Cellulose nanocrystal enhanced solvent response type double-layer hydrogel actuator as well as preparation method and application thereof

Abstract

The invention discloses a cellulose nanocrystal enhanced solvent response type double-layer hydrogel actuator as well as a preparation method and application thereof. The cellulose nanocrystal enhanced solvent response type double-layer hydrogel actuator is characterized in that a first hydrogel layer and a second hydrogel layer are prepared from the following raw materials: an acrylamide monomer, a cellulose nanocrystal suspension, a cross-linking agent and a photoinitiator; the concentration of the cellulose nanocrystal suspension used by the first hydrogel layer is 1-6 wt%, and the concentration of the cellulose nanocrystal suspension used by the second hydrogel layer is 0.5 wt%; according to the preparation method, the cellulose nanocrystals with different contents are added into the double-layer hydrogel, and the double-layer hydrogel structure with a heterostructure is realized by utilizing ultraviolet polymerization, so that the preparation method has the advantages that the mechanical property and the adjustable bending angle of the hydrogel are remarkably improved, and the double-layer hydrogel actuator with the reversible solvent response characteristic is realized.

Description

Technical Field

[0001] The present invention relates to a double-layer hydrogel actuator, in particular to a cellulose nanocrystal-enhanced solvent-responsive double-layer hydrogel actuator and a preparation method and application thereof. Background Art

[0002] Hydrogel is a material with a three-dimensional network structure formed by cross-linking hydrophilic polymer chains. With its excellent water retention, biocompatibility and stimulus-responsive properties, it has become an important functional material in the fields of flexible actuation, soft robotics and smart devices. Hydrogel actuators are devices composed of hydrogels that can respond to specific environments. Their power mainly comes from the polymer chains' repulsion or absorption of water molecules. The changes in water molecules drive changes in the macroscopic shape, converting other forms of energy into mechanical energy. If the hydrogel actuator exhibits uniform swelling or contraction in all directions, it can only achieve simple dimensional changes and cannot complete complex deformations. Currently, the method of making hydrogels achieve complex deformations is usually to use non-homogeneous excitation methods such as local electric fields or light to act on isotropic hydrogels, or to design hydrogels with double-layer heterogeneous structures, cross-linked gradient structures, etc., to generate deformation driving forces through swelling / contraction differences.

[0003] Cellulose nanocrystals (CNCs) have attracted widespread attention due to their low cost, strong bioaffinity, and high mechanical strength. Their good hydrophilicity also lays the foundation for their combination with hydrogels. By introducing cellulose nanocrystals into hydrogels, they can form a three-dimensional network with the hydrogel through the action of hydrogen bonds. On the one hand, different amounts of cellulose nanocrystals introduced will affect the density of the three-dimensional network. By introducing different cellulose nanocrystal contents into the hydrogel structure, hydrogel actuators with different functions can be realized. On the other hand, the introduction of cellulose nanocrystals can greatly improve the mechanical strength of the hydrogel and solve the problem of poor mechanical properties of the hydrogel actuator. Most hydrogel actuators with heterogeneous structures are expensive and have complex manufacturing processes. At present, there are no reports on research on hydrogel actuators with double-layer heterogeneous structures based on cellulose nanocrystals. Summary of the Invention

[0004] The purpose of the present invention is to address the above shortcomings and provide a cellulose nanocrystal-reinforced solvent-responsive double-layer hydrogel actuator with high mechanical strength, reversible solvent responsiveness and controllable bending angle, as well as its preparation method and application.

[0005] The technical solution adopted by the present invention to solve the above technical problems is: A cellulose nanocrystal-enhanced, solvent-responsive bilayer hydrogel actuator comprises a first and second cross-linked hydrogel layer. Both the first and second hydrogel layers are prepared from acrylamide monomer, a cellulose nanocrystal suspension, a cross-linking agent, and an initiator. The bilayer hydrogels contain varying amounts of cellulose nanocrystals, resulting in heterogeneous bilayer hydrogel actuators with varying network densities.

[0006] Preferably, the concentration of the cellulose nanocrystal suspension used in the first layer of hydrogel is 1 wt%-6 wt%, and the concentration of the cellulose nanocrystal suspension used in the second layer of hydrogel is 0.5 wt%.

[0007] Preferably, the thickness ratio of the first hydrogel layer to the second hydrogel layer is 1:1.

[0008] Preferably, the crosslinking agent is N,N-methylenebisacrylamide (Bis), and the initiator is a photoinitiator: 2-hydroxy-4'-(2-hydroxyethoxy)-2-methylpropiophenone (Irgacure 2959).

[0009] The present invention also provides a method for preparing the above-mentioned cellulose nanocrystal-enhanced solvent-responsive double-layer hydrogel actuator, which is characterized by comprising the following steps: 1. A certain amount of acrylamide (AM) monomer, crosslinker and photoinitiator were added to cellulose nanocrystal suspensions of different concentrations and stirred evenly using a magnetic stirrer to prepare a prepolymer solution of hydrogel; 2. The prepolymer solution of the first hydrogel layer is injected into a homemade mold and cured by ultraviolet (UV) light to form the first hydrogel layer; 3. Pour the second layer of prepolymer solution onto the first layer of hydrogel prepared in step S2, and use UV light to cure it again to form a second hydrogel layer, thereby obtaining a cellulose nanocrystal-enhanced solvent-responsive double-layer hydrogel actuator.

[0010] Preferably, in the hydrogel prepolymer solution in step S1, the added amount of the polyacrylamide monomer is 10% of the mass of the cellulose nanocrystal suspension, and the added amount of the cross-linking agent and the photoinitiator are both 3% of the mass of the polyacrylamide monomer.

[0011] The present invention also provides the use of the above-mentioned cellulose nanocrystal-enhanced solvent-responsive double-layer hydrogel actuator as an intelligent gripper in different liquids.

[0012] Preferably, the shape of the intelligent gripper is cross-shaped; it is composed of two strip-shaped double-layer hydrogel actuators (80mm×10mm×2mm), the two double-layer hydrogel actuators are cross-perforated at the center position, and the shape is fixed by two upper and lower silicone films.

[0013] Preferably, the intelligent gripper swells in water and deswells in ethanol, and utilizes the swelling characteristics in different solvents to achieve the intelligent gripping function.

[0014] The basic principles of the present invention are explained as follows: 1) By adding different contents of cellulose nanocrystals into two layers of polyacrylamide (PAM) hydrogel to construct an asymmetric structure, there is a difference in the pore size between the two layers of the solidified double-layer hydrogel. This difference results in different swelling rates of the double-layer hydrogel in water and deswelling rates in ethanol. When the double-layer hydrogel that is completely swollen in water is transferred to ethanol, the non-uniform deswelling and shrinkage on both sides induces bending deformation. 2) Cellulose nanocrystals act as a nano-reinforcement phase, and their surface hydroxyl groups form a cross-linked network with polymer chains, effectively improving the cross-linking density and mechanical strength of the hydrogel. 3) By regulating the difference in cellulose nanocrystal content between the two layers of hydrogel, the controllability of the bending angle can be achieved.

[0015] Compared with the prior art, the advantages of the present invention are: 1. The present invention introduces cellulose nanocrystal ribbons into acrylamide hydrogels, which not only significantly improves the mechanical properties of acrylamide-containing hydrogels, but also has potential applications in soft robots, intelligent grippers and other fields based on its excellent mechanical strength and reversible driving characteristics.

[0016] 2. The double-layer hydrogel actuator prepared by the present invention also gives the double-layer hydrogel network pore size differences along the thickness direction by changing the content of cellulose nanocrystals between the two layers. The pore size difference leads to different swelling rates and deswelling rates, and has bending response characteristics in ethanol and aqueous solutions.

[0017] 3. The present invention adopts a layer-by-layer polymerization method for preparation, and the double-layer structure has the same material, which has the advantages of simple preparation and strong interface bonding.

[0018] 4. The present invention achieves a widely controllable bending angle in ethanol by adjusting the concentration difference of cellulose nanocrystals between the two layers. The specific bending angle depends on the concentration difference of cellulose nanocrystals between the two layers.

[0019] 5. The double-layer hydrogel actuator prepared by the present invention can be bent to achieve reversible deformation in ethanol and water, and the double-layer hydrogel actuator has no delamination or performance degradation after multiple bending / recovery cycles. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] Figure 1 Figure 2 shows the microstructure of PAM hydrogels with different CNC contents, where (a) is a CNC suspension with a concentration of 6 wt%, (b) is a CNC suspension with a concentration of 3 wt%, and (c) is a CNC suspension with a concentration of 0.5 wt%. Figure 2 Swelling rate (a) and water retention rate (b) curves of hydrogels with different CNC contents in water; Figure 3 are the tensile stress-strain curves of different CNC hydrogels; Figure 4 The bending angle of the double-layer hydrogel actuator in ethanol changes with the CNC concentration of the first hydrogel layer. The horizontal axes 1-6 in the figure represent Examples 1-6 respectively; Figure 5 Bending / recovery cycle test of double-layer hydrogel actuator in ethanol / deionized water; Figure 6 The double-layer hydrogel actuator is used as a smart gripper to capture objects in an ethanol environment. DETAILED DESCRIPTION

[0021] The present invention will be described in further detail below with reference to the embodiments of the accompanying drawings. These examples are only for illustrating the present invention and are not intended to limit the scope of protection of the present invention.

[0022] 1. Specific Example: Preparation of solvent-responsive double-layer hydrogel actuators reinforced with cellulose nanocrystals.

[0023] Example 1: A method for preparing a cellulose nanocrystal-enhanced solvent-responsive double-layer hydrogel actuator, comprising the following steps: 1. Weigh 1 g of acrylamide monomer, 0.03 g of cross-linker N,N'-methylenebisacrylamide, and 0.03 g of photoinitiator 2-hydroxy-4'-(2-hydroxyethoxy)-2-methylpropiophenone, dissolve them in 10 g of 1 wt% cellulose nanocrystal suspension, and stir them evenly with a magnetic stirrer to obtain a prepolymer solution for the first layer of hydrogel; 2. Weigh 1 g of acrylamide monomer, 0.03 g of cross-linker N,N'-methylenebisacrylamide, and 0.03 g of photoinitiator 2-hydroxy-4'-(2-hydroxyethoxy)-2-methylpropiophenone, dissolve them in 10 g of 0.5 wt% cellulose nanocrystal suspension, and stir them evenly with a magnetic stirrer to obtain a prepolymer solution for the second layer of hydrogel; 3. Transfer the prepolymer solution of the first hydrogel layer into a silicone mold and irradiate with UV light for 4 minutes to obtain the first hydrogel layer; 4. The prepolymer solution of the second hydrogel layer was transferred to the upper layer of the first hydrogel layer. After irradiation with UV light for 4 minutes, the second hydrogel layer was formed, and the cellulose nanocrystal-enhanced solvent-responsive double-layer hydrogel actuator was obtained, which was recorded as C1A / C 0.5 A.

[0024] Example 2 is the same as Example 1, except that in step S1, the concentration of the cellulose nanocrystal suspension in the process of preparing the prepolymer solution of the first layer of hydrogel is 2 wt %. Finally, a cellulose nanocrystal-enhanced solvent-responsive double-layer hydrogel actuator is obtained, which is denoted as C2A / C 0.5 A.

[0025] Example 3 is the same as Example 1, except that in step S1, the concentration of the cellulose nanocrystal suspension in the process of preparing the prepolymer solution of the first layer of hydrogel is 3 wt %. Finally, a cellulose nanocrystal-enhanced solvent-responsive double-layer hydrogel actuator is obtained, which is denoted as C3A / C. 0.5 A.

[0026] Example 4 is the same as Example 1, except that in step S1, the concentration of the cellulose nanocrystal suspension in the process of preparing the prepolymer solution of the first layer of hydrogel is 4 wt %. Finally, a cellulose nanocrystal-enhanced solvent-responsive double-layer hydrogel actuator is obtained, which is denoted as C4A / C 0.5 A.

[0027] Example 5 is the same as Example 1, except that in step S1, the concentration of the cellulose nanocrystal suspension in the process of preparing the prepolymer solution of the first layer of hydrogel is 5 wt %. Finally, a cellulose nanocrystal-enhanced solvent-responsive double-layer hydrogel actuator is obtained, which is denoted as C5A / C 0.5 A.

[0028] Example 6 is the same as Example 1, except that in step S1, the concentration of the cellulose nanocrystal suspension in the process of preparing the prepolymer solution of the first layer of hydrogel is 6 wt %. Finally, a cellulose nanocrystal-enhanced solvent-responsive double-layer hydrogel actuator is obtained, which is denoted as C6A / C. 0.5 A.

[0029] The control group, a cellulose nanocrystal-reinforced hydrogel, was prepared by preparing only a single-layer hydrogel, including the following steps: 1. Weigh 1 g of acrylamide monomer, 0.03 g of cross-linker N,N'-methylenebisacrylamide, and 0.03 g of photoinitiator 2-hydroxy-4'-(2-hydroxyethoxy)-2-methylpropiophenone, dissolve them in 10 g of cellulose nanocrystal suspension with a concentration of x wt% (x = 0.5, 1, 2, 3, 4, 5, 6), and stir evenly with a magnetic stirrer to obtain a hydrogel prepolymer solution.

[0030] 2. Transfer the prepolymer solution of the hydrogel to a silicone mold and place it under UV light for 4 minutes to obtain a single layer of hydrogel, which is recorded as C. x A, where x = 0.5, 1, 2, 3, 4, 5, 6.

[0031] 2. Characterization and performance testing.

[0032] 1. Microstructure of single-layer hydrogel.

[0033] The monolayer hydrogel prepared in the control group was immersed in deionized water for 2 days to remove unreacted monomers and achieve a fully swollen state. After freeze-drying, its cross-sectional morphology was observed using a scanning electron microscope: Figure 1 The CNC content was 6 wt% ( Figure 1 a), 3wt% ( Figure 1 b) and 0.5wt% ( Figure 1 (c) Microscopic image of the hydrogel. The hydrogel exhibits a honeycomb structure. Furthermore, the pore size of the hydrogel's three-dimensional network decreases with increasing CNC content, indicating that the crosslinking density of the hydrogel network increases with increasing CNC content.

[0034] 2. Reswelling and deswelling kinetics of monolayer hydrogels.

[0035] The monolayer hydrogel prepared in the control group was immersed in deionized water for 2 days and then transferred to an ethanol solution for deswelling kinetics testing. After the deswelling kinetics testing, the hydrogel was immersed in deionized water again for reswelling kinetics testing.

[0036] The deswelling dynamics process is described by the water retention rate (WR), which is defined as follows: WR=(W t de -W d ) / (W s -W d )×100%. Among them, W t de is the mass of the hydrogel at time t after being soaked in ethanol, W s is the mass of the hydrogel after it is fully swollen in deionized water, W d is the mass of the hydrogel after swelling equilibrium in ethanol.

[0037] In the study of reswelling kinetics, the swelling ratio is defined as follows: SR = (W t re -W d ) / W d ×100%. Among them, W t re is the mass of the hydrogel at a certain point in time during the reswelling process, W d is the mass of the hydrogel at swelling equilibrium in ethanol.

[0038] Figure 2 The swelling behavior of hydrogels with different CNC contents in deionized water is shown ( Figure 2 a) and deswelling behavior in ethanol ( Figure 2 b). It can be found that with the increase of CNC content, the hydrogel exhibits a lower swelling ratio and a slower deswelling rate. Figure 1 It can be concluded that the larger the pore size of the three-dimensional network structure of PAM hydrogel, the higher the hydrogel swelling rate and the faster the deswelling rate.

[0039] 3. Mechanical properties test of single-layer hydrogel.

[0040] The tensile samples were prepared using the same prepolymer solution as the control group. After being formed into a special dumbbell-shaped mold, they were soaked in deionized water for 2 days and then subjected to tensile testing using a soft material mechanical testing machine.

[0041] Its tensile stress-strain curve is as follows Figure 3 As shown, from Figure 3 It can be seen from the graph that with the increase of CNC content, the strength of the hydrogel increases and the toughness decreases.

[0042] 4. Solvent-responsive behavior of the double-layer hydrogel actuator.

[0043] The double-layer hydrogel prepared in Example 1-6 was cut into 35 mm × 7 mm strip samples, one end of which was fixed and immersed in an ethanol solution. When the hydrogel reached desolvation equilibrium in ethanol, its bending angle was recorded.

[0044] Figure 4 The figure below shows the bending angle of a bilayer hydrogel actuator in ethanol as a function of the CNC concentration in the second hydrogel layer. It can be seen that the higher the CNC concentration in the first hydrogel layer—that is, the greater the CNC concentration difference between the two layers—the larger the bending angle of the hydrogel. This demonstrates that the bilayer hydrogel actuator exhibits excellent solvent responsiveness and a wide adjustable bending angle range.

[0045] 5. Cyclic test of double-layer hydrogel actuator in ethanol / deionized water The double-layer hydrogel prepared in Example 3 was cut into 35 mm × 7 mm strip samples, and then subjected to 10 ethanol / deionized water alternating immersion cycle tests, and the bending angle after reaching the swelling / de-swelling equilibrium was recorded each time.

[0046] Figure 5 The researchers demonstrated the change in bending angle of the bilayer hydrogel actuator during 10 swelling-contraction cycles. The actuator showed no delamination or bending angle decay during the 10 swelling-contraction cycles, demonstrating tight interfacial bonding, excellent reusability, and reversible bending deformation.

[0047] 3. Application example: Grasping objects in ethanol environment.

[0048] The double-layer hydrogel actuator C3A / C in Example 3 0.5 A cross-shaped gripper was constructed to grasp objects in ethanol solution.

[0049] The cross-shaped gripper consists of two strips of double-layer hydrogel (80mm×10mm×2mm). The two hydrogels are cross-perforated at the center and the shape is fixed by two upper and lower silicone films.

[0050] Figure 6 The complete operation of a grasping device based on this double-layer hydrogel actuator in an ethanol environment was recorded. The experimental results demonstrate that the driving force generated by the double-layer hydrogel actuator's shrinkage in ethanol solution effectively drives the device to complete the grasping action. During the test, the device successfully and stably grasped a wax block in ethanol, fully demonstrating its practical operability in ethanol media and expanding the application of this double-layer hydrogel actuator.

[0051] In summary, the cellulose nanocrystal-enhanced solvent-responsive double-layer hydrogel actuator provided by the present invention has reversible solvent responsiveness, tight interfacial bonding, and excellent mechanical properties. The difference in swelling and shrinkage between the two layers caused by the CNC concentration gradient, especially the driving force generated by water loss shrinkage, can drive the hydrogel to move in ethanol. At the same time, by changing the difference in CNC concentration between the two layers, the bending angle of the actuator can be effectively adjusted. Based on the above performance, the hydrogel actuator is expected to be used in fields such as soft robots and intelligent grippers.

[0052] The above description is not intended to limit the present invention, and the present invention is not limited to the above examples. Any changes, modifications, additions or substitutions made by persons of ordinary skill in the art within the spirit and scope of the present invention shall also fall within the scope of protection of the present invention.

Claims

1. A cellulose nanocrystal-enhanced solvent-responsive double-layer hydrogel actuator, characterized by: A double-layer hydrogel with a heterogeneous structure is achieved through polymerization and cross-linking; the raw materials for preparing the first hydrogel layer and the second hydrogel layer are: acrylamide monomer, cellulose nanocrystal suspension, cross-linking agent and initiator.

2. The concentration of the cellulose nanocrystal suspension used in the first hydrogel layer according to claim 1 is 1 wt%-6 wt%, and the concentration of the cellulose nanocrystal suspension used in the second hydrogel layer is 0.5 wt%.

3. The cellulose nanocrystal-enhanced solvent-responsive double-layer hydrogel actuator according to claim 1, characterized in that: The crosslinking agent is N,N-methylenebisacrylamide, the initiator is a photoinitiator 2-hydroxy-4'-(2-hydroxyethoxy)-2-methylpropiophenone, and the polymerization method is ultraviolet polymerization.

4. The cellulose nanocrystal-enhanced solvent-responsive double-layer hydrogel actuator according to claim 1, characterized in that: The thickness ratio of the first hydrogel layer to the second hydrogel layer is 1:

1.

5. A method for preparing the cellulose nanocrystal-enhanced solvent-responsive double-layer hydrogel actuator according to claim 1, characterized in that The following steps are involved: S1. Adding a certain amount of acrylamide monomer, crosslinking agent and photoinitiator to cellulose nanocrystal suspensions of different concentrations, stirring uniformly using a magnetic stirrer to prepare a prepolymer solution of hydrogel; S2. Injecting a prepolymer solution of a first layer of hydrogel into a mold and curing it by ultraviolet light to form a first hydrogel layer, wherein the concentration of the cellulose nanocrystal suspension used in the prepolymer solution of the first layer of hydrogel is 1 wt%-6 wt%; S3. Pour the second layer of prepolymer solution onto the first layer of hydrogel prepared in step S2, and use ultraviolet light to cure it again to form a second hydrogel layer, thereby obtaining a cellulose nanocrystal-enhanced solvent-responsive double-layer hydrogel actuator. The concentration of the cellulose nanocrystal suspension used in the second hydrogel layer is 0.5 wt%.

6. The method for preparing a cellulose nanocrystal-enhanced solvent-responsive double-layer hydrogel actuator according to claim 5, characterized in that: In the prepolymer solution of the hydrogel described in step S1, the added amount of the polyacrylamide monomer is 10% of the mass of the cellulose nanocrystal suspension, and the added amount of the cross-linking agent and the photoinitiator are both 3% of the mass of the polyacrylamide monomer.

7. Use of the solvent-responsive double-layer hydrogel actuator enhanced by cellulose nanocrystals according to any one of claims 1 to 6 as an intelligent gripper in an ethanol solution.

8. The use according to claim 7, characterized in that: The intelligent gripper is cross-shaped; it is composed of two strip-shaped double-layer hydrogel actuators, which are cross-perforated at the center and fixed in shape by two upper and lower silicone films.

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