| Hydorgels with a unique 3D network structure have great potential applications in fields of flexible energy storage,smart wearables,and so on.However,the high-water content of the hydrogels results in the poor mechanical properties and anti-freezing performance,which seriously affects the applications of hydrogels.Recently,many strategies have been developed to enhance the mechanical properties or anti-freezing performance of hydrogels,such as introducing a second cross-linked network,increasing the interactions of the polymer chains,and adding fillers,and so on.While these strategies fail to completely solve the problems of hydrogels exciting in truly utilizing.Therefore,it is necessary to develop facile and effective strategies to strengthen hydrogels and improve their anti-freezing ability,which is conducive to realizing the application of hydrogels in real life and industry.To enhance the mechanical properties and anti-freezing performance of the hydrogels,this thesis aims to improve the energy dissipation ability of the hydrogels and destroy the ordered structure of free water in the hydrogels by regulating the preparation process of the hydrogels and changing the mechanism of the system media and ions.Additionally,the hydrogel is also endowed some functional characteristics such as ionic conductivity to expand its applications.Finally,the application prospects of the hydrogel in flexible energy storage and smart wearable are initially explored.Based on the above research ideas and processes,this thesis uses two common polymers polyvinyl alcohol（PVA）and sodium alginate（Alg）,and conducts the following four aspects of research:（1）An enhanced double-network structure of hydrogel was constructed by freezing-thawing,soaking and annealing method.Freeze-thawing and soaking are used to form PVA crystal regions and Alg-Ca ionic bonds crosslinked,respectively,which constructs the double-network structure of the hydrogel.Annealing is used to promote PVA crystallization,increasing the entanglement of polymer chains,thus forming an enhanced double-crosslinked network to improve the mechanical properties.Thus,the hydrogel exhibits well mechanical properties（breaking strength 1.94 MPa,breaking elongation 607%,Young’s modulus 0.45 MPa）.During the annealing process,the free water content of the hydrogels is reduced,thereby synergistically improving the anti-freezing ability of the hydrogel（the freezing point is-32℃）.（2）Based on the construction of the enhanced double-network structure,a conjoined-network hydrogel is constructed through a simple two-step strategy of freezing-thawing and soaking.PVA/Alg pre-hydrogel prepared by freezing-thawing was soaked in Ca Cl2 glyceol aqueous solution,forming the Alg-Ca ionic crosslinked network.At the same time,the conjoined-network hydrogel was fabricated by diffusing glyceol into the PVA/Alg pre-hydrogel to connect the PVA crystal network formed in freezing-thawing process and the Alg-Ca network.Under the synergistic effect of salt and glyceol solution,the constructed conjoined-network hydrogel shows well comprehensive performance of mechanical properties with tensile strength 2.11 MPa,elongation at break 580%,and Young’s modulus 0.42 MPa.Moreover,under the action of the colligative property of Ca Cl2 solution and the hydrogen bond of glycerol,the free water content in the hydrogel is reduced,while the anti-freezing perfoamance of the hydrogel is improved,displaying excellent anti-freezing ability with the freezing point of-42.1℃,well anti-drying ability（exposure to air for 72hours without drying）,and great ionic conductivity of 1.69 S m-1.Compared with the enhanced double-network hydrogel,the mechanical properties and antifreeze properties of conjoined-network hydrogel both have been improved.A simple sensing determination of finger bending shows the hydrogels can sensitively detect small activities,have good potential in the field of flexible sensing.（3）The construction mechanism of the conjoined-network hydrogels is explored by using different small organic molecules（ethylene glycol,glycerol,and sorbitol）as bridging molecules.The results shows that more numbers of the hydroxyl groups in the bridging molecules could be conducive to the better entanglement of polymer chains than that with water.The less numbers of the hydroxyl groups in the bridging molecules are just the opposite.Subsequently,a flexible supercapacitor was constructed by using the conjoined-network hydrogel as electrolyte.It exhibits a wellspecific capacitance of 140.75 m F cm-2 at 1.0 m A cm-2 and could output energy stably under bending and different temperatures,showing certain application prospects in the field of flexible energy storage under extreme environments.（4）A homogeneous network structure hydrogel is constructed by improving the preparation strategy of the conjoined-network hydrogel.By introducing a relatively poor solvents and kosmotropic ions into the soaking solution,the PVA pre-hydrogel prepared by freezing-thawing method forming crystalline regions and hydrophobic entanglement regions under the action of solvent displacement and Hofmeister effect.Under the connection of the PVA polymer chains,the two crosslinked regions tangle to form a homogeneous network.The energy is uniformly dissipated in the system,thereby improving the mechanical properties of the hydrogel.Through the efficient energy dissipation mechanism of the structure,the prepared hydrogel exhibits well mechanical properties with tensile strength 16.54 MPa,elongation at break 1203%,and toughness 111.21 MJ m-3.In addition,the hydrogel also exhibits a well anti-freezing ability under the colligative effect of kosmotropic ions（NH4）2SO4.Subsequently,a flexible supercapacitor was assembled using the hydrogel as the electrolyte.The supercapacitor displays an excellent area specific capacitance of 156.50 m F cm-2 at the current density of 1.0 m A cm-2.Moveover,the supercapacitor exhibits high resistance to mechanical stimuli such as bending and pressure and can well maintain stable energy output even being dynamically deformed,showing great potential in the wearable applications. |
