,so that the cost is greatly increased.So,inspired by the idea that natural organisms can heal themselves,the use of elastomers would be greatly enhanced if they were given an interesting ability to heal themselves.It has been reported that self-healing of elastomers can be achieved by introducing dynamic covalent or non-covalent sites into the internal network of elastomers.However,elastomers used as flexible sensors are still faced with many problems.For example,in the process of giving the elastomer conductivity and constructing a continuous conductive path,it is easy to appear that the conductive filler is incompatible with the elastomer matrix or the contact between the conductive interface and the matrix interface is unstable,resulting in the conductive filler falling off.As a result,the sensitivity of the final prepared sensor is low,the response signal is unstable and not timely.In addition,in terms of the relationship between mechanical properties and structure,the traditional elastomers are difficult to achieve high elongation at break while maintaining high resilience due to their dense cross-linking sites and single dynamic sites,and the serious stress relaxation phenomenon is also one of the problems to be solved.Therefore,in order to overcome the above problems,it is urgent to develop self-healing conductive elastomers with high tensile properties,high resilience,fatigue resistance and electrical signal stability.Based on this,this paper carries out the following research work:(1)Dithiourea complex crosslinking agent SSH was synthesized by nucleophilic addition reaction between cysteamine and ethyl 2-methacrylate isocyanate.Then,the synthesized SSH was used as a crosslinking agent and n-butyl acrylate(n-BA)monomer through simple photoinduced free radical copolymerization to prepare a self-healing elastomer with dual dynamic(hydrogen bond and disulfide bond)network structure,and then silver nanowires were uniformly coated on the surface of the elastomer by a simple and quick coating method as a conductive filler.Finally,selfadhesive and self-healing conductive elastomers(PBA/SSH)were prepared.In the double dynamic cross-linking network constructed by SSH crosslinking agent,the urea-based group can provide non-covalent strong hydrogen bonding,while the disulfide bond contained in cysteamine provides strong dynamic covalent bonding for the elastomer network.Thanks to this,the elastomer PBA/SSH has satisfactory tensile properties(～1300%),structural stability of the network(tolerance frequency1-100 rad s-1)and ideal healing efficiency(94.4%)at high temperature(80 ℃).PBA/SSH showed ideal resilience and fatigue resistance(energy dissipation coefficient was 0.363).PBA/SSH also shows excellent adhesion to different contact interfaces(such as steel,glass,rubber,etc.).PBA/SSH can be made into a flexible sensor used to detect the activities of different joints of the human body,showing high sensitivity(GF 2.57),fast response(120 ms),regular response of electrical signals and other advantages.(2)A novel strategy is developed here for the preparation of high-performance conductive elastomers with long-functional polymer crosslinkers.By the mercaptan/acrylate reaction between PEGDA(polyethylene glycol diacrylate)and borate(BDB)containing dimercaptan,a macromolecular crosslinking agent PEG-BDB with polyborate bond in main chain was designed.PBA/PEG-BDB elastomers were obtained by using the obtained PEG-BDB as crosslinking agent and n-butyl acrylate(n-BA)monomer through simple photoinduced free radical copolymerization.The existence of PEG-BDB optimizes the originally dense cross-linked network and greatly improves the mobility of polymer chains in the network.Moreover,the long functional chain PEGBDB skeleton contains multiple borate ester bonds,providing multiple dynamic sites for the system.The resultant PBA/PEG-BDB elastomers exhibit ideal properties of high tensile(1950%),low Young’s modulus(29 k Pa),low hysteretic(energy loss coefficient 0.039)and excellent healing efficiency(96%)at room temperature.To demonstrate its practical application,PBA/PEG-BDBbased sensors were successfully attached to different joints(wrists,elbows and fingers)of puppets for real-time motion detection.In addition,elastomeric sensors are also good for recognizing other human activities,such as writing.This work provides a new design strategy for future flexible sensors by optimizing the cross-linking size of long functional polymer chains as cross-linkers.