Mechanically discoloring pure polymer hydrogel and application thereof

Abstract

The invention discloses a pure polymer hydrogel with mechanical discoloration and application, and the preparation method comprises the following steps: mixing a polymer gel monomer, water, an initiator, a cross-linking agent and an accelerant according to a ratio, and carrying out polymerization reaction to obtain the pure polymer hydrogel with stretching and compression bidirectional mechanical discoloration. The pure polymer hydrogel with the stretching and compressing two-way mechanical discoloration function does not contain any color response pigment, a pre-assembling step is not needed, and the preparation method is simple; when the prepared polymer hydrogel material is compressed or stretched, the polymer hydrogel material can be gradually converted from colorless and transparent to colored under a polaroid and cyclically changes according to the sequence of white, yellow, dark purple, peacock blue, sapphire blue, bright yellow, bright powder, green and light powder, the birefringence reaches up to 1.4 * 10 <-4 >, the mechanical color changing stability is high, and the polymer hydrogel material has good application prospects in the intelligent fields of strain sensing and the like.

Description

Technical Field

[0001] The present invention relates to the technical field of hydrogel applications, in particular to a hydrogel with mechanical color change, a preparation method and an application thereof. Background Art

[0002] Mechanically chromic hydrogels can respond to external mechanical stimuli by changing their optical properties (such as transparency, color, and luminescence intensity), which makes them have broad application potential in sensors, smart displays, anti-counterfeiting materials, and biomedicine. However, the design of most mechanically chromic hydrogels currently relies on the precise assembly of nanostructures inside the gel, which usually requires the pre-preparation of nanoparticles or nanotemplates, which not only increases the complexity of the preparation, but also may lead to problems with the consistency and stability of the material. In addition, the color of the reported mechanically chromic hydrogels is usually angle-dependent and highly sensitive to defects inside the material, which seriously limits its reliability and universality in practical applications. Therefore, the development of a hydrogel material with a stable, adjustable and predictable mechanically chromic response remains a key scientific issue that needs to be urgently addressed in this field.

[0003] This invention innovatively proposes constructing a mechanochromic pure polymer hydrogel composed of polymer and water. By adjusting the ratio of each component during the polymerization process, the polymer chain content and crosslink density in the polymer hydrogel are controlled, resulting in a tunable interference color upon stretching or compression. This invention eliminates the need for additional functional color components and simplifies the preparation process by adjusting the polymer network alone, providing a new, efficient and practical approach for the development of mechanochromic hydrogels. Summary of the Invention

[0004] Purpose of the invention: In view of the problems that the preparation process of mechanical color-changing hydrogel materials is complicated and the mechanical color-changing performance is unstable due to external factors, one of the purposes of the present invention is to provide a pure polymer hydrogel with mechanical color-changing properties.

[0005] In order to achieve the above-mentioned object of the invention, the technical solution adopted by the present invention is as follows:

[0006] According to one aspect of the present invention, a pure polymer hydrogel with mechanical color change is provided, which is prepared by the following steps: polymer gel monomer, water, initiator, cross-linker and accelerator are mixed in proportion, and then subjected to ultraviolet polymerization reaction to obtain a transparent pure polymer hydrogel with bidirectional mechanical color change under tension and compression.

[0007] Furthermore, the polymer gel monomer is any one of acrylic acid, methacrylic acid, hydroxyethyl acrylate, hydroxyethyl methacrylate, butyl acrylate, butyl methacrylate, acrylamide, methacrylamide, and N-isopropylacrylamide, or a combination of at least two thereof.

[0008] Further, the initiator is any one or a combination of at least two of: 2-hydroxy-4'-(2-hydroxyethoxy)-2-methylpropiophenone, phenylbis(2,4,6-trimethylbenzoyl)phosphine oxide, ammonium persulfate, and potassium persulfate.

[0009] Further, the crosslinking agent is any one or a combination of at least two of: dimethylacrylamide, dipropylene glycol diacrylate, N,N'-methylenebisacrylamide, and methacrylic anhydride gelatin.

[0010] Further, the ratio of the polymer gel monomer to water is: 10 - 50 wt%, the mass fraction of the initiator in the polymer gel monomer is 0.5 - 1 wt%, the mass fraction of the crosslinking agent in the polymerizable monomer is 0.04 - 0.8 wt%, and the accelerator is 0.1 - 0.5 wt% of the polymerizable monomer; the prepared pure polymer hydrogel has compression-tensile mechanochromic properties.

[0011] Further, the ratio of the polymer gel monomer to water is: 26 - 50 wt%, the mass fraction of the initiator in the polymer gel monomer is 0.5 - 1 wt%, the mass fraction of the crosslinking agent in the polymerizable monomer is 0.50 - 0.80 wt%, and the accelerator is 0.1 - 0.5 wt% of the polymerizable monomer; the prepared pure polymer hydrogel has compression chromic properties.

[0012] Further, during the compression strain range of 0 - 60% of the hydrogel, it can be observed under a polarizer that the hydrogel gradually changes from colorless and transparent to colored, and in the order of white, yellow, dark purple, peacock blue, sapphire blue, bright yellow, bright pink, green, light pink, and its birefringence index is 0.4×10 -4 ~1.2×10 -4 ; the compression response range of the hydrogel is: the load is 5.78 - 30.68 N, the distance is 2.89 - 7.98 mm, and the stress is 17.63 - 93.84 KPa.

[0013] Further, after immersing the pure polymer hydrogel with compression chromic properties in a 2 - 5 wt% initiator solution, a pre-polymerized hydrophobic monomer is coated, and a hydrophobic layer is formed by thermal polymerization at 60 - 100 °C to obtain a mechanically chromic pure polymer hydrogel with a hydrophobic coating; the initiator is preferably azobisisobutyronitrile; the pre-polymerized hydrophobic monomer is any one or a combination of at least two of epoxidized soybean methacrylate, lauryl acrylate, stearyl acrylate, lauryl methacrylate, and stearyl methacrylate.

[0014] Further, the mechanically chromic pure polymer hydrogel with a hydrophobic coating has excellent mechanical chromic stability, can be placed in air and water for 5 - 20 days, maintains 95 - 110% of the original weight, and maintains the original mechanical chromic property.

[0015] Furthermore, the ratio of the polymer gel monomer to water is: 10-35 wt%, the mass fraction of the initiator in the polymer gel monomer is 0.5-1 wt%, the mass fraction of the crosslinking agent in the polymerizable monomer is 0.04-0.50 wt%, and the accelerator is 0.1-0.5 wt% of the polymerizable monomer; the prepared pure polymer hydrogel has the property of tensile color change.

[0016] Furthermore, in the process of the tensile strain range of the hydrogel being 0-2000%, it can be observed under a polarizer that the hydrogel gradually changes from colorless and transparent to colored, and changes in the order of orange, dark purple, peacock blue, sapphire blue, bright yellow, and light violet, and the birefringence index is 0.5×10 -4 ~1.4×10 -4 , and the mechanical response range of the hydrogel is: the load is 0.23-1.57 N, the distance is 12.28-140.33 mm, and the stress is 12.46-53.21 KPa.

[0017] Furthermore, the hydrogel has mechanical color change stability in cyclic stretching and still maintains the original mechanical color change property after 200-500 stretching cycles.

[0018] According to another aspect of the present invention, there is provided an application of a pure polymer hydrogel having mechanical color change, such as the application of the cellulose nanofiber hydrogel having mechanical color change in the fields of intelligent sensing, electronic skin, and biomedicine.

[0019] The beneficial effects of the present invention are as follows:

[0020] (1) By precisely regulating the polymer network structure, the present invention can achieve tunable interference color change without adding any functional nanoparticles or dyes, simplifies the preparation process and improves the material purity, and avoids the compatibility problems introduced by functional color additives.

[0021] (2) The hydrogel prepared by the present invention simultaneously has tensile and compressive response capabilities, exhibits a wide range (strain 0-2000%), high-contrast and reversible color changes, and breaks through the limitations of the single response mode of traditional mechanically color-changing materials.

[0022] (3) The hydrogel prepared by the present invention has excellent anti-swelling, anti-drying and mechanical stability, and maintains stable mechanical and optical properties in complex environments such as air and underwater for a long time, expanding its practical value in the fields of flexible sensing, electronic skin, etc. Description of the Drawings

[0023] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the following will briefly introduce the drawings required for use in the embodiments. Obviously, the drawings in the following description are only some embodiments of the present invention. For those of ordinary skill in the art, without creative efforts, other drawings can also be obtained based on these drawings.

[0024] Figure 1 Birefringence photos, polarized light microscope photos, and birefringence of the hydrogel during compression in a pure polymer hydrogel with mechanochromism.

[0025] Figure 2 Birefringence photos, polarized light microscope photos, and birefringence of the hydrogel during stretching in a pure polymer hydrogel with mechanochromism.

[0026] Figure 3 Birefringence photos and mechanical properties during compression of a pure polymer hydrogel with mechanochromism prepared by replacing acrylamide with acrylic acid.

[0027] Figure 4 Compressive mechanical properties of gels with different crosslinking densities in a pure polymer hydrogel with mechanochromism.

[0028] Figure 5 Tensile mechanical properties of gels with different degrees of crosslinking in a pure polymer hydrogel with mechanochromism.

[0029] Figure 6 Optical microscope photos of the hydrogel with a hydrophobic coating before and after the preparation of the hydrophobic coating.

[0030] Figure 7 Swelling ratio of the hydrogel with a hydrophobic coating in water and water loss rate after standing in air.

[0031] Figure 8 Comparison photos of the hydrogel with a hydrophobic coating after being soaked in water for 5 days and standing in air for 5 days and the uncoated hydrogel.

[0032] Figure 9 Compressive mechanochromism photos of the hydrogel with a hydrophobic coating after being soaked in water for 5 days and standing in air for 5 days. Detailed implementation manners

[0033] Next, in combination with the accompanying drawings, technical process steps, specific implementation conditions and materials in the embodiments of the present invention, the technical solutions in the embodiments of the present invention will be clearly and completely described. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative efforts shall fall within the protection scope of the present invention.

[0034] To better understand the technical solution of the present invention, the following will provide a detailed description of specific embodiments of the present invention.

[0035] Among them, the cellulose nanofiber hydrogel with mechanochromism provided by the present invention can flexibly regulate the quality of the hydrogel according to factors such as the reagent use environment, mold size, and test requirements. In the following embodiments, 4 g is used as the standard for hydrogel preparation to illustrate the present invention.

[0036] Example 1

[0037] Using acrylamide as the raw material, acrylamide monomer (2.6 g, 26 wt% relative to the weight of water), MBA cross-linking agent (0.013 g, 0.5 wt% relative to acrylamide), 2-hydroxy-4'-(2-hydroxyethoxy)-2-methylpropiophenone (rgacure2959) initiator (0.013 g, 0.5 wt% relative to acrylamide) and tetramethylethylenediamine (TEMED) (13 μL, 0.5 wt% relative to acrylamide) were dissolved in 10.4 mL of distilled water at room temperature (the hydrogel was named "XPAM" Y ", where X represents the polymer content, defined as the weight percentage of acrylamide relative to water, and Y represents the cross-linking degree, defined as the weight percentage of MBA relative to acrylamide. Therefore, it is named 26PAM here 0.5 ). Thereafter, the solution was transferred to a glass mold. Then the mold was placed under an ultraviolet lamp (16 W, 365 nm, the distance between the sample and the lamp was 2.0 cm) for 30 minutes to polymerize into a mechanochromic pure polymer hydrogel. The pure polymer hydrogel was placed under polarized light in the air for compression testing, and the compression range was 0-60%. At the same time, the changes in the birefringence and mechanical properties of the hydrogel were recorded to characterize its mechanochromic properties. The hydrogel gradually changed from colorless and transparent to colored, and changed in the order of white, yellow, dark purple, peacock blue, sapphire blue, bright yellow, bright pink, green, and light pink during compression. Its birefringence index during compression was 0.4×10 -4 ~1.2×10 -4 . The mechanical response range was: the load was 5.78-30.68 N, the distance was 2.89-7.98 mm, and the stress was 17.63-93.84 KPa. The number of compression cycles was 500-2000 times.

[0038] Example 2

[0039] Based on Example 1, this example investigated the effect of accelerator content on the mechanochromism of pure polymer hydrogels. The only difference is that the accelerator concentration used in the present invention is not limited to 0.5 wt%, and any concentration between 0.1 and 0.6 wt% is selected. The preparation process of the hydrogel and the analysis conditions of the compression mechanochromic properties are the same as those in Example 1.

[0040] Table 1 briefly lists the preparation and mechanochromic properties of pure polymer hydrogels with different accelerator concentrations involved in the present invention.

[0041] Table 1

[0042]

[0043] The accelerator directly affects the polymerization kinetics and final structure of acrylamide hydrogels by regulating the free radical generation rate. As can be seen from Table 1, at low accelerator concentrations, the polymerization efficiency of the hydrogel is too low to reach the network density required for color interference. When the accelerator concentration is too high, it causes local overheating during polymerization, resulting in an uneven gel without a uniform interference color. Therefore, it was found that the optimal accelerator concentration range for compression is 0.1 - 0.5 wt% of tetramethylethylenediamine.

[0044] Example 3

[0045] This example uses the same preparation method as Example 1. The only difference is that the accelerator is replaced by dimethylethylenediamine. The preparation process of the hydrogel and the analysis conditions of the mechanochromic properties are the same as those in Example 1. Dissolve acrylamide monomer (2.6 g, 26 wt% relative to the weight of water), dimethacrylamide crosslinker (0.013 g, 0.5 wt% relative to acrylamide), 2-hydroxy-4'-(2-hydroxyethoxy)-2-methylpropiophenone (rgacure 2959) initiator (0.013 g, 0.5 wt% relative to acrylamide) and dimethylethylenediamine (DMED) (13 μL, 0.5 wt% relative to acrylamide) in 10 mL of distilled water at room temperature. Thereafter, transfer the solution to a glass mold. Then place the mold under an ultraviolet lamp (16 W, 365 nm, the distance between the sample and the lamp is 2.0 cm) for 30 minutes to polymerize into a mechanochromic pure polymer hydrogel. In addition, due to the difference in chemical structure, the catalytic efficiency of dimethylethylenediamine is reduced, and the mechanical strength and mechanochromic properties of the gel are weaker than those in Example 1. The specific comparison is as follows: When compressed, it changes in the order of white, yellow, dark purple, peacock blue, and sapphire blue. The mechanical response range is: the load is 4.33 - 21.52 N, and the stress is 10.46 - 58.43 KPa.

[0046] Example 4

[0047] This example uses the same preparation method as Example 1. The only difference is that the crosslinking agent is replaced from MBA crosslinking agent to dimethylacrylamide. The preparation process of the hydrogel and the analysis conditions of the mechanochromic properties are the same as those in Example 1. Dissolve acrylamide monomer (2.6 g, 26 wt% relative to the weight of water), dimethylacrylamide crosslinking agent (0.013 g, 0.5 wt% relative to acrylamide), 2-hydroxy-4'-(2-hydroxyethoxy)-2-methylpropiophenone (rgacure 2959) initiator (0.013 g, 0.5 wt% relative to acrylamide) and tetramethylethylenediamine (TEMED) (13 μL, 0.5 wt% relative to acrylamide) in 10 mL of distilled water at room temperature. Thereafter, transfer the solution to a glass mold. Then place the mold under an ultraviolet lamp (16 W, 365 nm, the distance between the sample and the lamp is 2.0 cm) for 30 minutes to polymerize into a mechanochromic pure polymer hydrogel. In addition, the methyl substituents of dimethylacrylamide increase the steric hindrance and reduce the crosslinking efficiency, and the mechanical strength and mechanochromic properties of the gel are weaker than those in Example 1. The specific comparison is as follows: when compressed, it changes in the order of white, yellow, dark purple, peacock blue, sapphire blue, and bright yellow. The mechanical response range is: the load is 5.29 - 27.74 N, and the stress is 12.47 - 76.19 KPa.

[0048] Example 5

[0049] This example uses the same preparation method as Example 1. The only difference is that the initiator is replaced from 2-hydroxy-4'-(2-hydroxyethoxy)-2-methylpropiophenone (rgacure 2959) initiator to ammonium persulfate. The polymerization method of the hydrogel changes from ultraviolet polymerization in Example 1 to thermal polymerization. Dissolve acrylamide monomer (2.6 g, 26 wt% relative to the weight of water), MBA crosslinking agent (0.013 g, 0.5 wt% relative to acrylamide), ammonium persulfate initiator (0.013 g, 0.5 wt% relative to acrylamide) and tetramethylethylenediamine (TEMED) (13 μL, 0.5 wt% relative to acrylamide) in 10 mL of distilled water at room temperature. Thereafter, transfer the solution to a glass mold. Then place the mold at 80 °C for 20 minutes to polymerize into a mechanochromic pure polymer hydrogel. In addition, the mechanical properties and the number of recyclable times of the hydrogel are also the same as those in Example 1.

[0050] Example 6

[0051] This example uses the same preparation method as Example 1. The only difference is that the monomer is replaced from acrylamide with acrylic acid. The preparation process of the hydrogel and the analysis conditions of the mechanochromic properties are the same as those in Example 1. Acrylic acid monomer (2.6 g, 26 wt% relative to the weight of water), MBA crosslinking agent (0.013 g, 0.5 wt% relative to acrylamide), 2-hydroxy-4'-(2-hydroxyethoxy)-2-methylpropiophenone (rgacure2959) initiator (0.013 g, 0.5 wt% relative to acrylamide), and tetramethylethylenediamine (TEMED) (13 μL, 0.5 wt% relative to acrylamide) are dissolved in 10 mL of distilled water at room temperature (the hydrogel is named "XPAA Y ", where X represents the polymer content, defined as the weight percentage of acrylic acid relative to water, and Y represents the crosslinking degree, defined as the weight percentage of MBA relative to acrylic acid. Therefore, it is named 26PAA 0.5 ). In addition, at the same polymer concentration and crosslinking density, compared with acrylamide-based hydrogels, due to chain breakage in the network dominated by brittle ions of acrylic acid, acrylic acid-based hydrogels have higher strength and lower toughness. At the same time, they will suffer irreversible damage under repeated loading. The number of compression cycles is 210 - 400 times.

[0052] Example 7

[0053] This example uses the same preparation method as Example 1. The only difference is that the monomer is replaced from acrylamide with a mixture of 2-hydroxyethyl methacrylate, butyl acrylate, and acrylamide, and the mass ratio of the three is 1:1:1. The preparation process of the hydrogel and the analysis conditions of the mechanochromic properties are the same as those in Example 1. 2-Hydroxyethyl methacrylate, butyl acrylate, and acrylamide are mixed in a ratio of 1:1:1 (a total of 2.6 g, 26 wt% relative to the weight of water), MBA crosslinking agent (0.013 g, 0.5 wt% relative to the total amount of monomers), 2-hydroxy-4'-(2-hydroxyethoxy)-2-methylpropiophenone (rgacure 2959) initiator (0.013 g, 0.5 wt% relative to the total amount of monomers), and tetramethylethylenediamine (TEMED) (13 μL, 0.5 wt% relative to the total amount of monomers) are dissolved in 10 mL of distilled water at room temperature. In addition, the hydroxyl group of 2-hydroxyethyl methacrylate enhances the hydrogen bond crosslinking density, and the hydrophobic long chain of butyl acrylate forms dynamic physical crosslinks, synergistically improving the toughness and compressive strength of the material. At the same time, the dynamic crosslinking network endows it with excellent self-healing ability, and the number of compression cycles is 1500 - 2000 times.

[0054] Example 8

[0055] Using acrylamide as a raw material, acrylamide monomer (2.6 g, 26 wt% relative to the weight of water), MBA crosslinking agent (0.001 g, 0.04 wt% relative to acrylamide), 2-hydroxy-4'-(2-hydroxyethoxy)-2-methylpropiophenone (rgacure2959) initiator (0.013 g, 0.5 wt% relative to acrylamide) and tetramethylethylenediamine (TEMED) (13 μL, 0.5 wt% relative to acrylamide) were dissolved in 10 mL of distilled water at room temperature (this hydrogel was named 26AM 0.04 ). Thereafter, the solution was transferred to a glass mold. Then the mold was placed under an ultraviolet lamp (16 W, 365 nm, the distance between the sample and the lamp was 2.0 cm) for 30 minutes to polymerize into a mechanochromic pure polymer hydrogel. When stretched, it changes in the order of orange, dark purple, peacock blue, light green, yellow, light purple, dark green, pink. The birefringence index during stretching is 0.5×10 -4 ~1.4×10 -4 . The mechanical response range is: the load is 0.23 - 1.57 N, the distance is 12.28 - 140.33 mm, and the stress is 12.46 - 53.21 KPa. The number of stretching cycles is 210 - 500 times.

[0056] Example Nine

[0057] Based on Example One, this example investigated the effect of monomer concentration on the compression mechanochromism of the pure polymer hydrogel. The only difference is that the monomer concentration used in the present invention is not limited to 26 wt%, and any concentration between 5 - 50 wt% is selected. The preparation process of the hydrogel and the analysis conditions of the compression mechanochromic properties are the same as those in Example One.

[0058] Table 2 briefly lists the preparation of pure polymer hydrogels with different monomer concentrations involved in the present invention and their mechanochromic properties.

[0059] Table 2

[0060]

[0061] As can be seen from Table 2, at low polymer concentrations, the sparse polymer network in the hydrogel cannot reach the network density required for colored interference colors even after compression alignment. When the polymer concentration is too high, the internal network of the gel will be highly entangled and fixed in some areas, preventing the necessary movement of polymer chains. Therefore, when the polymer concentration is too high, the hydrogel fails to exhibit uniform color changes, and the optimal polymer concentration range for compression is found to be 26 - 50 wt% acrylamide.

[0062] Example Ten

[0063] On the basis of Example 1, this example investigated the effect of the concentration of the crosslinking agent on the compression mechanochromism of the pure polymer hydrogel. The only difference is that the concentration of the crosslinking agent used in the present invention is not limited to 0.5 wt%, and any concentration between 0.04 and 0.8 wt% is selected to prepare the pure polymer hydrogel. The preparation process of the hydrogel and the analysis conditions of the compression mechanochromism performance are the same as those in Example 1.

[0064] Table 3 briefly lists the preparation of pure polymer hydrogels with different crosslinking agent concentrations involved in the present invention and their mechanochromism performance.

[0065] Table 3

[0066]

[0067] As can be seen from Table 3, as the crosslinking degree increases, the hydrogel exhibits richer colors. It is found that the optimal crosslinking agent concentration for compression is 0.5 wt%.

[0068] Table 4 briefly lists the mechano-mechanical properties of the hydrogel (26PAM 0.5 ) with the optimal monomer concentration and crosslinking agent concentration involved in the present invention.

[0069] Table 4

[0070] Serial number Strain / % Distance / mm Load / N Stress / KPa Color 1 20 2.89 5.78 17.63 White 2 25 3.29 6.69 24.11 Yellow 3 30 3.99 8.65 31.14 Deep purple 4 35 4.63 10.73 38.63 Peacock blue 5 40 5.26 13.25 47.48 Jewel blue 6 45 5.96 16.50 59.39 Bright yellow 7 50 6.63 20.27 72.94 Bright pink 8 55 7.30 25.85 89.44 Green 9 60 7.98 30.68 93.84 Light pink

[0071] Example 11

[0072] On the basis of Example 8, this example investigated the effect of the concentration of the monomer and the crosslinking agent on the tensile mechanochromism of the pure polymer hydrogel. The only difference is that the monomer concentration used in the present invention is not limited to 26 wt%, and any concentration between 10 and 35 wt% is selected; the crosslinking agent concentration is not limited to 0.04 wt%, and any concentration between 0.04 and 0.5 wt% is selected. The preparation process of the hydrogel and the analysis conditions of the tensile mechanochromism performance are the same as those in Example 8.

[0073] Table 5 briefly lists the preparation of pure polymer hydrogels with different monomer concentrations and different crosslinking agent concentrations involved in the present invention and their tensile mechanochromism performance.

[0074] Table 5

[0075]

[0076] As can be seen from Table 5, at low polymer concentrations, the sparse polymer network in the hydrogel cannot achieve the network density required for color interference colors even after compression alignment. When the polymer concentration is too high, the gel strength is too strong and it is easily broken during stretching, unable to achieve many interference colors; as the crosslinking density increases, the strength of the hydrogel also becomes higher and it is easily broken during stretching, unable to achieve many interference colors; therefore, it is found that the optimal polymer concentration range for compression is 26 wt%. It is found that the optimal crosslinking agent concentration for compression is 0.04 wt%.

[0077] Example Twelve

[0078] On the basis of Example One, this example examines the influence of different polymerization methods on the mechanochromism of pure polymer hydrogels. The difference is that the polymerization method is not limited to ultraviolet light initiation, and ultraviolet light initiation can be replaced by either white light initiation or thermal initiation. The initiators provided by the present invention can be I2959, phenylbis(2,4,6-trimethylbenzoyl)phosphine oxide (PBPO), ammonium persulfate, potassium persulfate, etc. According to different polymerization methods, the polymerization time can be replaced with any time within the range of 15 - 40 min instead of 30 min. This example selects the same monomer concentration and crosslinking agent concentration as in Example One to prepare the hydrogel. In addition, the analysis conditions for the compression mechanochromic properties of the pure polymer hydrogel are the same as those in Example One.

[0079] Table 6 briefly lists the compression mechanical properties of pure polymer hydrogels prepared by ultraviolet light initiation, thermal initiation, and white light initiation.

[0080]

[0081] Table 6

[0082] As can be seen from Table 6, the initiation efficiencies of different initiators are different, and the required gelation times vary.

[0083] Example Thirteen

[0084] On the basis of Example One, in this example, the pure polymer hydrogel is immersed in an azobisisobutyronitrile solution of the initiator so that it adsorbs on the surface. The mass fraction of the initiator in the solvent is 4 wt%; the solvent is removed by heating, and the initiator remains on the surface; a prepolymerized lauryl acrylate solution is coated and polymerized by heating at 90 °C for 30 minutes to form a hydrophobic layer. Finally, a hydrogel with a hydrophobic coating is obtained (the gel with a hydrophobic layer is named C-MPPHs, and the gel without a hydrophobic layer is named MPPHs).

[0085] A pure polymer hydrogel with a hydrophobic coating was placed in water for 5 days and in air for 20 days. During this period, a compression test was carried out under polarized light, and the compression range was 0-60%. At the same time, the water loss and swelling of the hydrogel were recorded. During the compression process, the hydrogel gradually changed from colorless and transparent to colored, and the order was white, yellow, dark purple, peacock blue, sapphire blue, bright yellow, bright pink, green, light pink when compressed.

[0086] Example 14

[0087] This example uses the same preparation method as Example 13. The only difference is that the monomer is replaced by any one or at least two combinations of epoxidized soybean methacrylate, octadecyl acrylate, lauryl methacrylate, and octadecyl methacrylate. The preparation process of the hydrophobic layer of the hydrogel and the analysis conditions of the compression mechanochromic performance are the same as those in Example 7. The results show that different hydrophobic monomers can achieve the coating of the mechanochromic hydrogel without affecting its mechanochromic performance.

[0088] Example 15

[0089] Based on Example 13, this example investigated the influence of different polymerization times on the formation of the hydrophobic layer of the hydrogel. The only difference is that the polymerization time of the prepolymerized lauryl acrylate solution used in the present invention is not limited to 30 min, and any polymerization time between 20 and 50 min is selected. The preparation process of the hydrogel and the analysis conditions of the mechanochromic performance are the same as those in Example 13.

[0090] Table 7 gives a brief list of the preparation of pure polymer hydrogels with hydrophobic coatings with different polymerization times involved in the present invention and their dry and swelling resistance properties.

[0091] Table 7

[0092]

[0093]

[0094] As can be seen from Table 7, the longer the polymerization time, the thicker the hydrophobic monomer layer obtained. While the dry and swelling resistance effect is improved, if the hydrophobic monomer layer is too thick, it will block the color presentation. Therefore, the optimal polymerization time is found to be 30 min.

[0095] Example 16

[0096] Based on Example 13, this example investigated the influence of different initiator concentrations on the formation of the hydrophobic layer of the hydrogel. The only difference is that the initiator concentration during the soaking of the hydrogel used in the present invention is not limited to 4 wt%, and any initiator concentration between 3 and 5 wt% is selected. The preparation process of the hydrogel and the analysis conditions of the mechanochromic performance are the same as those in Example 13.

[0097] The preparation of pure polymer hydrogels with hydrophobic coatings at different polymerization times involved in the present invention and their dry and swelling resistance properties are briefly listed in Table 8.

[0098] Table 8

[0099]

[0100]

[0101] As can be seen from Table 8, the higher the initiator concentration, the more initiator remains on the surface of the hydrogel after soaking, and the thicker the hydrophobic monomer layer obtained after polymerization. While the dry and swelling resistance effect is improved, if the hydrophobic monomer layer is too thick, it will block the color presentation. Therefore, it is found that the optimal initiator concentration is 4 wt%.

[0102] In summary, through the combined regulation of the polymer concentration and the crosslinking density of the hydrogel, pure polymer hydrogels with uniform mechanochromism can be prepared within a certain crosslinking degree and concentration range. Further, a hydrophobic monomer coating is prepared on the surface of the hydrogel, and a transparent and uniform hydrophobic monomer layer that does not affect color presentation can be obtained through the combined regulation of the polymerization time and the initiator concentration. Hereinafter, comparative examples will be described based on the implementation technical solutions and examples of the present invention to describe the situations where pure polymer hydrogels with uniform mechanochromism cannot be formed. The described comparative examples are only a part of the comparative examples of the present invention, rather than all of the comparative examples.

[0103] The key to preparing a hydrogel with uniform mechanochromism for the present invention lies in the combined regulation of the polymer concentration and the crosslinking density of the hydrogel. Among them, the ratio of the polymer gel monomer to water is: 10 - 50 wt%, the mass fraction of the initiator in the polymer gel monomer is 0.5 - 1 wt%, the mass fraction of the crosslinking agent in the polymerizable monomer is 0.04 - 0.8 wt%; the accelerator is 0.1 - 0.5 wt% of the polymerizable monomer. The following Comparative Example 1 to Comparative Example 3 illustrate the situations where uniform mechanochromism cannot be formed in the present invention due to different combinations of polymer concentrations and hydrogel crosslinking densities.

[0104] Comparative Example 1

[0105] The acrylamide monomer (2.6 g, 5 wt% relative to the weight of water), MBA crosslinker (0.013 g, 0.5 wt% relative to acrylamide), Irgacure 2959 initiator (0.013 g, 0.5 wt% relative to acrylamide), and TEMED (13 μL, 0.5 wt% relative to acrylamide) were dissolved in 52 mL of distilled water at room temperature. Thereafter, the solution was transferred to a glass mold. Then the mold was placed under an ultraviolet lamp (16 W, 365 nm, the distance between the sample and the lamp was 2.0 cm) for 30 minutes. And the pure polymer hydrogel was placed under polarized light in air for tensile testing.

[0106] Due to the too low polymer concentration, the sparse polymer network in the hydrogel could not reach the network density required for the colored interference color even after stretching alignment.

[0107] Comparative Example 2

[0108] The acrylamide monomer (2.6 g, 55 wt% relative to the weight of water), MBA crosslinker (0.013 g, 0.5 wt% relative to acrylamide), Irgacure 2959 initiator (0.013 g, 0.5 wt% relative to acrylamide), and TEMED (13 μL, 0.5 wt% relative to acrylamide) were dissolved in 4.7 mL of distilled water at room temperature. Thereafter, the solution was transferred to a glass mold. Then the mold was placed under an ultraviolet lamp (16 W, 365 nm, the distance between the sample and the lamp was 2.0 cm) for 30 minutes. And the pure polymer hydrogel was placed under polarized light in air for compression testing.

[0109] Due to the too high polymer concentration, the internal network of the gel would be highly entangled and fixed in some areas, thus preventing the necessary movement of the polymer chains. Therefore, the hydrogel failed to exhibit a uniform color change.

[0110] Comparative Example 3

[0111] The acrylamide monomer (2.6 g, 26 wt% relative to the weight of water), MBA crosslinker (0.026 g, 1 wt% relative to acrylamide), Irgacure 2959 initiator (0.013 g, 0.5 wt% relative to acrylamide), and TEMED (13 μL, 0.5 wt% relative to acrylamide) were dissolved in 10 mL of distilled water at room temperature. Thereafter, the solution was transferred to a glass mold. Then the mold was placed under an ultraviolet lamp (16 W, 365 nm, the distance between the sample and the lamp was 2.0 cm) for 30 minutes. And the pure polymer hydrogel was placed under polarized light in air for compression testing.

[0112] Due to the excessively high gel crosslinking degree, the hydrogel becomes too hard and brittle, resulting in gel fragmentation after stress application and the inability to exhibit uniform color changes.

[0113] Comparative Example 4

[0114] Dissolve acrylamide monomer (2.6 g, 26 wt% relative to the weight of water), MBA crosslinker (0.0005 g, 0.02 wt% relative to acrylamide), Irgacure 2959 initiator (0.013 g, 0.5 wt% relative to acrylamide), and TEMED (13 μL, 0.5 wt% relative to acrylamide) in 10 mL of distilled water at room temperature. Thereafter, transfer the solution to a glass mold. Then place the mold under an ultraviolet lamp (16 W, 365 nm, the distance between the sample and the lamp is 2.0 cm) for 30 minutes. And place the pure polymer hydrogel under polarized light in the air for tensile testing.

[0115] Due to the excessively low gel crosslinking degree, the polymer network in the hydrogel is loose, and even after stretching and alignment, it cannot reach the network density required for colored interference colors.

[0116] As can be seen from the above partial comparative examples, a polymer concentration below 10 wt% or above 50 wt% will result in the inability to form a polymer network with an appropriate crosslinking degree, and a crosslinker concentration below 0.04 wt% will result in an overly loose polymer network, which also cannot reach the network density required for colored interference colors, and a pure polymer hydrogel with uniform mechanochromism cannot be obtained.

[0117] Legend analysis:

[0118] Figure 1 These are interference color photos of the pure polymer hydrogel in Example 1 of the present invention under different compressive strains (a. Interference color photos of the pure polymer mechanochromic hydrogel under various compressive strains from 0% to 60%. b. POM images of the pure polymer hydrogel under different compressive strains. c. Birefringence of the pure polymer mechanochromic hydrogel under different compressive strains). As Figure 1 can be seen, 26PAM 0.5 The hydrogel exhibits rich and colorful interference patterns and more abundant colors. The order of color change corresponds to the Michel-Levy color chart. At a compressive strain of 60%, 26PAM 0.5 The hydrogel exhibits color changes from off-white, light yellow, dark purple, peacock blue, aquamarine, bright yellow to soft purple.

[0119] Figure 2This is the interference color photograph of the pure polymer hydrogel under different tensile strains in the eighth embodiment of the present invention (a. Interference color photographs of the pure polymer mechanochromic hydrogel under various tensile strains from 0% to 2000%. b. POM images of the pure polymer hydrogel under different tensile strains; c. And birefringence). Considering that the reversible deformation of the polymer network under applied compressive force leads to the color change of the hydrogel, the proposed mechanochromic pure polymer hydrogel is considered suitable for applications involving tensile response. After reducing the crosslinking density, its stretchability is significantly improved. As can be seen from Figure 2 a, as the tensile strain increases, the loose polymer network in the hydrogel is stretched horizontally, resulting in a higher degree of alignment and forming an ordered network structure. Thus, a uniform color change is formed.

[0120] Figure 3 This is the pure polymer hydrogel with mechanochromism prepared by replacing acrylamide with acrylic acid in the sixth embodiment of the present invention. The preparation process of the hydrogel and the analysis conditions of the mechanochromic properties are the same as those in Example 1. (a. Interference color photographs of the acrylic acid pure polymer hydrogel with different crosslinking densities under various compressive strains from 0% to 60%, b. Stress-strain curves and birefringence photographs before and after compression of the hydrogel during 500 compression / release cycles at 60% strain, c. Stress changes of the hydrogel during 500 compression / release cycles at 60% strain). As can be seen from Figure 3 this, the strategy of achieving mechanochromism by simply adjusting the crosslinking density of the polymer network is not limited to acrylamide hydrogels.

[0121] Figure 4 This is the compressive mechanical properties of the pure polymer hydrogels with mechanochromism and different crosslinking densities in the first and eighth embodiments of the present invention. (a. Stress-strain curves of the acrylamide-based pure polymer hydrogel during compression, b. Stress-strain curves and birefringence photographs before and after compression of the hydrogel during 1000 compression / release cycles at 60% strain, c. Stress changes of the hydrogel during 1000 compression / release cycles at 60% strain). As can be seen from Figure 4 this, as the crosslinking density of the hydrogel increases, the compressive performance of the hydrogel continuously increases. And the hydrogel undergoes 1000 compression cycles at 60% strain. At this strain level, the cyclic stress-strain curves completely overlap, indicating excellent stability and fatigue resistance. In addition, the stable single-component structure enables efficient stress transfer perpendicular to the compression direction, significantly reducing energy dissipation and enhancing the elasticity in the compression direction.

[0122] Figure 5is the tensile mechanical properties of pure polymer hydrogels with mechanical color change and different crosslinking densities in Example 1 and Example 8 of the present invention. (a. Stress-strain curves of acrylamide-based pure polymer hydrogels during stretching, b. Stress-strain curves and birefringence photographs before and after compression of the hydrogel during 500 compression / release cycles at 500% strain, c. Stress changes of the hydrogel during 500 stretching / release cycles at 500% strain.) It can be seen from Figure 5 that the 25PAM0.04 hydrogel exhibits a tensile strain of up to 1700% in the stretching direction. The 25PAM0.04 hydrogel withstood 500 compression cycles at 500% strain. The cyclic stress-strain curves almost overlapped after the first cycle, showing excellent fatigue resistance and long-term stability.

[0123] Figure 6 are optical microscope photographs of the hydrogel with a hydrophobic coating before and after the preparation of the hydrophobic coating in Example 13 of the present invention (optical microscope photographs of the (a1) edge and (b1) surface of the gel before the hydrophobic monomer coating, and the (a2) edge and (b2) surface of the hydrogel after the hydrophobic monomer coating). A uniform and smooth 100-μm hydrophobic polymer layer was observed after coating.

[0124] Figure 7 are the swelling rate of the hydrogel with a hydrophobic coating in water and the water loss rate after standing in air in Example 1 of the present invention (a. Swelling properties of the uncoated hydrogel and the hydrogel with a hydrophobic coating in water (25°C), b. Drying resistance of the uncoated hydrogel and the hydrogel with a hydrophobic coating in air (25°C). It can be seen from Figure 7 that the hydrogel with a hydrophobic coating exhibits excellent anti-swelling performance, and there is no obvious change in volume after long-term immersion in water. After 5 days, its swelling rate is only 8%, while the swelling rate of the uncoated hydrogel is 139%. Similarly, when the hydrogel with a hydrophobic coating is placed in air at 25°C for 5 days, it only loses 4.0 ± 0.05 wt% of its weight, while the uncoated hydrogel loses all its moisture (68.2 ± 0.3 wt%). Therefore, the hydrogel with a hydrophobic coating has stable anti-drying and anti-swelling properties, broadening its application environment.

[0125] Figure 8 are comparison photographs of the hydrogel with a hydrophobic coating after soaking in water for 5 days and standing in air for 5 days and the uncoated hydrogel in Example 13 of the present invention (a. Images of the hydrogel with a hydrophobic coating and the hydrogel without a hydrophobic coating after soaking in water for 5 days, b. Images of the hydrogel with a hydrophobic coating and the hydrogel without a hydrophobic coating after standing in air for 5 days). It can be seen from Figure 8It can be seen that when the hydrogel with a hydrophobic coating is placed in water at 25 °C for 5 days, no obvious size change is shown, while its uncoated counterpart significantly swells within 1 day until the volume increases by 4 times. Similarly, when the hydrogel with a hydrophobic coating is placed in air at 25 °C for 5 days, no obvious size change is shown, while its uncoated counterpart significantly shrinks and completely dries within 2 days.

[0126] Figure 9 It is the compressive mechanochromic photograph of the hydrogel with a hydrophobic coating according to Embodiment XIII of the present invention after being soaked in water for 5 days and standing in air for 5 days. After being soaked in water for 5 days or placed in air at 25 °C for 5 days, at a strain of 55%, the hydrogel still exhibits a wide mechanochromic change from white to purple.

Claims

1. A mechanically color-changing pure polymer hydrogel, characterized in that, It is prepared by the following steps: After mixing a polymer gel monomer, water, an initiator, a crosslinking agent, and an accelerator in proportion, a transparent pure polymer hydrogel with bidirectional mechanochromism in tension and compression is obtained through ultraviolet photopolymerization reaction; the polymer gel monomer is any one or a combination of at least two of acrylic acid, methacrylic acid, hydroxyethyl acrylate, hydroxyethyl methacrylate, butyl acrylate, butyl methacrylate, acrylamide, methacrylamide, N-isopropylacrylamide; the initiator is any one or a combination of at least two of 2-hydroxy-4'-(2-hydroxyethoxy)-2-methylpropiophenone (I2959), phenylbis(2,4,6-trimethylbenzoyl)phosphine oxide, ammonium persulfate, and potassium persulfate; the crosslinking agent is any one or a combination of at least two of polyethylene glycol diacrylate, dipropylene glycol diacrylate, N,N'-methylenebisacrylamide, and gelatin methacrylate; the accelerator is any one or a combination of at least two of dimethylethylenediamine, tetramethylethylenediamine, and ascorbic acid; the ratio of the polymer gel monomer to water is 10-50 wt%, the mass fraction of the initiator in the polymer gel monomer is 0.5-1 wt%, the mass fraction of the crosslinking agent in the polymerizable monomer is 0.04-0.8 wt%, and the mass fraction of the accelerator in the polymerizable monomer is 0.1-0.5 wt%.

2. The pure polymer hydrogel with mechanical color change according to claim 1, characterized in that: The ratio of the polymer gel monomer to water is 26-50 wt%, the mass fraction of the initiator in the polymer gel monomer is 0.5-1 wt%, the mass fraction of the crosslinking agent in the polymerizable monomer is 0.50-0.80 wt%, and the mass fraction of the accelerator in the polymerizable monomer is 0.1-0.5 wt%.

3. The mechanically color-changing pure polymer hydrogel according to claim 2, wherein: After immersing the pure polymer hydrogel in a 2-5 wt% initiator solution, a pre-polymerized hydrophobic monomer is coated, and a hydrophobic layer is formed through thermal polymerization at 60-100 °C to obtain a pure polymer hydrogel with a hydrophobic coating and bidirectional mechanochromism in tension and compression; the pre-polymerized hydrophobic monomer is any one or a combination of at least two of epoxidized soybean methacrylate, lauryl acrylate, stearyl acrylate, lauryl methacrylate, and stearyl methacrylate.

4. The mechanically color-changing pure polymer hydrogel according to claim 2, characterized in that, During the compression strain range of 0-60%, the hydrogel can be observed to gradually change from colorless and transparent to colored under a polarizer, and in the order of white, yellow, dark purple, peacock blue, sapphire blue, bright yellow, bright pink, green, light pink, and its birefringence index is 0.4×10 -4 ~1.2×10 -4 ; The compression response range of the hydrogel is: the load is 5.78-30.68 N, the distance is 2.89-7.98 mm, and the stress is 17.63-93.84 KPa.

5. A purely polymeric hydrogel with mechanical color change according to claim 1, characterized in that: The ratio of the polymer gel monomer to water is 10-35 wt%, the mass fraction of the initiator in the polymer gel monomer is 0.5-1 wt%, the mass fraction of the crosslinking agent in the polymerizable monomer is 0.04-0.50 wt%, and the mass fraction of the accelerator in the polymerizable monomer is 0.1-0.5 wt%.

6. The pure polymer hydrogel with mechanical color change according to claim 5, characterized in that, During the stretching strain range of 0-2000%, it can be observed under a polarizer that the hydrogel gradually changes from colorless and transparent to colored, and changes in the order of orange, dark purple, peacock blue, sapphire blue, bright yellow, and light violet. The birefringence index is 0.5×10 -4 ~1.4×10 -4 , and the mechanical response range of the hydrogel is: the load is 0.23-1.57 N, the distance is 12.28-140.33 mm, and the stress is 12.46-53.21 KPa.

7. A pure polymer hydrogel with mechanical color change according to claim 3, characterized in that, The hydrogel has excellent mechanochromic stability and can be placed in air and water for 5-20 days, maintaining 95-110% of the original weight and maintaining the original mechanochromism.

8. A mechanically color-changing pure polymer hydrogel according to any one of claims 2, 3, 4, and 7, characterized in that, The hydrogel has mechanochromic stability under cyclic compression and still maintains the original mechanochromism after being compressed 500-2000 times.

9. A mechanically color-changing pure polymer hydrogel according to claim 5 or 6, characterized in that, The hydrogel has mechanochromic stability under cyclic tension and still maintains the original mechanochromism after being stretched cyclically 200-500 times.

10. Application of a mechanically color-changing pure polymer hydrogel according to any one of claims 1-9 in the fields of intelligent sensing, electronic skin, and biomedicine.

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