Poly(Vinyl Alcohol)-based Functional Gels:Design,Fabrication And Performance

The development of soft robotics,flexible electronic devices,and artificial intelligence in recent years has boosted research on the fabrication strategy and the performance optimization of gels.Nevertheless,some obstacles still remain so far:（1）the preparation process of stimuli-responsive gels is often complicated,especially the thermosensitive hydrogels that are subject to polymer types and time-consuming synthesis process;（2）traditional gels using water,organic solvent or ionic liquid as continuous phases are facing the problems of poor temperature-resistance,low conductivity,bio-incompatibility and high cost;（3）it is rather challenging to simultaneously integrate multiple functions in one gel system for achieving high efficiency of functions in ever-changing environments.Therefore,it is of profound significance to develop novel multifunctional gels to solve these issues through simple and green approaches,which will greatly promote the leap-forward development of functional gel materials.In order to achieve the above objectives,in this dissertation we utilize polyvinyl alcohol（PVA）that owns regular structures,non-toxicity and low price as the building unit,and have fabricated three PVA-based functional gels from the perspective of gelator construction,dispersion medium optimization,and introduction of functional components.In addition,their potential applications in the fields of smart devices,flexible electronics,and adhesives have been explored in depth.The major results are as follows:1.A NIR light-induced thermogel PVA/Cu（OAc）₂was prepared by simply mixing PVA with copper acetate[Cu（OAc）₂]in water,which exhibited a photothermal conversion of?37.8%and the reversible thermal-induced sol-gel transition.By studying the formation mechanism of PVA/Cu（OAc）₂,it found that the Cu（OH）₂clusters generated by the hydrolysis of Cu²⁺ions could co-assemble with PVA to form clathrate complexes mainly driven by hydrophobic interactions,where the flexible chains of PVA projected hydroxyl groups outside while Cu（OH）₂clusters were located in the center of the tangled chains,thus inducing the sol-gel transition.Moreover,because of the changes in gel transmittance and conductivity upon NIR light,a smart window and a photoresistor have been fabricated to adjust the indoor temperature and control the current in the circuit respectively.This work is the first to create a thermogel with intrinsic NIR light response by means of a non-thermosensitive polymer,which breaks the limitation of thermogels prepared from thermosensitive polymers,and provides an alternative strategy to develop temperature-responsive functional gels.2.Taking advantages of the distribution difference of PVA in water and deep eutectic solvents（DESs）,the high-strength and high-toughness PVA eutectogels have been prepared through a simple and green solvent exchange strategy with the tensile stress of～20.2 MPa and the toughness～62.7 MJ/m³.It was found that the exchange of DESs with water can re-strengthen the interactions between PVA chains which were weakened in water,thus inducing PVA to crystallize and constructing uniform and strong polymer networks with high mechanical properties and crack propagation resistance.Benefiting from the good comorehesive performance and the simple preparation process of PVA eutectogels,high-strength adhesives and continuous fibers have been developed,respectively,thereby realizing their applications in the fields of bonding and electronic sensing.This work proposed a strategy to regulate the spatiotemporal expression of non-covalent interactions between PVA chains at the molecular scale,which gives a guidance for the development of high-strength physical eutectogels,and provides an approach to solve problems of poor temperature-resistance,low conductivity,bio-incompatibility,and high cost of traditional gels.3.By introducing natural glycyrrhizic acid with good assembly and reducibility into the PVA crystalline network,a physical double-network hydrogel sensor with biocompatibility,freezing-resistance and drying-resistance has been fabricated in the glycerol/water solvent.Followed by the reduction reaction assisted by ultraviolet radiation,a gradient silver nanoparticles（Ag NPs）layer formed in situ on the hydrogel surface.By regulating the thickness of Ag NPs layer with illumination time,the hydrogel sensor could not only reflect and absorb all ultraviolet rays,but also maintain a visible light transmittance of 80%.Moreover,Ag NPs allowed the hydrogel to have a superior antibacterial activity that makes the long-term touch of human skin with the hydrogel sensor much safer.Meanwhile,the hydrogel sensor exhibited good sensitivity（gauge factor～2.84）and durability,which was able to take real-time and effective monitoring of human motion under multiple environmental conditions.This work simultaneously integrated all above-mentioned functions in one gel system for the first time,giving strong material support for the development of multifunctional hydrogel flexible sensors.