| Flexible force-sensitive sensors are a class of electronic devices that convert mechanical stimuli such as pressure,tension,stress and strain into electrical signals for easy information processing,and have broad application prospects in the fields of intelligent interaction,biological engineering,unmanned aerial vehicle,and even cultural relic protection.Due to its unique viscoelasticity,polymer hydrogels can be deformed under external forces,which are expected to be used for flexible force-sensitive sensors and give real-time response of electrical information under external force deformation,and is a highly promising electrode material for flexible force-sensitive sensors.However,conventional polymer hydrogels usually exhibit mechanical brittleness,low energy dissipation and susceptibility to stress concentration when subjected to external mechanical stimuli,making the hydrogels usually exhibit low mechanical properties and deformation resistance,which largely limit the application of polymer hydrogels as flexible force-sensitive sensing materials.Therefore,this thesis focuses on the design and construction of polymer nanocomposite hydrogels with three-dimensional ordered network structure,and through the investigation of the relationship between the composition and three-dimensional ordered network structures of nanocomposite hydrogels and their mechanical elasticity and force-sensitive sensing properties,so as to obtain new-type polymer nanocomposite hydrogels with high mechanical strength and high toughness for flexible force-sensitive sensor devices.The following research advances were achieved in this thesis:（1）A strategy of freeze casting-assisted in-situ polymerization was developed to induce the enrichment of composite aqueous solutions containing polyvinyl alcohol（PVA）molecular chains,acrylic acid（AA）monomers and initiators at the skeleten of the oriented ice crystal structures formed by the oriented growth and in-situ polymerization to achieve the first oriented structure by using the volume exclusion effect generated by the oriented growth of ice crystals.At the same time,the two-dimensional montmorillonite（MMT）nanosheets added in the precursor solution were simultaneously oriented in the oriented hydrogel skeleton to form the second oriented structures by the spatial restriction of the oriented structures,and finally the dual-oriented nanocomposite hydrogel material（BNCH）with dual oriented inorganic nanomaterials and polymer walls was obtained.Subsequently,the molecular chain entanglement and secondary aggregation of hydrophilic PAA polymer chains were induced by the salting out effect to obtain the nanocomposite hydrogel materials with a high dual-oriented structure（s-BNCH）.The s-BNCH materials exhibited significant anisotropic properties with high mechanical properties parallel to the orientation direction(e.g.,high fracture strength:～10 MPa,high fracture toughness:～41 MJ m-3,and high puncture resistance:（29）50 N)and high stretchability in the direction perpendicular to the orientation（～1000%）.Thanks to the microcrystalline network structures formed by the crystallization of PVA molecular chains and the formation of secondary bonds（hydrogen bonds）between PVA,PAA and MMT,the s-BNCH exhibited good energy dissipation properties with an energy dissipation coefficient of 0.8.The s-BNCH also had high frost resistance with an ionic conductivity of 1.05 S m-1 at room temperature and an ionic conductivity of 0.46 S m-1 at a low temperature of-20°C.The s-BNCH could be used as a freeze-resistant and highly robust flexible force-sensitive sensing material for real-time detection of human motions and physiological signals.The flexible force-sensitive sensor exhibited high sensitivity(1.49-2.49 MPa-1),good cycling stability（no performance degradation after 100 tensile/release cycles at 50%strain）,and high puncture resistance（almost no degradation of mechanical sensing signals after 100 cycles of needle puncture test）over a wide strain range.（2）By the strategy of freeze casting-assisted polymerization-induced phase separation,the mixed aqueous dispersion formed by two-dimensional MMT,PVA,and hydroxyethyl methacrylate（HEMA）was induced to the orderly aligns along the growth direction of ice crystals by using the volume exclusion effect generated by the directional growth of ice crystals,during which HEMA could in-situ polymerize to form poly（hydroxyethyl methacrylate）（PHEMA）,and because the formed PHEMA polymer chains have certain hydrophobic properties,they would form a unique nanoglobule structures in aqueous solution after the microphase separation,and finally the MMT/PVA dual-oriented composite hydrogels（GNCH）with the decorated PHEMA nanoglobules was obtained.The salting out effect could induce the crystallization of PVA,which gave good mechanical strength to the GNCH and resulted in a more compact and ordered PHEMA nanoglobule-decorated dual-oriented composite hydrogels（s-GNCH）.The s-GNCH had high mechanical strength（1.9MPa）and high fracture toughness(2.3 MJ m-3),and also showed high resistance varies and stability under compression（for at least 100 cpmpression cycles）.The s-GNCH can effectively change the ion transport paths in hydrogels under deformation conditions due to the significant change in the contact areas between the sphere-decorated layers during the compression deformation,which could significantly improve the sensitivity of hydrogels to the resistance changes.Therefore,the s-GNCH could be used as a flexible force-sensitive sensing material with high mechanical strength and high sensitivity,which exhibits a high sensitivity factor（GF value of 1.7 for the strain of<40%,and GF value of 2.28 for the strain between 40%and 100%）,ultra-fast sensing response（<120 ms）and extremely low detection limit（1%strain）during the deformation,thus enabling monitoring of subtle human movements（writing,joint movements,etc.）and physiological signals（swallowing,etc.）. |
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