Design,Syntheses And Properties Of POMs-PVA Anti-freezing Conductive Hydrogels

Hydrogels are a class of three-dimensional reticulated materials made of cross-linked polymers with high water content,excellent mechanical properties and tunable physicochemical properties.Conductive hydrogels,as an important branch of hydrogel materials,combine the mechanical properties of hydrogel matrix and the high electrical conductivity of conductive fillers,and have received a lot of attention from researchers in basic science and advanced technology.Polyoxometalates（POMs）are excellent proton conductors with unique structural characteristics and functional properties,and have high proton conductivity under high humidity conditions.The introduction of POMs into the hydrogel system is beneficial to combine the advantages of both to obtain multifunctional hydrogel materials with good proton conductivity even under ambient humidity.However,the high water content causes conductive hydrogels to freeze at sub-zero temperatures,leading to a decrease or even loss of their electrical conductivity and other properties（flexibility,transparency,etc.）,which greatly limits the practical application of such materials.Currently,how to improve the tolerance of conductive hydrogel materials under low temperature conditions is an important research topic.In this thesis,antifreeze conductive hydrogels based on POMs and PVA were prepared by introducing POMs into poly（vinyl alcohol）（PVA）,a hydrogel matrix with low price and low toxicity,and using glycerol as antifreeze agent.The details of the study are as follows:（1）A series of antifreeze proton-conducting hydrogels PVA/Gly/HPW-SA-x（x represents the mass of HPW added,x=1.6,1.8,2.0,2.2 g）were prepared by introducing both phosphotungstic acid(H₃PW₁₂O_40,HPW)and sulfamic acid（NH₂SO₃H,SA）into PVA/Gly hydrogel system by the one-pot method.The acid-base pair formed between HPW and SA lowers the energy barrier for proton hopping and facilitates the fast proton transport,and the proton conductivity of PVA/Gly/HPW-SA-2.0 can reach 2.78×10^-1 S·cm^-1 at 25°C and ambient humidity.Since Gly can form abundant hydrogen bonds with H₂O and thus inhibit the freezing of water,the resulting hydrogel has good transparency,flexibility,stretchability,and ultra-high proton conductivity at low temperatures.The proton conductivity of PVA/Gly/HPW-SA-2.0 at-30°C is 3.33×10^-2 S·cm^-1,which exceeds most low-temperature proton-conducting materials.We have assembled it into a supercapacitor and a strain sensor,and the test results show that the supercapacitor can still reach a specific capacitance of 56 m F·cm^-2 at-20°C.The strain sensor can be used to effectively monitor some human activities（joint flexion,swallowing,and speaking different words）.（2）A new inorganic-organic hybrid based on POMs,H[Zn（Hapca）₃][PW₁₂O₄₀]·7H₂O 1（Hapca=3-aminopyrazine-2-carboxylic acid）,was synthesized.In 1,Zn（Hapca）₃²⁺is connected by hydrogen bonds to form a two-dimensional layer with hexagonal pores,and PW₁₂O₄₀^3-is embedded in the pores as a guest.Due to the abundance of proton sources and proton hopping sites in the structure of 1,we investigated the proton conductivity of 1.At 55°C and 95%RH,the proton conductivity of 1 reached its highest value(1.16×10^-4 S·cm^-1).To obtain proton conductive materials with good mechanical properties,we prepared hydrogel composites1@PVA/Gly-x（where x represents the percentage by mass of compound 1,x=20%,30%and40%）by introducing 1 into the PVA/Gly hydrogel system.At ambient humidity,the proton conductivity of 1@PVA/Gly-40%can reach 1.07×10^-4,1.61×10^-3 and 1.11×10^-2 S·cm^-1 at-30,25 and 75°C,respectively.Given the good elasticity and fast resistive response of 1@PVA/Gly-40%,we assembled it into a strain sensor,which can be used to effectively monitor human movements（joint flexion,finger tapping and swallowing）.