An ultralight porous hydrogel based on the salting-out effect and its preparation method

Abstract

This invention discloses an ultralight porous hydrogel based on the salting-out effect and its preparation method. The ultralight porous hydrogel based on the salting-out effect has the following structural formula: [Structure formula would be inserted here]. This invention utilizes hydrogen generated by the in-situ redox reaction between gallium-based liquid metal (LM) and an aqueous medium to drive the rapid foaming of the system to form a porous structure. Combined with hydrophobic association induced by the salting-out effect, and under the multiple physicochemical crosslinking effects of dynamic borate ester bonds, metal ion coordination bonds, and hydrogen bonds, an LMCNF porous hydrogel with high strength, high toughness, and high conductivity is prepared. It exhibits excellent mechanical strength; high electrical conductivity; sensitive, stable, and reversible resistance response; a high strain coefficient (GF up to 1.68); and a wide operating range; and excellent cyclic compression stability and fatigue resistance.

Description

Technical Field

[0001] This invention relates to an ultralight porous hydrogel based on the salting-out effect and its preparation method, belonging to the field of hydrogel materials technology. Background Technology

[0002] Porous hydrogels are hydrogel materials with a porous internal structure. They not only possess the high water content and flexibility of traditional hydrogels but also exhibit good biocompatibility and high porosity. Their uniformly distributed pore structure endows the material with unique advantages. The porous structure design helps to significantly reduce the material's density, making it particularly suitable for lightweight applications such as flexible electronic devices. Introducing closed-pore structures can effectively disperse stress under compressive or tensile loads, preventing stress concentration and thus enhancing the material's structural stability. The high specific surface area provides porous hydrogels with abundant active sites and efficient mass transport channels, making them exhibit great application potential in catalysis, adsorption, and drug delivery.

[0003] Currently, researchers have proposed various methods to prepare porous hydrogels, such as template methods, freeze-drying methods, gas foaming methods, and phase separation methods. However, due to their porous structure and high water content, these hydrogels typically exhibit poor mechanical properties. Under compressive or shear stress, the pore walls of the hydrogel are prone to irreversible plastic deformation, resulting in permanent damage and weakening their elastic recovery ability. Furthermore, their relatively high density severely limits the application of porous hydrogels in many fields. Therefore, how to obtain a porous structure, reduce density, and simultaneously improve the strength and toughness of hydrogels remains a significant challenge for porous gel materials. Summary of the Invention

[0004] To address the shortcomings of existing hydrogels, such as poor mechanical properties and low toughness, this invention provides an ultra-lightweight porous hydrogel based on the salting-out effect and its preparation method. The hydrogel of this application exhibits excellent mechanical strength; excellent electrical conductivity; sensitive, stable and reversible electrical resistance response; high strain coefficient and wide working range; excellent cyclic compression stability and fatigue resistance; and ultra-low density, meeting the lightweight requirements of high-end products.

[0005] To solve the above-mentioned technical problems, the technical solution adopted by the present invention is as follows:

[0006] An ultralight porous hydrogel based on the salting-out effect has the following structural formula:

[0007] .

[0008] The terms "Cations" and "Anions" refer to cations (positive ions) and "Hydrogen bonding" and "Ionic coordination."

[0009] The aforementioned porous hydrogel based on the salting-out effect is composed of a mixture of LM / TOCNF / PVA / borax.

[0010] In this application, LM is gallium-based liquid metal. TOCNF is TEMPO oxidized cellulose nanoparticles, wherein TEMPO is 2,2,6,6-tetramethylpiperidine-1-oxy radical. PVA is polyvinyl alcohol.

[0011] The aforementioned ultralight porous hydrogel based on the salting-out effect has a wet density ≤0.029 g / cm³. 3 The porous hydrogel exhibits a sensitive, stable and reversible resistance response during compression, with a maximum GF of 1.68.

[0012] This application presents a porous hydrogel based on the salting-out effect, which significantly reduces density while maintaining excellent mechanical properties. The tensile strength reaches 1009.47 kPa, the toughness reaches 1520.6 kJ / m³, the elongation at break reaches 266.06%, and the electrical conductivity reaches 8.39 mS / cm. During compression, it exhibits a sensitive, stable, and reversible resistance response, with a high strain coefficient (GF up to 1.68) and a wide operating range. It also possesses excellent cyclic compression stability and fatigue resistance, recovering its initial shape after 100 cycles at 70% compressive strain. This makes it suitable for flexible electronic devices, smart sensors, and health monitoring.

[0013] A method for preparing an ultralight porous hydrogel based on the salting-out effect involves thoroughly mixing gallium-based liquid metal (LM), TEMPO oxidized cellulose nanofibers (TOCNF), polyvinyl alcohol (PVA), and borax, followed by freeze-thaw cycles, limited foaming, and salting-out treatment to obtain an ultralight porous hydrogel with an internal three-dimensional porous network structure.

[0014] To ensure the overall performance of the resulting hydrogel, the mass content of gallium-based liquid metal after limited foaming and before salting-out treatment is 2.5–12.5 wt%, preferably 7.5–10 wt%. The mass content of PVA after limited foaming and before salting-out treatment is 12–18 wt%, preferably 14–16 wt%.

[0015] The above salting-out treatment involves soaking in a sodium citrate solution with a concentration of 10-30% for 24-48 hours.

[0016] The gallium-based liquid metal (LM) mentioned above is composed of 75 wt% gallium and 25 wt% indium.

[0017] As a specific implementation scheme, the above-mentioned method for preparing ultralight porous hydrogels based on the salting-out effect includes the following steps:

[0018] 1) Liquid metal (LM) and TEMPO oxidized cellulose nanofibers (TOCNF) were ultrasonically dispersed in water under ice-water bath conditions to obtain a uniformly dispersed LM dispersion.

[0019] 2) Dissolve polyvinyl alcohol (PVA) in water and add it to the LM dispersion obtained in step 1). Stir the mixture evenly at a temperature of 70~100℃ to obtain a viscous liquid.

[0020] 3) Add borax (borax is soluble in water) to the viscous liquid obtained in step 2) and knead it thoroughly at a temperature of 70~100℃. Cool it to room temperature and perform a freeze-thaw cycle. Then, perform limited foaming for 10~14h. Finally, soak it in a 10~30% sodium citrate aqueous solution for 12~24h to obtain an ultra-lightweight porous hydrogel.

[0021] The above-mentioned method utilizes hydrogen generated by an in-situ redox reaction between gallium-based liquid metal (LM) and an aqueous medium to drive rapid foaming and form a porous structure. Combined with hydrophobic association induced by the salting-out effect, and through multiple physicochemical crosslinking processes including dynamic borate ester bonds, metal ion coordination bonds, and hydrogen bonds, a porous LMCNF hydrogel with high strength, high toughness, and high conductivity is prepared. It exhibits excellent mechanical strength; high electrical conductivity; a sensitive, stable, and reversible electrical resistance response; a high strain coefficient (GF up to 1.68); a wide operating range; and excellent cyclic compression stability and fatigue resistance.

[0022] The above method involves immersing the hydrogel in a 10-30 wt% sodium citrate solution to form a porous hydrogel with high strength, high toughness, and high conductivity through the salting-out effect.

[0023] In order to improve the mechanical properties of the obtained porous hydrogel while taking into account the requirement of lightweight, in step 1), the mass ratio of gallium-based liquid metal and TEMPO oxidized cellulose nanofiber is (0.1~0.8):1.

[0024] To further ensure the mechanical properties of the obtained porous hydrogel, in steps 1) and 2) above, the mass ratio of gallium-based liquid metal (LM) to polyvinyl alcohol (PVA) is (0.12~1.12):1, preferably (0.45~0.53):1. In steps 1) and 2), the mass ratio of gallium-based liquid metal (LM) to borax is (80~465):1.

[0025] In step 2) above, the mass ratio of PVA to the water used for its dissolution is (0.2~0.4):1.

[0026] In step 3) above, the mass ratio of borax to water is (2.5~3):800.

[0027] In step 3) above, the mass concentration of the sodium citrate aqueous solution is 15-25 wt%, more preferably 15-20 wt%.

[0028] In step 3) above, the freeze-thaw cycle is as follows: the material is placed in a cryogenic reactor at -70±20℃ for 10-14 hours to freeze, then thawed at room temperature, and this cycle is repeated 2-5 times. The material is then transferred to a constant temperature drying oven for limited foaming for 10-14 hours to obtain the hydrogel. More preferably, the freezing temperature is -70±5℃.

[0029] In step 3) above, the conditions for limited foaming are: foaming time of 10~14h in a constant temperature drying oven at 37℃ and 60% RH.

[0030] The LM content in this application refers to the mass content of LM relative to the hydrogel after limited foaming and before soaking in sodium citrate solution; the PVA content refers to the mass content of PVA relative to the hydrogel after limited foaming and before soaking in sodium citrate solution.

[0031] Any techniques not mentioned in this invention are based on existing technologies.

[0032] This invention relates to an ultra-lightweight porous hydrogel based on the salting-out effect. It utilizes hydrogen generated by the in-situ redox reaction between gallium-based liquid metal (LM) and an aqueous medium to drive the rapid foaming of the system and form a porous structure. Combined with hydrophobic association induced by the salting-out effect, a porous hydrogel with high strength, high toughness, and high conductivity is prepared under the multiple physicochemical crosslinking effects of dynamic borate ester bonds, metal ion coordination bonds, and hydrogen bonds. Characterization results show that by adjusting the amount of LM added, the PVA content, and the concentration of sodium citrate solution, the porous hydrogel achieves a tensile strength of 1009.47 kPa, a toughness of 1520.6 kJ / m³, an elongation at break of 266.06%, and an electrical conductivity of 8.39 mS / cm. Furthermore, this hydrogel exhibits a sensitive, stable, and reversible resistance response during compression, possesses a high strain coefficient (GF up to 1.68), and a wide operating range. It also demonstrates excellent cyclic compression stability and fatigue resistance, recovering its initial shape after 100 cycles at 70% compressive strain. With its ultra-low density, it can be used in flexible electronic devices, smart sensors, and health monitoring. Attached Figure Description

[0033] Figure 1The reaction process of the porous hydrogel prepared in Example 3 (Ion-dipole interaction; Ion-ion interaction; Hydrogen bonding; Ionic coordination).

[0034] Figure 2 This is a schematic diagram of the preparation process of the porous hydrogel prepared in Example 3 (Borax; cations; anions; Sonication; Freeze-thaw-cycle; Transfer to mold; Foaming; Salting out).

[0035] Figure 3 The infrared analysis results (transmittance; wavenumber) of LM dispersions prepared by mixing different masses of LM with TOCNF in Examples 1-5 are shown.

[0036] Figure 4 These are scanning electron microscope (SEM) images of the porous hydrogel prepared in Example 3 at different stages.

[0037] Figure 5 (a) Morphological changes of the hydrogel prepared in Example 3 with and without salt precipitation under 90% compressive strain; (b) Lightweight properties; (c) Schematic diagram of the hydrogel bearing a 10 kg load (Compress).

[0038] Figure 6 The stress-strain curves (tensile stress and tensile strain) of the hydrogel materials prepared in Examples 1 to 5 are shown.

[0039] Figure 7 The hydrophobic compressive strength and Young's modulus (YM) of aqueous solutions of LM, PVA and sodium citrate with different contents prepared in Examples 1 to 5 are given.

[0040] Figure 8The cyclic compressive stress-strain curves (Compress stress; Compressive strain) for the porous hydrogel prepared in Example 3 were recorded at 30%, 50%, and 70% compressive strain after 500, 200, and 100 loading-unloading cycles, respectively.

[0041] Figure 9 Example 3 shows the conductivity of hydrogels containing different amounts of LM, PVA, and Na3Cit.

[0042] Figure 10 The change in relative resistance of the hydrogel prepared in Example 3 during pressure is shown.

[0043] Figure 11 The changes in relative resistance of the hydrogel prepared in Example 3 under different compression rates are shown.

[0044] Figure 12 The changes in the relative resistance of the hydrogel under stress conditions of 1%, 3%, 5%, and 7% are shown in Example 3; the changes in the relative resistance of the hydrogel under stress conditions of 10%, 30%, 50%, and 70% are shown in Example 3; and the changes in the relative resistance of the hydrogel during 500 compression cycles at a fixed stress of 30% (x-axis: time in seconds). Detailed Implementation

[0045] To better understand the present invention, the following embodiments further illustrate the content of the present invention, but the content of the present invention is not limited to the following embodiments.

[0046] In all cases, unless otherwise specified, the operation was carried out at room temperature (15~25℃); unless otherwise specified, the stirring speed was 200 rpm; the molds used in all cases were cylindrical with a diameter of 40 mm and a height of 40 mm.

[0047] Example 1

[0048] A method for preparing porous hydrogels based on the salting-out effect includes the following steps:

[0049] 2.24 g of liquid metal LM (composed of 75 wt% gallium and 25 wt% indium, purity 99.99%, purchased from Sichuan High Purity Materials Technology Co., Ltd.) and 18 g of oxidized nanocellulose (TOCNF, 1.26 wt%, purchased from Tianjin Wood Elf Biotechnology Co., Ltd.) were mixed and ultrasonically treated for 60 min in an ice-water bath using an ultrasonic cell disruptor in pulse mode (running cycle 1 s, intermittent cycle 2 s, ultrasonic power 600 W) to obtain an LM dispersion. Subsequently, PVA aqueous solutions prepared by mixing 11.18 g, 13.18 g, 15.46 g, and 17.82 g of polyvinyl alcohol (PVA, type 1799, degree of alcoholysis 98% ~ 99%, purchased from Shanghai Aladdin Biochemical Technology Co., Ltd.) with 48 g of deionized water were added, and the mixture was mechanically stirred at 95 ℃ for 2 h to obtain a viscous liquid (hydrogel). Add 0.027 g of borax and 8 g of water to a mixture and knead thoroughly. After the system cools to room temperature, pour it into molds (each mold contains 5 g of sample). Then, freeze the mixture in a -70 ℃ cryogenic reactor for 12 h, thaw it at room temperature, and repeat the cycle three times. Next, transfer the mixture to a constant temperature drying oven at 37 ℃ and 60% RH for limited foaming for 12 h. Finally, completely immerse the resulting hydrogels in sodium citrate solutions (Na3Cit, analytical grade, 98%, purchased from Shanghai Aladdin Biochemical Technology Co., Ltd.) with mass fractions of 0 wt%, 15 wt%, 20 wt%, 25 wt%, and 30 wt% for 24 h to obtain porous hydrogels.

[0050] Example 2

[0051] 4.60 g of liquid metal LM (composed of 75 wt% gallium and 25 wt% indium, purity 99.99%, purchased from Sichuan High Purity Materials Technology Co., Ltd.) and 18 g of TOCNF oxidized nanocellulose (TOCNF, 1.26 wt%, purchased from Tianjin Wood Elf Biotechnology Co., Ltd.) were mixed and ultrasonically treated for 60 min in an ice-water bath using an ultrasonic cell disruptor in pulse mode (running cycle 1 s, intermittent cycle 2 s, ultrasonic power 600 W) to obtain an LM dispersion. Subsequently, PVA aqueous solutions prepared by mixing 11.18 g, 13.18 g, 15.46 g, and 17.82 g of polyvinyl alcohol (PVA, type 1799, degree of alcoholysis 98% ~ 99%, purchased from Shanghai Aladdin Biochemical Technology Co., Ltd.) with 48 g of deionized water were added, and the mixture was mechanically stirred at 95 ℃ for 2 h to obtain a viscous liquid (hydrogel). Add 0.027 g of borax and 8 g of water to a mixture and knead thoroughly. After the system cools to room temperature, pour it into molds (each mold contains 5 g of sample). Freeze the mixture in a -70 ℃ cryogenic reactor for 12 h, then thaw it at room temperature. Repeat this freezing and thawing cycle three times. Then, transfer it to a constant temperature drying oven at 37 ℃ and 60% RH for limited foaming for 12 h. Finally, completely immerse the resulting hydrogel in sodium citrate solutions (Na3Cit, analytical grade, 98%) with mass fractions of 0 wt%, 15 wt%, 20 wt%, 25 wt%, and 30 wt% (purchased from Shanghai Aladdin Biochemical Technology Co., Ltd.) for 24 h to obtain porous hydrogels.

[0052] Example 3

[0053] 7.09 g of liquid metal LM (composed of 75 wt% gallium and 25 wt% indium, purity 99.99%, purchased from Sichuan High Purity Materials Technology Co., Ltd.) and 18 g of TOCNF oxidized nanocellulose (TOCNF, 1.26 wt%, purchased from Tianjin Wood Elf Biotechnology Co., Ltd.) were mixed and ultrasonically treated for 60 min in an ice-water bath using an ultrasonic cell disruptor in pulse mode (running cycle 1 s, intermittent cycle 2 s, ultrasonic power 600 W) to obtain an LM dispersion. Subsequently, PVA aqueous solutions prepared by mixing 11.18 g, 13.18 g, 15.46 g, and 17.82 g of polyvinyl alcohol (PVA, type 1799, degree of alcoholysis 98% ~ 99%, purchased from Shanghai Aladdin Biochemical Technology Co., Ltd.) with 48 g of deionized water were added, and the mixture was mechanically stirred at 95 ℃ for 2 h to obtain a viscous liquid (hydrogel). Add 0.027 g of borax and 8 g of water to a mixture and knead thoroughly. After the system cools to room temperature, pour it into molds (each mold contains 5 g of sample). Freeze the mixture in a -70 ℃ cryogenic reactor for 12 h, then thaw it at room temperature. Repeat this freezing and thawing cycle three times. Then, transfer the mixture to a constant temperature drying oven at 37 ℃ and 60% RH for limited foaming for 12 h. Finally, completely immerse the resulting hydrogels in sodium citrate solutions (Na3Cit, analytical grade, 98%, purchased from Shanghai Aladdin Biochemical Technology Co., Ltd.) with mass fractions of 0 wt%, 15 wt%, 20 wt%, 25 wt%, and 30 wt% for 24 h to obtain porous hydrogels.

[0054] Example 4

[0055] 9.72 g of liquid metal LM (composed of 75 wt% gallium and 25 wt% indium, purity 99.99%, purchased from Sichuan High Purity Materials Technology Co., Ltd.) and 18 g of TOCNF oxide nanoparticles (TOCNF, 1.26 wt%, purchased from Tianjin Wood Elf Biotechnology Co., Ltd.) were mixed and ultrasonically treated for 60 min in an ice-water bath using an ultrasonic cell disruptor in pulse mode (running cycle 1 s, intermittent cycle 2 s, ultrasonic power 600 W) to obtain an LM dispersion. Subsequently, PVA aqueous solutions prepared by mixing 11.18 g, 13.18 g, 15.46 g, and 17.82 g of polyvinyl alcohol (PVA, type 1799, degree of alcoholysis 98% ~ 99%, purchased from Shanghai Aladdin Biochemical Technology Co., Ltd.) with 48 g of deionized water were added, and the mixture was mechanically stirred at 95 ℃ for 2 h to obtain a viscous liquid (hydrogel). Add 0.027 g of borax and 8 g of water to a mixture and knead thoroughly. After the system cools to room temperature, pour it into molds (each mold contains 5 g of sample). Freeze the mixture in a -70 ℃ cryogenic reactor for 12 h, then thaw it at room temperature. Repeat this freezing and thawing cycle three times. Then, transfer it to a constant temperature drying oven at 37 ℃ and 60% RH for limited foaming for 12 h. Finally, completely immerse the resulting hydrogels in sodium citrate solutions (Na3Cit, analytical grade, 98%, purchased from Shanghai Aladdin Biochemical Technology Co., Ltd.) with mass fractions of 0 wt%, 15 wt%, 20 wt%, 25 wt%, and 30 wt% for 24 h to obtain porous hydrogels.

[0056] Example 5

[0057] 12.49 g of liquid metal LM (composed of 75 wt% gallium and 25 wt% indium, purity 99.99%, purchased from Sichuan High Purity Materials Technology Co., Ltd.) and 18 g of TOCNF oxidized nanocellulose (TOCNF, 1.26 wt%, purchased from Tianjin Wood Elf Biotechnology Co., Ltd.) were mixed and ultrasonically treated for 60 min in an ice-water bath using an ultrasonic cell disruptor in pulse mode (running cycle 1 s, intermittent cycle 2 s, ultrasonic power 600 W) to obtain an LM dispersion. PVA aqueous solutions prepared by mixing 11.18 g, 13.18 g, 15.46 g, and 17.82 g of polyvinyl alcohol (PVA, type 1799, degree of alcoholysis 98% ~ 99%, purchased from Shanghai Aladdin Biochemical Technology Co., Ltd.) with 48 g of deionized water were added, and the mixture was mechanically stirred at 95 ℃ for 2 h to obtain a viscous liquid (hydrogel). Add 0.027 g of borax and 8 g of water to a mixture and knead thoroughly. After the system cools to room temperature, pour it into molds (each mold contains 5 g of sample). Freeze the mixture in a -70 ℃ cryogenic reactor for 12 h, then thaw it at room temperature. Repeat this freezing and thawing cycle three times. Then, transfer it to a constant temperature drying oven at 37 ℃ and 60% RH for limited foaming for 12 h. Finally, completely immerse the resulting hydrogels in sodium citrate solutions (Na3Cit, analytical grade, 98%, purchased from Shanghai Aladdin Biochemical Technology Co., Ltd.) with mass fractions of 0 wt%, 15 wt%, 20 wt%, 25 wt%, and 30 wt% for 24 h to obtain porous hydrogels.

[0058] Figure 3 Infrared spectroscopy results are shown for LM dispersions prepared by mixing different masses of LM and TOCNF during the hydrogel preparation process in Examples 1-5. The interaction between LM and TOCNF was investigated in the figures; as the LM content increased, the concentration at 1652 cm⁻¹ increased. -1 The C=O stretching vibration peak at 1329 cm⁻¹ and the peak at 1329 cm� -1 The intensities of the COO-H bending vibration peaks at various locations all show a synchronous decreasing trend, while the OH stretching vibration peak near 3300 cm⁻¹ exhibits a significant redshift, indicating that more hydrogen bonds and coordination bonds have formed between LM and TOCNF. In the figure, 0 indicates oxidized cellulose nanoparticles without the addition of liquid metal LM.

[0059] Figure 4The scanning electron microscope (SEM) images of the hydrogel prepared in Example 3 (LM content 7.5 wt%, PVA content 16 wt%, sodium citrate concentration 20 wt%) are shown. Image 4(a) shows the initial morphology of the porous hydrogel without any treatment (after adding borax and kneading, without freeze-thaw cycles). It exhibits numerous and uniformly distributed pores, but with a small average pore size, presenting an overall dense porous structure. After entering the limited foaming stage, the internal pore structure of the hydrogel undergoes significant changes. This process is accompanied by gas release, a significant increase in pore size, thinning of local pore walls, and a tendency towards irregular pore morphology. Figure 4 (b)). From Figure 4 In (c), a large pore that is expanding can be clearly observed, with several raised small pores distributed around it; among them, (b) and (c) are taken during the foaming process, with (b) being a macroscopic view and (c) being a local view. Figure 4 (d) illustrates the microstructure of the hydrogel after salting-out treatment. Salting-out not only effectively locks in the macroporous structure generated during the foaming stage, preventing shrinkage or collapse, but also promotes the formation of new pores, ultimately resulting in a denser honeycomb network structure. Note: In this application, the LM content refers to the mass content of LM relative to the hydrogel after limited foaming and before immersion in sodium citrate solution; the PVA content refers to the mass content of PVA relative to the hydrogel after limited foaming and before immersion in sodium citrate solution.

[0060] Figure 5 The hydrogel prepared in Example 3 (LM content 7.5wt%, PVA content 16wt%, sodium citrate concentration 20wt%): (a) morphological changes of the hydrogel with and without salt precipitation under 90% compressive strain; (b) lightweight properties; (c) schematic diagram of the hydrogel bearing a 10 kg load. Figure 5 As shown in (a). Figure 5 (a1) is before compression. Figure 5 (a2) The left image shows the LMCNF porous hydrogel after salting out. Under 90% compression deformation, its recovery rate exceeds 95%, and the recovery process is very rapid, reaching maximum recovery within approximately 10 seconds. Figure 5 (a2) The right image shows a stark contrast to the untreated hydrogel. The honeycomb-like porous structure formed inside the hydrogel significantly increases the material's volume, resulting in a higher specific surface area and lower density (0.029 g / cm³) while maintaining its toughness. 3 This achieves lightweighting. Figure 5 (b)). Figure 5 (c) It shows that the LMCNF porous hydrogel (a cylinder with a diameter of 3 cm and a height of 4 cm) remained intact and did not break when subjected to a 10 kg load.

[0061] Figure 6 The stress-strain curves of the hydrogel materials prepared in Examples 1-5 are shown. With increasing LM content (PVA content 16 wt%, sodium citrate solution concentration 20 wt%), the elongation at break of the hydrogel first increases and then decreases, with the material using 10 wt% LM achieving an elongation at break of 225.88%. The effect of PVA content variation on the mechanical strength of the hydrogel was further investigated under conditions of 7.5 wt% LM content and 20 wt% sodium citrate concentration. Figure 6 As shown in (b), with the increase of PVA mass fraction, the tensile strength first increases and then decreases, increasing from 346.73 kPa to 1009.47 kPa, and achieving a maximum elongation at break of 266.06%. At this point, the PVA content is 16 wt%. Further increases in PVA mass fraction lead to a decrease in both tensile strength and elongation at break. The inventors believe this is mainly due to the fact that the increase in PVA content significantly increases the crosslinking density of the hydrogel network, making the network structure more rigid and thus suppressing the flexibility and deformation capacity of the chain segments. Figure 6 As shown in (c) (using 7.5 wt% LM and 16 wt% PVA), the tensile strength of the hydrogel initially increases and then decreases with increasing salt concentration. At a salt concentration of 20%, the maximum tensile strength is approximately 6.3 times higher than that of the unsalted hydrogel. The inventors believe this is because salting-out weakens the hydration layer around the polymer chains, causing the chain segments to approach and aggregate, leading to enhanced inter-chain interactions. This process makes the hydrogel's network structure more compact and stable, thus significantly improving its strength and toughness.

[0062] Figure 7 The compressive strength and Young's modulus of hydrogels prepared in Examples 1-5 with different contents of LM, PVA, and sodium citrate solution concentration are shown. When the LM content is 7.5 wt%, the PVA content is 16 wt%, and the sodium citrate solution concentration is 20 wt%, the hydrogel exhibits the highest compressive strength of 20.72 MPa (95% strain) and modulus of 51.42 kPa. Figure 7 (a) and (d)). However, further increasing the LM content resulted in a decrease in the compressive properties of the hydrogel. The inventors believe this is mainly because increasing the LM content accelerates the limited foaming process, leading to a significant increase in pore size, thinning of pore walls, a decrease in the relative density of the porous skeleton, and a reduction in the effective load-bearing cross-sectional area. This makes the material more prone to buckling and collapse under pressure, resulting in stress concentration, thus limiting further improvements in compressive strength and modulus. Figure 7As shown in d, when the PVA content is 16%, the LM content is 7.5 wt%, and the sodium citrate solution concentration is 20 wt%, the compressive modulus reaches a peak value of 67.8 kPa. Figure 7 e). Figure 7 Figures c and f show the compressibility of the hydrogel under different salt solution concentrations. As can be seen from the figure, the compressive modulus is positively correlated with the salt solution concentration. When the PVA content is 16%, the LM content is 7.5 wt%, and the sodium citrate solution concentration is 30%, the compressive modulus reaches a maximum of 136.27 kPa, which is about 5.9 times higher than that of the LMCNF porous hydrogel without salting out.

[0063] Figure 8 The porous hydrogel prepared in Example 3 was subjected to cyclic compressive stress-strain curves at 30%, 50%, and 70% compressive strains, with 500, 200, and 100 loading-unloading cycles recorded, respectively. When the LM content was 7.5 wt%, the PVA content was 16 wt%, and the sodium citrate solution concentration was 20 wt%, after 500 and 200 loading-unloading cycles at 30% and 50% low compressive strains, respectively, the maximum compressive stress of the porous hydrogel did not decrease but instead showed a gradual increasing trend. The inventors believe this is mainly due to the effect of water loss during cyclic loading. During repeated compression deformation, free water in the porous hydrogel network is gradually squeezed out, leading to an increase in solid content and network structure shrinkage. The changes in the hysteresis curve also support this mechanism; as the number of cycles increases, the hysteresis loop area gradually decreases, indicating reduced energy dissipation and a gradual increase in material rigidity. When the compressive strain increases to 70%, the maximum compressive stress of the hydrogel shows a continuous decreasing trend from the beginning of the initial cycle, eventually decaying to 85% of the initial value. Despite the stress decay to some extent, the compression cycle curve remains continuous and stable, without structural collapse or obvious permanent deformation, indicating that the material has good structural stability and fatigue resistance.

[0064] Figure 9 The conductivity of porous hydrogels with different contents of LM, PVA, and Na3Cit is shown. When the PVA content is 16 wt% and the sodium citrate solution concentration is 20 wt%, the conductivity of the hydrogel is positively correlated with the amount of LM added. Figure 9 a) The inventors believe this is primarily due to the higher LM content leading to a denser conductive network within the hydrogel and enhancing the quantum tunneling effect. Figure 9As shown in Figure b, when the LM content is 7.5 wt% and the sodium citrate solution concentration is 20 wt%, the conductivity of the hydrogel initially increases and then decreases with increasing PVA concentration. The inventors believe that this is because when the PVA content is low, the polymer network is relatively loose, which is not conducive to the formation of continuous conductive pathways. As the PVA content appropriately increases, the polymer network structure gradually improves, promoting the construction of conductive pathways and thus increasing the conductivity of the hydrogel. However, when the PVA content further increases, the overly dense polymer network restricts the effective contact between LM conductive particles and may also adversely affect ion migration, ultimately leading to a decrease in conductivity. Salting out can further affect the conductivity of porous hydrogels by changing the ion concentration and polymer structure in the system. When the content of LM was 7.5 wt% and the content of PVA was 16 wt%, the conductivity of the LMCNF porous hydrogel significantly increased after salting-out treatment, from 0.985 mS / cm before salting-out to 4.68 mS / cm after salting-out (using a sodium citrate solution concentration of 15 wt%). The inventors believe that this increase is mainly attributed to the large number of sodium ions introduced during the salting-out process, which significantly increased the ion concentration in the system. However, with further increases in the concentration of sodium citrate solution, the crosslinking density remained unchanged, the ion concentration reached saturation, and the conductivity remained constant. It is worth noting that at higher PVA concentrations, the dense polymer network may restrict ion migration, and the introduction of a porous structure helps to mitigate this effect. However, excessively high porosity may affect the stability of the internal structure of the hydrogel; therefore, it is necessary to appropriately optimize the porosity to balance mechanical and electrical properties and obtain the best overall performance. The LMCNF porous hydrogel prepared under optimal conditions had an electrical conductivity of approximately 8.39 mS / cm (LM content was 7.5 wt%, PVA content was 16 wt%, and sodium citrate solution concentration was 20 wt%), which was significantly higher than the electrical conductivity of similar structure (PVA-LMPs) hydrogels reported in the literature. When the sodium citrate solution concentration was 25 wt%, the electrical conductivity increased to approximately 9.36 mS / cm.

[0065] Figure 10The changes in relative resistivity of the hydrogel prepared with 7.5 wt% LM, 16 wt% PVA, and 20 wt% sodium citrate solution during compression are shown. In the strain range of 0–30%, the GF value of the hydrogel is 1.68, indicating high sensitivity at low strain, i.e., the rate of change of relative resistivity is greater than the rate of change of strain. However, in the strain range of 30%–60%, the GF value decreases to 0.73, indicating that the sensitivity of the hydrogel decreases with increasing compressive strain, i.e., the rate of change of relative resistivity is slightly lower than the rate of change of strain. When the compressive strain continues to increase to 60%–80%, the GF value of the hydrogel drops to 0.26, indicating that the material's resistance response is significantly weakened in the high strain range, and the rate of change of relative resistivity is significantly lower than the rate of change of strain. The change in resistance during compression is mainly due to the change in the internal conductive pathway structure. When external pressure is applied, the porous framework shrinks, the pore volume decreases, the spacing between the LM conductive fillers shortens, and the effective contact area increases, thereby improving the connectivity of the conductive pathways and causing changes in contact resistance. Simultaneously, the compression process alters the spatial distribution of water-containing ion transport channels within the hydrogel, thereby affecting charge migration behavior. Therefore, the resistance change is more significant in the low-strain stage, while in the high-strain stage, the conductive network gradually approaches saturation, and the resistance change becomes more gradual.

[0066] Figure 11 The changes in relative electrical resistance of the hydrogels prepared at different compression rates are shown when the content of LM is 7.5 wt%, the content of PVA is 16 wt%, and the concentration of sodium citrate solution is 20 wt%. Under the condition of applying a constant 50% compressive strain, the hydrogels exhibit a basically consistent resistance response and good durability.

[0067] Figure 12 The graph shows the changes in relative resistivity of the hydrogel under stress conditions of 1%, 3%, 5%, and 7% when the LM content is 7.5 wt%, the PVA content is 16 wt%, and the sodium citrate solution concentration is 20 wt%; the changes in relative resistivity of the hydrogel under stress conditions of 10%, 30%, 50%, and 70%; and the changes in relative resistivity of the hydrogel after 500 compression cycles at a fixed stress of 30% (x-axis: time in seconds). The porous hydrogel also exhibits a wide operating range, showing stable resistance changes not only at large strains (e.g., 10% – 70%). Figure 12 a) It also exhibits sensitive and repeatable response characteristics in lower strain ranges (e.g., 1%–7%). Figure 12(b) Finally, the LMCNF porous hydrogel was subjected to 500 cycles of loading-unloading tests at 30% compressive strain (12c). The results show that even after multiple cycles, the hydrogel maintains a stable signal-to-noise ratio and periodic electrochemical response, exhibiting excellent sensing performance.

[0068] Comparative Example 1

[0069] The difference from Example 3 is that LM was omitted. The PVA solution concentration was 16 wt%, and the sodium citrate solution concentration was 20 wt%. All other parameters were the same as in Example 3. Results: The resulting hydrogel could still form, but its electrical conductivity decreased significantly, by 12% compared to Example 3, its toughness decreased by 31.0%, and its tensile strength decreased by 22.7%.

[0070] Comparative Example 2

[0071] The difference from Example 3 is that TOCNF was omitted. The PVA solution concentration was 16 wt%, and the sodium citrate solution concentration was 20 wt%. All other parameters were the same as in Example 3. Results: The obtained samples failed to form a stable porous framework, exhibiting structural collapse and poor foaming, resulting in a significant decrease in mechanical properties. This may be related to the lack of nanofiber framework support, weakened interfacial interactions, and decreased network stability after omitting TOCNF.

[0072] Comparative Example 3

[0073] The difference from Example 3 is that the PVA solution was omitted. The sodium citrate solution concentration was 20 wt%. All other parameters were the same as in Example 3. Results: A stable porous structure failed to form, and foaming ultimately failed. This may be because omitting PVA significantly reduced the viscoelasticity and structural support of the precursor, and the borax lacked a key cross-linking agent, making it difficult to establish a stable network and hindering bubble formation and retention.

[0074] Comparative Example 4

[0075] Unlike Example 3, borax was omitted. The PVA solution concentration was 16 wt%, and the sodium citrate solution concentration was 20 wt%. All other parameters were the same as in Example 3. Results: The system's adhesion and plasticity decreased significantly, the kneading and molding process was significantly limited, the resulting green body had poor support, and subsequent foaming was ineffective, with a 45% increase in density. This may be because the system struggles to form an effective dynamic cross-linking network, and the pre-cross-linking degree of the precursor is insufficient, which is detrimental to green body forming and pore structure stability.

Claims

1. An ultralight porous hydrogel based on the salting-out effect, characterized in that: Its structural formula is: 。 2. The ultralight porous hydrogel based on the salting-out effect according to claim 1, characterized in that: Its wet density is ≤0.029 g / cm³ 3 The porous hydrogel exhibits a sensitive, stable and reversible resistance response during compression, with a maximum GF of 1.

68.

3. A method for preparing an ultralight porous hydrogel based on the salting-out effect, characterized in that: An ultra-lightweight porous hydrogel with a three-dimensional porous network structure was prepared by thoroughly mixing gallium-based liquid metal, TEMPO oxidized cellulose nanofibers, polyvinyl alcohol, and borax, followed by freeze-thaw cycles, limited foaming, and salting-out treatment.

4. The preparation method according to claim 3, characterized in that: After limited foaming and before salting out, the mass content of gallium-based liquid metal is 2.5–12.5 wt%.

5. The preparation method according to claim 3 or 4, characterized in that: After limited foaming and before salting out, the mass content of PVA is 12-18 wt%.

6. The preparation method according to claim 3 or 4, characterized in that: Salting out involves soaking the sample in a 10-30% sodium citrate solution for 24-48 hours.

7. The preparation method according to claim 3 or 4, characterized in that: Includes the following steps: 1) Gallium-based liquid metal and TEMPO oxidized cellulose nanofibers were ultrasonically dispersed in water under ice-water bath conditions to obtain an LM dispersion; 2) Dissolve polyvinyl alcohol in water and add it to the LM dispersion obtained in step 1). Stir the mixture evenly at a temperature of 70~100℃ to obtain a viscous liquid. 3) Add a mixture of borax and water to the viscous liquid obtained in step 2), knead thoroughly at a temperature of 70~100℃, cool to room temperature, perform a freeze-thaw cycle, then perform limited foaming for 10~14h, and finally soak in a 10~30% sodium citrate aqueous solution for 12~24h to obtain an ultra-lightweight porous hydrogel.

8. The preparation method according to claim 3 or 4, characterized in that: In step 1), the mass ratio of gallium-based liquid metal to TEMPO oxidized cellulose nanofibers is (0.1~0.8):

1.

9. The preparation method according to claim 3 or 4, characterized in that: In steps 1) and 2), the mass ratio of gallium-based liquid metal to polyvinyl alcohol is (0.12~1.12):1; in steps 1) and 2), the mass ratio of gallium-based liquid metal to borax is (80~465):

1.

10. The preparation method according to claim 3 or 4, characterized in that: In step 3), the freeze-thaw cycle is as follows: freeze in a low-temperature reactor at -70±20℃ for 10-14h, then thaw at room temperature, repeat the cycle 2-5 times, and then transfer to a constant temperature drying oven for limited foaming for 10-14h to obtain hydrogel; In step 3), the conditions for limited foaming are: foaming time of 10~14h in a constant temperature drying oven at 37℃ and 60% RH.

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