A method for preparing carbon dot-based super-strong hydrogel

Abstract

The present invention belongs to the technical field of hydrogel materials, and in particular, relates to a method for preparing a carbon-dot-based super-strong hydrogel. The carbon-dot-based super-strong hydrogel of the present invention is prepared by using specific carbon dots (citric acid carbon dots, tannic acid carbon dots, malic acid carbon dots, or bark carbon dots) as nanofillers in the hydrogel and salting-out assisted progressive stretching. By utilizing the specific carbon dots as nanofillers and the Hofmeister effect-sensitive properties of polyvinyl alcohol, the carbon dots and salting-out assisted progressive stretching are combined to construct a super-strong hydrogel with a simple preparation process and excellent performance.

Description

Technical Field

[0001] The present invention relates to the technical field of hydrogel materials, and in particular to a method for preparing a carbon-dot-based super-strong hydrogel. Background Art

[0002] Hydrogels are widely used in wearable electronics, soft robotics, nano-friction generators, biomedicine, and solid electrolytes due to their unique conductivity, flexibility, and three-dimensional network structure. However, traditional hydrogels are composed of a single hydrophilic polymer network, lack energy dissipation mechanisms, and are susceptible to stress concentration. Most of them exhibit the disadvantages of weak mechanical properties and toughness. In addition, there is an inherent contradiction between the strength and toughness of hydrogels. Improving the mechanical strength often leads to material hardening, making it difficult for hydrogels to have both high toughness and high strength. This poor mechanical property greatly limits the application of such hydrogels. Recently, high-strength hydrogels have been developed through strategies such as constructing double networks, organic-inorganic hybridization, densification, multiple interactions and directional freezing. The hydrogels constructed by these methods exhibit better mechanical strength, but despite this, it is still difficult for the materials to achieve a fracture strength of 50 MPa and a conductivity of 200 MJ m -3 Therefore, it is necessary to design and develop high-strength and high-toughness hydrogels.

[0003] Nanofillers (carbon nanotubes, graphene, and MXene, etc.) have attracted great attention due to their interfacial effects with polymers. However, nanofillers generally have poor compatibility with the matrix, resulting in poor dispersion, which undermines the effectiveness of this reinforcement strategy. Moreover, this interfacial interaction is usually based on weak physical interactions and limited chemical bonding, which has limited improvement in the mechanical properties of hydrogels. In addition, nanofillers are expensive and cumbersome to prepare. Subsequently, carbon dots gradually came into people's attention. As a new member of the "carbon family", carbon dots (CDs) are carbon nanoparticles with a size of less than 10 nm. They were accidentally discovered in 2004 during the preparation and purification of single-walled carbon nanotubes by electrophoresis. In 2006, luminescent carbon was prepared by surface passivation. Since then, due to their excellent optical properties, conductivity, low toxicity and high biocompatibility, this field has attracted many researchers. However, carbon dots have rarely been used to improve the mechanical properties of composite materials. Summary of the Invention

[0004] The purpose of the present invention is to provide a method for preparing a carbon dot-based super-strong hydrogel to solve the problems existing in the above-mentioned prior art and realize the preparation of super-strong hydrogel.

[0005] To achieve the above object, the present invention provides the following solutions:

[0006] One of the technical solutions of the present invention is to provide a method for preparing a carbon dot-based super hydrogel, comprising the following steps:

[0007] Using carbon source as raw material and water as solvent, the carbon dot solution is prepared through firing and cooling;

[0008] The carbon dot solution and polyvinyl alcohol are used as reactants, and subjected to high temperature reaction and freeze-thaw cycles to obtain a carbon dot-polyvinyl alcohol hydrogel;

[0009] The carbon dot-polyvinyl alcohol hydrogel is subjected to salting-out assisted progressive stretching and then immersed in a sodium citrate solution to obtain the carbon dot-based super-strong hydrogel;

[0010] The carbon source is at least one of citric acid, tannic acid, malic acid and bark.

[0011] Furthermore, the ratio of the carbon source to water is 10 g:100 mL.

[0012] Furthermore, the firing temperature is 160° C. and the firing time is 8 hours.

[0013] Furthermore, the mass ratio of the carbon dot solution to polyvinyl alcohol is 17:3.

[0014] Furthermore, the temperature of the high temperature reaction is 90° C. and the time is 3 hours.

[0015] Furthermore, the freeze-thaw cycle is freezing at -20°C for 8 hours, then thawing at 20°C for 3 hours, and the freeze-thaw cycle is repeated at least once.

[0016] Furthermore, the step of salting-out assisted progressive stretching is:

[0017] The carbon dot-polyvinyl alcohol hydrogel was immersed in a sodium citrate solution for 12 hours and then subjected to gradual stretching.

[0018] Preferably, the concentration of the sodium citrate solution is 2 mol / L.

[0019] Preferably, the deformation amount of the progressive stretching is 300%.

[0020] During the salting-out assisted progressive stretching process, the hydrogel becomes more compact, achieving the purpose of enhancing mechanical properties.

[0021] Furthermore, the immersion time in the sodium citrate solution is 12 hours, and the concentration of the sodium citrate solution is 2 mol / L.

[0022] In order to improve the mechanical properties of hydrogels, the present invention designs a method in which specific carbon dots (citric acid carbon dots, tannic acid carbon dots, malic acid carbon dots or bark carbon dots) act as nanofillers and salting-out assisted progressive stretching. 2The crystal nucleus can give the material good interfacial interactions (such as ππ stacking, CH-π, etc.). Moreover, after carbonization, the functional groups of the carbonized precursor can be well retained and have good dispersibility. In addition, the mechanical properties of the hydrogel are enhanced through non-covalent bonds between specific carbon points, polyethylene and sodium ions, including π-π, CH-π, van der Waals force and ionic bonds.

[0023] The second technical solution of the present invention is to provide a carbon dot-based super-strong hydrogel prepared by the above preparation method.

[0024] The present invention discloses the following technical effects:

[0025] The specific carbon dot-based super-strong hydrogel of the present invention is prepared by using citric acid carbon dots, tannic acid carbon dots, malic acid carbon dots or bark carbon dots as nanofillers and salting-out assisted progressive stretching, and utilizing the Hofmeister effect sensitive characteristics of polyvinyl alcohol (PVA) to combine carbon dots and progressive stretching to construct a hydrogel with super-strong mechanical properties.

[0026] The specific carbon dots used in the present invention (citric acid carbon dots, tannic acid carbon dots, malic acid carbon dots or bark carbon dots) not only have attractive sp 2 The carbonized citric acid, tannic acid, malic acid, and bark retain the functional groups of the carbonized precursors, resulting in excellent dispersibility and the introduction of a large number of physical crosslinking reaction sites into the polyvinyl alcohol hydrogel, which imparts ultra-high tensile strength and toughness. Furthermore, the salt solution introduced during the compacting process imparts excellent antifreeze properties to the hydrogel. The hydrogel is simple to prepare and exhibits excellent performance. BRIEF DESCRIPTION OF THE DRAWINGS

[0027] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the following briefly introduces the drawings required for use in the embodiments. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying any creative work.

[0028] Figure 1 The stress-strain curves of the hydrogels prepared in Example 1 and Comparative Examples 6-7;

[0029] Figure 2 The stress-strain curves of the hydrogels prepared from different carbon sources in Examples 1 to 4;

[0030] Figure 3 This is the stress-strain curve of the hydrogel prepared in Comparative Example 1;

[0031] Figure 4 This is the stress-strain curve of the hydrogel prepared in Comparative Example 2;

[0032] Figure 5 This is the stress-strain curve of the hydrogel prepared in Comparative Example 3;

[0033] Figure 6 This is the stress-strain curve of the hydrogel prepared in Comparative Example 4;

[0034] Figure 7 This is the stress-strain curve of the hydrogel prepared in Comparative Example 5;

[0035] Figure 8 The DSC graphs of the hydrogels prepared in Example 1 and Comparative Examples 1 to 5 are shown;

[0036] Figure 9 Schematic diagram of the process flow of Example 1. DETAILED DESCRIPTION

[0037] Various exemplary embodiments of the present invention will now be described in detail. This detailed description should not be considered as limiting the present invention, but rather as a more detailed description of certain aspects, features, and embodiments of the present invention.

[0038] It should be understood that the terms described herein are intended only to describe particular embodiments and are not intended to limit the present invention. In addition, for numerical ranges herein, it should be understood that each intermediate value between the upper and lower limits of the range is also specifically disclosed. The intermediate value within any stated value or stated range, and each smaller range between any other stated value or intermediate value within the stated range, is also encompassed within the present invention. The upper and lower limits of these smaller ranges may be independently included or excluded within the scope.

[0039] Unless otherwise indicated, all technical and scientific terms used herein have the same meaning as commonly understood by those skilled in the art. Although only preferred methods and materials are described herein, any methods and materials similar or equivalent to those described herein may also be used in the practice or testing of the present invention. All documents mentioned in this specification are incorporated by reference to disclose and describe the methods and / or materials associated with the documents. In the event of any conflict with any incorporated document, the contents of this specification shall prevail.

[0040] It will be apparent to those skilled in the art that various modifications and variations may be made to the specific embodiments described herein without departing from the scope or spirit of the invention. Other embodiments will be apparent to those skilled in the art from the description of the invention. The description and examples are intended to be illustrative only.

[0041] The words “include,” “including,” “have,” “contain,” etc. used in this document are open-ended terms, meaning including but not limited to.

[0042] Unless otherwise specified, the room temperature in the specific embodiments of the present invention refers to 25±5°C.

[0043] The raw materials and reagents used in the specific embodiments of the present invention are all commercially available products.

[0044] Example 1

[0045] The preparation steps of citric acid carbon dot-based super hydrogel are as follows:

[0046] S1. Add 10 g of citric acid to 100 mL of deionized water, place the mixture in a pressure-resistant reaction bottle, heat to 160° C., and react for 8 h. The reaction vessel is then naturally cooled to room temperature to obtain a citric acid carbon dot solution.

[0047] S2, citric acid carbon dot solution (34 g) and polyvinyl alcohol (6 g) were mixed and reacted at 90°C for 3 h. After the mixed solution was cooled to room temperature, the obtained reaction product was subjected to freeze-thaw cycle treatment (freeze-thaw cycle was frozen at -20°C for 8 h, then thawed at 20°C for 3 h, and freeze-thaw was repeated twice, for a total of 3 freeze-thaw cycles) to obtain citric acid carbon dot-polyvinyl alcohol hydrogel;

[0048] S3. The citric acid carbon dot-polyvinyl alcohol hydrogel obtained in step S2 was soaked in a 2 mol / L sodium citrate solution for 12 h, then progressively stretched to 300% deformation, and soaked in a 2 mol / L sodium citrate solution again for 12 h to obtain a citric acid carbon dot-based super hydrogel, recorded as PVA-CDYSY.

[0049] Figure 9 Schematic diagram of the process flow of Example 1.

[0050] Example 2

[0051] The preparation steps of eucalyptus bark carbon dot-based super hydrogel are as follows:

[0052] S1. Add 10 g of eucalyptus bark to 100 mL of deionized water, place the mixture in a pressure-resistant reaction bottle, heat to 160° C., and react for 8 h. The reaction vessel is then naturally cooled to room temperature to obtain a eucalyptus bark carbon dot solution.

[0053] S2, eucalyptus bark carbon dot solution (34 g) and polyvinyl alcohol (6 g) were mixed and reacted at 90°C for 3 h. After the mixed solution was cooled to room temperature, the obtained reaction product was subjected to freeze-thaw cycle treatment (freeze-thaw cycle was frozen at -20°C for 8 h, then thawed at 20°C for 3 h, and freeze-thaw was repeated twice, for a total of three freeze-thaw cycles) to obtain eucalyptus bark carbon dot-polyvinyl alcohol hydrogel;

[0054] S3. The eucalyptus bark carbon dot-polyvinyl alcohol hydrogel obtained in step S2 is soaked in a 2 mol / L sodium citrate solution for 12 hours, then progressively stretched to 300% deformation, and soaked in a 2 mol / L sodium citrate solution again for 12 hours to obtain a eucalyptus bark carbon dot-based super-strong hydrogel.

[0055] Example 3

[0056] The preparation steps of tannic acid carbon dot-based super hydrogel are as follows:

[0057] S1. Add 10 g of tannic acid to 100 mL of deionized water, place the mixture in a pressure-resistant reaction bottle, heat to 160° C., and react for 8 h. The reaction vessel is then naturally cooled to room temperature to obtain a tannic acid carbon dot solution.

[0058] S2, tannic acid carbon dot solution (34 g) and polyvinyl alcohol (6 g) were mixed and reacted at 90° C. for 3 h. After the mixed solution was cooled to room temperature, the obtained reaction product was subjected to freeze-thaw cycle treatment (freeze-thaw cycle was frozen at -20° C. for 8 h, then thawed at 20° C. for 3 h, and freeze-thaw was repeated twice, for a total of three freeze-thaw cycles) to obtain tannic acid carbon dot-polyvinyl alcohol hydrogel;

[0059] S3. The tannic acid carbon dot-polyvinyl alcohol hydrogel obtained in step S2 is soaked in a 2 mol / L sodium citrate solution for 12 h, then progressively stretched to 300% deformation, and soaked in a 2 mol / L sodium citrate solution again for 12 h to obtain a tannic acid carbon dot-based super-strong hydrogel.

[0060] Example 4

[0061] The preparation steps of malic acid carbon dot-based super hydrogel are as follows:

[0062] S1. Add 10 g of malic acid to 100 mL of deionized water, place the mixture in a pressure-resistant reaction bottle, heat to 160° C., and react for 8 h. The reaction vessel is then naturally cooled to room temperature to obtain a malic acid carbon dot solution.

[0063] S2, malic acid carbon dot solution (34 g) and polyvinyl alcohol (6 g) were mixed and reacted at 90°C for 3 h. After the mixed solution was cooled to room temperature, the obtained reaction product was subjected to freeze-thaw cycle treatment (freeze-thaw cycle was frozen at -20°C for 8 h, then thawed at 20°C for 3 h, and freeze-thaw was repeated twice, for a total of 3 freeze-thaw cycles) to obtain malic acid carbon dot-polyvinyl alcohol hydrogel;

[0064] S3. The malic acid carbon dot-polyvinyl alcohol hydrogel obtained in step S2 was soaked in a 2 mol / L sodium citrate solution for 12 h, then progressively stretched to 300% deformation, and soaked in a 2 mol / L sodium citrate solution again for 12 h to obtain a malic acid carbon dot-based super-strong hydrogel.

[0065] Comparative Example 1

[0066] The preparation steps of polyvinyl alcohol hydrogel are as follows:

[0067] Polyvinyl alcohol (6 g) was mixed with deionized water (34 g) and reacted at 90°C for 3 h. After the reaction product was cooled to room temperature, it was subjected to freeze-thaw cycle treatment (the freeze-thaw cycle was frozen at -20°C for 8 h, then thawed at 20°C for 3 h, and the freeze-thaw cycle was repeated twice, for a total of 3 freeze-thaw cycles) to obtain polyvinyl alcohol hydrogel, which was recorded as PVA.

[0068] Comparative Example 2

[0069] The preparation steps of citric acid-polyvinyl alcohol hydrogel are as follows:

[0070] Polyvinyl alcohol (6 g), citric acid (4 g) and deionized water (30 g) were mixed and reacted at 90°C for 3 h. After the reaction product was cooled to room temperature, it was subjected to freeze-thaw cycle treatment (the freeze-thaw cycle was frozen at -20°C for 8 h, then thawed at 20°C for 3 h, and the freeze-thaw cycle was repeated twice, for a total of 3 freeze-thaw cycles) to obtain citric acid-polyvinyl alcohol hydrogel, which was recorded as PVA-CA.

[0071] Comparative Example 3

[0072] The preparation steps of citric acid carbon dot-polyvinyl alcohol hydrogel are as follows:

[0073] S1. Add 10 g of citric acid to 100 mL of deionized water, place the mixture in a pressure-resistant reaction bottle, heat to 160° C., and react for 8 h. The reaction vessel is then naturally cooled to room temperature to obtain a citric acid carbon dot solution.

[0074] S2, citric acid carbon dot solution (34 g) and polyvinyl alcohol (6 g) were mixed and reacted at 90°C for 3 h. After the mixed solution was cooled to room temperature, the reaction product was subjected to freeze-thaw cycle treatment (freeze-thaw cycle was freezing at -20°C for 8 h, then thawing at 20°C for 3 h, and freezing-thawing was repeated 2 times, for a total of 3 freeze-thaw cycles) to obtain citric acid carbon dot-polyvinyl alcohol hydrogel, recorded as PVA-CD.

[0075] Comparative Example 4

[0076] The preparation steps of citric acid carbon dot-polyvinyl alcohol hydrogel after salting out treatment are as follows:

[0077] S1. Add 15 g of citric acid to 100 mL of deionized water, place the mixture in a pressure-resistant reaction bottle, heat to 160° C., and react for 8 h. The reaction vessel is then naturally cooled to room temperature to obtain a citric acid carbon dot solution.

[0078] S2, citric acid carbon dot solution (34 g) and polyvinyl alcohol (6 g) were mixed and reacted at 90°C for 3 h. After the mixed solution was cooled to room temperature, the obtained reaction product was subjected to freeze-thaw cycle treatment (freeze-thaw cycle was frozen at -20°C for 8 h, then thawed at 20°C for 3 h, and freeze-thaw was repeated twice, for a total of 3 freeze-thaw cycles) to obtain citric acid carbon dot-polyvinyl alcohol hydrogel;

[0079] S3. Soak the citric acid carbon dot-polyvinyl alcohol hydrogel obtained in step S2 in a 2 mol / L sodium citrate solution for 12 h to obtain a citric acid carbon dot-polyvinyl alcohol hydrogel after salting out, which is recorded as PVA-CDY.

[0080] Comparative Example 5

[0081] The preparation steps of citric acid carbon dot-polyvinyl alcohol hydrogel after salting-out assisted progressive stretching are as follows:

[0082] S1. Add 10 g of citric acid to 100 mL of deionized water, place the mixture in a pressure-resistant reaction bottle, heat to 160° C., and react for 8 h. The reaction vessel is then naturally cooled to room temperature to obtain a citric acid carbon dot solution.

[0083] S2, citric acid carbon dot solution (34 g) and polyvinyl alcohol (6 g) were mixed and reacted at 90°C for 3 h. After the mixed solution was cooled to room temperature, the obtained reaction product was subjected to freeze-thaw cycle treatment (freeze-thaw cycle was frozen at -20°C for 8 h, then thawed at 20°C for 3 h, and freeze-thaw was repeated twice, for a total of 3 freeze-thaw cycles) to obtain citric acid carbon dot-polyvinyl alcohol hydrogel;

[0084] S3. The citric acid carbon dot-polyvinyl alcohol hydrogel obtained in step S2 was soaked in a 2 mol / L sodium citrate solution for 12 h, and then progressively stretched to 300% deformation to obtain the citric acid carbon dot-polyvinyl alcohol hydrogel after salting-out assisted progressive stretching, which was recorded as PVA-CDYS.

[0085] Comparative Example 6

[0086] The preparation steps of citric acid carbon dot-based super hydrogel sintered at 140°C are as follows:

[0087] S1. Add 10 g of citric acid to 100 mL of deionized water, place the mixture in a pressure-resistant reaction bottle, heat to 140° C., and react for 8 h. The reaction vessel is then naturally cooled to room temperature to obtain a citric acid carbon dot solution.

[0088] S2, citric acid carbon dot solution (34 g) and polyvinyl alcohol (6 g) were mixed and reacted at 90°C for 3 h. After the mixed solution was cooled to room temperature, the obtained reaction product was subjected to freeze-thaw cycle treatment (freeze-thaw cycle was frozen at -20°C for 8 h, then thawed at 20°C for 3 h, and freeze-thaw was repeated twice, for a total of 3 freeze-thaw cycles) to obtain citric acid carbon dot-polyvinyl alcohol hydrogel;

[0089] S3. The citric acid carbon dot-polyvinyl alcohol hydrogel obtained in step S2 is soaked in a 2 mol / L sodium citrate solution for 12 h, then progressively stretched to 300% deformation, and soaked in a 2 mol / L sodium citrate solution again for 12 h to obtain a citric acid carbon dot-based super-strong hydrogel.

[0090] Comparative Example 7

[0091] The preparation steps of citric acid carbon dot-based super hydrogel sintered at 180°C are as follows:

[0092] S1. Add 10 g of citric acid to 100 mL of deionized water, place the mixture in a pressure-resistant reaction bottle, heat to 180° C., and react for 8 h. The reaction vessel is then naturally cooled to room temperature to obtain a citric acid carbon dot solution.

[0093] S2, citric acid carbon dot solution (34 g) and polyvinyl alcohol (6 g) were mixed and reacted at 90°C for 3 h. After the mixed solution was cooled to room temperature, the obtained reaction product was subjected to freeze-thaw cycle treatment (freeze-thaw cycle was frozen at -20°C for 8 h, then thawed at 20°C for 3 h, and freeze-thaw was repeated twice, for a total of 3 freeze-thaw cycles) to obtain citric acid carbon dot-polyvinyl alcohol hydrogel;

[0094] S3. The citric acid carbon dot-polyvinyl alcohol hydrogel obtained in step S2 is soaked in a 2 mol / L sodium citrate solution for 12 h, then progressively stretched to 300% deformation, and soaked in a 2 mol / L sodium citrate solution again for 12 h to obtain a citric acid carbon dot-based super-strong hydrogel.

[0095] Test example

[0096] The mechanical properties of the hydrogels prepared in Examples 1 to 4 and Comparative Examples 1 to 7 were tested and the toughness was calculated using the following methods. The results are shown in Table 1.

[0097] The hydrogels prepared in Examples 1-4 and Comparative Examples 1-7 were cut into rectangular specimens (4 cm × 2 mm × 1 mm) and subjected to tensile testing at room temperature. Uniaxial tensile testing was performed using an electronic universal testing machine (Suntech Technology Co., Ltd., Shenzhen, China) at an extension speed of 80 mm / min.

[0098] Toughness is calculated based on the area under the stress-strain curve using the following formula:

[0099] △U=σdε

[0100] where σ and ε are the stress and strain of the hydrogel, respectively.

[0101] Table 1

[0102]

[0103]

[0104] It can be seen from the data in Table 1 that the tensile strength and toughness of the hydrogel prepared in Example 1 are the highest, reaching 156 MPa and 225.2 MJ / m 3 From the data changes in Comparative Examples 1 to 5, it can be seen that the mechanical properties of the hydrogel gradually enhance, indicating that the use of citric acid carbon dots as nanofillers in polyvinyl alcohol hydrogels, salting-out assisted progressive stretching, and soaking after progressive stretching all have a certain impact on the mechanical properties of the hydrogel. Similarly, carbon dots have universal applicability. Hydrogels prepared from carbon dots sintered with eucalyptus bark, malic acid, and tannic acid all have superb mechanical properties. At the same time, citric acid carbon dots sintered at different temperatures also have good mechanical properties.

[0105] Figure 1 The stress-strain curves of the hydrogels prepared in Example 1 and Comparative Examples 6-7 are shown in FIG. Figure 1 As can be seen, the mechanical properties of hydrogels prepared from citric acid carbon dots sintered at 140°C, 160°C, and 180°C increased from 110.61 MPa to 156 MPa and then decreased to 125.03 MPa. However, the deformation showed a positive correlation with temperature. This is because the structure and surface properties of the carbon dots may change with increasing sintering temperature. The carbon dots sintered at 140°C have a lower degree of graphitization and a higher number of functional groups, resulting in a weaker interaction with the polyvinyl alcohol hydrogel, which reduces the mechanical properties. The carbon dots sintered at 160°C have a higher degree of graphitization and more optimized surface properties, which strengthen their bond with the hydrogel and improve the mechanical properties. Further increasing the temperature to 180°C leads to excessive graphitization or structural changes of the carbon dots, which reduces their interaction with the hydrogel, resulting in a slight decrease in mechanical properties but a significant increase in deformation. This may be because the high-temperature carbon dots have a greater impact on the network structure of the hydrogel, resulting in increased flexibility.

[0106] Figure 2 The stress-strain curves of the hydrogels prepared from different carbon sources in Examples 1 to 4 are shown in Table 1. Figure 2It can be seen that the tensile strength and toughness of the hydrogels prepared based on citric acid carbon dots, eucalyptus bark carbon dots, tannic acid carbon dots and malic acid carbon dots are 150 MPa, 85.85 MPa, 52.23 MPa, 129.37 MPa and 225.2 MJ / m, respectively. 3 、148.6MJ / m 3 、86.5MJ / m 3 、202.8MJ / m 3 This is because the preparation of citric acid carbon dots involves the chemical reaction of citric acid to generate carbon dots, which usually have good dispersibility and surface functionalization, and can form strong cross-linking with polyvinyl alcohol, thereby improving the tensile strength and toughness of the hydrogel. Eucalyptus bark carbon dots are derived from plant extracts and contain complex natural organic components. The influence of these components leads to their poor dispersibility and interfacial binding ability in hydrogels, resulting in relatively low tensile strength. Tannic acid carbon dots have a complex structure and contain multiple phenolic hydroxyl groups, which leads to poor cross-linking effect in hydrogels, thereby affecting mechanical properties and toughness. Malic acid carbon dots have moderate dispersibility and interaction with polyvinyl alcohol, which leads to the tensile strength and toughness of their hydrogels being intermediate among these carbon dots.

[0107] Figure 3 is the stress-strain curve of the hydrogel prepared in Comparative Example 1, Figure 3 It can be seen that the mechanical properties of polyvinyl alcohol hydrogel are 0.27MPa and 0.65MJ / m 3 This is because polyvinyl alcohol hydrogel has a high water content, which makes it flexible and absorbent, but also reduces the mechanical strength of the material. The presence of water weakens the mechanical strength of the PVA chain, causing the hydrogel to deform and break easily under tension or pressure.

[0108] Figure 4 is the stress-strain curve of the hydrogel prepared in Comparative Example 2, Figure 4 It can be seen that the mechanical properties of citric acid-polyvinyl alcohol hydrogel are 1.06MPa and 1.5MJ / m 3 , which is a certain improvement over the mechanical properties of polyvinyl alcohol hydrogel. This is because citric acid has three carboxyl groups that can undergo cross-linking reactions with the hydroxyl groups in the PVA molecules. Citric acid forms cross-linking points with PVA molecules through these functional groups, which helps to form a tighter network structure, thereby improving the strength of the hydrogel. Compared with pure PVA, citric acid can form cross-linking points between PVA chains, enhancing the stability and strength of the network structure. Although the cross-linking effect may not be as good as some specialized cross-linking agents, it is still better than pure PVA hydrogel without cross-linking.

[0109] Figure 5 is the stress-strain curve of the hydrogel prepared in Comparative Example 3, Figure 5 It can be seen that the mechanical properties of citric acid carbon dots-polyvinyl alcohol hydrogel are 11.55MPa and 71.6MJ / m 3 This is because the citric acid carbon dots can form more cross-linking points in the PVA hydrogel, making the hydrogel network more stable and compact. At the same time, the nanometer size and excellent dispersibility of the citric acid carbon dots enhance the network density and cross-linking effect of the PVA hydrogel. This improved network structure can effectively disperse and withstand external loads, thereby improving tensile strength and toughness.

[0110] Figure 6 is the stress-strain curve of the hydrogel prepared in Comparative Example 4, Figure 6 It can be seen that the mechanical properties of citric acid carbon dots-polyvinyl alcohol hydrogel after salting out treatment are improved to 18.04MPa and 72.38MJ / m 3 This is because sodium citrate enhances the network structure of the hydrogel by increasing the ionic strength and promoting the formation of cross-linking points. Secondly, the interaction between sodium ions and the functional groups of PVA and carbon dots further improves the cross-linking density and network stability. On the other hand, in the salt solution, osmotic pressure is formed inside and outside the PVA, making the PVA denser and enhancing its mechanical properties.

[0111] Figure 7 is the stress-strain curve of the hydrogel prepared in Comparative Example 5, Figure 7 It can be seen that the mechanical properties of the citric acid carbon dot-polyvinyl alcohol hydrogel after salting-out assisted progressive stretching are 52.7 MPa and 72.87 MJ / m 3 However, its elongation rate is greatly reduced compared with that of the citric acid carbon dot-polyvinyl alcohol hydrogel after salting-out treatment. The reason is that during the gradual stretching process, the long PVA chains in the hydrogel will gradually orient under the action of external force, thereby improving the mechanical properties. However, this gradual stretching will reduce the relative slippage between the material chain segments, resulting in a decrease in strain.

[0112] Figure 8 The DSC graphs of the hydrogels prepared in Example 1 and Comparative Examples 1 to 5 are shown in FIG. Figure 8As can be seen from DSC testing, the freezing points of the hydrogels are 2.18°C for PVA, 1.98°C for PVA-CA, -2.8°C for PVA-CD, -8.23°C for PVA-CDY, -13.77°C for PVA-CDYS, and -12.81°C for PVA. Their enthalpy changes are -136.4 J / g, -112.5 J / g, -102.4 J / g, -47.28 J / g, -23.84 J / g, and -21.99 J / g, respectively. Citric acid, as an organic acid, may slightly lower the freezing point when dissolved in PVA. The introduction of citric acid carbon dots significantly lowers the freezing point. This is because the carbon dots increase the internal dispersion and heterogeneity of the hydrogel, potentially promoting the formation of more ice nuclei and leading to a freezing point depression. During the salting-out process, salt ions cause changes in the interactions between water molecules, lowering the freezing point. The stretched hydrogel likely undergoes structural rearrangement, resulting in a tighter arrangement of water molecules and a significant decrease in the freezing point. This suggests that the combined treatment of stretching and salting-out has a significant effect on the freezing point. After multiple salting-out treatments, the freezing point, although slightly elevated, remains lower than that of untreated PVA. This suggests that multiple salting-out treatments further enhance the effect of salt on the behavior of water molecules, but the overall effect tends to stabilize. Regarding the enthalpy change, the addition of citric acid reduces it because the interaction between citric acid and PVA alters the behavior of water molecules during solidification, thereby affecting energy release. Furthermore, the presence of carbon dots increases the network complexity, reducing energy release during solidification. The carbon dots introduce more heterogeneous nuclei, complicating ice crystal formation and melting. Subsequent salting-out results in more ions and a modified structure in the hydrogel, all of which influence energy release during solidification. Salting-out alters the thermodynamic properties of the solidification process, reducing the enthalpy change. The stretching process causes more significant changes in the hydrogel's internal structure, leading to altered alignment of water molecules and making the energy changes during solidification more complex and less significant. After multiple salting-out cycles, the hydrogel's structural changes stabilize, and the energy release during solidification decreases and approaches equilibrium.

[0113] The embodiments described above are merely descriptions of preferred embodiments of the present invention and are not intended to limit the scope of the present invention. Without departing from the spirit of the present invention, various modifications and improvements made to the technical solutions of the present invention by persons skilled in the art should fall within the scope of protection defined by the claims of the present invention.

Claims

1. A method for preparing a carbon dot-based super hydrogel, characterized in that the steps include: Using carbon source as raw material and water as solvent, the carbon dot solution is prepared through firing and cooling; The carbon dot solution and polyvinyl alcohol are used as reactants, and subjected to high temperature reaction and freeze-thaw cycles to obtain a carbon dot-polyvinyl alcohol hydrogel; The carbon dot-polyvinyl alcohol hydrogel is subjected to salting-out assisted progressive stretching and then immersed in a sodium citrate solution to obtain the carbon dot-based super-strong hydrogel; The carbon source is at least one of citric acid, tannic acid, malic acid and bark; The mass ratio of the carbon dot solution to polyvinyl alcohol is 17:3; the temperature of the high temperature reaction is 90° C. and the time is 3 hours; The freeze-thaw cycle is freezing at -20°C for 8 hours, then thawing at 20°C for 3 hours, and the freeze-thaw cycle is repeated at least once; The steps of salting-out assisted progressive stretching are: The carbon dot-polyvinyl alcohol hydrogel was immersed in a sodium citrate solution for 12 hours and then subjected to gradual stretching.

2. The preparation method according to claim 1, characterized in that The ratio of the carbon source to water is 10 g:100 mL; the firing temperature is 160° C., and the firing time is 8 h.

3. The preparation method according to claim 1, characterized in that The concentration of the sodium citrate solution is 2 mol / L; the deformation amount of the progressive stretching is 300%.

4. The preparation method according to claim 1, characterized in that The immersion time in the sodium citrate solution is 12 hours, and the concentration of the sodium citrate solution is 2 mol / L.

5. A carbon dot-based super-strong hydrogel prepared by the preparation method according to any one of claims 1 to 4.

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