Dynamic hydrogen bond enhanced double-layer hydrogel and interface toughening preparation method thereof

Abstract

The invention provides dynamic hydrogen bond enhanced double-layer hydrogel and an interface toughening preparation method thereof, and belongs to the technical field of hydrogel material preparation. Comprising 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 cross-linking agent, and uniformly stirring to obtain a CNF solution; mixing the CNF solution and the polymerization precursor solution in equal proportion, stirring, putting into a mold, and carrying out photocuring to obtain single-layer hydrogel; then, the substrate layer and the crack layer are soaked in a zirconium ion solution at different times respectively, and the substrate layer and the crack layer are obtained; preparing an interface layer above the cracking layer; and finally, placing the substrate layer above the interface layer, and carrying out photocuring again to obtain the dynamic hydrogen bond enhanced double-layer hydrogel. According to the method, the dynamic hydrogen bond network is constructed through the cellulose nanofibers, and the interface layer gradient design is combined, so that the synergistic enhancement of crack tip stress relaxation and energy dissipation is realized, and a new path is provided for the design of a high-performance flexible material.

Description

Technical Field

[0001] The present invention relates to the technical field of hydrogel material preparation, in particular to a dynamic hydrogen bond enhanced double-layer hydrogel and an interface toughening preparation method thereof. Background Art

[0002] As an important class of soft materials, hydrogels have shown broad application prospects in the fields of biomedicine and flexible electronics due to their high water content, excellent biocompatibility and adjustable mechanical properties. For example, in biomedicine, hydrogels can be used as tissue engineering scaffolds to provide a growth microenvironment for cells, or as smart carriers to achieve controlled release of drugs; 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 biomimetic robots to achieve complex deformations. However, the low modulus characteristics of hydrogels make them prone to crack initiation and propagation when subjected to dynamic loads such as stretching and bending, resulting in insufficient fracture toughness, which seriously restricts their application in long-term service scenarios.

[0003] At present, the existing hydrogel toughening methods mainly include two categories: the first category is to introduce rigid fillers (such as nanoparticles) or construct a double network structure to disperse the stress at the crack tip or increase the energy dissipation during the fracture process. For example, the existing technology dissipates energy by designing a double network hydrogel by breaking covalent bonds, but it relies on an irreversible static energy dissipation mechanism, and its efficiency gradually decays under dynamic loads, making it difficult to meet the requirements of repeated deformation. The second category is to enhance interfacial adhesion through multi-layer structural design to inhibit crack propagation along the interface. For example, the existing technology prepares double-layer hydrogels by layer-by-layer polymerization, attempting to improve 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 interlayer synergistic toughening effects.

[0004] Furthermore, existing methods for enhancing interfacial properties through hydrogen bonding can enhance interfacial adhesion through host-guest hydrogen bonding or cation-π interactions. However, these methods often rely on a single hydrogen bond type or static bonding structure, failing to form a dynamically reversible hydrogen bond network and thus making it difficult to achieve sustained energy dissipation during crack propagation. Furthermore, the intrinsic correlation between the cellulose nanofiber (CNF) content, moisture content, and thickness of the interfacial layer and the interfacial adhesion energy and energy release rate remains unclear, resulting in a lack of theoretical guidance for the quantitative regulation of interfacial toughening effects.

[0005] The above findings demonstrate the following challenges with existing methods for toughening hydrogels: a) Inadequate adhesion or irrational modulus gradients can lead to heterogeneous interfaces within multilayer structures becoming pathways for rapid crack propagation, leading to a breakdown in interlayer synergy and an inability to effectively suppress crack initiation and propagation. b) Traditional static energy dissipation mechanisms (such as covalent bond breakage) are irreversible and unable to dissipate energy continuously under cyclic loading. This makes the material susceptible to fatigue failure and makes it difficult to meet the complex mechanical requirements of practical applications.

[0006] In summary, the existing technology has not yet achieved the synergistic optimization of interfacial adhesion energy enhancement and dynamic energy dissipation. There is an urgent need for a hydrogel interface toughening preparation method that can take into account high toughness, high interfacial stability and dynamic load adaptability. Summary of the Invention

[0007] In order to overcome the shortcomings of the existing technology, the purpose of the present invention is to provide a dynamic hydrogen bond enhanced double-layer hydrogel and its interface toughening preparation method, construct a dynamic hydrogen bond network by cellulose nanofibers, and combine the interface layer gradient design to achieve the synergistic enhancement of crack tip stress relaxation and energy dissipation, providing a new path for the design of high-performance flexible materials and promoting their application value in the fields of biomedicine and flexible electronics.

[0008] To achieve the above object, the present invention provides the following solutions:

[0009] A method for preparing an interface toughening of a dynamic hydrogen bond-enhanced double-layer hydrogel comprises the following steps:

[0010] S1. Place the dried microcrystalline cellulose in a solution consisting of LiOH and urea, stir evenly, and then stand overnight at low temperature to completely dissolve the microcrystalline cellulose, and stir again to obtain a transparent microcrystalline cellulose solution;

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

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

[0013] S4, soaking the single-layer hydrogel in a zirconium ion solution for different time periods to obtain a base layer and a cracked layer, and introducing pre-cracks in the cracked layer;

[0014] S5. Add the mixed solution obtained by mixing the CNF solution and the polymerization precursor solution dropwise onto the surface of the cracked layer to form an interface layer; and place the base layer on top of the interface layer, and photocuring again to obtain a dynamic hydrogen bond enhanced double-layer hydrogel.

[0015] Preferably, in S1, the drying condition of the microcrystalline cellulose is: drying in an environment at 60° C. for 24 hours to remove moisture from the microcrystalline cellulose.

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

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

[0018] Preferably, in S3, the polymer precursor solution comprises acrylamide AAM, acrylic acid AAc, a cross-linking agent N,N-methylenebisacrylamide MBAA, and a photoinitiator phenyl (2,4,6-trimethylbenzoyl).

[0019] Preferably, in S3, the photocuring conditions are: using a 30W, 365nm ultraviolet lamp, irradiating for 30 minutes at 25°C; the single-layer hydrogel obtained by photocuring is a CNF-PAM / AA hydrogel.

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

[0021] Preferably, in S5, the thickness of the interface layer is controlled by a mold, and the thickness of the mold is 0.1 mm to 1 mm; the conditions for the secondary photocuring are: using a 30W, 365nm ultraviolet lamp, and irradiating at 25°C for 30 minutes.

[0022] The present invention also provides a dynamic hydrogen bond enhanced double-layer hydrogel prepared by the above-mentioned interface toughening preparation method of the dynamic hydrogen bond enhanced double-layer hydrogel. The dynamic hydrogen bond enhanced double-layer hydrogel includes a cracked layer, an interface layer and a base layer. The interface layer is located between the cracked layer and the base layer. The cellulose nanofiber CNF in the interface layer forms a dynamic hydrogen bond network through hydroxyl groups with the acrylic acid carboxyl groups in the cracked layer and the base layer to achieve interface toughening.

[0023] The present invention also provides an application of the above-mentioned dynamic hydrogen bond enhanced double-layer hydrogel in the fields of biomedicine and flexible electronics.

[0024] According to the specific embodiments provided by the present invention, the present invention discloses the following technical effects:

[0025] (1) The present invention achieves a significant improvement in interfacial adhesion performance by introducing cellulose nanofibers (CNF) to construct a dynamic hydrogen bond network and combining it with an interfacial gradient design. Specifically, at a water content of 68.63%, the interfacial adhesion energy of the intermediate layer containing 0.63% CNF reaches 307 J / m 2 , which is 11.9 times higher than that of the CNF-free system. The mechanism is that the hydroxyl groups of CNFs form a dense hydrogen bond network with the carboxyl groups of the polymer chains, and enhance the mechanical interlocking through physical entanglement, which solves the debonding problem of traditional multilayer hydrogel interfaces and provides guarantee for the long-term reliability of flexible electronic devices.

[0026] (2) The present invention significantly enhances crack propagation resistance by optimizing the modulus gradient and synergizing dynamic hydrogen bonding. Experimental results show that the driving force for crack propagation is reduced by 29.7%, and the energy release rate is reduced to 4.3% of that in a system without an interface layer. The main mechanism is that the CNF hydrogen bond network dissipates energy during the stretching process through dynamic fracture and recombination. At the same time, the modulus gradient between the substrate and the cracked layer disperses the stress at the crack tip, mimicking the interlayer interface effect of biological nacre, effectively inhibiting the instability of crack propagation.

[0027] (3) The present invention achieves controllable interfacial energy release behavior by regulating the threshold effect of water content and interlayer thickness. When the water content exceeds 64.33%, the interfacial lateral 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 tough bridging. By adjusting the interlayer thickness and CNF content, the differentiated mechanical requirements of biological tissue engineering scaffolds and wearable sensors can be matched. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the following briefly introduces the drawings required for use in the embodiments. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying any creative work.

[0029] Figure 1 The SEM images of the hydrogels provided by the present invention at different scales are shown below; Figure 1 (a) is the SEM image of PAM / AA hydrogel at 5.0 μm. Figure 1 (b) is the SEM image of PAM / AA hydrogel at 2.0 μm. Figure 1 (c) is the SEM image of CNF-PAM / AA hydrogel at 5.0 μm. Figure 1 (d) is the SEM image of CNF-PAM / AA hydrogel at 2.0 μm;

[0030] Figure 2 A comparison chart of the tensile properties of CNF-PAM / AA hydrogels with different soaking times and different CNF contents provided by the present invention;

[0031] Figure 3 This is a comparison chart of the tensile properties of the hydrogels provided by the present invention; wherein, Figure 3 (a) is a comparison of the tensile properties of PAM / AA hydrogels. Figure 3 (b) is a comparison curve of the tensile properties of CNF-PAM / AA hydrogel;

[0032] Figure 4 This is a graph showing the test results of the adhesion performance of the hydrogel to glass provided by the present invention; wherein, Figure 4 (a) shows the adhesion test results of PAM / AA hydrogels with different water contents to glass. Figure 4 (b) shows the adhesion test results of CNF-PAM / AA hydrogels with different water contents to glass;

[0033] Figure 5 This is a data graph of the pure shear performance results of the double-layer hydrogel provided by the present invention; wherein, Figure 5 (a) is a schematic diagram of the internal network structure between CNF-PAM / AA hydrogel layers. Figure 5 (b) shows the tensile properties of CNF-PAM / AA hydrogels with different interface layer thicknesses. Figure 5 (c) shows the pure shear test results of double-layer CNF-PAM / AA hydrogel with or without interface layer. Figure 5 (d) is a schematic diagram of the crack propagation process of the double-layer CNF-PAM / AA hydrogel. Figure 5 (e) is a schematic diagram of the interface layer gradient connection mechanism mediated by dynamic hydrogen bonds;

[0034] Figure 6 This is a data graph of the peeling performance results of the double-layer hydrogel provided by the present invention; wherein, Figure 6 (a) is the adhesion performance result of PAM / AA hydrogel. Figure 6 (b) is the adhesion performance result of CNF-PAM / AA hydrogel. Figure 6 (c) is the infrared test spectrum of the PAM / AA hydrogel interface layer. Figure 6 (d) is the infrared test spectrum of the CNF-PAM / AA hydrogel interface layer;

[0035] Figure 7 The energy release rate of lateral debonding of the double-layer CNF-PAM / AA hydrogel provided by the present invention;

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

[0037] The following will clearly and completely describe the technical solutions in the embodiments of the present invention in conjunction with the accompanying drawings. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative efforts are within the scope of protection of the present invention.

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

[0039] The sources of the experimental materials used in this invention: Acrylamide (AAM), acrylic acid (AAc), N,N-methylenebisacrylamide (MBAA), microcrystalline cellulose (MCC), and zirconyl chloride octahydrate (0.5 mol / L, Aladdin) were purchased from Shanghai Aladdin Biochemical Technology Co., Ltd. The particle size of MCC was approximately 147 μm. Phenyl (2,4,6-trimethylbenzoyl), lithium hydroxide (LiOH), and urea were purchased from Beijing Honghu United Chemical Products Co., Ltd. All reagents in this experiment were used directly without further purification. Unless otherwise specified, other raw materials were commercially available products in the field.

[0040] Example 1

[0041] This embodiment provides a method for preparing an interface toughening of a dynamic hydrogen bond-enhanced double-layer hydrogel, comprising the following steps:

[0042] S1. Place the dried microcrystalline cellulose in a solution consisting of LiOH and urea, stir evenly, and then stand overnight at low temperature to completely dissolve the microcrystalline cellulose, and stir again to obtain a transparent microcrystalline cellulose solution;

[0043] S2, adding a chemical crosslinking agent to the microcrystalline cellulose solution and stirring 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 the mixed solution evenly, placing the solution in a mold, and photocuring the solution to obtain a single-layer hydrogel;

[0045] S4, soaking the single-layer hydrogel in a zirconium ion solution for different time periods to obtain a base layer and a cracked layer, and introducing pre-cracks in the cracked layer;

[0046] S5. Add the mixed solution obtained by mixing the CNF solution and the polymerization precursor solution dropwise onto the surface of the cracked layer to form an interface layer; and place the base layer on top of the interface layer, and photocuring again to obtain a dynamic hydrogen bond enhanced double-layer hydrogel.

[0047] Specifically, in S1, the drying conditions of the microcrystalline cellulose are: drying for 24 hours in an environment at 60°C to remove moisture from 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°C. In S2, the chemical crosslinker 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, crosslinker N,N-methylenebisacrylamide 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 of 25°C; the single-layer hydrogel obtained by photocuring is CNF-PAM / AA hydrogel. In S4, the zirconium ion solution is a 0.5 mol / L zirconium oxychloride octahydrate solution. The base layer is immersed for 24 hours, and the cracked layer is immersed for 12 hours. The pre-crack is 20 mm long and is formed by vertically cutting a blade into the middle of the cracked layer. In S5, the thickness of the interface layer is controlled by a mold, and the mold thickness is 0.1 mm to 1 mm. The secondary light curing conditions are: irradiation with a 30W, 365nm UV lamp at 25°C for 30 minutes.

[0048] The dynamic hydrogen bond-enhanced double-layer hydrogel includes a cracked layer, an interface layer and a base layer. The interface layer is located between the cracked layer and the base layer. The cellulose nanofibers CNF in the interface layer form a dynamic hydrogen bond network with the acrylic acid carboxyl groups in the cracked layer and the base layer through hydroxyl groups to achieve interface toughening.

[0049] The difference in adhesion energy between the interlayer with and without cellulose (thickness 0.1-1 mm) was compared by the controlled variable method, and the crack propagation behavior was analyzed by peeling test and tensile test.

[0050] 1. Scanning electron microscope test

[0051] Conductive carbon glue was applied to the sample stage, and the sample was picked up with tweezers and adhered to the conductive carbon glue. The sample stage with the sample was quickly frozen in liquid nitrogen slush for 30 seconds. Then, a cryogenic freezing preparation transfer system was used to transfer the sample to the sample preparation chamber under vacuum for sublimation gold plating. The sample was sublimated at -90°C for 10 minutes and then sputter-coated with gold at a current of 10 mA for 60 seconds. Finally, the processed sample was microscopically observed and analyzed using a scanning electron microscope (Hitachi SU8010, PP3000T, Japan).

[0052] 2. Fourier transform infrared absorption spectroscopy test

[0053] After drying and grinding, the sample was uniformly mixed with potassium bromide (KBr) powder at a mass ratio of 1:100 and a transparent sheet was prepared by tableting. The prepared sample sheet was placed on the sample holder of the infrared spectrometer to ensure that the sample surface was flat and free of bubbles or impurities. The instrument parameters were set, including 32 sample scans, 32 background scans, and a resolution of 4.000 cm. -1 , sampling gain of 1.0, mirror speed of 0.4747, and aperture of 100.00. The test was performed using a Thermo Fisher Scientific Nicolet iS50 Fourier transform infrared spectrometer, with a DTGS KBr detector, a KBr beam splitter, and an infrared light source. During the test, the instrument automatically records the infrared absorption spectrum of the sample, typically in the wavenumber range of 4000 to 400 cm -1 By analyzing the position, intensity and shape changes of characteristic absorption peaks, the existence of hydrogen bonds in the sample and the changes in their intensity can be determined.

[0054] 3. Mechanical testing

[0055] All mechanical tests were performed at room temperature using an IBTC-300 micro in-situ mechanical testing machine with a 500N sensor to perform pure shear (PS) tests to measure the fracture properties of the hydrogels. The pure shear test was originally proposed by Rivlin and Thomas to test the fracture of rubber samples and has recently been used to characterize gel fracture. Both cracked and cracked samples were used. The uncracked hydrogel had a size of 50mm×50mm×1mm and was glued between two clamps for uniaxial tensile testing. The sample size was 50mm×10mm×1mm (H=10mm). The cracked sample was prepared by introducing a crack of c=20mm in the middle of the sample with a blade. The tensile λ is defined as the current length divided by the initial length of the specimen. The nominal stress is defined as the applied force divided by the cross-sectional area of ​​the specimen in the undeformed state.

[0056] To measure fracture toughness, cracked and uncracked specimens were stretched at a rate of 50 mm / min. Fracture toughness was calculated as Γ = HW(λc), where H is the height of the specimen and W(λc) is the integral of the stress-stretch curve of the uncracked specimen from λ = 1 to the critical stretch λ = λc, where λc is defined as the stretch at which crack growth begins. The energy release rate was calculated as G = HW(λc). s ), λ s Defined as the stretch before crack growth begins.

[0057] In order to measure the elastic modulus, the crack-free specimen was stretched at a speed of 50 mm / min, and the slope of the stress-stretch curve at small deformation was the elastic modulus E.

[0058] 4. Peel test

[0059] One end of a bilayer hydrogel sample was fixed to a fixture in a testing apparatus, and the other end was connected to another fixture. The testing apparatus was controlled to apply tension at a constant rate, stretching the sample at a speed of 50 mm / min to gradually separate the two layers. During this process, the variation of tension with displacement was recorded, reflecting the interaction between the two hydrogel layers during the peeling process. Analysis of the force-displacement curve allowed the calculation of the interfacial adhesion energy (w = F / ω), where F is the peeling force and w is the sample width. The interfacial adhesion energy is a key parameter measuring the adhesion strength between two hydrogel layers; it represents the energy required to completely separate the two layers. To enhance the accuracy and reliability of the experimental results, multiple replicates were used for each experiment. This approach reduces experimental error and avoids bias in the results due to the idiosyncrasies of individual samples. Statistical analysis of the experimental data from these replicates, including calculation of the mean and standard deviation, more accurately reflects the interfacial properties of the bilayer 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 test results are as follows:

[0061] 1. Microstructure of two hydrogels with and without cellulose interlayer:

[0062] Among them, the one without cellulose middle layer is PAM / AA hydrogel, and the one with cellulose middle layer is CNF-PAM / AA hydrogel. Figure 1 (a) and (c), (b) and (d) in the equation can be obtained. Figure 1(c) and (d) show an interlaced and highly porous network. The porous structure is attributed to the steric hindrance caused by the rigid CNFs, which prevents the close packing of polymer chains, thus forming a porous structure within the hydrogel. The coordination bond interaction between CNFs and PAAM chains inhibits the movement of polymer chains, preventing them from freely arranging during the freeze-drying process, resulting in a porous structure with low shrinkage after drying.

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

[0064] The mechanical properties of PAM / AA hydrogels and CNF-PAM / AA hydrogels with different CNF contents and immersion time were tested to evaluate their performance indicators such as strength, Young's modulus and elongation. The results are as follows: Figure 2 shown.

[0065] The cracking layer needs to be able to crack when subjected to external forces, so its mechanical properties need to be moderate, with both a certain strength and the ability to deform. The base layer, on the other hand, needs to provide stable support and requires higher strength and modulus. Analysis of the stress-elongation curves shows that the four hydrogel samples exhibit different mechanical responses.

[0066] After 12 hours of immersion and a CNF content of 0.63%, the hydrogel exhibited a strength of 2.39 MPa and a Young's modulus of 1.28 MPa. These mechanical properties allow the hydrogel to deform slightly when subjected to external forces, resulting in cracks, while preventing it from becoming brittle and completely rupturing. The hydrogel under these conditions had a water content of 68.51%. This high water content imparts excellent flexibility and biocompatibility to the hydrogel, facilitating the formation and propagation of cracks. After 24 hours of immersion and a CNF content of 0.63%, the hydrogel exhibited a strength of 3.145 MPa and a Young's modulus of 1.47 MPa. These high mechanical properties enable the basal layer to withstand greater external forces and provide stable support. The hydrogel under these conditions, at a water content of 68.47%, slightly lower than the cracking layer, still maintained excellent flexibility and biocompatibility. This moderate water content helps the basal layer provide support without becoming brittle due to excessive drying.

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

[0068] Based on the above content, the CNF-PAM / AA hydrogel soaked for 12 hours and with a CNF content of 0.63% was determined as the crack layer, and the CNF-PAM / AA hydrogel soaked for 24 hours and with a CNF content of 0.63% was determined as the base layer to optimize the performance and application effect of the double-layer cellulose fiber hydrogel.

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

[0070] The interface layer is solidified with the same monomer content and CNF content, but different water content. The water content increases gradually from 18g to 33.85g. The experiment is divided into two parts: first, the tensile properties of 12 different interface layers of the CNF-free system and the CNF-containing system are tested, and then the adhesion of these 12 groups of interface layers between the double-layer hydrogel is tested. The experiment fixes the CNF content (0.63%) and the monomer ratio, regulates the water content of the interface layer (56.05% to 68.65%), and systematically studies its effect on the hydrogen bond density and interface toughening performance.

[0071] The tensile properties of the hydrogels without CNF system and with CNF system (not soaked in zirconium ion solution, as the interface layer) were tested, and the results were as follows: Figure 3 As shown in (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 drops sharply from 527.93 kPa to 130.36 kPa. This indicates that the increase in water content leads to a decrease in the strength of the cellulose-free hydrogel, an increase in flexibility, but a significant decrease in stiffness.

[0072] For the CNF-containing system, as the water content increases from 56.08% to 68.63%, the stress first decreases from 0.0209MPa to 0.12895MPa, the elongation continues to increase from 3.1203 to 5.7502, and the Young's modulus gradually decreases from 339.65kPa to 70.17kPa. The results are as follows: Figure 3 This trend indicates that, within a certain range, increasing water content strengthens the interaction between cellulose and polymer chains, leading to a slight increase in stress. However, as the water content further increases, the overall structure of the system becomes diluted, and the stress gradually decreases. Simultaneously, increasing water content provides more space for polymer chain movement, resulting in a continuous increase in elongation, while Young's modulus decreases due to structural weakening.

[0073] By comparing and analyzing the stress, elongation and Young's modulus properties of hydrogels with and without CNF systems, significant differences were found between the two. At the stress level, the system without CNF showed a significant strength attenuation of 59% with increasing water content, while the decrease in the system with CNF narrowed to 37%, indicating that the auxiliary network formed by CNF through hydrogen bond crosslinking and physical entanglement effectively delayed the weakening of the main network. It is worth noting that when the water content is lower than 60%, the strength of the CNF system is lower than that of the control group, which is attributed to the stress concentration caused by the local agglomeration of CNF under the influence of limited water content; when the water content exceeds 60%, sufficient free water promotes the uniform dispersion of CNF, so that the strength of the system is reversed, verifying the key role of the solvation effect in network reconstruction.

[0074] In terms of elongation performance, the CNF-containing system achieved an 84% increase in elongation, a 22% improvement. Dynamic mechanical analysis revealed that the dynamic hydrogen bond network of CNFs undergoes a break-recombination cycle during stretching, delaying crack initiation through a reversible energy dissipation mechanism. The trend in Young's modulus further reveals the dual-phase effect of CNFs: at low water content (<55%), CNFs act as a rigid filler, increasing the initial modulus by 21%. However, at high water content (>65%), phase separation between CNFs and the polymer matrix causes a 79% drop in modulus, a 4% increase in the drop, confirming the interfacial slip mechanism dominated by the water plasticization effect.

[0075] 4. Adhesion performance test results:

[0076] The adhesion performance of two hydrogels, one without CNF and one with CNF, was tested by adhering them to glass. The results are shown in the following table. Figure 4 As shown in (a) and (b) in the CNF-free system, as the water content increases, the adhesion force gradually increases from 0.1673N to 0.51494N, and the adhesion energy increases from 8.635J / m 2 Gradually increased to 25.747 J / m 2 This indicates that the increase in water content helps to improve the adhesion properties of CNF-free hydrogels, possibly because the increase in water content enhances the interaction between the polar groups on the hydrogel surface and the glass surface, thereby improving the adhesion force and adhesion energy.

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

[0078] In addition, the pure shear performance, peeling performance, energy release rate and fitting coefficient analysis of the double-layer hydrogel prepared above were performed. The results are shown below, including:

[0079] 1. Pure shear performance results

[0080] Figure 5 Figure (a) shows the three-dimensional network structure formed by hydrogen-bonding interactions between CNF and polyacrylic acid-acrylamide copolymer. Here, CNF (chain structure) and P(AAc-AAm) (flexible polymer chains) are dynamically cross-linked via hydrogen bonds, forming a composite hydrogel system with interpenetrating network properties. Water molecules (H2O), acting as a solvent and hydrogen bonding medium, are uniformly distributed throughout the network, promoting intermolecular interactions and imparting excellent swelling properties and structural stability.

[0081] In order to determine the optimal thickness of the interface layer between the double-layer hydrogels, double-layer hydrogels with interface layer thicknesses of 0.1 mm, 0.5 mm, and 1 mm were prepared, and their mechanical properties were tested. The experimental results are shown in Figure 2. Figure 5 As shown in (b) in . Figure 5 (b) shows the effect of different water content of the interface layer on the pure shear properties of the bilayer hydrogel under the conditions of fixed CNF content (0.63%) and interface layer thickness (0.1mm). Experimental data show that when the water content increases from 56.08% to 68.63%, the peak stress of the system increases from 0.48MPa to 1.02MPa, an increase of 113%, while the elongation increases from 5.68 to 9.68, showing a significant synergistic effect of strength and toughness. This phenomenon can be attributed to the following: at high water content (>64%), free water molecules act as plasticizers to promote the dynamic reorganization of hydrogen bonds between CNF and PAM chains, forming an energy dissipation network with a gradient entanglement density; as the water content increases, the modulus of the interface layer decreases from 0.34MPa to 0.07MPa, and the matching degree with the modulus of the base layer (0.47MPa) is significantly improved.

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

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

[0084] Pure shear experiments were conducted on bilayer hydrogels containing CNF interface layers and bilayer hydrogels without interface layers. The experimental results are shown in Figure 2. Figure 5 As shown in (c) in . Figure 5 Figure (d) shows the crack propagation process in the interface layer of a bilayer hydrogel under stress. From left to right, the figure shows the crack initiation, propagation, passivation, and final stabilization process in the interface layer. In the initial stage of stress, the crack gradually forms and propagates in the interface layer. As the stress continues, the hydrogen bond network in the interface layer begins to function, dissipating some energy through the breaking and reforming of hydrogen bonds, thereby slowing further crack propagation. Ultimately, the crack reaches a stable state in the interface layer, effectively improving the fracture toughness of the bilayer hydrogel.

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

[0086] It is worth noting that the elongation of the control group without interface layer (λ = 2.29) is only 23.6% of that of the system with interface layer, and the fracture energy (0.45MJ / m 3 ) dropped to 4.3% of the optimal system, fully verifying the necessity of interface layer design.

[0087] 2. Peeling performance test results

[0088] Here, a peeling experiment was conducted on the double-layer hydrogel to further explore the adhesion properties of different hydrogel systems as the interface layer, and then analyze the interface toughening effect of the CNF system. The results were obtained by referring to Figure 6 .in, Figure 6 (a) and (b) reveal the significant regulatory effect of cellulose nanofibers (CNF) on interfacial adhesion properties and their water content dependence. For the CNF-containing system, when the water content increases from 56.08% to 68.63%, the adhesion force gradually increases from 3.08N to 6.14N, and the adhesion energy increases from 154J / m 2 Increased to 307J / m 2 . Compared with the system without CNF, under the same water content conditions, the adhesion force and adhesion energy of the system containing CNF are significantly higher. This is mainly attributed to the unique properties and functions of CNF. CNF has abundant polar groups such as hydroxyl groups. These polar groups can form various interactions such as hydrogen bonds and electrostatic effects with 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. This structure helps to improve the mechanical interlocking effect between the hydrogel and the double-layer gel, further enhancing the adhesion performance.

[0089] In addition, the peeling behavior of bilayer hydrogels at different interfaces (gel-glass and gel-gel) was analyzed in depth to reveal the differences in their adhesion properties and their underlying mechanisms. The experimental results showed that the adhesion force and adhesion energy at the gel-gel interface were significantly higher than those at the gel-glass interface, both in the CNF-free and CNF-containing systems. Further analysis revealed that the adhesion energy of both systems increased with increasing water content, but the rate of increase was faster in the CNF-containing system. This suggests that an appropriate amount of water can make the polymer chains more flexible, allowing them to better conform to the substrate surface microtopography and enhance effective contact. However, excessive water can lead to excessive swelling and weaken interfacial bonding strength. The presence of CNFs can lock in some water molecules through hydrogen bonds, slowing uncontrolled swelling. Furthermore, at low water content, the adhesion energy of the CNF-containing system was significantly higher than that of the CNF-free system, indicating that the polar groups of CNFs play a dominant role in interfacial bonding. At high water content, the adhesion energy of the CNF-containing system increased more rapidly, suggesting that CNFs may mitigate the damage to interfacial bonding caused by swelling by locking in water.

[0090] From the perspective of the dominant dissipation mechanism, the gel-glass interface dissipates energy primarily through hydrogen bond breakage (-COOH of AAc and Si-OH of glass) and segment slippage, while the gel-gel interface dissipates energy through segment interpenetration (polymer chains of the two gel layers diffuse into each other at the contact interface, forming a physical entanglement network) and dynamic bond reorganization (-OH of cellulose and -COOH of AAc). In terms of failure mode, the gel-glass interface exhibits interfacial 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 interfacial debonding, enhancing segment interpenetration, and providing dynamic sacrificial bonds.

[0091] W=W0+ΔW dynamic ;

[0092] W0 is the intrinsic adhesion energy without dynamic hydrogen bonds (contributed by main chain chemical bonds), ΔW dynamic is the additional energy contributed by dynamic hydrogen bond dissipation, which is related to the hydrogen bond density (N), the breakage energy of a single hydrogen bond (∈), and the recombination rate (f): ΔW dynamic ∝N·∈·f. CNF significantly improves ΔW by increasing the hydrogen bond density (N) dynamic , thereby increasing the adhesion energy.

[0093] Quantitative experimental data demonstrates that the adhesion energy of the CNF-containing systems increases by 15.6% (gel-glass) and 18.5% (gel-gel), respectively, consistent with theoretically predicted contributions from dynamic dissipation. This suggests that CNF significantly increases the additional energy dissipated by dynamic hydrogen bonds by increasing hydrogen bond density, thereby enhancing adhesion energy.

[0094] Combine Figure 5 (b) and Figure 6 (b) found that both groups of curves showed obvious "first three groups - last three groups" grouping characteristics, revealing the existence of a significant threshold effect in 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 effect 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 in the system as hydrogen bond donors / acceptors is significantly enhanced, promoting the optimized construction of the hydrogen bond network in the interface layer, making the interfacial adhesion effect dominant, and thus 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 interface 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 the dynamic breaking and reorganization of hydrogen bonds, and ultimately achieving a phased leap in interface performance.

[0095] In order to verify the correctness of the above analysis, Fourier transform infrared absorption spectroscopy tests were carried out on the interface layers of the two systems.

[0096] For cellulose-free systems (refer to Figure 5 (c)) At low water content (52.13%-59.22%), the OH peak is located at a higher wave number (~3336cm -1 ), the peak shape is narrow and the absorbance is low (0.02-0.18), indicating that the hydrogen bond network is sparse and the hydroxyl groups mainly exist in the form of free or weak hydrogen bonds. As the water content increases (62.03%-67.19%), the OH peak gradually shifts to ~3191cm -1 The absorbance increased slightly (0.20-0.30), but the change was gentle, indicating that water molecules supplemented some interactions as hydrogen bond donors but did not form a dense network. The C=O peak is located at a higher wave number (1665-1680cm -1 ), the absorbance is low (0.08-0.20), and the wave number only shifts slightly with the increase of water content (~1655cm -1 ), indicating that the C=O group mainly forms weak hydrogen bonds with itself or a small amount of water molecules.

[0097] For cellulose-containing systems (ref. Figure 5 (d)) Under all water content conditions, the OH peak is significantly broadened and has a higher absorbance (0.24-0.42), and the wave number is significantly lower than that of the CNF-free system (e.g., 3191 cm -1 vs3336cm -1 ), indicating that the hydroxyl groups of CNF form a dense hydrogen bond network with water molecules and polymer chains. As the water content increases (56.08%-68.63%), the OH peak further shifts to ~3150 cm -1 , and the migration rate is accelerated after the critical water content (~64.33%), and the absorbance increases nonlinearly (0.32-0.42), which is consistent with the adhesion energy growth trend, confirming the reconstruction of the hydrogen bond network under the threshold effect. The C=O peak wave number is significantly reduced (1650-1665cm -1 ), the absorbance increased significantly (0.22-0.38), and further shifted to ~1640 cm with increasing water content. -1 This phenomenon is attributed to the formation of strong hydrogen bonds between the hydroxyl groups of CNF and the C=O groups, and the dynamic bonding network optimizes the bonding density with the increase of water content.

[0098] The shift amplitude of OH and C=O peaks in the CNF-containing system is significantly greater than that in the CNF-free system, especially after the critical water content (~64%), which is consistent with the trend of the adhesion energy jump, verifying the threshold mechanism of hydrogen bond network reconstruction. The OH peak absorbance and peak width of the CNF-containing system are higher than those in the CNF-free system at all water contents, and increase nonlinearly, which is consistent with the dynamic hydrogen bond dissipation contribution (ΔW dynamicThe infrared spectroscopy results are highly consistent with the mechanical properties data (adhesion and adhesion energy increase with increasing water content), indicating that CNF significantly enhances interfacial interactions by increasing hydrogen bond density and dynamic bonding networks.

[0099] 3. Energy release rate analysis

[0100] The energy release rate is a key parameter for evaluating a material's crack growth behavior, reflecting the energy released per unit area of ​​crack expansion. In this example, the energy release rates of bilayer hydrogels with and without an interfacial layer were analyzed to reveal the influence of the interfacial layer on crack growth behavior. The experimental results show that the energy release rate of the bilayer CNF-PAM / AA hydrogel with an interfacial layer is significantly lower than that of the system without an interfacial layer, and exhibits a regular decreasing trend with increasing water content. This phenomenon reveals the critical role of the interfacial layer in energy dissipation: CNFs within the interfacial layer form a strong interaction with the hydrogel matrix through a hydrogen bond network. When subjected to stress, the dynamic hydrogen bond breakage and reformation process continuously dissipates energy, effectively reducing the overall energy release rate of the system. This indicates that the presence of the interfacial layer effectively hinders crack growth and enhances the toughening properties of the material. Further analysis revealed that the energy release rate gradually increases with increasing water content. This is likely because an appropriate amount of water enhances the flexibility of the polymer chains, thereby improving the material's ductility and energy dissipation capacity. However, when the water content exceeds a certain threshold, excessive water may lead to a decrease in the material's strength, consistent with the threshold effect discussed previously.

[0101] The results of the energy release rate of the interface lateral debonding, such as Figure 7 As shown, it decreases with increasing water content and exhibits a clear negative correlation with adhesion energy—the higher the adhesion energy, the lower the transverse debonding energy release rate. This indicates that at higher water contents, the debonding resistance of the interfacial layer decreases, likely due to the weakening effect of water on the adhesion properties of the interfacial layer. However, the addition of CNF mitigates this trend to some extent, increasing the adhesion energy and debonding resistance of the interfacial layer by increasing hydrogen bond density and forming a physical entanglement network. This echoes the previously mentioned role of cellulose in improving adhesion properties. This finding, further analyzed in the transverse debonding energy release rate, further validates the interfacial toughening mechanism: the high adhesion energy interfacial layer suppresses energy release at the crack tip by enhancing energy dissipation at the interface, thereby hindering crack propagation. These results are highly consistent with the theoretical framework of interfacial toughening and confirm that the strategy of introducing a CNF-containing interfacial layer into a bilayer hydrogel can achieve efficient interfacial toughening by precisely controlling the relationship 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, the experimental data were fitted to obtain a series of fitting coefficients with important physical significance. These coefficients not only reflect the performance characteristics of the double-layer cellulose fiber hydrogel under different parameters, but also provide key clues for a deeper understanding of its interfacial toughening mechanism. Specifically:

[0104]

[0105] First, the Dundurs parameters α and β are calculated, which are used to describe the elastic mismatch between two heterogeneous materials. For the cracked layer and the base layer materials in this embodiment, α = -0.069 ≈ 0 and β = 0 are calculated with reference to formulas (2) and (3). 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 base layer materials are similar, and the coefficient Z(α, β) = 1.976 is determined.

[0106] For a double-layer material without an interface, the coefficient n is added to formula (1) to obtain formula (4):

[0107]

[0108] By fitting the experimental data, such as Figure 8 As shown in (a), the coefficient n = 11.25, indicating that the crack propagation is mainly dominated by the intrinsic toughness of the matrix.

[0109] For different interface layers, use formula (5):

[0110]

[0111] By fitting the experimental data, such as Figure 8 As shown in (b) of Figure 1, the coefficients t are 43.56, 36.42, 31.9, 30.28, 29.62, and 28.75, respectively. The decrease in these coefficients t indicates that as the moisture content increases, the interfacial layer's resistance to crack propagation weakens, and the energy release rate decreases. This indicates that increasing moisture content weakens the interfacial bond strength, but the addition of cellulose mitigates this effect to some extent.

[0112] For the case of lateral debonding at the interface, use formula (6):

[0113]

[0114] By fitting the experimental data, such as Figure 8 As shown in (c), the coefficient Z is obtained dThey 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. d The decrease in shows that at higher water content, the lateral debonding resistance of the interface layer increases and the energy release rate G d The lower the moisture content, the easier it is for cracks to propagate, and the interface dissipation ratio increases by 21%. This further confirms the significant effect of moisture content on the performance of the interface layer, and also reflects the important role of cellulose in improving the stability of the interface layer.

[0115] At the same time, the decreasing trend of the interface transverse debonding coefficient (4.2→0.477) is also directly related to the transformation of the failure mode: at low water content (56.08%), Z d =4.2 corresponds to brittle debonding, and energy dissipation is mainly due to the breakage of the main chain; when the water content is high (68.63%), Z d = 0.477, reflecting that the tough fiber bridging is dominant, and the dynamic bond reorganization and CNF bridging contribute an additional 52% energy dissipation.

[0116] For the bilayer hydrogel without an interface, the fitting coefficient n was 11.24518. However, for the bilayer hydrogel with an interface, the coefficient t gradually decreased with increasing water content, from 43.55646 (56.08% water content) to 28.75201 (68.63% water content). This trend suggests that increasing water content leads to a decrease in the adhesion of the interfacial layer, thereby reducing the energy release rate. In other words, increasing water content weakens the interfacial bond strength, although the addition of CNF mitigates this effect to some extent.

[0117] Moisture content and adhesion energy are key factors influencing the coefficients: With increasing water content, the fitting coefficient for the interfacial lateral debonding energy release rate shows a significant decreasing trend, from (4.2±1.6) to (0.477±0.293). This change essentially reflects the regulatory mechanism of water content on the interfacial hydrogen bond network and adhesion energy. High water content promotes the participation of water molecules as hydrogen bond donors / acceptors in the construction of the interfacial structure, enhancing interfacial adhesion energy, and thus suppressing crack propagation through enhanced energy dissipation, ultimately manifesting as a regular decrease in the fitting coefficient.

[0118] Traditional energy release rate formulas often focus on single material systems. However, this application reveals the synergistic effect of water content and adhesion energy on interfacial toughening in CNF-reinforced bilayer hydrogels, expanding the formula's applicability to complex interfacial systems. In summary, changes in the fitting coefficients reflect the combined influence of factors such as adhesion energy and water content on the energy release rate. By combining experimental and theoretical approaches, this application deeply reveals the mechanisms of these factors, providing a solid theoretical foundation and practical guidance for the optimized design of bilayer hydrogel materials.

[0119] In summary, CNF-containing bilayer hydrogels exhibit higher stress, elongation, and Young's modulus, and their adhesion energy is approximately 18.5% higher than that of cellulose-free systems. This is primarily attributed to the CNF-containing system exhibiting superior ductility due to the breakage-reorganization mechanism of the dynamic hydrogen bond network, and the CNFs forming an auxiliary network through hydrogen bonding and physical entanglement, which enhances the material's mechanical properties and interfacial stability. At the same time, an appropriate amount of water can make the polymer chains more flexible and improve effective contact, but excessive water can cause swelling and weaken the interfacial bond strength. By optimizing the thickness of the interfacial layer, it was found that an interfacial layer thickness of 0.1 mm can achieve the best mechanical properties for the bilayer hydrogel, and there is a threshold effect for the water content of the interfacial layer. High water content causes the hydrogen bond network to dominate interfacial adhesion, significantly improving the mechanical response of the bilayer structure.

[0120] Analysis of the energy release rate further confirmed the superior performance of the CNF-containing bilayer hydrogel. Its energy release rate was significantly lower than that of the CNF-free system, and the trend of the fitting coefficient was closely related to factors such as adhesion energy and water content. This suggests that the addition of cellulose not only increases adhesion energy but also effectively reduces the energy release rate and slows crack propagation by increasing hydrogen bond density and forming a physical entanglement network. The bilayer hydrogel containing the interfacial layer effectively reduces the energy release rate by enhancing adhesion energy. The fitting coefficient shows a clear correlation with water content and adhesion energy, revealing a quantitative interface toughening mechanism of "water content-hydrogen bond network-adhesion energy-energy release rate."

[0121] Therefore, the above-mentioned dynamic hydrogen bond enhanced double-layer hydrogel and its interface toughening preparation method are adopted to construct a dynamic hydrogen bond network through cellulose nanofibers, and combined with the interface layer gradient design to achieve the synergistic enhancement of crack tip stress relaxation and energy dissipation, providing a new path for the design of high-performance flexible materials and promoting their application value in the fields of biomedicine and flexible electronics.

[0122] This document uses specific examples to illustrate the principles and implementation methods of the present invention. The above examples are only intended to help understand the method and core concept of the present invention. At the same time, those skilled in the art will find that the specific implementation methods and application scopes may vary based on the concept of the present invention. In summary, the contents of this specification should not be construed as limiting the present invention.

Claims

1. A method for preparing an interface toughening of a dynamic hydrogen bond-enhanced double-layer hydrogel, characterized in that: The following steps are involved: S1. Place the dried microcrystalline cellulose in a solution consisting of LiOH and urea, stir evenly, and then stand overnight at low temperature to completely dissolve the microcrystalline cellulose, and stir again to obtain a transparent microcrystalline cellulose solution; S2, adding a chemical crosslinking agent to the microcrystalline cellulose solution and stirring to obtain a CNF solution; S3, mixing the CNF solution and the polymer precursor solution in equal proportions to obtain a mixed solution, stirring the mixed solution evenly, placing the solution in a mold, and photocuring the solution to obtain a single-layer hydrogel; S4, soaking the single-layer hydrogel in a zirconium ion solution for different time periods to obtain a base layer and a cracked layer, and introducing pre-cracks in the cracked layer; S5. Add the mixed solution obtained by mixing the CNF solution and the polymerization precursor solution dropwise onto the surface of the cracked layer to form an interface layer; and place the base layer on top of the interface layer, and photocuring again to obtain a dynamic hydrogen bond enhanced double-layer hydrogel.

2. The method for preparing an interface toughening of a dynamic hydrogen bond-enhanced double-layer hydrogel according to claim 1, characterized in that: In S1, the drying condition of the microcrystalline cellulose is: drying at 60° C. for 24 hours to remove moisture from the microcrystalline cellulose.

3. The method for preparing an interface toughening of a dynamic hydrogen bond-enhanced double-layer hydrogel according to claim 2, characterized in that: In S1, the solution consisting of LiOH and urea contains LiOH and urea in a mass ratio of 4.6:15, the solvent is deionized water, and the low temperature environment is -15°C.

4. The method for preparing an interface toughening of a dynamic hydrogen bond-enhanced double-layer hydrogel according to claim 1, characterized in that: In S2, the chemical cross-linking agent is epichlorohydrin, and the mass ratio of microcrystalline cellulose to epichlorohydrin in the microcrystalline cellulose solution is 2:

1.

5. The method for preparing an interface toughening of a dynamic hydrogen bond enhanced double-layer hydrogel according to claim 1, characterized in that: In S3 , the polymer precursor solution includes acrylamide AAM, acrylic acid AAc, a cross-linking agent N,N-methylenebisacrylamide MBAA, and a photoinitiator phenyl (2,4,6-trimethylbenzoyl).

6. The method for preparing an interface toughening of a dynamic hydrogen bond-enhanced double-layer hydrogel according to claim 5, characterized in that: In S3, the photocuring conditions are: using a 30W, 365nm ultraviolet lamp, irradiating at 25°C for 30 minutes; the single-layer hydrogel obtained by photocuring is CNF-PAM / AA hydrogel.

7. The method for preparing an interface toughening of a dynamic hydrogen bond enhanced double-layer hydrogel according to claim 1, characterized in that: In S4, the zirconium ion solution is a 0.5 mol / L zirconium oxychloride octahydrate solution, the immersion time of the base layer is 24 hours, and the immersion time of the crack layer is 12 hours; the length of the pre-crack is 20 mm, and it is formed by vertically cutting the middle position of the crack layer with a blade.

8. The method for preparing an interface toughening of a dynamic hydrogen bond-enhanced double-layer hydrogel according to claim 1, characterized in that: In S5, the thickness of the interface layer is controlled by a mold, and the thickness of the mold is 0.1 mm to 1 mm; the conditions for the secondary photocuring are: using a 30W, 365nm ultraviolet lamp, irradiating at 25°C for 30 minutes.

9. A dynamic hydrogen bond enhanced double-layer hydrogel prepared by the method for preparing a dynamic hydrogen bond enhanced double-layer hydrogel by interface toughening according to any one of claims 1 to 8, characterized in that: The dynamic hydrogen bond-enhanced double-layer hydrogel includes a cracked layer, an interface layer and a base layer. The interface layer is located between the cracked layer and the base layer. The cellulose nanofibers CNF in the interface layer form a dynamic hydrogen bond network with the acrylic acid carboxyl groups in the cracked layer and the base layer through hydroxyl groups to achieve interface toughening.

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

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