A dynamic hydrogen bond enhanced double-layer hydrogel and an interfacial toughening preparation method thereof

Abstract

The application provides a dynamic hydrogen bond enhanced double-layer hydrogel and an interfacial toughening preparation method thereof, and belongs to the technical field of hydrogel material preparation. The application comprises the following steps: placing dried microcrystalline cellulose in a solution composed of LiOH and urea to obtain a transparent microcrystalline cellulose solution; then adding a chemical crosslinking agent and stirring uniformly to obtain a CNF solution; mixing the CNF solution with a polymer precursor solution at equal proportions, stirring and placing in a mold, and performing light curing to obtain a single-layer hydrogel; then immersing the single-layer hydrogel in a zirconium ion solution for different times to obtain a base layer and a crack layer; preparing an interfacial layer above the crack layer; finally placing the base layer above the interfacial layer and performing light curing again to obtain the dynamic hydrogen bond enhanced double-layer hydrogel. The application constructs a dynamic hydrogen bond network through cellulose nanofibers, and realizes the synergistic enhancement of crack tip stress relaxation and energy dissipation through gradient design of the interfacial layer, thereby providing a new path for the design of high-performance flexible materials.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of hydrogel material preparation, and particularly relates to a dynamic hydrogen bond enhanced double-layer hydrogel and an interfacial toughening preparation method thereof. BACKGROUND

[0002] As an important class of soft materials, hydrogels exhibit a wide range of applications in biomedical and flexible electronics due to their high water content, excellent biocompatibility and tunable mechanical properties. For example, in the biomedical field, hydrogels can provide a growth microenvironment for cells as tissue engineering scaffolds, or achieve controlled drug release as intelligent carriers. In the field of flexible electronics, they can be used to prepare wearable sensors to sense human motion signals, or as flexible joint materials for bionic robots to achieve complex deformation. However, the low modulus of hydrogels makes them prone to crack initiation and propagation under dynamic loads such as stretching and bending, resulting in insufficient fracture toughness and severely restricting their application in long-term service scenarios.

[0003] Currently, existing methods for toughening hydrogels mainly include two categories: the first category is to introduce rigid fillers (such as nanoparticles) or construct a double network structure to disperse crack tip stress or increase energy dissipation during fracture. For example, existing technologies use a double network hydrogel to dissipate energy through covalent bond rupture, but they rely on irreversible static energy dissipation mechanisms that gradually decay under dynamic loads, making it difficult to meet the requirements of repeated deformation. The second category is to enhance interfacial adhesion through multi-layer structure design to inhibit crack propagation along the interface. For example, existing technologies prepare double-layer hydrogels through layer-by-layer polymerization to try to enhance toughness through interfacial synergy, but the interfacial adhesion strength between heterogeneous materials is insufficient or the elastic modulus is mismatched, resulting in widespread interfacial debonding and limited interfacial synergistic toughening effect.

[0004] In addition, existing technologies also use hydrogen bond interactions to improve interfacial performance, which can enhance interfacial adhesion through hydrogen bond host-guest interactions or cation-π interactions, but rely on a single hydrogen bond type or static bonding structure, and cannot form a dynamic and reversible hydrogen bond network, making it difficult to achieve continuous energy dissipation during crack propagation. At the same time, the internal relationship between the content of cellulose nanofiber (CNF), water content, thickness of the interfacial layer, and interfacial adhesion energy, energy release rate has not been clearly defined, resulting in a lack of theoretical guidance for the quantitative control of interfacial toughening effect.

[0005] According to the above, it is illustrated that the method for toughening hydrogel in the prior art has the following problems: a) the heterogeneous interface of the multi-layer structure is easy to become the path for the rapid expansion of the crack due to the insufficient adhesion strength or the unreasonable modulus gradient, resulting in the invalidation of the interlayer synergistic effect and the inability to effectively inhibit the crack initiation and expansion. b) the traditional static energy dissipation mechanism (such as covalent bond rupture) is irreversible and cannot continuously dissipate energy under cyclic loading, and the material is easy to fail due to fatigue, which is difficult to meet the demand of complex mechanical environment in actual application.

[0006] In summary, the prior art has not realized the synergistic optimization of the interface adhesion energy improvement and the dynamic energy dissipation, and there is an urgent need for a hydrogel interface toughening preparation method that can consider high toughness, high interface stability and dynamic load adaptability. SUMMARY

[0007] In order to overcome the shortcomings of the prior art, the purpose of the present application is to provide a dynamic hydrogen bond enhanced double-layer hydrogel and an interface toughening preparation method thereof, which realizes the synergistic enhancement of crack tip stress relaxation and energy dissipation by constructing a dynamic hydrogen bond network with cellulose nanofiber and combining with interface layer gradient design, provides a new path for the design of high-performance flexible materials, and promotes the application value in the fields of biomedical and flexible electronics.

[0008] To achieve the above purpose, the present application provides the following scheme:

[0009] An interface toughening preparation method of a dynamic hydrogen bond enhanced double-layer hydrogel, comprising the following steps:

[0010] S1, placing dried microcrystalline cellulose in a solution composed of LiOH and urea, stirring uniformly, and then treating in a low-temperature environment overnight for static placement to make the microcrystalline cellulose completely dissolved, and then stirring again to obtain a transparent microcrystalline cellulose solution;

[0011] S2, adding a chemical crosslinking agent to the microcrystalline cellulose solution to obtain a CNF solution after uniform stirring;

[0012] S3, mixing the CNF solution and a polymer precursor solution in equal proportions to obtain a mixed solution, stirring uniformly, and then placing in a mold to obtain a single-layer hydrogel through photocuring;

[0013] S4, immersing the single-layer hydrogel in a zirconium ion solution for different times to obtain a base layer and a crack layer, and introducing a pre-crack in the crack layer;

[0014] S5, dropping the mixed solution obtained by mixing the CNF solution and the polymer precursor solution on the surface of the crack layer to form an interface layer, and placing the base layer above the interface layer, and then photocuring again to obtain a dynamic hydrogen bond enhanced double-layer hydrogel.

[0015] Preferably, in S1, the dry condition of the microcrystalline cellulose is drying in an environment at 60℃ for 24h to remove the water in the microcrystalline cellulose.

[0016] Preferably, in S1, the solution consisting of LiOH and urea contains LiOH and urea with a mass ratio of 4.6:15, and the solvent is deionized water; the low-temperature environment is-15℃.

[0017] Preferably, in S2, the chemical cross-linking agent is epichlorohydrin, and the mass ratio of the microcrystalline cellulose in the microcrystalline cellulose solution to epichlorohydrin is 2:1.

[0018] Preferably, in S3, the polymerization precursor solution contains acrylamide AAM, acrylic acid AAc, cross-linking agent N,N-methylene bisacrylamide MBAA, and photoinitiator phenyl(2,4,6-trimethylbenzoyl).

[0019] Preferably, in S3, the light curing condition is irradiation for 30 minutes under an environment at 25℃ using a 30W, 365nm ultraviolet lamp; and the single-layer hydrogel obtained by light curing is a CNF-PAM / AA hydrogel.

[0020] Preferably, in S4, the zirconium ion solution is a 0.5mol / L zirconyl chloride octahydrate solution, the soaking time of the base layer is 24 hours, and the soaking time of the crack layer is 12 hours; the pre-crack has a length of 20mm and is formed by vertically cutting the middle position of the crack layer with a blade.

[0021] Preferably, in S5, the thickness of the interface layer is controlled by a mold, and the thickness of the mold is 0.1mm-1mm; and the re-light curing condition is irradiation for 30 minutes under an environment at 25℃ using a 30W, 365nm ultraviolet lamp.

[0022] The application further provides a dynamic hydrogen bond enhanced double-layer hydrogel prepared by the interface toughening preparation method of the dynamic hydrogen bond enhanced double-layer hydrogel.

[0023] The application further provides an application of the dynamic hydrogen bond enhanced double-layer hydrogel in the biomedical and flexible electronic fields.

[0024] According to the specific embodiments of the application, the following technical effects are achieved.

[0025] (1) The present application realizes the significant improvement of the interfacial adhesion performance by introducing cellulose nanofiber (CNF) to construct a dynamic hydrogen bond network and combining with the interfacial layer gradient design. Specifically, under the condition of 68.63% water content, the interfacial adhesion energy of the intermediate layer containing 0.63% CNF reaches 307J / m 2 , which is 11.9 times higher than that of the system without CNF. The mechanism is that the hydroxyl groups of CNF form a dense hydrogen bond network with the carboxyl groups of the polymer chain, and the mechanical interlocking is enhanced through physical entanglement, solving the problem of interfacial debonding of traditional multilayer hydrogel, and providing protection for the long-term reliability of flexible electronic devices.

[0026] (2) The present application realizes the significant enhancement of crack propagation resistance by optimizing the modulus gradient and the synergistic effect of dynamic hydrogen bond. Based on the experimental surface, the crack propagation driving force is reduced by 29.7%, and the energy release rate is reduced to 4.3% of the system without interfacial layer. The main mechanism is that the CNF hydrogen bond network dissipates energy through dynamic fracture-recombination during the stretching process, and the modulus gradient of the substrate layer and the cracked layer disperses the stress at the crack tip, effectively inhibiting the unstable propagation of the crack, simulating the interfacial effect of biological nacre.

[0027] (3) The present application realizes the controllability of interfacial energy release behavior by regulating the threshold effect of water content and intermediate layer thickness. When the water content exceeds 64.33%, the interfacial transverse debonding energy release rate coefficient decreases from 4.2 to 0.477, triggering the reconstruction of the hydrogen bond network and promoting the transition of the failure mode from brittle debonding to ductile bridging. By adjusting the thickness of the intermediate layer and the content of CNF, the difference in mechanical requirements of biological tissue engineering scaffolds and wearable sensors can be matched. BRIEF DESCRIPTION OF DRAWINGS

[0028] In order to more clearly illustrate the technical solutions of the embodiments of the present application or the prior art, the following will briefly introduce the drawings needed in the embodiments. Obviously, the drawings described below are only some embodiments of the present application, and other drawings can also be obtained by those skilled in the art without creative labor.

[0029] Figure 1 The present application provides hydrogel SEM images under different scales; wherein, Figure 1 (a) of (a) is the SEM image of PAM / AA hydrogel under 5.0μm, Figure 1 (b) of (b) is the SEM image of PAM / AA hydrogel under 2.0μm, Figure 1 (c) of (c) is the SEM image of CNF-PAM / AA hydrogel under 5.0μm, Figure 1 (d) of (d) is the SEM image of CNF-PAM / AA hydrogel under 2.0μm;

[0030] Figure 2 The comparison chart of the tensile property results of the CNF-PAM / AA hydrogel with different soaking time and different CNF content provided by the present application is shown in the following figure:

[0031] Figure 3 The comparison chart of the tensile property results of the hydrogel provided by the present application is shown in the following figure: Figure 3 (a) in the figure is the comparison chart of the tensile property results of the PAM / AA hydrogel, Figure 3 (b) in the figure is the comparison chart of the tensile property results of the CNF-PAM / AA hydrogel;

[0032] Figure 4 The test result chart of the adhesion property of the hydrogel to glass provided by the present application is shown in the following figure: Figure 4 (a) in the figure is the test result of the adhesion property of the PAM / AA hydrogel with different water content to glass, Figure 4 (b) in the figure is the test result of the adhesion property of the CNF-PAM / AA hydrogel with different water content to glass;

[0033] Figure 5 The data chart of the pure shear property results of the double-layer hydrogel provided by the present application is shown in the following figure: Figure 5 (a) in the figure is the schematic diagram of the internal network structure of the CNF-PAM / AA hydrogel layer, Figure 5 (b) in the figure is the tensile property result chart of the CNF-PAM / AA hydrogel with different interface layer thickness, Figure 5 (c) in the figure is the pure shear experiment result of the double-layer CNF-PAM / AA hydrogel with or without interface layer, Figure 5 (d) in the figure is the schematic diagram of the crack propagation process of the double-layer CNF-PAM / AA hydrogel, Figure 5 (e) in the figure is the schematic diagram of the dynamic hydrogen bond-mediated interface layer gradient connection mechanism;

[0034] Figure 6 The data chart of the peeling property results of the double-layer hydrogel provided by the present application is shown in the following figure: Figure 6 (a) in the figure is the adhesion property result chart of the PAM / AA hydrogel, Figure 6 (b) in the figure is the adhesion property result chart of the CNF-PAM / AA hydrogel, Figure 6 (c) in the figure is the infrared test spectrum chart of the interface layer of the PAM / AA hydrogel, Figure 6 (d) in the figure is the infrared test spectrum chart of the interface layer of the CNF-PAM / AA hydrogel;

[0035] Figure 7 The interface transverse debonding energy release rate of the double-layer CNF-PAM / AA hydrogel provided by the present application is shown in the following figure:

[0036] Figure 8The energy release rate fitting curve provided by the present application; wherein, Figure 8 (a) in the CNF-PAM / AA hydrogel without interface layer is an energy release rate fitting curve, Figure 8 (b) in the CNF-PAM / AA hydrogel containing interface layer is an energy release rate fitting curve, Figure 8 (c) in the interface transverse debonding is an energy release rate fitting curve. DETAILED DESCRIPTION

[0037] The technical solutions in the embodiments of the present application will be clearly and completely described below with reference to the drawings in the embodiments of the present application. Obviously, the described embodiments are only part of the embodiments of the present application, rather than all the embodiments. Based on the embodiments in the present application, all other embodiments obtained by those skilled in the art without creative labor fall within the scope of the present application.

[0038] In order to make the above-mentioned purposes, features and advantages of the present application more obvious and easy to understand, the present application will be further described in detail below with reference to the drawings and specific embodiments.

[0039] The source of the experimental material used in the present application: acrylamide (AAM), acrylic acid (AAc), N,N methylene bisacrylamide (MBAA), microcrystalline cellulose (MCC), zirconyl chloride octahydrate (0.5 mol / L, Aladdin) are purchased from Shanghai Aladdin Biochemical Technology Co., Ltd. The particle size of MCC is about 147 µm. Phenyl (2,4,6-trimethylbenzoyl), lithium hydroxide (LiOH), urea are purchased from Beijing Honghu Joint Chemical Products Co., Ltd. All reagents used in this experiment are used directly without further purification. If not specified, other raw materials are conventional commercially available products in the art.

[0040] Example 1

[0041] The present embodiment provides an interfacial toughening preparation method of dynamic hydrogen bond enhanced double-layer hydrogel, comprising the following steps:

[0042] S1, dry microcrystalline cellulose is placed in a solution composed of LiOH and urea, stirred uniformly, and then treated with overnight standing in a low temperature environment to make the microcrystalline cellulose completely dissolved, and then stirred again to obtain a transparent microcrystalline cellulose solution;

[0043] S2, a chemical crosslinking agent is added to the microcrystalline cellulose solution, and stirred uniformly to obtain a CNF solution;

[0044] S3, mixing the CNF solution and the polymer precursor solution in equal proportions to obtain a mixed solution, stirring uniformly and then placing in a mold, and then obtaining a single-layer hydrogel by photocuring;

[0045] S4, immersing the single-layer hydrogel in a zirconium ion solution for different times to obtain a substrate layer and a crack layer, and introducing a pre-crack in the crack layer;

[0046] S5, dropping the mixed solution obtained by mixing the CNF solution and the polymer precursor solution on the surface of the crack layer to form an interface layer, and placing the substrate layer above the interface layer, and then obtaining a dynamic hydrogen bond enhanced double-layer hydrogel by photocuring again.

[0047] Specifically, in S1, the drying conditions of the microcrystalline cellulose are: drying in an environment at 60℃ for 24h to remove the water in the microcrystalline cellulose. The solution composed of LiOH and urea contains LiOH and urea in a mass ratio of 4.6:15, and the solvent is deionized water; the low-temperature environment is-15℃. In S2, the chemical crosslinking agent is epichlorohydrin, and the mass ratio of microcrystalline cellulose to epichlorohydrin in the microcrystalline cellulose solution is 2:1. In S3, the polymer precursor solution contains acrylamide AAM, acrylic acid AAc, crosslinking agent N,N-methylene bisacrylamide MBAA, and photoinitiator phenyl(2,4,6-trimethylbenzoyl). In S3, the photocuring conditions are: using a 30W, 365nm ultraviolet lamp, irradiating for 30 minutes in an environment at 25℃; the single-layer hydrogel obtained by photocuring is a CNF-PAM / AA hydrogel. In S4, the zirconium ion solution is a 0.5mol / L zirconium oxychloride octahydrate solution, the immersion time of the substrate layer is 24 hours, and the immersion time of the crack layer is 12 hours; the length of the pre-crack is 20mm, which is formed by vertically cutting the middle position of the crack layer with a blade. In S5, the thickness of the interface layer is controlled by a mold, and the thickness of the mold is 0.1mm~1mm; the re-photocuring conditions are: using a 30W, 365nm ultraviolet lamp, irradiating for 30 minutes in an environment at 25℃.

[0048] The dynamic hydrogen bond enhanced double-layer hydrogel includes a crack layer, an interface layer and a substrate layer, the interface layer is located between the crack layer and the substrate layer, and the cellulose nanofiber CNF in the interface layer forms a dynamic hydrogen bond network with the acrylic acid carboxyl in the crack layer and the substrate layer, achieving interface toughening.

[0049] The peel experiment and tensile test are used to analyze the crack propagation behavior by comparing the adhesion energy difference of the cellulose intermediate layer (thickness 0.1~1mm) with / without cellulose.

[0050] 1. Scanning electron microscope test

[0051] The conductive carbon glue was applied on the sample stage, and the sample was picked up with tweezers and stuck on the conductive carbon glue. After the sample stage with the sample was quickly frozen in liquid nitrogen snow for 30 seconds, it was transferred to the sample preparation chamber for sublimation gold plating treatment using a low-temperature cryogenic preparation transfer system in a vacuum state. After sublimation at -90°C for 10 minutes, the sample was sputtered with gold at a current of 10 mA for 60 seconds. Finally, the treated sample was observed and analyzed in terms of morphology by scanning electron microscopy (Japan Hitachi, Hitachi SU8010, PP3000T).

[0052] 2. Fourier Transform Infrared Absorption Spectroscopy Test

[0053] After the sample was dried and ground, it was uniformly mixed with potassium bromide (KBr) powder at a mass ratio of 1:100. The transparent thin sheet was prepared by the tabletting method. The prepared sample sheet was placed on the sample holder of the infrared spectrometer, ensuring that the sample surface was flat and free of bubbles or impurities. The instrument parameters were set, including 32 scans for the sample, 32 scans for the background, a resolution of 4.000 cm -1 -1, a moving mirror speed of 0.4747, and an aperture of 100.00. The test was performed using a Fourier transform infrared spectrometer (model: Thermo Scientific Nicolet iS50) from Thermo Fisher Scientific. The detector was DTGS KBr, the beam splitter was KBr, and the light source was infrared light. During the test, the instrument automatically recorded the infrared absorption spectrum of the sample, with a wave number range of 4000 to 400 cm -1 . By analyzing the position, intensity, and shape changes of the characteristic absorption peaks, the presence and strength changes of hydrogen bonds in the sample were determined.

[0054] 3. Mechanical Test

[0055] All mechanical tests were performed at room temperature using an IBTC-300 micro-in situ mechanical tester with a 500 N sensor for pure shear (PS) tests to measure the fracture properties of the hydrogel. Pure shear testing was originally proposed by Rivlin and Thomas for testing the fracture of rubber samples and has recently been used to characterize gel fracture. Both unnotched and notched samples were used, with the unnotched hydrogel having dimensions of 50 mm x 50 mm x 1 mm and being stuck between two grips for uniaxial tensile testing with a sample size of 50 mm x 10 mm x 1 mm (H = 10 mm). The notched sample was prepared by introducing a c = 20 mm crack in the middle of the sample using a knife. The tensile λ was defined as the current length divided by the initial length of the sample. The nominal stress was defined as the applied force divided by the cross-sectional area of the sample in the undeformed state.

[0056] To measure the fracture toughness, both notched and unnotched samples were stretched at a rate of 50 mm / min. The fracture toughness was calculated as where H is the height of the sample, is the integral of the stress-strain curve of the unnotched sample from to the critical strain , and is defined as the strain at which crack propagation begins. The energy release rate was calculated as , , defined as the strain before crack propagation begins.

[0057] To measure the elastic modulus, unnotched samples were stretched at a rate of 50 mm / min, and the slope of the stress-strain curve at small deformations was the elastic modulus E.

[0058] 4. Peeling test

[0059] One end of the double-layer hydrogel sample was fixed on the clamp of the testing equipment, and the other end was connected to the other clamp. By controlling the testing equipment, a pulling force was applied at a certain rate, and the two layers of hydrogel were gradually separated at a rate of 50 mm / min. During this process, the data of the change of pulling force with displacement were recorded, which reflected the interaction force between the two layers of hydrogel during the peeling process. By analyzing the pulling force-displacement curve, the interfacial adhesion energy was calculated, where F is the size of the peeling force, and w is the width of the sample. The interfacial adhesion energy is an important parameter to measure the adhesion strength between the two layers of hydrogel, which represents the energy consumed to completely separate the two layers of hydrogel. In order to improve the accuracy and reliability of the experimental results, multiple parallel samples were set for each group of experiments. The setting of parallel samples can reduce experimental errors and avoid the deviation of experimental results caused by the particularity of individual samples. Statistical analysis of the experimental data of multiple parallel samples can more accurately reflect the interfacial performance characteristics of the double-layer CNF-PAM / AA hydrogel. Through this experimental design and data analysis method, the interfacial adhesion energy and crack propagation behavior of the double-layer CNF-PAM / AA hydrogel can be comprehensively and deeply studied, providing strong experimental support for further revealing its interfacial toughening mechanism.

[0060] The experimental results are as follows:

[0061] 1. Microstructure of two kinds of hydrogels with / without cellulose intermediate layer:

[0062] Among them, the PAM / AA hydrogel without cellulose intermediate layer and the CNF-PAM / AA hydrogel with cellulose intermediate layer. By comparing Figure 1 (a) and (c), (b) and (d) in Figure 1(c) and (d) in FIG. 6 show the interlaced and highly porous network, whose porous structure is attributed to the steric hindrance caused by rigid CNFs, which prevents the close packing of polymer chains, thus forming a porous structure inside the hydrogel. The coordination bond interaction between CNFs and PAAM chains inhibits the movement of polymer chains, which cannot freely arrange during the freeze-drying process, thus producing a porous structure with low shrinkage after drying.

[0063] 2. Mechanical performance test results of CNF-PAM / AA hydrogel:

[0064] Among them, the mechanical properties of PAM / AA hydrogel and CNF-PAM / AA hydrogel with different CNF contents and soaking time were tested to evaluate the performance indicators such as strength, Young's modulus and elongation, and the results are shown in FIG. 6. Figure 2

[0065] The crack layer needs to be able to produce cracks when subjected to external force, so its mechanical properties need to be moderate, both with certain strength and able to produce deformation; while the substrate layer needs to provide stable support, requiring higher strength and modulus. Through stress-elongation curve analysis, it can be known that the four kinds of hydrogel samples show different mechanical responses.

[0066] The strength of the hydrogel with CNF content of 0.63% after soaking for 12 hours is 2.39 MPa, and the Young's modulus is 1.28 MPa. This mechanical property allows the hydrogel to produce certain deformation when subjected to external force, thus forming a crack, but it will not completely break due to being too fragile; the water content of the hydrogel under this treatment condition is 68.51%, which makes the hydrogel have good flexibility and biocompatibility, which is conducive to the formation and expansion of the crack. The strength of the hydrogel with CNF content of 0.63% after soaking for 24 hours is 3.145 MPa, and the Young's modulus is 1.47 MPa. This higher mechanical property allows the substrate layer to withstand greater external force and provide stable support; the water content of the hydrogel under this treatment condition is 68.47%, which is slightly lower than that of the crack layer, but still maintains good flexibility and biocompatibility, and the moderate water content helps the substrate layer to provide support while not becoming too dry and fragile.

[0067] From the perspective of bionics, in terms of gradient modulus design, the modulus difference (1.07 vs 1.17 MPa) between the crack layer and the substrate layer forms a continuous mechanical gradient, dispersing the stress concentration at the crack tip, similar to the interfacial effect of the nacreous layer of a shell; in terms of dynamic bond synergy, the high water content of the crack layer and the cross-linking density of the substrate layer work together to achieve dynamic energy dissipation at the "soft-hard" interface, imitating the multi-level energy dissipation mechanism of biological tissues (such as the sliding of collagen fibers in tendons). ​

[0068] According to the above, the CNF-PAM / AA hydrogel with 0.63% CNF content soaked for 12 hours is determined as the cracking layer, and the CNF-PAM / AA hydrogel with 0.63% CNF content soaked for 24 hours is determined as the substrate layer, so as to optimize the performance and application effect of the double-layer cellulose hydrogel.

[0069] 3. Tensile property test results of two hydrogels with / without cellulose intermediate layer:

[0070] Among them, the interface layer is cured by the same monomer content and CNF content, but different water content. The water content is gradually increased from 18 g to 33.85 g. The experiment is divided into two parts: first, test the tensile properties of 12 different interface layers of CNF-free system and CNF-containing system, and then test the adhesion of the 12 groups of interface layers between the double-layer hydrogels. By fixing the CNF content (0.63%) and monomer ratio, the water content of the interface layer (56.05%-68.65%) is adjusted to systematically study its effect on hydrogen bond density and interfacial toughening performance.

[0071] The tensile properties of the hydrogels of CNF-free system and CNF-containing system (not soaked in zirconium ion solution as interface layer) are tested, and the results are shown in Figure 3 (a). In the CNF-free system, as the water content gradually increases from 52.13% to 67.19%, the stress gradually decreases from 0.2884 MPa to 0.1176 MPa, the elongation gradually increases from 2.6732 to 4.3357, and the Young's modulus sharply decreases from 527.93 kPa to 130.36 kPa. This indicates that the increase of water content will lead to the decrease of strength and the increase of flexibility of the CNF-free system hydrogel, but the stiffness will significantly decrease.

[0072] For the CNF-containing system, as the water content increases from 56.08% to 68.63%, the stress gradually decreases from 0.0209 MPa to 0.12895 MPa, the elongation continuously increases from 3.1203 to 5.7502, and the Young's modulus gradually decreases from 339.65 kPa to 70.17 kPa, and the results are shown in Figure 3 (b). This trend indicates that within a certain range, the increase of water content enhances the interaction between cellulose and polymer chains, resulting in an increase in stress, but as the water content further increases, the overall structure of the system is diluted, and the stress gradually decreases. At the same time, the increase of water content provides more space for the movement of polymer chains, resulting in a continuous increase in elongation, while the Young's modulus decreases due to the weakening of the structure.

[0073] By comparing the stress, elongation and Young's modulus of the hydrogel with and without CNF system, it is found that there are significant differences between the two. In terms of stress, the strength of the system without CNF decreases by 59% as the water content increases, while the decrease of the system with CNF is narrowed to 37%, indicating that the auxiliary network formed by CNF through hydrogen bond crosslinking and physical entanglement effectively delays the weakening of the main network. It is worth noting that when the water content is less than 60%, the strength of the system with CNF is lower than that of the control group, which is due to the stress concentration caused by the local aggregation of CNF under the influence of limited water content; while the water content is more than 60%, sufficient free water promotes the uniform dispersion of CNF, so that the strength of the system is reversed, which verifies the key role of the solvation effect on network reconstruction.

[0074] In terms of elongation performance, the elongation of the system with CNF increases by 84%, which is increased by 22%. Dynamic mechanical analysis shows that the dynamic hydrogen bond network of CNF undergoes a cycle of fracture-recombination during stretching, which delays crack initiation through a reversible energy dissipation mechanism. The trend of Young's modulus further reveals the dual-phase effect of CNF: at low water content stage (<55%), CNF acts as a rigid filler to increase the initial modulus by 21%; while at high water content stage (>65%), the phase separation of CNF and polymer matrix leads to a sharp decrease of 79% in modulus, which is enlarged by 4%, proving the interface slip mechanism dominated by the water plasticizing effect.

[0075] 4. Adhesion test results:

[0076] The adhesion of the two kinds of hydrogels, namely the hydrogel without CNF and the hydrogel with CNF, to glass was tested, and the results are shown in Figure 4 (a) and (b). In the system without CNF, as the water content increases, the adhesion force increases from 0.1673 N to 0.51494 N, and the adhesion energy increases from 8.635 J / m 2 to 25.747 J / m 2 . This shows that the increase of water content helps to improve the adhesion performance of the hydrogel without CNF, which may be because the increase of water content enhances the interaction between the polar groups on the surface of the hydrogel and the glass surface, thereby improving the adhesion force and adhesion energy.

[0077] In the system with CNF, as the water content increases from 56.08% to 68.63%, the adhesion force increases from 0.23556 N to 0.5953 N, and the adhesion energy increases from 11.778 J / m 2 to 29.765 J / m 2Similar to the CNF-free system, the increase of water content also promotes the adhesion of the CNF-containing hydrogel. Meanwhile, the adhesion force and adhesion energy of the CNF-containing system are higher than those of the CNF-free system at the same water content. This is mainly because CNF has a large number of polar groups such as hydroxyl groups, which can form more hydrogen bonds and other interactions with the glass surface, thereby significantly improving the adhesion of the hydrogel. In addition, the addition of CNF can also change the surface roughness and microstructure of the hydrogel, further enhancing the mechanical interlocking effect between the hydrogel and the glass surface, and improving the adhesion performance.

[0078] In addition, the double-layer hydrogel prepared according to the above is subjected to pure shear performance, peeling performance, energy release rate analysis and fitting coefficient analysis, and the results are as follows, which specifically include:

[0079] 1. Pure shear performance results

[0080] Figure 5 (a) in FIG. 1 shows a three-dimensional network structure formed by CNF and polyacrylic acid-acrylamide copolymer through hydrogen bond interaction. In the figure, CNF (chain structure) and P(AAc-AAm) (flexible polymer chain) are dynamically cross-linked through hydrogen bond, constructing a composite hydrogel system with interpenetrating network characteristics. Water molecules (H2O) as solvent and hydrogen bond forming medium are uniformly distributed in the network, not only promoting intermolecular interaction, but also endowing the material with excellent swelling performance and structural stability.

[0081] In order to determine the optimal interface layer thickness between the double-layer hydrogel, double-layer hydrogels with interface layer thicknesses of 0.1 mm, 0.5 mm and 1 mm were prepared respectively, and their mechanical properties were tested, and the experimental results are shown in Figure 5 (b) in FIG. 1. Figure 5 (b) in FIG. 1 shows the influence of different water content interface layers on the pure shear performance of the double-layer hydrogel under the conditions of fixed CNF content (0.63%) and interface layer thickness (0.1 mm). The experimental data show that when the water content increases from 56.08% to 68.63%, the peak stress of the system increases from 0.48 MPa to 1.02 MPa, with an increase of 113%, and the elongation increases from 5.68 to 9.68, showing a significant toughening effect. This phenomenon can be attributed to: under high water content (>64%), free water molecules as plasticizers promote the dynamic recombination of CNF and PAM chains through hydrogen bonds, forming an energy dissipation network with gradient entanglement density; with the increase of water content, the modulus of the interface layer decreases from 0.34 MPa to 0.07 MPa, and the matching degree with the modulus of the substrate layer (0.47 MPa) is significantly improved.

[0082] When the interfacial layer thickness is 0.1 mm, the double-layer hydrogel exhibits the best mechanical properties. The stress-strain curve shows that the material can withstand a higher stress and has a larger elongation during the stretching process, which means that the material has both good strength and excellent ductility. In contrast, as the thickness of the interfacial layer increases, the mechanical properties of the double-layer hydrogel gradually decrease. The stress-strain curves corresponding to the interfacial layer thicknesses of 0.5 mm and 1 mm show that both the maximum stress and the elongation of the material decrease, indicating a decrease in the mechanical properties.

[0083] This phenomenon can be attributed to the stress field gradient matching mechanism. The modulus of the 0.1 mm interfacial layer (0.31 MPa) is between that of the substrate layer (0.47 MPa) and the crack layer (0.18 MPa), forming a continuous modulus gradient that effectively disperses the stress concentration at the crack tip. When the thickness of the interfacial layer increases to 1 mm, both the strength and toughness of the system significantly deteriorate. This is due to the excessive aggregation of CNFs in the thick interfacial layer, leading to increased phase separation and a water plasticization effect dominating the failure mode. Due to the dynamic bond characteristics of CNFs, the optimal thickness (0.1 mm) of this system is one order of magnitude lower than that of traditional composites, providing a new paradigm for ultra-thin interfacial design. In the following tests of double-layer hydrogels, double-layer hydrogels with an interfacial layer thickness of 0.1 mm are selected for testing.

[0084] The double-layer hydrogels with CNF interfacial layers and double-layer hydrogels without interfacial layers were subjected to pure shear experiments, and the experimental results are shown in Figure 5 (c). Figure 5 (d) of Figure 5 shows the crack propagation process of the interfacial layer of the double-layer hydrogel under stress. From left to right, the figure shows the initiation, propagation, blunting, and final stabilization of the crack in the interfacial layer. In the early stage of stress, the crack gradually forms and propagates in the interfacial layer. With the continuous action of stress, the hydrogen bond network in the interfacial layer begins to play a role, dissipating part of the energy through the breaking and recombination of hydrogen bonds, thereby slowing down the further propagation of the crack. Finally, the crack reaches a stable state in the interfacial layer, effectively improving the fracture toughness of the double-layer hydrogel.

[0085] Figure 5 (e) of Figure 5 reveals the multi-scale synergistic effect of the interfacial layer in connecting the film and the substrate through a dynamic hydrogen bond network. In the interfacial layer, the hydroxyl groups (-OH) of cellulose nanofibers (CNF) form dynamic hydrogen bonds with the acrylic acid (AAc) carboxyl groups (-COOH) of the upper film, constructing a gradient bonding network. This design mimics the "soft-hard" transition structure of biological interfaces (such as tendon-bone junctions). During the crack propagation process, the dynamic dissociation and recombination mechanism of hydrogen bonds endows the interface with energy dissipation ability. This structure not only realizes the synergistic mechanical response of the upper and lower layers of materials, but also regulates the stress transfer path through the dynamic interaction of hydrogen bonds, effectively inhibiting the unstable expansion of the crack.

[0086] Notably, the elongation of the no-interface layer control group was only 23.6% of the interface layer-containing system, and the fracture energy (0.45 MJ / m 3 ) dropped to 4.3% of the optimal system, fully verifying the necessity of the interface layer design.

[0087] 2. Peeling performance test results

[0088] Here, the double-layer hydrogel was subjected to peeling experiments, aiming to further explore the adhesion performance of different systems as interface layers, and to analyze the interface toughening effect of the CNF-containing system. The results are referred to Figure 6 . Among them, Figure 6 (a) and (b) in the above table reveal the significant regulatory effect of cellulose nanofiber (CNF) on the adhesion performance of the interface and its water content-dependent characteristics. For the CNF-containing system, when the water content increases from 56.08% to 68.63%, the adhesion force gradually rises from 3.08 N to 6.14 N, and the adhesion energy increases from 154 J / m 2 to 307 J / m 2 . Compared with the no-CNF system, the adhesion force and adhesion energy of the CNF-containing system are significantly higher under the same water content conditions. This is mainly due to the unique properties and effects of CNF. CNF has abundant polar groups such as hydroxyl groups, which can form various interactions such as hydrogen bonds and electrostatic interactions with the atoms or groups on the surface of the double-layer gel, thereby enhancing the adhesion of the interface. At the same time, the addition of CNF changes the microstructure of the hydrogel, forming a more complex and compact network structure, which helps to improve the mechanical interlocking between the hydrogel and the double-layer gel, further enhancing the adhesion performance.

[0089] In addition, the peeling behavior of the double-layer hydrogel under different interfaces (gel-glass and gel-gel) was further analyzed, aiming to reveal the differences in adhesion performance and their mechanisms. The experimental results show that, whether it is a no-CNF or a CNF-containing system, the adhesion force and adhesion energy of the gel-gel interface are significantly higher than those of the gel-glass interface. Further analysis found that the adhesion energy of both systems increased with increasing water content, but the CNF-containing system had a faster growth rate. This indicates that an appropriate amount of water can make the polymer chains more flexible and more easily conform to the microtopography of the substrate surface, improving the effective contact. However, excessive water may lead to excessive swelling, weakening the interface bonding strength, but the presence of CNF can lock some water molecules through hydrogen bonds, delaying the uncontrolled swelling. In addition, at low water content, the adhesion energy of the CNF-containing system is significantly higher than that of the no-CNF system, indicating that the polar groups of CNF play a dominant role in interface bonding; at high water content, the adhesion energy of the CNF-containing system increases more rapidly, indicating that CNF may reduce the destruction of interface bonding by swelling through water locking.

[0090] From the perspective of dominant dissipation mechanism, the gel-glass interface mainly dissipates energy through hydrogen bond (AAc's -COOH and glass's Si-OH) breakage and chain segment slippage, while the gel-gel interface dissipates energy through chain segment interpenetration (polymer chains of two layers of gel diffuse into each other at the contact interface, forming a physical entanglement network) and dynamic bonding recombination (cellulose's -OH and AAc's -COOH). In terms of failure mode, the gel-glass interface exhibits interface debonding (brittle failure), while the gel-gel interface exhibits cohesive failure (ductile failure). CNF plays a role in increasing the hydrogen bond density in both systems, thereby delaying interface debonding, enhancing chain segment interpenetration, and providing dynamic sacrificial bonds.

[0091] ;

[0092] is the intrinsic adhesion energy (contributed by backbone chemical bonds) when there is no dynamic hydrogen bond, is the additional energy contributed by dynamic hydrogen bond dissipation, which is related to hydrogen bond density and the breakage energy of a single hydrogen bond ( ), recombination rate . CNF significantly increases , thereby increasing the adhesion energy.

[0093] Quantitative experimental data show that the adhesion energy of the CNF-containing system increases by 15.6% (gel-glass) and 18.5% (gel-gel), which is consistent with the theoretical prediction of dynamic dissipation contribution. This indicates that CNF significantly increases the additional energy of dynamic hydrogen bond dissipation by increasing the hydrogen bond density, thereby increasing the adhesion energy.

[0094] Combining (b) in Figure 5 with (b) in Figure 6 , it is found that both groups of curves exhibit a clear "first three groups - last three groups" grouping feature, revealing a significant threshold effect within the system. When the water content is at a low level, the number of hydrogen bonds in the interface layer is insufficient, and the interfacial adhesion has not yet become the dominant mechanism, resulting in a relatively weak mechanical response of the double-layer structure. When the water content exceeds the critical threshold, the role of water molecules as hydrogen bond donors / acceptors is significantly enhanced, promoting the optimal construction of the hydrogen bond network in the interface layer, making interfacial adhesion dominant, and thereby significantly improving the mechanical properties of the double-layer hydrogel. This threshold effect is essentially a reflection of the regulation of water content on the interfacial toughening mechanism: under low water content conditions, the toughening contribution of the hydrogen bond network is limited; while high water content triggers the evolution of the hydrogen bond network, forming a more efficient energy dissipation path through dynamic breakage and recombination of hydrogen bonds, ultimately realizing the phased leap of interface performance.

[0095] ​To verify the correctness of the above analysis, the interfacial layer of the two groups of systems was tested by Fourier transform infrared absorption spectroscopy.

[0096] For the cellulose-free system (refer to Figure 5 (c) in the above): at low water content (52.13%-59.22%), the O-H peak is located at a higher wavenumber (~3336 cm -1 ), the peak shape is narrower and the absorbance is lower (0.02-0.18), indicating that the hydrogen bond network is sparse and the hydroxyl group mainly exists in the form of free or weak hydrogen bond. With the increase of water content (62.03%-67.19%), the O-H peak gradually shifts to ~3191 cm -1 , and the absorbance slightly rises (0.20-0.30), but the change is gentle, indicating that water molecules as hydrogen bond donors supplement part of the interaction, but do not form a dense network. The C=O peak is located at a higher wavenumber (1665-1680 cm -1 ), and the absorbance is lower (0.08-0.20), and with the increase of water content, the wavenumber only moves slightly (~1655 cm -1 ), indicating that the C=O group mainly forms weak hydrogen bonds with itself or a small amount of water molecules.

[0097] For the cellulose-containing system (refer to Figure 5 (d) in the above): under all water content conditions, the O-H peak is significantly widened and the absorbance is higher (0.24-0.42), and the wavenumber is significantly lower than that of the CNF-free system (e.g. 3191 cm -1 vs 3336 cm -1 ), indicating that the hydroxyl group of CNF forms a dense hydrogen bond network with water molecules and polymer chains. With the increase of water content (56.08%-68.63%), the O-H peak further shifts to ~3150 cm -1 , and after the critical water content (~64.33%), the moving rate accelerates, and the absorbance nonlinearly rises (0.32-0.42), which is consistent with the trend of the adhesion energy, confirming the reconstruction of the hydrogen bond network under the threshold effect. The C=O peak wavenumber significantly decreases (1650-1665 cm -1 ), and the absorbance significantly increases (0.22-0.38), and further shifts to ~1640 cm -1 with the increase of water content. This phenomenon is attributed to the formation of O-H···O=C strong hydrogen bonds between the hydroxyl group of CNF and the C=O group, and the dynamic bonding network optimizes the bonding density with the increase of water content.

[0098] The O-H and C=O peak shift amplitudes of CNF-containing systems are significantly larger than those of CNF-free systems, especially after the critical water content (~64%), which is consistent with the jump trend of adhesion energy and verifies the threshold mechanism of hydrogen bond network reconstruction. The O-H peak absorbance and peak width of CNF-containing systems are higher than those of CNF-free systems at all water contents, and increase nonlinearly, which is directly related to the enhancement of dynamic hydrogen bond dissipation contribution (G The infrared spectrum results are highly consistent with the mechanical property data (adhesion force and adhesion energy increase with water content), indicating that CNF significantly improves the interfacial interaction by increasing the hydrogen bond density and dynamic bonding network.

[0099] 3. Energy release rate analysis

[0100] Energy release rate is an important parameter for evaluating the crack propagation behavior of materials, which reflects the energy released by the system per unit area of crack propagation. In this embodiment, the energy release rate of double-layer hydrogels with and without interfacial layers is analyzed to reveal the influence of interfacial layers on crack propagation behavior. The experimental results show that the energy release rate of double-layer CNF-PAM / AA hydrogels with interfacial layers is significantly lower than that of systems without interfacial layers, and shows a regular decreasing trend with increasing water content. This phenomenon reveals the key role of the interfacial layer in energy dissipation: the CNF in the interfacial layer forms strong interactions with the hydrogel matrix through hydrogen bond networks, and the dynamic hydrogen bond breaking-recombination process continuously dissipates energy under stress, effectively reducing the overall energy release rate of the system. This shows that the presence of the interfacial layer effectively hinders the propagation of cracks and improves the toughening performance of the material. Further analysis shows that the energy release rate gradually increases with increasing water content, which may be because an appropriate amount of water can enhance the flexibility of polymer chains, thereby improving the ductility and energy dissipation capacity of the material. However, when the water content exceeds a certain threshold, excessive water may lead to a decrease in the strength of the material, which is consistent with the threshold effect discussed earlier.

[0101] The results of interfacial lateral debonding energy release rate are shown in Figure 7As shown, it decreases with the increase of water content, showing a clear negative correlation with the adhesion energy - the greater the adhesion energy, the lower the interfacial lateral debonding energy release rate. This indicates that the debonding resistance of the interfacial layer decreases at a higher water content, which may be related to the weakening effect of water on the adhesion properties of the interfacial layer. However, the addition of CNF to some extent alleviates this trend, which increases the adhesion energy and debonding resistance of the interfacial layer by increasing the hydrogen bond density and forming a physical entanglement network. This echoes the role of cellulose in improving adhesion properties mentioned earlier. This finding further analyzes the interfacial lateral debonding energy release rate The results further verify the interfacial toughening mechanism: the interfacial layer with high adhesion energy strengthens the energy dissipation at the interface, inhibits the energy release at the crack tip, and hinders crack propagation. The results of this study are highly consistent with the interfacial toughening theoretical framework, confirming that the strategy of introducing a CNF-containing interfacial layer in a double-layer hydrogel can achieve efficient interfacial toughening by precisely regulating the correlation between adhesion energy and energy release rate.

[0102] 4. Fitting coefficient analysis

[0103] Based on the existing relationship between energy release rate and various parameters, a series of fitting coefficients with important physical significance were obtained by fitting the experimental data. These coefficients not only reflect the performance characteristics of double-layer cellulose fiber hydrogel under different parameters, but also provide key clues for in-depth understanding of its interfacial toughening mechanism. Specifically:

[0104] (1)

[0105] (2)

[0106] (3)

[0107] First, calculate Dundurs parameters and , which are used to describe the elastic mismatch between two heterogeneous materials. For the cracked layer and the substrate layer materials in this embodiment, the Dundurs parameters are calculated according to formulas (2) and (3) ≈0 and . These values indicate that the elastic mismatch between the two materials is small, which is consistent with the experimental design of this application, because the elastic modulus and Poisson's ratio of the cracked layer and the substrate layer materials are similar, thereby determining the coefficient =1.976.

[0108] For double-layer materials without interfaces, add the coefficient to formula (1) to obtain formula (4):

[0109] (4)

[0110] By fitting experimental data, such as Figure 8 As shown in (a), the coefficients are obtained. =11.25, indicating that crack propagation is mainly dominated by the intrinsic toughness of the matrix.

[0111] For layers with different interfaces, use formula (5):

[0112] (5)

[0113] By fitting experimental data, such as Figure 8 As shown in (b) above, the coefficients are obtained. The coefficients are 43.56, 36.42, 31.9, 30.28, 29.62, and 28.75, respectively. The decrease in moisture content indicates that as water content increases, the interfacial layer's resistance to crack propagation weakens, and the energy release rate decreases. In other words, increased water content weakens interfacial bonding strength, but the addition of cellulose mitigates this effect to some extent.

[0114] For cases where the interface decouples laterally, use formula (6):

[0115] (6)

[0116] By fitting experimental data, such as Figure 8 As shown in (c), the coefficients are obtained. The coefficients are 4.2±1.6, 2.55±0.95, 1.65±0.65, 1.18±0.325, 0.845±0.155, and 0.477±0.293, respectively. The decrease indicates that at higher water content, the lateral debonding resistance of the interface layer increases, and the energy release rate decreases. As moisture content decreases, crack propagation becomes more likely, and the proportion of interfacial dissipation increases by 21%. This further confirms the significant impact of moisture content on interfacial layer performance and also reflects the important role of cellulose in improving interfacial layer stability.

[0117] Meanwhile, the decreasing trend of the interfacial lateral deadhesion coefficient (4.2→0.477) is also directly related to the change in failure mode: at low water content (56.08%) =4.2 corresponds to brittle debonding, with energy dissipation mainly through main chain breakage; at high water content (68.63%) =0.477 reflects the dominance of tough fiber bridging, with dynamic bond reorganization and CNF bridging contributing an additional 52% of energy dissipation.

[0118] For interface-free bilayer hydrogels, the fitting coefficient is... The coefficient is 11.24518. For bilayer hydrogels with interfaces, the coefficient is... It gradually decreased from 43.55646 (56.08% water content) to 28.75201 (68.63% water content) with the increase of water content. This trend indicates that the increase of water content leads to the decrease of the adhesion performance of the interfacial layer, thereby reducing the energy release rate. That is, the increase of water content will weaken the interfacial bonding strength, although the addition of CNF alleviates this effect to some extent.

[0119] Water content and adhesion energy are the core factors affecting the coefficient: with the increase of water content, the fitting coefficient of the interfacial transverse debonding energy release rate decreases significantly from (4.2±1.6) to (0.477±0.293), which essentially reflects the regulation mechanism of water content on the interfacial hydrogen bond network and adhesion energy. High water content promotes water molecules as hydrogen bond donors / acceptors to participate in the construction of the interfacial structure, enhances the interfacial adhesion energy, and then inhibits crack propagation by strengthening energy dissipation, ultimately reflected in the regular decrease of the fitting coefficient.

[0120] Traditional energy release rate formula focuses on single material system, while the present application reveals the synergistic effect of water content and adhesion energy on interfacial toughening in CNF reinforced double-layer hydrogel, which expands the applicability of the formula in complex interfacial systems. In summary, the change of the fitting coefficient reflects the comprehensive influence of adhesion energy, water content and other factors on the energy release rate. Through the combination of experiments and theories, the application reveals the action mechanism of these factors in depth, providing a solid theoretical basis and practical guidance for the optimization design of double-layer hydrogel materials.

[0121] In summary, the double-layer hydrogel containing CNF exhibits higher stress, elongation and Young's modulus, and its adhesion energy is increased by about 18.5% compared with the non-cellulose system. This is mainly due to the fact that the CNF system exhibits better ductility due to the breaking-recombination mechanism of the dynamic hydrogen bond network, and CNF forms an auxiliary network through hydrogen bonding and physical entanglement, enhancing the mechanical properties and interfacial stability of the material. At the same time, appropriate water can make the polymer chain more flexible and improve the effective contact, but excessive water will lead to swelling and weaken the interfacial bonding strength. By optimizing the interfacial layer thickness, it is found that the double-layer hydrogel can achieve the best mechanical properties when the interfacial layer thickness is 0.1 mm, and there is a threshold effect of water content in the interfacial layer. High water content promotes the hydrogen bond network to dominate the interfacial adhesion, significantly improving the mechanical response of the double-layer structure.

[0122] The analysis of energy release rate further confirms the superior performance of the CNF-containing double-layer hydrogel. Its energy release rate is significantly lower than that of the system without CNF, and the change trend of the fitting coefficient is closely related to factors such as adhesion energy and water content. This shows that the addition of cellulose not only increases the adhesion energy, but also effectively reduces the energy release rate and delays the crack propagation by increasing the hydrogen bond density and forming a physical entanglement network. The double-layer hydrogel containing the interfacial layer effectively reduces the energy release rate by enhancing the adhesion energy, and the fitting coefficient is clearly related to the water content and adhesion energy, revealing the "water content-hydrogen bond network-adhesion energy-energy release rate" interfacial toughening quantification mechanism.

[0123] Therefore, by using the above-mentioned dynamic hydrogen bond enhanced double-layer hydrogel and its interfacial toughening preparation method, a dynamic hydrogen bond network is constructed by using cellulose nanofiber, and the gradient design of the interfacial layer is combined to realize the synergistic enhancement of crack tip stress relaxation and energy dissipation, thereby providing a new path for the design of high-performance flexible materials and promoting the application value in the fields of biomedical and flexible electronics.

[0124] The principles and implementation modes of the present application are described by applying specific examples in this paper, and the above examples are only used to help understand the method of the present application and its core idea; at the same time, for those skilled in the art, according to the idea of the present application, the specific implementation mode and application range will be changed. In summary, the content of the specification should not be understood as a limitation of the present application.

Claims

1. An interfacial toughening method for the preparation of dynamic hydrogen bond enhanced double network hydrogels, characterized in that, The method comprises the following steps: S1, placing the dried microcrystalline cellulose in a solution composed of LiOH and urea, stirring uniformly, and then placing in a low-temperature environment overnight, the low-temperature environment being-15℃, so that the microcrystalline cellulose is completely dissolved, and then stirring again to obtain a transparent microcrystalline cellulose solution; S2, adding a chemical crosslinking agent to the microcrystalline cellulose solution, and stirring uniformly to obtain a CNF solution; S3, mixing the CNF solution and a polymer precursor solution in equal proportions to obtain a mixed solution, stirring uniformly, and then placing in a mold, and then performing light curing to obtain a single-layer hydrogel; S4, immersing the single-layer hydrogel in a zirconium ion solution for different times to obtain a base layer and a crack layer, and introducing a pre-crack in the crack layer; S5, dropping the mixed solution obtained by mixing the CNF solution and the polymer precursor solution on the surface of the crack layer to form an interface layer, and placing the base layer above the interface layer, and then performing light curing again to obtain a dynamic hydrogen bond enhanced double-layer hydrogel; In S3, the polymer precursor solution comprises acrylamide AAM, acrylic acid AAc, crosslinking agent N, N-methylene bisacrylamide MBAA, and a photoinitiator.

2. The method according to claim 1, wherein the method is characterized by, In S1, the drying conditions of the microcrystalline cellulose are: drying in an environment at 60℃ for 24h to remove the water in the microcrystalline cellulose.

3. The method according to claim 2, wherein the method is characterized by, In S1, the solution composed of LiOH and urea comprises LiOH and urea in a mass ratio of 4.6:15, and the solvent is deionized water.

4. The method according to claim 1, wherein the method is characterized by, In S2, the chemical crosslinking agent is epichlorohydrin, and the mass ratio of the microcrystalline cellulose in the microcrystalline cellulose solution to epichlorohydrin is 2:

1.

5. The method according to claim 1, wherein In S3, the light curing conditions are: using a 30W, 365nm ultraviolet lamp, irradiating in an environment at 25℃ for 30min; and the single-layer hydrogel obtained by light curing is a CNF-PAM / AA hydrogel.

6. The method according to claim 1, wherein In S4, the zirconium ion solution is a 0.5mol / L zirconium oxychloride octahydrate solution, the immersion time of the base layer is 24h, and the immersion time of the crack layer is 12h; the length of the pre-crack is 20mm, and the pre-crack is formed by vertically cutting the middle position of the crack layer with a blade.

7. The method according to claim 1, wherein the method is characterized by, In S5, the thickness of the interface layer is controlled by a mold, the thickness of the mold is 0.1mm-1mm, and the light curing conditions are: using a 30W, 365nm ultraviolet lamp, irradiating in an environment at 25℃ for 30min.

8. The dynamic hydrogen-bond enhanced double-layer hydrogel prepared by the interfacial toughening method according to any one of claims 1-7, characterized in that, The dynamic hydrogen bond enhanced double-layer hydrogel comprises a crack layer, an interface layer, and a base layer, the interface layer is located between the crack layer and the base layer, the cellulose nanofiber CNF in the interface layer forms a dynamic hydrogen bond network with the acrylic acid carboxyl groups in the crack layer and the base layer through the hydroxyl groups, and interface toughening is achieved.

9. Application of the dynamic hydrogen bond enhanced double-layer hydrogel according to claim 8 in the fields of biomedicine and flexible electronics.

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