Construction Of Strong And Tough CS-PHEAA Physically Crosslinked Double Network Hydrogels And Application In Flexible Sensors

Conductive hydrogels exhibit excellent ductility and adjustable physicochemical property.They have been concerned widely in the field of flexible wearable sensors.However,traditional hydrogels are difficult to be widely used because of their intrinsic weak freezing resistance and poor mechanical properties including inferior strength,self-recoverability and mechanical stability.Over the past years,although some hydrogel-based sensors have been developed,it is difficult to meet the actual need of flexible wearable sensors due to their single detection type.In order to solve these problems,two kinds of multifunctional,strong and tough chitosan-polyhydroxyethylacrylamide physically-physically crosslinked network hydrogels were prepared by simple immersion method,and their mechanical properties,freezing resistance and sensing performance were systematically investgated.The main research contents and results are as follows:（1）Construction and mechanical property of a strong and tough chitosan-polyhydroxyethylacrylamide（CS-PHEAA）physically-physically crosslinked network hydrogel.Firstly,CS-PHEAA composite hydrogel was formed based on hydrogen bonding and exhibited excellent ductility.Then,the composite hydrogel was post-crosslinked in sodium sulfate solution（Na₂SO₄）to obtain chitosan-polyhydroxyethylacrylamide fully physically linked double-network hydrogel（DN-Sul hydrogel）.Upon deformation,dissociation of CS ionic network and PHEAA hydrogen bond network were capable of dispersing stress and dissipating energy.Due to the synergy effect of the two networks,DN-Sul hydrogel exhibited excellent mechanical properties including high tensile strength（σ_b=4.48 MPa）,high elastic modulus（E=1.42 MPa）,ultra-high toughness（T=22.28 MJ/m³）and compressibility（σ_c=46.50 MPa,ε_c=98%）.In addition,the reconstructability of the physical cross-linking network provided a recoverable energy dissipation mechanism for the DN-Sul hydrogel,which contributed to the rapid self-recovery ability and excellent mechanical stability.When the DN-Sul hydrogel was placed at room temperature for 20 min,the recovery efficiency of maximum tensile stress,elastic modulus,and dissipation energy achieved 99.2%,94.3%,and 83.1%,respectively.The mechanical properties of DN-Sul hydrogel（maximum stress and dissipated energy）remained stability under both tensile and compressive tests,which provided a foundation for application in flexible wearable sensors,brakes and soft robots.（2）Fabrication of multifunctional,strong and tough ionic conductive hydrogel and its application in flexible sensors.A high-mechanical,anti-freezing and ionic-conductive double network hydrogel（DN-Cit hydrogel）was obtained by post-crosslinking CS-PHEAA composite hydrogel in sodium citrate solution（Na₃Cit）.The immobilized ions participated in the construction of the CS ionic network and enhanced the mechanical property of DN-Cit hydrogel.Compared to DN-Sul hydrogel,DN-Cit hydrogel exhibited superior mechanical performance:tensile strength:5.56 MPa,elastic modulus:1.70 MPa,toughness:23.23 MJ/m³ and compressive stress:71.03 MPa,which resulted from a more denser ionic network structure.In addition,the DN-Cit hydrogel exhibited excellent ionic conductivity by ion migration and significant freezing resistance（-50 ^oC）by means of hindering the formation of hydrogen bonds between water molecules in the hydrogel.The sensor assembled from DN-Cit hyrogel exhibited high strain response sensitivity（GF=6.91,ε_t=400%）and good cycle stability（300 cycles）,which could be used to monitor various types of deformation and pressure（extension,bending,a wide range of compressive strain and stress）,as well as the subtle expressions and joint movements of the human body.The strain response sensitivity and the monitoring range of compressive strain and stress of the hydrogel sensor have surpassed most ionic conductive hydrogel sensors.In addition,due to the excellent freezing resistance and mechanical stability of DN-Cit hydrogel,the sensors of composite DN-Cit hydrogel and DN-Cit hydrogel/aluminum hybrid combination still maintained good responsiveness and stability at low temperatures.