| Polymer gels show great potential applications in the fields of agriculture,industry,biomedicine,and wearable electronics due to their distinctive tree-dimensional network structure and broadly tunable physicochemical properties.However,conventional polymer gels,particularly hydrogels,have a single structure and lack efficient energy dissipation thus having poor mechanical properties,which makes them unsuitable for practical applications in continuous loading conditions.Therefore,the preparation of polymer gels with excellent mechanical properties is essential for engineering applications.Polyvinyl alcohol（PVA）,a synthetic polymer with good biocompatibility and variable crystallinity,has been widely used for the construction of strong polymer gels.Traditional PVA gels are mainly generated based on a freeze-thaw cycle strategy,which is a straightforward but time-consuming and energy-intensive preparation process,and because of the inhomogeneous distribution of crystalline domains within them,traditional PVA gels have low mechanical strength.In recent years,benefiting from the development of materials manufacturing technology,a series of techniques based on advanced manufacturing technologies including freeze casting and 3D printing have emerged to build robust PVA gels,which have considerably enhanced the mechanical properties of PVA gels.Nevertheless,it is challenging to apply these advanced manufacturing techniques for large-scale production since they are excessively expensive and complex in terms of parameter design and process management during use.Consequently,it would be advantageous to develop an effective and straightforward strategy for constructing strong PVA gels for actual production and applications.Herein,in order to improve the mechanical strength and toughness of physically cross-linked PVA gels,the gelation process and subsequent toughening process are regulated to induce a dense and uniform distribution of crystalline domains within the gel network.Furthermore,the PVA gels are endowed with multifunctional properties such as ionic conductivity,freezing and drying resistance,etc.,to explore their application prospects in the field of flexible energy storage.The following three particular studies were carried out:1.A novel carboxylic acid induction strategy was proposed to prepare PVA physical gels with high mechanical properties by introducing small molecule carboxylic acids into the PVA/dimethyl sulfoxide（DMSO）system to break the hydrogen bonds between PVA and DMSO,which induced the restoration of intra/inter-chain hydrogen bonds of PVA chains and induced the formation of crystalline domains of a certain scale.Meanwhile,the mechanism of carboxylic acid-induced gel formation and toughening was elucidated,and the impacts of carboxylic acid structure,concentration,and induction duration on the characteristics of PVA physical gels were further verified.The prepared PVA physical gels had a maximum breaking strength of 2.2 MPa,an elongation at break of 720%,and a toughness of 7.7 MJ/m3.In addition,the physical gels have fast thermal reversibility and high transparency.2.Based on the carboxylic acid induction strategy,a hydrogel network with dense and homogeneous structural microdomains was constructed by synergizing it with the salting-out effect and solvent replacement.Specifically,acetic acid was used to induce the formation of PVA pre-gel precursors,which were subsequently immersed in sodium sulfate solution for salting-out and solvent replacement,driving the rearrangement and crystallization of PVA polymer chains.The PVA hydrogel constructed by synergistic strategy exhibited unparalleled mechanical properties with a strength of 9.66 MPa,elongation at break of 1763%,and toughness of 85.80 MJ/m3.In addition,the synergistic PVA hydrogel had excellent ionic conductivity(18.02 m S cm-1)due to the introduction of free ions.The solid-state supercapacitor constructed with the synergistic PVA hydrogel as the solid-state electrolyte demonstrated a high area specific capacitance of 73.13 m F cm-2 at a current density of 0.5 m A cm-2,while maintaining a stable energy output under external force and deformation,indicating a promising potential for application in the field of flexible energy storage devices.3.The carboxylic acid induction strategy was combined with the dual-network strategy and ionic cross-linking to construct a dual-network hydrogel with abundant hydrogen bonding and ionic bonding.Specifically,PVA/alginate（Alg）pre-gels produced by acetic acid induction were submerged in a glycerol/water mixture solution containing calcium chloride.During the immersion process,calcium chloride and glycerol entered the gel network for ionic cross-linking and hydrogen bonding cross-linking,respectively.Owing to multiple physical cross-linking,the constructed PVA-Alg organohydrogel exhibited excellent mechanical properties with a breaking strength of 1.55 MPa,elongation at break of 940%,and toughness of 6.99MJ/m3,while the organohydrogel showed outstanding anti-freezing property（freezing point of-62.3℃）and high temperature drying resistance due to hydrogen bonding between glycerol and water as well as hydration of ions.The flexible supercapacitor constructed by using PVA-Alg organohydrogel as a solid electrolyte demonstrated an area specific capacitance of 72.3 m F cm-2 at a current density of 0.5 m A cm-2 and was capable of maintaining stable electrochemical properties under external forces and over a wide temperature range（-20 to 60℃）,which greatly expands the application prospects of the gel material in harsh environments.To sum up,physically cross-linked PVA gels with superb mechanical properties were prepared by modulating the cross-linking mode and the distribution of the structural microdomains in the gel network based on the carboxylic acid induction strategy.Besides,the application of the prepared physically cross-linked PVA gels in the field of flexible energy storage is also explored,which provides fresh perspectives for the design of flexible energy storage devices that can withstand harsh environments. |
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