Preparation Of Multifunctional High-strength And Tough Supramolecular Hydrogels And Characterization Of Their Mechanical Sensing Performance

Due to the dynamicity and reversibility of non-covalent bonds,high-strength and tough supramolecular hydrogels based on non-covalent bonds have stretchability,mechanical toughness,and excellent self-recovery properties,making them become an ideal material for flexible sensors.However,due to insufficient functionalities,supramolecular hydrogels usually lack sensing characteristics and the application range of them is limited.In this study,based on multiple non-covalent interactions such as hydrophobic associations,coordination bonds and hydrogen bonds,a series of multifunctional and high-strength supramolecular hydrogels were designed and constructed,and their mechanical properties and related molecular mechanisms were studied in detail,and then shape memory,thermoplasticity,antifreezing property and electrical conductivity of them were also evaluated,and finally the application of them in wearable mechanical sensors was explored.Based on dual-cross-linked strategy,a novel type of dual-crosslinked supramolecular（DS-）hydrogel crosslinked by hydrophobic associations and metal coordination bonds was successfully synthesized via micellar copolymerization of stearyl methacrylate（SMA）,acrylamide（AAm）and acrylic acid（AAc）and subsequent introduction of Fe³⁺.First,the mechanical properties of hydrogels were evaluated in detail.Results suggest that the mechanical properties of hydrogels could be effectively adjusted by controlling the densities of two cross-links.Under the optimal condition,the DS-hydrogel had a tensile strain of up to 1000%,a tensile strength of 6.8 MPa,a toughness of 53 MJ m^-3,excellent self-recovery properties and good fatigue resistance because of the synergistic effect of hydrophobic association and metal coordination bonds.Furthermore,this hydrogel also exhibited a triple shape memory effect owing to the stimuli responsiveness of non-covalent interactions.Based on the stability of hydrophobic association in the water environment,a conductive CNTs/HAPAAm nanocomposite supramolecular hydrogel was prepared by integrating hydrophobic carbon nanotubes（CNTs）into hydrophobically associated polyacrylamide（HAPAAm）hydrogel.The results of the physical and chemical characterization,mechanical and electrical conductivity tests suggest that the formation of multiple hydrophobic associations and the uniform dispersion of CNTs in hydrogel network made the CNTs/HAPAAm hydrogel exhibit excellent mechanical performances and good electrical conductivity.The testing of sensing performances demonstrates the CNTs/HAPAAm hydrogel had high strain sensitivity（GF=4.32）in the 1000%strain range and high linear sensitivity(0.127 kPa^-1)in a large pressure region within 0-50 kPa,and good sensing stability,which could be used as wearable mechanical sensors to detect different types of human activities.In order to expand the application of supramolecular hydrogels in low temperature environments,in this study,an antifreezing gelatin supramolecular organohydrogel was prepared via a simple strategy of immersing a gelatin pre-hydrogel in sodium citrate（Na₃Cit）water-glycerol mixture solution.Results of the mechanical test suggest that the organohydrogel had a tensile strength of 2.06 MPa,a tensile strain of 690%,rapid self-recovery and good fatigue resistance.The low-temperature stability of the organohydrogel was evaluated through low-temperature rheology,DSC,and low-temperature tensile tests.Results indicate that the organohydrogel exhibited excellent antifreezing property and could maintain its flexibility even at-80℃.Further investigation finds that the organohydrogels also had multiple functions such as good conductivity,self-healing ability,thermoplasticity and adhesion.Based on the mechanical flexibility and conductivity of organohydrogel under ultra-low temperature conditions,flexible stretchable sensors with ultra-low operating temperature were prepared.Results of sensing performance test suggest that the organohydrogel sensor had high linear sensitivity（GF=1.5）in the 300%strain range and good sensing stability.In addition,due to the reversibility of the non-covalent crosslinked structure,the organohydrogel sensor was fully recyclable.The organohydrogel sensor after repeated recycling still exhibited good stretchability,conductivity and strain sensing characteristics.The accurate detection of human motion at normal and low temperatures by organohydrogel sensors confirms its wide operating temperature range.