A force-induced color-changing nanocomposite hydrogel and its preparation method and application

Abstract

The present invention belongs to the technical field of hydrogel preparation, and specifically relates to a force-induced color-changing nanocomposite hydrogel, its preparation method and application. The force-induced color-changing nanocomposite hydrogel is composed of covalently cross-linked polyacrylamide and rod-shaped cellulose nanocrystals. Among them, polyacrylamide forms a gel matrix with a cross-linked network structure, and the rod-shaped cellulose nanocrystals are interspersed in the gel matrix and arranged in a chiral nematic structure; the pitch is 1.9 - 3.0 μm; the nanocomposite hydrogel is colorless and transparent under a pressure < 150 kPa, and under a pressure ≥ 150 kPa, the nanocomposite hydrogel presents visible structural colors. Compared with the current sensitive hydrogels, the nanocomposite hydrogel needs to be under the action of at least 150 kPa of pressure to present structural colors, which can meet the requirements of some special pressure sensing fields.

Description

Technical Field

[0001] The present invention belongs to the technical field of hydrogel preparation, and particularly relates to a force-induced color-changing nanocomposite hydrogel, a preparation method thereof, and an application thereof. Background Art

[0002] Existing force-induced color-changing hydrogels are all relatively sensitive to force. For example, the cellulose nanocrystal hydrogel prepared by the DMSO swelling method in the prior art can produce color changes under a force of less than 100 kPa; another example is that the mechanically color-changing elastic photonic hydrogel prepared by magnetic nanoclusters in the prior art can trigger the force-induced color-changing performance of the hydrogel under a pressure of less than 1 kPa. Although highly sensitive force-induced color-changing gels have potential application values in fields such as mechanical measurement and medical sensors, there are also the following problems: 1. Prone to false triggering and false detection: Due to the high sensitivity to low pressure, the pressure-induced color-changing hydrogel may be affected by minor environmental changes (such as wind, vibration, slight contact, etc.) and present structural color, thus leading to false triggering and false detection, which will have a greater adverse impact on applications that require precise pressure measurement and response. 2. Prone to interference from noise, irrelevant force sources, etc.: In practical applications, the hydrogel may be interfered by mechanical noise or other irrelevant force sources, affecting its accuracy and reliability. 3. Prone to aging: The false triggering caused by environmental factors will also accelerate the aging of the gel sensing performance at the same time, because after repeated low-pressure stimuli, the hydrogel may not be able to fully return to its initial state, affecting the reliability of its repeated use. 4. Limited scope of application: Highly sensitive pressure-induced color-changing hydrogels are more suitable for detecting minor pressure changes, but may perform poorly in application scenarios that require large pressure detection. Therefore, there is an urgent need to obtain a force-induced color-changing hydrogel with lower sensitivity to be applicable to fields where sensitive hydrogels cannot meet the requirements.

[0003] In addition, the preparation method of the existing force-induced color-changing cellulose nanocrystal hydrogel generally adopts the following methods: (1) obtaining a dry-state assembled film by evaporating an aqueous solution of cellulose nanocrystals; (2) swelling with organic solvents (DMSO, NMMO, etc.); (3) permeating polymer monomers into the ordered structure of the swollen cellulose nanocrystals; (4) polymerization of the polymer monomers. It can be seen that its preparation steps are quite cumbersome and a large amount of organic solvents are used, which is not conducive to environmental protection requirements. There is an urgent need for a preparation method of a force-induced color-changing nanocomposite hydrogel that is simple to operate and environmentally friendly. Summary of the Invention

[0004] The purpose of the present invention is to provide a slightly insensitive force-induced color-changing nanocomposite hydrogel, that is, compared with the current sensitive hydrogels, the nanocomposite hydrogel needs to present structural color under the action of a pressure of at least 150 kPa, which can meet the requirements of some special pressure sensing fields.

[0005] The technical solution of the present invention is: a mechanochromic nanocomposite hydrogel, which is composed of covalently cross-linked polyacrylamide and rod-shaped cellulose nanocrystals, wherein the polyacrylamide forms a gel matrix with a cross-linked network structure, and the rod-shaped cellulose nanocrystals are interspersed in the gel matrix and arranged in a chiral nematic structure. The pitch is 1.9-3.0 μm.

[0006] The nanocomposite hydrogel is colorless and transparent under a pressure of less than 150 kPa, and exhibits visible structural color under a pressure of ≥150 kPa.

[0007] In the present invention, the mechanochromic nanocomposite hydrogel exhibits red color when the pressure is 150 kPa≤≤220 kPa; exhibits green color when the pressure is 220 kPa<≤288 kPa; and exhibits blue color when the pressure is 288 kPa<≤380 kPa.

[0008] In the present invention, the initial reflection wavelength of the mechanochromic nanocomposite hydrogel is 650-700 nm, and under 80% strain, the reflection wavelength moves to 450-490 nm.

[0009] The toughness of the nanocomposite hydrogel is 0.9-1.65 MJ / m 3 , breaking strength is 400-565kPa, tensile strain is 350-480%, and compressive strength is 340-460kPa.

[0010] In the present invention, the raw materials of the mechanochromic nanocomposite hydrogel include polyethylene glycol, sodium chloride, acrylamide, a crosslinking agent, a photoinitiator and a cellulose nanocrystal suspension with a mass concentration of 10%-15%.

[0011] The mass ratio of cellulose nanocrystals to acrylamide is 1:1.5-4.5; the mass ratio of cellulose nanocrystals to polyethylene glycol is 9-12:1; the amount of sodium chloride added is 0.75-1% of the mass of the cellulose nanocrystals; the amount of the crosslinking agent added is 0.5%-2% of the mass of the acrylamide; and the amount of the photoinitiator added is 0.5%-2% of the mass of the acrylamide.

[0012] The cellulose nanocrystal suspension is obtained by acid hydrolysis of cellulose raw materials. The acid hydrolysis process is as follows: acid hydrolyze the cellulose raw materials, add deionized water to dilute and terminate the reaction, let it stand and centrifuge to obtain a precipitate, wash the precipitate by centrifugation, and dialyze it until the pH is close to neutral to obtain a cellulose nanocrystal suspension.

[0013] Preferably, the cellulose is cotton or cellulose pulp or microcrystalline cellulose.

[0014] Preferably, the acid used for acid hydrolysis is hydrochloric acid, sulfuric acid or phosphoric acid. The mass fraction of hydrochloric acid is 5-37%, the mass fraction of sulfuric acid is 5-70%, and the mass fraction of phosphoric acid is 5-80%. During the acid hydrolysis operation, the acid hydrolysis temperature is 10-60°C, and the acid hydrolysis time is 0.2-24 hours.

[0015] In the present invention, for the force-responsive color-changing nanocomposite hydrogel, the number-average molecular weight of polyethylene glycol in its raw materials is 17,000-22,000; preferably, the number-average molecular weight of polyethylene glycol is 20,000.

[0016] The cross-linking agent in the raw materials of the nanocomposite hydrogel is polyethylene glycol diacrylate; the photoinitiator is 2-hydroxy-2-methylpropiophenone; the cellulose nanocrystals are rod-shaped, with a diameter of 15-20 nm and an average length of 100-200 nm.

[0017] The preparation method of the above-mentioned force-responsive color-changing nanocomposite hydrogel includes the following steps:

[0018] (1) Add polyethylene glycol and sodium chloride to the cellulose nanocrystal suspension, and stir evenly to obtain a premixed solution.

[0019] (2) Add acrylamide to the premixed solution obtained in step (1). After ultrasonic treatment and magnetic stirring, then add the photoinitiator and cross-linking agent, and stir evenly to obtain a hydrogel precursor solution.

[0020] (3) Pour the hydrogel precursor solution obtained in step (2) into a mold, and place it in an incubator for evaporation self-assembly at 20-35°C and a humidity RH = 50-60%. Control the content of cellulose nanocrystals in the hydrogel precursor solution to be 15-20%, and stop evaporation to obtain a hydrogel precursor.

[0021] (4) Seal and store the hydrogel precursor obtained in step (3), and horizontally stand it in a dark and light-proof environment for 2-7 days for equilibration.

[0022] (5) After the equilibration is completed, place the sealed hydrogel precursor at an inclination angle α of 30°-90°, and continue to stand it in a dark and light-proof environment for 10-14 days.

[0023] In this step, a dynamic change process occurs to the cellulose nanocrystals in the cross-linked network of polyacrylamide. The rod-shaped cellulose nanocrystals will undergo secondary rearrangement: the ordered chiral nematic phase structure obtained from equilibration first rearranges to form a disordered local conical symmetry nematic structure; then the local conical symmetry nematic structure rearranges again to form an ordered chiral nematic phase structure. During this dynamic change process, the pitch P remains unchanged, and the hydrogel precursor is prone to intermediate layer flow.

[0024] (6) After the static treatment is completed, irradiate the hydrogel precursor after static treatment in step (5) with an ultraviolet lamp having a wavelength of 365 nm for 15 - 30 s to obtain the force - induced color - changing nanocomposite hydrogel.

[0025] In the present invention, in the preparation method of the force - induced color - changing nanocomposite hydrogel, when the hydrogel precursor is placed obliquely at an angle of 30° ≤ α < 45° in step (5) and subjected to static treatment for 10 - 14 days, the finally obtained nanocomposite hydrogel in step (6) is colorless and transparent under a pressure < 150 kPa; and presents a structural color under a pressure ≥ 150 kPa.

[0026] In the present invention, in the preparation method of the force - induced color - changing nanocomposite hydrogel, when the hydrogel precursor is placed obliquely at an angle of 45° ≤ α < 60° in step (5) and subjected to static treatment for 10 - 14 days, the finally obtained nanocomposite hydrogel in step (6) is colorless and transparent under a pressure < 170 kPa, and presents a structural color under a pressure ≥ 170 kPa.

[0027] In the present invention, in the preparation method of the force - induced color - changing nanocomposite hydrogel, when the hydrogel precursor is placed obliquely at an angle of 60° ≤ α < 90° in step (5) and subjected to static treatment for 10 - 14 days, the finally obtained nanocomposite hydrogel in step (6) is colorless and transparent under a pressure < 190 kPa, and presents a structural color under a pressure ≥ 190 kPa.

[0028] When the hydrogel precursor is placed vertically (α = 90°) in step (5) and subjected to static treatment for 10 - 14 days, the finally obtained nanocomposite hydrogel in step (6) is colorless and transparent under a pressure < 220 kPa, and presents a structural color under a pressure ≥ 220 kPa.

[0029] Application of the above - mentioned force - induced color - changing nanocomposite hydrogel or the force - induced color - changing nanocomposite hydrogel prepared by the above - mentioned preparation method in sensing, anti - counterfeiting, and optical devices.

[0030] The beneficial effect of the present invention is that: compared with traditional hydrogels which are sensitive to force stimuli and are prone to quickly respond to color changes, the color - showing force intensity of the force - induced color - changing nanocomposite hydrogel of the present invention is increased by 33 - 150 times, that is, a greater pressure is required to make it present a structural color. In this way, it can meet the requirements of preventing accidental touch in large - pressure detection in the fields of mechanical detection or mechanical measurement in harsh environments. Description of the Drawings

[0031] Figure 1 It is a cross - sectional scanning electron micrograph of the force - induced color - changing nanocomposite hydrogel prepared in Example 1.

[0032] Figure 2 It is a cross - sectional scanning electron micrograph of the force - induced color - changing nanocomposite hydrogel prepared in Example 7.

[0033] Figure 3 SEM image of the cross-section of the mechanochromic nanocomposite hydrogel prepared in Example 8.

[0034] Figure 4 SEM image of the cross-section of the mechanochromic nanocomposite hydrogel prepared in Comparative Example 1.

[0035] Figure 5 SEM image of the cross-section of the mechanochromic nanocomposite hydrogel prepared in Example 4.

[0036] Figure 6 SEM image of the cross-section of the mechanochromic nanocomposite hydrogel prepared in Example 6.

[0037] Figure 7 Mechanochromic optical photograph of the nanocomposite hydrogel prepared in the present invention. Detailed Description of the Invention

[0038] The technical solutions of the present invention will be described in detail below with reference to the accompanying drawings.

[0039] 1. Acrylamide (AM) was purchased from Tianjin Damao Chemical Reagent Co., Ltd., and the purity was analytical pure.

[0040] The photoinitiator 2-hydroxy-4-(2-hydroxyethoxy)-2-methylpropiophenone and the crosslinking agent polyethylene glycol diacrylate (PEGDA) were purchased from Shanghai Macklin Biochemical Co., Ltd. All chemicals were not further purified. Deionized water was used in all experiments.

[0041] Polyethylene glycol (PEG20000) and sodium chloride were purchased from Sinopharm Chemical Reagent Co., Ltd., and the purity was analytical pure.

[0042] 2. The specific steps for preparing the cellulose nanocrystal suspension by the sulfuric acid hydrolysis method are as follows: First, the cellulose nanocrystal raw materials, such as cotton pulp, wood pulp, etc., were pulverized and reserved. Under the water bath condition of 45 - 50 °C, the cellulose raw material was mixed and stirred with 64 wt% H2SO4 for acid hydrolysis. After 30 minutes, a large amount of deionized water was added to terminate the reaction. After standing for precipitation, the supernatant was taken for centrifugation. Then, the centrifuged product was dialyzed until neutral. Finally, the cellulose nanocrystal suspension was centrifuged again and ultrasonically dispersed to obtain a well-dispersed CNCs suspension, which was stored in the refrigerator at low temperature for later use.

[0043] 3. All tests were conducted at room temperature. The mechanical properties of the hydrogels were evaluated using a texture analyzer (TA). Dumbbell-shaped (3 cm in length and 4 mm in width at the center) hydrogel strips were cut from the hydrogels, and tensile tests were performed at a speed of 30 mm / min. Cyclic tests were carried out at the same speed (10 times), and the tensile stress-strain curves of the cellulose nanocrystal hydrogels with different mass ratios were plotted. The toughness was calculated by the average area under the stress-strain curve. For accuracy, all tests were conducted 3 times using the same sample throughout the process, and no dwell time was set between tensile cycles.

[0044] 4. Cross-sectional scanning electron microscopy images: A Regulus 8220 field emission scanning electron microscope (FE-SEM, Hitachi, Japan) of Hitachi, Japan was used.

[0045] Example 1

[0046] For the force-responsive color-changing nanocomposite hydrogel described above, the raw material composition is as follows: 1 g of a 15% cellulose nanocrystal suspension, 0.3 g of acrylamide (cellulose nanocrystals: acrylamide = 1:2), 0.0166 g of polyethylene glycol 20000 (cellulose nanocrystals: polyethylene glycol = 9:1), 0.0015 g of sodium chloride (1% of the mass of cellulose nanocrystals), 0.0015 g of the photoinitiator 2-hydroxy-2-methylpropiophenone (1% of the mass of cellulose nanocrystals), and 0.0015 g of the crosslinking agent polyethylene glycol diacrylate (1% of the mass of cellulose nanocrystals). The cellulose nanocrystals are rod-shaped, with a diameter of 15 nm and an average length of 150 nm.

[0047] The preparation method of the force-responsive color-changing nanocomposite hydrogel is as follows:

[0048] (1) Polyethylene glycol and sodium chloride were added to the cellulose nanocrystal suspension and stirred evenly to obtain a premixed solution.

[0049] (2) Acrylamide was added to the premixed solution obtained in step (1). First, it was sonicated for 1 h at an output power of 40%, then magnetically stirred at 25 °C for 12 h, and then the photoinitiator and crosslinking agent were added, and stirring was continued for 12 h to obtain a hydrogel precursor solution.

[0050] (3) The hydrogel precursor solution obtained in step (2) was poured into a mold and placed in an incubator for evaporation self-assembly at 25 °C and a humidity of RH = 55%. The content of cellulose nanocrystals in the hydrogel precursor solution was controlled to be 20%, and evaporation was stopped to obtain a hydrogel precursor.

[0051] (4) The hydrogel precursor obtained in step (3) was sealed and stored, horizontally statically placed in a dark and light-shielded environment, and equilibrated for 2 days.

[0052] (5) After the balance is completed, place the sealed hydrogel precursor at an inclined angle α of 30°, and continue to stand for 14 days in a dark and light-shielded environment;

[0053] (6) After the standing balance is completed, irradiate the balanced hydrogel precursor with a UV lamp with a wavelength of 365 nm for 10 s to obtain the nanocomposite hydrogel.

[0054] Through Figure 1 It can be intuitively seen that the cellulose nanocrystals inside the obtained nanocomposite hydrogel are arranged in a chiral nematic structure.

[0055] Through Figure 7 It can be seen that the nanocomposite hydrogel obtained by the present invention can exhibit a color change process from colorless to red and then to blue under different acting forces.

[0056] Example 2

[0057] The difference from Example 1 is that the raw material composition of the force-responsive color-changing nanocomposite hydrogel in this example is as follows: 1 g of a cellulose nanocrystal suspension with a mass concentration of 15%, 0.6 g of acrylamide (cellulose nanocrystals: acrylamide = 1:4), 0.0166 g of polyethylene glycol 20000 (cellulose nanocrystals: polyethylene glycol = 9:1), 0.0015 g of sodium chloride (1% of the mass of cellulose nanocrystals), 0.0015 g of photoinitiator 2-hydroxy-2-methylpropiophenone (1% of the mass of cellulose nanocrystals), and 0.0015 g of crosslinking agent polyethylene glycol diacrylate (1% of the mass of cellulose nanocrystals). The cellulose nanocrystals are rod-shaped, with a diameter of 15 nm and an average length of 150 nm.

[0058] Others are the same as in Example 1.

[0059] Example 3

[0060] The difference from Example 1 is that the raw material composition of the force-responsive color-changing nanocomposite hydrogel in this example is as follows: 1 g of a cellulose nanocrystal suspension with a mass concentration of 10%, 0.3 g of acrylamide (cellulose nanocrystals: acrylamide = 1:3), 0.0083 g of polyethylene glycol 20000 (cellulose nanocrystals: polyethylene glycol = 12:1), 0.0015 g of sodium chloride (1% of the mass of cellulose nanocrystals), 0.002 g of photoinitiator 2-hydroxy-2-methylpropiophenone (2% of the mass of cellulose nanocrystals), and 0.002 g of crosslinking agent polyethylene glycol diacrylate (2% of the mass of cellulose nanocrystals). The cellulose nanocrystals are rod-shaped, with a diameter of 15 nm and an average length of 150 nm.

[0061] In addition, in step (3) of the preparation method, the content of cellulose nanocrystals in the hydrogel precursor solution is controlled to be 10%, and the evaporation is stopped.

[0062] Others are the same as in Example 1.

[0063] Example 4

[0064] The difference from Example 1 is that in step (5) of the preparation method described in this example, after the balance is completed, the sealed hydrogel precursor is placed at an inclination angle α of 45°.

[0065] Others are the same as in Example 1.

[0066] Through Figure 5 It can be intuitively seen that the chiral nematic alignment structure of cellulose nanocrystals inside the nanocomposite hydrogel obtained in this example. And it can be seen that during the process of placing at an inclination of 45°, the cellulose nanocrystals (CNC) undergo secondary rearrangement under the action of gravity, changing from the original chiral nematic phase structure to a symmetric structure, and then changing back to the chiral nematic phase structure as the balance time increases.

[0067] Example 5

[0068] The difference from Example 1 is that in step (5) of the preparation method described in this example, after the balance is completed, the sealed hydrogel precursor is placed at an inclination angle α of 60°.

[0069] Others are the same as in Example 1.

[0070] Example 6

[0071] The difference from Example 1 is that in step (5) of the preparation method described in this example, after the balance is completed, the sealed hydrogel precursor is placed at an inclination angle α of 90°.

[0072] Others are the same as in Example 1.

[0073] Through Figure 6 It can be intuitively seen that the chiral nematic alignment structure of cellulose nanocrystals inside the nanocomposite hydrogel obtained in this example. And it can be seen that during the process of placing at an inclination of 90°, the cellulose nanocrystals (CNC) undergo secondary rearrangement under the action of gravity, changing from the original chiral nematic phase structure to a symmetric structure, and then changing back to the chiral nematic phase structure as the balance time increases.

[0074] Example 7

[0075] The difference from Example 1 is that in step (4) of the preparation method described in this example, the obtained hydrogel precursor is sealed and stored, horizontally statically placed in a dark and light-proof environment, and balanced for 4 days.

[0076] Others are the same as in Example 1.

[0077] Through Figure 2 The chiral nematic alignment structure of cellulose nanocrystals inside the nano-composite hydrogel obtained in this example can be visually observed.

[0078] Example 8

[0079] The difference from Example 1 is that in step (4) of the preparation method described in this example, the obtained hydrogel precursor is sealed and stored, horizontally statically placed in a dark and light-proof environment, and balanced for 7 days.

[0080] Others are the same as in Example 1.

[0081] Through Figure 3 The chiral nematic alignment structure of cellulose nanocrystals inside the nano-composite hydrogel obtained in this example can be visually observed.

[0082] Example 9

[0083] The difference from Example 1 is that in step (5) of the preparation method described in this example, the sealed hydrogel precursor is placed at an inclined angle α of 30°, and in a dark and light-proof environment, it is continuously statically treated for 10 days.

[0084] Others are the same as in Example 1.

[0085] Comparative Example 1

[0086] The preparation method of this comparative example is as follows:

[0087] (1) Polyethylene glycol and sodium chloride are added to the cellulose nanocrystal suspension, and stirred evenly to obtain a premixed solution;

[0088] (2) Acrylamide is added to the premixed solution obtained in step (1), first ultrasonicated for 1 h with an output power of 40%, then magnetically stirred at 25 °C for 12 h, and then a photoinitiator and a crosslinking agent are added, and stirring is continued for 12 h to obtain a hydrogel precursor solution;

[0089] (3) The hydrogel precursor solution obtained in step (2) is poured into a mold, and placed in an incubator for evaporation self-assembly at 25 °C and a humidity RH = 55%, controlling the content of cellulose nanocrystals in the hydrogel precursor solution to be 20 wt%, and stopping evaporation to obtain a hydrogel precursor;

[0090] (4) Directly irradiate the hydrogel precursor obtained in step (3) with a UV lamp having a wavelength of 365 nm for 10 s without horizontally standing and equilibrating it in a dark and light-shielded environment to obtain the nanocomposite hydrogel.

[0091] From Figure 4 It can be seen that for the un-equilibrated gel, the cellulose nanocrystals (CNCs) are in a long-range disordered and short-range ordered arrangement structure and cannot exhibit color change under the action of force.

[0092] Comparative Example 2

[0093] The difference from Example 1 is that in step (5) of the preparation method described in this comparative example, the sealed hydrogel precursor is placed horizontally (i.e., α = 0°) and continuously standing treatment is carried out for 14 days in a dark and light-shielded environment.

[0094] Others are the same as in Example 1.

[0095] The obtained nanocomposite hydrogel can respond to color under the action of a relatively small force and is sensitive to force.

[0096] Comparative Example 3

[0097] The difference from Example 1 is that in step (4) of the preparation method described in this comparative example, the horizontal standing and equilibration are not carried out in the dark.

[0098] The standing treatment in step (5) is also not carried out in the dark.

[0099] Others are the same as in Example 1.

[0100] If not in a dark environment (not light-shielded), the nanocomposite hydrogel may undergo self-polymerization, and the gel will become very hard and have a large rigidity.

[0101] Comparative Example 4

[0102] The difference from Example 1 is that in step (4) of the preparation method described in this comparative example, the horizontal standing and equilibration are not carried out in a sealed manner.

[0103] The standing treatment in step (5) is also not carried out in a sealed manner.

[0104] Others are the same as in Example 1.

[0105] If the equilibration is not carried out in a sealed manner, the hydrogel precursor will continue to evaporate, the water content will decrease, and the mechanical properties of the gel will decline.

[0106] Comparative Example 5

[0107] The difference from Example 1 is that in step (3) of the preparation method described in this comparative example, the hydrogel precursor solution is placed in an incubator and undergoes evaporation self-assembly at 50 °C and a humidity of RH = 25%.

[0108] The others are the same as in Example 1.

[0109] Comparative Example 6

[0110] The difference from Example 1 is that in step (3) of the preparation method described in this comparative example, the content of cellulose nanocrystals in the hydrogel precursor solution is controlled to be 25 wt%, and the evaporation is stopped. The others are the same as in Example 1.

[0111] The relevant indexes of each example and comparative example are summarized in Table 1 below.

[0112] Table 1

[0113]

[0114]

[0115] Note: The pitch in Table 1 refers to the pitch relative to the tilt angle.

Claims

1. A method for preparing a mechanochromic nanocomposite hydrogel, characterized in that: The following steps are involved: (1) Adding polyethylene glycol and sodium chloride to the cellulose nanocrystal suspension and stirring evenly to obtain a premixed solution; (2) adding acrylamide to the premixed solution obtained in step (1), and then adding a photoinitiator and a crosslinking agent after ultrasonic treatment and magnetic stirring, and stirring evenly to obtain a hydrogel precursor solution; (3) pouring the hydrogel precursor solution obtained in step (2) into a mold, placing it in an incubator at 20-35° C. and humidity RH=50-60% for evaporation self-assembly, controlling the content of cellulose nanocrystals in the hydrogel precursor solution to be 15-20wt%, stopping evaporation, and obtaining a hydrogel precursor; (4) The hydrogel precursor obtained in step (3) is sealed and stored, and placed horizontally in a dark and light-proof environment for 2-7 days; (5) After the equilibrium is completed, the sealed hydrogel precursor is placed at an inclination angle α of 30°-90° in a dark environment for a further 10-14 days; (6) After the static treatment is completed, the hydrogel precursor after the static treatment in step (5) is irradiated with an ultraviolet lamp with a wavelength of 365 nm for a time of 15-30 seconds to obtain a mechanochromic nanocomposite hydrogel; When the hydrogel precursor is placed at an angle of 30°≤α<45° in step (5) and left to stand for 10-14 days, the nanocomposite hydrogel obtained in the final step (6) is colorless and transparent at a pressure of <150 kPa; and exhibits structural color at a pressure of ≥150 kPa; When the hydrogel precursor is tilted at 45°≤α<60° in step (5) and left to stand for 10-14 days, the nanocomposite hydrogel obtained in the final step (6) is colorless and transparent at a pressure of less than 170 kPa, and exhibits structural color at a pressure of ≥170 kPa; When the hydrogel precursor is tilted at 60°≤α<90° in step (5) and left to stand for 10-14 days, the nanocomposite hydrogel obtained in the final step (6) is colorless and transparent at a pressure of less than 190 kPa, and exhibits structural color at a pressure of ≥190 kPa; When the hydrogel precursor is tilted at α=90° in step (5) and left to stand for 10-14 days, the nanocomposite hydrogel obtained in the final step (6) is colorless and transparent at a pressure of less than 220 kPa, and exhibits structural color at a pressure of ≥220 kPa; The mechanochromic nanocomposite hydrogel is composed of covalently cross-linked polyacrylamide and rod-shaped cellulose nanocrystals, wherein the polyacrylamide forms a gel matrix with a cross-linked network structure, and the rod-shaped cellulose nanocrystals are interspersed in the gel matrix and arranged in a chiral nematic structure; the pitch is 1.9-3.0 μm.

2. The method for preparing the mechanochromic nanocomposite hydrogel according to claim 1, characterized in that: The initial reflection wavelength of the nanocomposite hydrogel is 650-700 nm, and at 80% strain, the reflection wavelength moves to 450-490 nm; The toughness of the nanocomposite hydrogel is 0.9-1.65 MJ / m 3 , the breaking strength is 400-565kPa, the tensile strain is 350-480%, and the compressive strength is 340-460kPa.

3. The method for preparing the mechanochromic nanocomposite hydrogel according to claim 1, characterized in that: The raw materials of the nanocomposite hydrogel include polyethylene glycol, sodium chloride, acrylamide, a crosslinking agent, a photoinitiator and a cellulose nanocrystal suspension with a mass concentration of 10%-15%; Among them, the mass ratio of cellulose nanocrystals: acrylamide is 1:1.5-4.5; the mass ratio of cellulose nanocrystals: polyethylene glycol is 9-12:1; the amount of sodium chloride added is 0.75-1% of the mass of cellulose nanocrystals; the amount of cross-linking agent added is 0.5%-2% of the mass of acrylamide, and the amount of photoinitiator added is 0.5%-2% of the mass of acrylamide.

4. The method for preparing the mechanochromic nanocomposite hydrogel according to claim 1, characterized in that: The number average molecular weight of polyethylene glycol in the nanocomposite hydrogel raw material is 17000-22000; The crosslinking agent in the hydrogel raw material is polyethylene glycol diacrylate; the photoinitiator is 2-hydroxy-2-methylpropiophenone; and the cellulose nanocrystals are rod-shaped with a diameter of 15-20nm and an average length of 100-200nm.

5. The method for preparing the mechanochromic nanocomposite hydrogel according to claim 4, characterized in that: The number average molecular weight of the polyethylene glycol is 20,000.

6. Application of the mechanochromic nanocomposite hydrogel prepared by the method for preparing the mechanochromic nanocomposite hydrogel according to any one of claims 1 to 5 in sensing, anti-counterfeiting and optical devices.

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