The Construction Of Self-healing Networks And Its Synergistic Adjustment Mechanism For Electrical Performance In Stretchable Conductive Composites

Stretchable electrical conductive composites（SECC）is a kind of stretchable composites that composed of elastic matrix and conductive network,which can be conductive under stretching,SECC has been widely applicated in flexible electrode,sensor,circuit and energy storage device and plays an important role in flexible electronics.In practical application,it will be suffered with the instability of elastic matrix and conductive network in the process of reciprocating deformation（stretching,bending,twisting,etc.）and the instability of electrical properties,complex preparation and high cost caused by the damage of the external environment（scraping,friction,impact and peeling etc.）.This severely limits the development and application of SECC.Based on this,the stability of SECC was improved by introducing self-healing characteristics in this work.The relationship between self-healing performance and electrical and mechanical properties,and the relationship between structure and performance were studied systematically.In this study,we introduced polycaprolactone microspheres（m-PCL）with low melting point into the system to deal with the problem that graphene（GE）and silver nanowires（Ag NWs）are easy to agglomerate in elastic matrix.The agglomeration will severely affect the percolation thresholds and the sensing performance of strain sensors.In the process,taking advantage of the adhesion nature of 2D materials,graphene oxide（GO）can wrap the m-PCL,forming GO@m-PCL.Then,Ag NWs are attracted onto the surface it because of the hydrogen bonding,constructing a well dispersed hybrid networks.After the liquid polysiloxane（PDMS）was poured into the hybrids and cured to prepare strain sensor,the existence of the hybrid network not only brought high electrical properties such as high conductivity（0.45 s/m）,sensitivity(0.26 rad^-1)and stability（more than 2400 stretching-releasing cycles）.More importantly,the introduction and hybridization of m-PCL also endowed the material with excellent electrical self-healing properties（healed with 80%efficiency in conductivity in 10min at 80℃）,and good stability.Therefore,this work constructs a m-PCL/GO/Ag NWs hybrid network,which reduced the threshold of the conductive network and improved the sensitivity,and was successfully applied in the body movement monitoring.In order to realize the response of force sensitive sensing materials to small or large strains in the human body or the nature,it is expected to develop the force sensitive sensing materials with high sensitivity and wide detection range.In this study,the low modulus and self-adhesion properties of hydrogels were innovatively proposed to ensure that hydrogels could make corresponding deformation and realize the response to the minimum strain.By using sodium tetraborate crosslinked polyvinyl alcohol（PVA）and polydopamine（PDA）blend,a ionic conductive hydrogel can be arbitrarily shaped can be obtained.The hydrogels have low modulus and self-adhesive properties,and exhibit the ability to respond to both minimal and large deformation.In addition,rapid（250 ms）electrical and mechanical self-healing can be achieved at room temperature by means of hydrogen and boron ester bonds between sodium tetraborate and polymer molecular chains.The self-healing ionic conductive network with hydrogen and boron ester bonds constructed in this work can well capture the extremely weak signals（strain>0.1%）in the human body,such as pulse beating,breathing and vocal chord vibration.At the same time,it can also detect large strain（up to500%）（such as knee bending,etc.）at the same time,which has a good application prospect in human health monitoring.At present,the preparation of elastomers with high stretchability,high mechanical strength and fast,simple and efficient self-healing is still challenging.In order to solve this problem,N,N’-ditert-butylenediamine（DTB）was used to introduce the hindered urea bond（HUB）and the dynamic hydrogen bond into the flexible matrix containing PDMS.The PDMS₁-DTB_0.5elastomer has excellent mechanical properties through balanceing the content of hydrogen bond and dynamic covalent bond.The tensile strength of PDMS₁-DTB_0.5elastomer can reach 2.9 MPa,and the elongation at break can reach 670%.Moreover,with the synergistic effect of HUB and hydrogen bond,the elastomer can achieve self-healing efficiency of 92%at 60℃in 30 min.Therefore,this material has great potential in using as SECC matrix.When the SECC is deformed,the conductive layer is easy to slip or even fall off from the matrix due to the high stress during the deformation of the elastomer matrix.In addition,the conductive layer may be peeled off or damaged by external influences such as scratches and impacts.In this study,it was found that the elastic matrix with dynamic covalent bonds can be well coated by the motion of the chain segment at topological freezing transition temperature,so as to obtain a stable conductive network.By spraying at 60℃on the prepared elastomer,we can obtain a conductive coating,which show high toughness under water rinsing,tape peeling and ultrasonic damage.Compared with Corning Sylgard 184,a silicone rubber widely used in SECC,the substrate can also gives SECC good electrical self-healing properties under similar mechanical properties.Experimental results show that the high stability and self-healing properties are caused by the synergistic network of dynamic urea bond and hydrogen bond.In addition,the above related properties have been verified in Ag NWs,carbon nanotubes,silver plates,liquid metal and other fillers by spraying,stencil printing and direct printing.Based on the above performance,the conductor is expected to be used in stretchable self-healing conductors and circuits.