A thermosensitive alkali-induced color-changing double-layer hydrogel and its preparation method and application

Abstract

The invention discloses a thermosensitive alkali-induced color-changing double-layer hydrogel and a preparation method and application thereof, and belongs to the technical field of water-absorbing materials. The double-layer hydrogel comprises a color-changing layer gel and a water-absorbing layer gel; the preparation monomers of the color-changing layer gel comprise acrylamide and 2-(1-(4-vinylbenzyl)pyridine-1-yl)-ethane-1-sulfonate, and the raw materials of the water-absorbing layer gel comprise methylcellulose, acrylic acid and a neutralizer. First, a polymerization reaction is performed to obtain the water-absorbing layer gel; then, the color-changing layer gel preparation monomers, a cross-linking agent and an initiator are dissolved in water to obtain a precursor solution, which is then placed on the surface of the water-absorbing layer gel, and the polymerization reaction is continued. The double-layer hydrogel prepared by the present invention has both thermosensitivity, high water absorption, salt tolerance and reversible color change induced by pH response, and can be used to monitor pH changes in saline-alkali environments, and can also be used in desertification prevention and control and saline-alkali land improvement, and has broad application prospects.

Description

Technical Field

[0001] The invention relates to the technical field of water-absorbing materials, and in particular to a temperature-sensitive alkali-induced color-changing double-layer hydrogel and a preparation method and application thereof. Background Art

[0002] Saline-alkali land is mainly distributed in arid and semi-arid areas with low precipitation and high evaporation. The fundamental reason for the formation of saline-alkali soil is the poor water condition, so in the early stage of improvement, the focus is generally on improving the water condition of the soil. Super absorbent polymers play a vital role in soil desertification control and saline-alkali land improvement, but the super absorbent polymers on the market today have great deficiencies in performance. Therefore, it is of great significance to prepare a material that has temperature sensitivity, high water absorption rate, good salt tolerance, and can detect the salinization level through pH-responsive color change effect for desertification and saline-alkali land control.

[0003] Most of the existing technologies for saline-alkali land improvement and salinity detection are used independently, and there are few multifunctional materials that combine the two. In addition, the chemical improvers commonly used in saline-alkali improvement mainly reduce the content of sodium ions, carbonate ions and bicarbonate ions in the soil pH value, but cannot improve the problem of drought and water shortage in saline-alkali land. Therefore, the development of an improver that combines saline-alkali land improvement and detection and has the ability to absorb and retain water is of great significance to the improvement of saline-alkali land. Summary of the invention

[0004] In view of the shortcomings of the current research status, the present invention provides a temperature-sensitive alkali-induced color-changing double-layer hydrogel and a preparation method and application thereof.

[0005] One of the technical solutions of the present invention is: a temperature-sensitive alkali-induced color-changing double-layer hydrogel, the hydrogel comprising a color-changing layer gel and a water-absorbing layer gel; the preparation monomers of the color-changing layer gel include acrylamide and 2-(1-(4-vinylbenzyl)pyridin-1-yl)-ethane-1-sulfonate, and the raw materials of the water-absorbing layer gel include methyl cellulose, acrylic acid and a neutralizer.

[0006] Furthermore, the monomer molar ratio of acrylamide and 2-(1-(4-vinylbenzyl)pyridin-1-yl)-ethane-1-sulfonate in the color-changing layer gel is 1:(0.5-1.5); the amount of methyl cellulose is 4%-8% of the mass of acrylic acid; preferably, the amount of methyl cellulose is 6%-8% of the mass of acrylic acid; more preferably, the amount of methyl cellulose is 7% of the mass of acrylic acid; the neutralizer is added according to the neutralization degree of acrylic acid of 45%-75%; preferably, the neutralizer is added according to the neutralization degree of acrylic acid of 45%-55%; more preferably, the neutralizer is added according to the neutralization degree of acrylic acid of 50%; the neutralizer includes sodium hydroxide and / or potassium hydroxide.

[0007] Technical solution 2 of the present invention: A method for preparing the thermosensitive alkali-induced color-changing double-layer hydrogel according to the above-mentioned method, wherein the color-changing layer gel and the water-absorbing layer gel are both obtained by free radical polymerization under the action of a crosslinking agent and an initiator, specifically comprising the following steps:

[0008] (1) dispersing methylcellulose with water to obtain a methylcellulose dispersion; mixing a neutralizer with acrylic acid for neutralization reaction to obtain an acrylic acid solution; adding a crosslinking agent, an initiator and the acrylic acid solution to the methylcellulose dispersion for polymerization reaction to obtain a water-absorbing layer gel;

[0009] (2) dissolving acrylamide, 2-(1-(4-vinylbenzyl)pyridin-1-yl)-ethane-1-sulfonate, a crosslinking agent and an initiator in water to obtain a color-changing layer gel precursor solution, which is then placed on the surface of the water-absorbing layer gel obtained in step (1), and the polymerization reaction is continued to obtain the temperature-sensitive alkali-induced color-changing double-layer hydrogel.

[0010] The anti-polyelectrolyte effect of zwitterionic polymers is used to improve salt resistance. The monomers used have a large number of hydrophilic groups such as carboxyl, hydroxyl, and sulfonic acid groups, which can greatly improve the water absorption performance of the polymer; the water-absorbing layer gel prepared by the present invention has a good three-dimensional network structure, which is conducive to the entry of water. According to tests, the water-absorbing layer gel can absorb more than 306 times its own weight of tap water, more than 1859 times of deionized water, and more than 131 times of 0.9% sodium chloride solution, and has temperature responsiveness, and the lower critical phase transition temperature is 40°C.

[0011] Furthermore, in step (1) and step (2), the cross-linking agent is N,N-methylenebisacrylamide, and the initiator is potassium persulfate and / or ammonium persulfate;

[0012] In step (1), the amount of the cross-linking agent is 0.03% to 0.1% by mass of acrylic acid; preferably, the amount of the cross-linking agent is 0.03% to 0.07% by mass of acrylic acid; more preferably, the amount of the cross-linking agent is 0.03% to 0.04% by mass of acrylic acid; the amount of the initiator is 0.2% to 1% by mass of acrylic acid; preferably, the amount of the initiator is 0.3% to 0.7% by mass of acrylic acid; more preferably, the amount of the initiator is 0.5% to 0.6% by mass of acrylic acid; in step (2), the amount of the cross-linking agent is 0.05% to 0.09% of the total mass of acrylamide and 2-(1-(4-vinylbenzyl)pyridin-1-yl)-ethane-1-sulfonate, and the amount of the initiator is 0.2% to 0.8% of the total mass of acrylamide and 2-(1-(4-vinylbenzyl)pyridin-1-yl)-ethane-1-sulfonate.

[0013] Furthermore, in step (1) and step (2), the polymerization reaction temperature is 65-80° C.; the polymerization reaction time in step (1) is 2.5-3.5 h, and the polymerization reaction time in step (2) is 3-4 h.

[0014] The present invention uses aqueous solution polymerization to prepare gel, so a water-soluble initiator is selected. The reaction temperature of the present invention is 65-80°C, which belongs to the medium temperature range, so a persulfate initiator is selected.

[0015] N,N-methylenebisacrylamide can undergo free radical copolymerization with acrylic monomers and introduce crosslinking points in the reaction to form a three-dimensional network structure. N,N-methylenebisacrylamide molecules contain two acrylamide groups, which can react with the double bonds in acrylic monomers to form a crosslinked structure. This structure can enhance the mechanical strength and chemical stability of the resin, while also controlling its water absorption and permeability. Therefore, N,N-methylenebisacrylamide is used as a crosslinking agent.

[0016] Methylcellulose (MC) is a cellulose methyl ether made by introducing methyl groups into cellulose through etherification, in which the hydroxyl (-OH) groups of cellulose are replaced by methoxy groups (-CH3O). It is derived from natural cellulose and thus has good water solubility, biodegradability and biocompatibility. Due to the hydrophobicity of the methoxy group, methylcellulose exhibits unique thermal responsiveness through hydrophobic interactions and has a low critical phase transition temperature (LCST). When the temperature is lower than its LCST, the polymer dissolves in the solution and forms a homogeneous phase, while when the temperature is higher than the LCST, the entropy of the system changes, the polymer undergoes phase separation, and the solution becomes milky or opaque. As the temperature changes, a solution-gel transition occurs. This temperature-sensitive sol-gel transition can be achieved by controlling the degree of substitution, molecular weight, concentration, and the type and amount of added salt. In addition, as a nonionic cellulose ether, its aqueous solution remains stable in the pH range of 2-12.

[0017] The structural feature of zwitterionic polymers is that in the same repeating unit, the positive charge and negative charge are simultaneously located on a pair of cationic and anionic groups. Therefore, zwitterionic polymers have a large dipole moment and a large number of charged groups. The coexistence of oppositely charged groups gives the polymers extremely high polarity and excellent hydrophilicity, and they are biocompatible and environmentally responsive. The electrical properties of the polymers can be further regulated by environmental pH and salt ions, and their unique anti-polyelectrolyte effect has made them important applications in the field of high-salt environments. Among them, 2-(1-(4-vinylbenzyl)pyridin-1-yl)-ethane-1-sulfonate is a pyridine-sulfonic acid type zwitterionic monomer, which contains styrene and pyridine groups in its structure, enabling it to undergo reversible intramolecular conversion between zwitterions and complex ions. There are three different intramolecular charge distributions between zwitterions and styrene groups, which trigger reversible pH-responsive color changes. In addition, it can also react with alkaline substances to form structural complexes, and after salinization, it can be degraded into water-soluble, nitrogen-rich components to provide nitrogen fertilizer for plant growth.

[0018] Furthermore, the method also includes the steps of washing with anhydrous ethanol, cutting into pieces and drying.

[0019] Technical solution three of the present invention: an application of the thermosensitive alkali-induced color-changing double-layer hydrogel described above in soil salinization detection.

[0020] Technical solution 4 of the present invention: A soil salinization detection indicator, comprising the above-mentioned temperature-sensitive alkali-induced color-changing double-layer hydrogel.

[0021] Technical solution five of the present invention: An application of the thermosensitive alkali-induced color-changing double-layer hydrogel described above in the improvement of saline-alkali land.

[0022] Technical solution six of the present invention: A saline-alkali land improver, comprising the temperature-sensitive alkali-chromic double-layer hydrogel as described in any one of claims 1 to 2.

[0023] The color-changing layer gel monomers of the double-layer hydrogel of the present invention are acrylamide and 2-(1-(4-vinylbenzyl)pyridin-1-yl)-ethane-1-sulfonate; the water-absorbing layer gel is made of methyl cellulose as a raw material and acrylic acid as a monomer. Under the action of a crosslinking agent and an initiator, both layers use deionized water as a reaction medium and are prepared by free radical polymerization.

[0024] In the present invention, the water-absorbing layer gel is obtained by aqueous solution polymerization using methyl cellulose as a temperature-sensitive raw material and acrylic acid as a monomer. The color-changing layer gel is obtained by copolymerization of acrylamide and 2-(1-(4-vinylbenzyl)pyridin-1-yl)-ethane-1-sulfonate as monomers. The double-layer hydrogel is prepared by adding a color-changing layer polymer precursor solution to the water-absorbing layer after complete polymerization and then polymerizing. The color-changing layer gel shows different shades of green in alkaline solution, and the color-changed gel can return to its original transparent and colorless state in an acidic or neutral solution.

[0025] The main reasons why the double-layer hydrogel prepared by the present invention can be used for saline-alkali land improvement are: 1. It can display the pH value of the soil through pH-induced reversible color change. 2. Water retention: maintain water circulation by absorbing and releasing water. 3. Temperature sensitivity: release water through a fast and reversible thermal regulation polymer network in response to the temperature difference between day and night. 4. Salt isolation and reduction of salinity and alkalinity: physical barrier and carboxylic acid groups and metal ions (Na + ,k + )'s ion chelation effect blocks salt ions and uses ion exchange and acid-base balance to reduce salinity.

[0026] Compared with the prior art, the present invention has the following beneficial effects:

[0027] The thermosensitive alkali-induced color-changing double-layer hydrogel provided by the present invention has a water absorption rate of up to 857 g / g in deionized water, is temperature sensitive, has a lower critical phase transition temperature of 40°C, and has a pH-induced reversible color change. It changes from a colorless and transparent state to green under alkaline conditions, and the color gradually deepens as the pH increases, while there is no color change under acidic or neutral conditions. The hydrogel can be used to monitor the real-time changes of pH in saline-alkali environments, and can also be used in desertification prevention and control and saline-alkali land improvement.

[0028] The thermosensitive alkali-chromic double-layer hydrogel provided by the present invention is a highly absorbent polymer, which can improve the water-saving and water-absorbing capacity of the soil while improving saline-alkali soil, and improve plant survival rate and water utilization rate. The thermosensitive highly absorbent polymer can release water through a fast and reversible thermal regulation polymer network in response to the temperature difference between day and night.

[0029] The thermosensitive alkali-induced color-changing double-layer hydrogel prepared by the invention has temperature sensitivity, high water absorption, salt resistance and response-induced reversible color change, good performance and broad application prospects. BRIEF DESCRIPTION OF THE DRAWINGS

[0030] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings required for use in the embodiments will be briefly introduced below. 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 creative work.

[0031] Figure 1 Schematic diagram of the free radical polymerization reaction mechanism during the preparation of the lower water-absorbing layer gel in Example 1;

[0032] Figure 2 This is a scanning electron microscope image of the lower water-absorbing layer gel obtained in step (2) of Example 1;

[0033] Figure 3 The color changes of the upper color-changing layer gel of the thermosensitive alkali-chromic double-layer hydrogel obtained in Example 2 at different pH values;

[0034] Figure 4 This is a physical picture of the temperature-sensitive alkali-chromic double-layer hydrogel prepared in Example 2 before changing color, changing color in an alkaline solution, and restoring its transparent color and swelling in an acidic solution and distilled water. DETAILED DESCRIPTION

[0035] Now, various exemplary embodiments of the present invention are described in detail, and this detailed description should not be considered as a limitation of the present invention, but should be understood as a more detailed description of certain aspects, characteristics and embodiments of the present invention. It should be understood that the terms described in the present invention are only for describing specific embodiments and are not used to limit the present invention.

[0036] In addition, for the numerical range in the present invention, it is understood that each intermediate value between the upper and lower limits of the range is also specifically disclosed. Each smaller range between the intermediate value in any stated value or stated range and any other stated value or intermediate value in the range is also included in the present invention. The upper and lower limits of these smaller ranges can be independently included or excluded in the scope.

[0037] Unless otherwise indicated, all technical and scientific terms used herein have the same meanings as those generally understood by those skilled in the art. Although the present invention describes only preferred methods and materials, any methods and materials similar or equivalent to those described herein may also be used in the implementation 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 a conflict with any incorporated document, the content of this specification shall prevail.

[0038] It will be apparent to those skilled in the art that various modifications and variations may be made to the specific embodiments of the present invention description without departing from the scope or spirit of the present invention. Other embodiments derived from the present invention description will be apparent to the skilled artisan. The present invention description and examples are exemplary only.

[0039] The words “include,” “including,” “have,” “contain,” etc. used in this document are open-ended terms, meaning including but not limited to.

[0040] In the following embodiments, normal temperature refers to 20-30° C., which will not be described again.

[0041] The invention provides a temperature-sensitive alkali-induced color-changing double-layer hydrogel, comprising a color-changing layer gel and a water-absorbing layer gel; the preparation monomers of the color-changing layer gel include acrylamide and 2-(1-(4-vinylbenzyl)pyridin-1-yl)-ethane-1-sulfonate, and the raw materials of the water-absorbing layer gel include methyl cellulose, acrylic acid and a neutralizer.

[0042] Methylcellulose, 2-(1-(4-vinylbenzyl)pyridin-1-yl)-ethane-1-sulfonate and acrylic acid can provide a large number of hydrophilic functional groups such as carboxyl, hydroxyl and sulfonic acid groups to improve the water absorption capacity.

[0043] Methylcellulose is thermoresponsive and provides temperature sensitivity to the absorbent layer gel.

[0044] The zwitterionic monomer 2-(1-(4-vinylbenzyl)pyridin-1-yl)-ethane-1-sulfonate provides pH-induced reversible color change and reverse polyelectrolyte effect for the color-changing layer gel.

[0045] In some embodiments, the molar ratio of acrylamide and 2-(1-(4-vinylbenzyl)pyridin-1-yl)-ethane-1-sulfonate monomers in the color-changing layer gel is 1:(0.5-1.5); and the amount of methyl cellulose used is 4%-8% of the mass of acrylic acid.

[0046] In some embodiments, the monomer molar ratio of acrylamide and 2-(1-(4-vinylbenzyl)pyridin-1-yl)-ethane-1-sulfonate in the color-changing layer gel is 1:1; the amount of methyl cellulose used is 6% to 8% of the mass of acrylic acid, and a neutralizer is added according to the neutralization degree of acrylic acid of 45% to 75%; the neutralizer includes sodium hydroxide and / or potassium hydroxide.

[0047] In some preferred embodiments, the amount of methyl cellulose used is 7% of the mass of acrylic acid, and a neutralizing agent is added based on the neutralization degree of acrylic acid being 45% to 55%.

[0048] In some more preferred embodiments, the neutralizing agent is added so that the neutralization degree of acrylic acid is 50%.

[0049] The present invention also provides a method for preparing the above-mentioned thermosensitive alkali-induced color-changing double-layer hydrogel, wherein the color-changing layer gel and the water-absorbing layer gel are both obtained by free radical polymerization under the action of a crosslinking agent and an initiator, and specifically comprises the following steps:

[0050] (1) dispersing methylcellulose with water to obtain a methylcellulose dispersion; mixing a neutralizing agent with acrylic acid for neutralization reaction to obtain an acrylic acid solution; adding a crosslinking agent, an initiator and the acrylic acid solution to the methylcellulose dispersion for polymerization reaction to obtain a water-absorbing layer gel;

[0051] (2) dissolving acrylamide, 2-(1-(4-vinylbenzyl)pyridin-1-yl)-ethane-1-sulfonate, a crosslinking agent and an initiator in water to obtain a color-changing layer gel precursor solution, which is then placed on the surface of the water-absorbing layer gel obtained in step (1), and the polymerization reaction is continued to obtain a thermosensitive alkali-induced color-changing double-layer hydrogel.

[0052] In some embodiments, in step (1) and step (2), the crosslinking agent is N, N-methylenebisacrylamide, and the initiator is potassium persulfate and / or ammonium persulfate. In the present invention, the crosslinking agent mainly plays the role of crosslinking molecular chains and increasing the stability of the polymer. The initiator mainly plays the role of initiating the reaction and controlling the reaction rate. In step (1), the amount of the crosslinking agent is 0.03% to 0.1% of the mass of acrylic acid; the amount of the initiator is 0.2% to 1% of the mass of acrylic acid; in step (2), the amount of the crosslinking agent is 0.05 to 0.09% of the total mass of acrylamide and 2-(1-(4-vinylbenzyl)pyridin-1-yl)-ethane-1-sulfonate, and the amount of the initiator is 0.2 to 0.8% of the total mass of acrylamide and 2-(1-(4-vinylbenzyl)pyridin-1-yl)-ethane-1-sulfonate.

[0053] In some preferred embodiments, in step (1), the amount of the crosslinking agent is 0.03% to 0.07% of the mass of acrylic acid, and the amount of the initiator is 0.3% to 0.7% of the mass of acrylic acid; in step (2), the amount of the crosslinking agent is 0.07% of the total mass of acrylamide and 2-(1-(4-vinylbenzyl)pyridin-1-yl)-ethane-1-sulfonate, and the amount of the initiator is 0.5% of the total mass of acrylamide and 2-(1-(4-vinylbenzyl)pyridin-1-yl)-ethane-1-sulfonate.

[0054] In some more preferred embodiments, in step (1), the amount of the cross-linking agent used is 0.03-0.04% by weight of the acrylic acid, and the amount of the initiator used is 0.5-0.6% by weight of the acrylic acid.

[0055] In some embodiments, in step (1) and step (2), the polymerization reaction temperature is 65-80° C.; the polymerization reaction time in step (1) is 2.5-3.5 h, and the polymerization reaction time in step (2) is 3-4 h.

[0056] In some embodiments, in step (1) and step (2), the polymerization reaction temperature is 70°C.

[0057] In some embodiments, the steps of washing with anhydrous ethanol, shredding and drying are also included.

[0058] In some embodiments, the drying temperature is 40 to 70° C.; the drying time is preferably 20 to 36 hours.

[0059] In some preferred embodiments, the drying temperature is 60° C. and the drying time is 24 hours.

[0060] The present invention also provides an application of the thermosensitive alkali-induced color-changing double-layer hydrogel in soil salinization detection.

[0061] The present invention also provides a soil salinization detection indicator, comprising the above-mentioned temperature-sensitive alkali-induced color-changing double-layer hydrogel.

[0062] The present invention also provides an application of the thermosensitive alkali-induced chromic double-layer hydrogel in improving saline-alkali land.

[0063] The present invention also provides a saline-alkali land improver, comprising the above-mentioned temperature-sensitive alkali-induced color-changing double-layer hydrogel.

[0064] Example 1

[0065] The preparation steps of the thermosensitive alkali-chromic double-layer hydrogel are as follows:

[0066] (1) 0.4809 g of methyl cellulose was dissolved in 10 mL of deionized water to obtain a methyl cellulose solution, which was stirred on a magnetic stirrer for 30 min; 2 g of sodium hydroxide was dissolved in 8 mL of water, and the obtained solution was mixed with 6.87 mL of acrylic acid and subjected to ultrasonic treatment to obtain a neutralized acrylic acid solution;

[0067] (2) Aqueous solution polymerization method was used. A magnetic bar was added to a beaker containing a methylcellulose solution. The mixture was placed in a water bath at 70° C. and stirred at a speed of 250 r / min until the methylcellulose was evenly dispersed in the water to obtain a dispersion. Then, the acrylic acid solution neutralized in step (1) was added in sequence. 0.0034 g of N,N-methylenebisacrylamide and 0.03435 g of potassium persulfate were dissolved in 1 mL of deionized water and treated with ultrasound. The total amount of deionized water added was 20 mL. Stirring was maintained during the process. The stirring speed was 250 r / min. After the viscosity of the material increased (it had viscosity but no climbing effect), the material was removed from the water bath, evenly divided into two molds, spread flat on the bottom of the molds, and placed in a forced air oven to continue the graft copolymerization reaction. The reaction temperature was 70° C. and the time was 210 min. The timing was started from the time when the material was removed from the water bath and placed in the forced air oven. A lower water-absorbing layer gel was obtained.

[0068] The reaction in the water bath makes the precursor solution of the water-absorbing layer gel have a certain viscosity, ensuring that the water-absorbing layer gel has better uniformity and maintains its fixed shape in the mold. Transfer to the oven to continue the reaction to complete the polymerization. The schematic diagram of the free radical polymerization reaction mechanism during the preparation of the water-absorbing layer gel is shown in Figure 1 The scanning electron microscope image of the water-absorbing layer gel is shown in Figure 2 As shown: the initiator in the reaction process first decomposes to produce primary free radicals, the primary free radicals act on the monomer molecules to produce monomer free radicals, the highly active monomer free radicals add to other monomers to form chain free radicals, and at the same time, interact with the cross-linking agent to form the final structure.

[0069] (3) Then, 0.7575 g of 2-(1-(4-vinylbenzyl)pyridin-1-yl)-ethane-1-sulfonate, 0.17769 g of acrylamide, 0.00065 g of crosslinking agent N,N-methylenebisacrylamide, and 0.0046 g of initiator potassium persulfate were weighed and dissolved in 12 mL of deionized water in sequence, and the color-changing layer polymer precursor solution was obtained after ultrasonic treatment. 6 mL of the precursor solution was placed on the surface of the water-absorbing layer gel, and the reaction was continued in an oven at 70°C for 180 min before being taken out. The obtained product was washed with anhydrous ethanol, cut into pieces, and dried at 60°C to obtain a temperature-sensitive alkali-induced color-changing double-layer hydrogel with the upper layer being the color-changing layer gel and the lower layer being the water-absorbing layer gel.

[0070] At room temperature, the water absorption performance of the water-absorbing layer gel obtained in step (2) was tested, and the results showed that the amount of tap water absorbed was 292 g / g, the amount of deionized water absorbed was 1281 g / g, and the amount of 0.9% sodium chloride solution absorbed was 92 g / g. In the range of pH = 4 to pH = 11 (using distilled water to dilute HCl (0.1 mol·L -1 ) or NaOH (0.1 mol·L -1) solution was obtained), and the water absorption rate was above 840g / g.

[0071] At room temperature, the water absorption performance of the thermosensitive alkali-chromic double-layer hydrogel obtained in this example was tested, and the results showed that the water absorption rate in deionized water was 525 g / g at most, the water absorption rate in tap water was 115 g / g, and the water absorption rate in 0.9% NaCl solution was 45.97 g / g.

[0072] Example 2

[0073] The same as Example 1, except that the mass of N,N-methylenebisacrylamide in step (2) is adjusted from "0.0034 g" to "0.0027 g".

[0074] In this embodiment, at room temperature, the water absorption performance of the water-absorbing layer gel obtained in step (2) is: 306 g / g of tap water, 1859 g / g of deionized water, and 131 g / g of 0.9% sodium chloride solution. In the range of pH = 4 to pH = 11, the water absorption rate is above 1335 g / g.

[0075] At room temperature, the water absorption performance of the thermosensitive alkali-chromic double-layer hydrogel obtained in this example was tested, and the results showed that the water absorption rate was as high as 857 g / g in deionized water, 143 g / g in tap water, and 51.63 g / g in 0.9% NaCl solution.

[0076] The color changing performance of the thermosensitive alkali-chromic double-layer hydrogel obtained in this example was tested. The specific method was as follows: the thermosensitive alkali-chromic double-layer hydrogel prepared in this example was placed in a solution with a pH of 3 to 12, and HCl (0.1 mol·L -1 ) or NaOH (0.1 mol·L -1 ) Adjust the pH value of the solution. After 60 minutes, Figure 3 As shown, Figure 3 The numbers represent the initial pH value of the solution. Figure 3 It can be seen that the upper color-changing layer of the double-layer hydrogel is pH-responsive and can change from colorless and transparent to green under alkaline conditions. As the pH increases, the color gradually deepens, while there is no color change under acidic or neutral conditions.

[0077] The double-layer hydrogel in pH 12 was taken out and placed in distilled water and pH 3 HCl solution for 60 min. Figure 4As shown in the figure, the numbers represent the initial pH value of the solution, and "original" represents the state of the prepared double-layer hydrogel in the air. It can be seen that the color change of the thermosensitive alkali-induced color-changing double-layer hydrogel prepared by the present invention is reversible. After changing from colorless and transparent to green under alkaline conditions, it will return to colorless and transparent when placed under acidic or neutral conditions again.

[0078] The temperature sensitivity of the thermosensitive alkali-induced color-changing double-layer hydrogel prepared in this example was tested. The specific method was as follows: (1) 7 portions of 0.1 g of the double-layer hydrogel were weighed and placed in a 250 ml beaker filled with distilled water, and placed in a constant temperature bath at 25, 30, 35, 37, 40, 45, 50, 55, and 60 ° C, respectively. After swelling for 3 hours, the hydrogel was taken out and weighed to calculate its water absorption rate; (2) 1 portion of 0.1 g of the gel was weighed and placed in a 250 ml beaker filled with distilled water, and the temperature was raised by a heat-collecting constant temperature heating magnetic stirrer at 25- The double-layer hydrogel was heated at a rate of 2°C / min within a range of 60°C, and the transmittance of the lower water-absorbing layer gel at different temperatures at 500nm was measured using an ultraviolet spectrophotometer (experiments show that under any temperature gradient, the upper color-changing layer of the double-layer hydrogel is white in deionized water, while the transmittance of the lower water-absorbing layer gel gradually decreases with increasing temperature. Therefore, when using the change in transmittance to measure the temperature sensitivity of the double-layer hydrogel, the double-layer hydrogel was heated at a rate of 2min / °C, and only the lower layer of gel was taken into a cuvette to measure the change in its transmittance). Results: (1) The water absorption of the gel decreased at 40°C. (2) The transmittance of the gel decreased at 40°C, and the gel changed from transparent and colorless to white and opaque. Table 1 shows the water absorption and transmittance of the lower water-absorbing layer gel at different temperatures.

[0079] Table 1 Water absorption and light transmittance of the lower water-absorbing layer gel of the double-layer hydrogel prepared in Example 2 at different temperatures

[0080]

[0081] Example 3

[0082] The same as Example 1, except that in step (2), the mass of potassium persulfate is adjusted from "0.03435 g" to "0.0412 g".

[0083] In this embodiment, at room temperature, the water absorption performance of the water-absorbing layer gel obtained in step (2) is: 297 g / g of tap water, 1381 g / g of deionized water, and 106 g / g of 0.9% sodium chloride solution. In the range of pH = 4 to pH = 11, the water absorption rate is above 937 g / g.

[0084] At room temperature, the water absorption performance of the thermosensitive alkali-chromic double-layer hydrogel obtained in this example was tested, and the results showed that the water absorption rate in deionized water was 631 g / g at most, the water absorption rate in tap water was 138 g / g, and the water absorption rate in 0.9% NaCl solution was 48.4 g / g.

[0085] Example 4

[0086] The same as Example 1, except that in step (1), the mass of methyl cellulose is adjusted from "0.4809g" to "0.2889g". In step (2), the "70°C water bath" is adjusted to "65°C water bath", the graft copolymerization reaction, the reaction temperature is adjusted to 65°C, and the time is 210min. In step (3), the amount of acrylamide is adjusted from "0.17769g" to "0.88825g", the reaction temperature is adjusted from 70°C to 65°C, and the time is adjusted from 180min to 240min.

[0087] In this embodiment, at room temperature, the water absorption performance of the water-absorbing layer gel obtained in step (2) is: 196 g / g of tap water, 970 g / g of deionized water, and 77 g / g of 0.9% sodium chloride solution. In the range of pH = 4 to pH = 11, the water absorption rate is more than 566 g / g.

[0088] At room temperature, the water absorption performance of the thermosensitive alkali-chromic double-layer hydrogel obtained in this example was tested, and the results showed that the water absorption rate was as high as 477 g / g in deionized water, 93 g / g in tap water, and 40.8 g / g in 0.9% NaCl solution.

[0089] Example 5

[0090] The same as Example 1, except that in step (1), the mass of methyl cellulose is adjusted from "0.4809g" to "0.5777g". In step (2), the "70°C water bath" is adjusted to "80°C water bath", the graft copolymerization reaction, the reaction temperature is adjusted to 80°C, and the time is 150min. In step (3), the amount of acrylamide is adjusted from "0.17769g" to "0.266475g", the reaction temperature is adjusted from 70°C to 80°C, and the time is adjusted from 180min to 210min.

[0091] In this embodiment, at room temperature, the water absorption performance of the water-absorbing layer gel obtained in step (2) is: 232 g / g of tap water, 1110 g / g of deionized water, and 82 g / g of 0.9% sodium chloride solution. In the range of pH = 4 to pH = 11, the water absorption rate is above 738 g / g.

[0092] At room temperature, the water absorption performance of the thermosensitive alkali-chromic double-layer hydrogel obtained in this example was tested, and the results showed that the water absorption rate in deionized water was 498 g / g at most, the water absorption rate in tap water was 104 g / g, and the water absorption rate in 0.9% NaCl solution was 43.64 g / g.

[0093] The color change performance, color change reversibility and temperature sensitivity of the thermosensitive alkali-induced color change double-layer hydrogels prepared in Examples 1 and 3 to 5 were tested, and the results obtained were consistent with those in Example 2.

[0094] Comparative Example 1

[0095] The same as Example 1, except that the mass of methyl cellulose in step (2) is adjusted from "0.4809 g" to "0" g.

[0096] In this embodiment, at room temperature, the water absorption performance of the water-absorbing layer gel obtained in step (2) is: the amount of tap water absorbed is 253 g / g, the amount of deionized water absorbed is 766 g / g, and the amount of 0.9% sodium chloride solution absorbed is 71 g / g. In the range of pH = 4 to pH = 11, the water absorption rate is more than 438 g / g, but lower than the water absorption in deionized water, that is, lower than 766 g / g. It can be seen that the water absorption rate of comparative example 1 without introducing methyl cellulose is lower than that of the embodiment, so the gel prepared in the embodiment has better practicality.

[0097] The color changing performance and color changing reversibility of the thermosensitive alkali-induced color changing double-layer hydrogel prepared in Comparative Example 1 were tested, and the results obtained were different from those in Example 1 in that the hydrogel was not temperature sensitive.

[0098] The lower water-absorbing layer gel prepared by the present invention can absorb more than 306 times of its own weight of tap water, more than 1859 times of deionized water and more than 131 times of 0.9% sodium chloride solution, and has temperature responsiveness, and the lower critical phase transition temperature is 40°C. The upper color-changing layer has pH responsiveness, and can change from colorless and transparent to green under alkaline conditions, and the color gradually deepens with the increase of pH, while there is no color change under acidic or neutral conditions, and can be used to monitor the real-time change of pH in saline-alkali environments. The water absorption rate of the double-layer hydrogel in deionized water is 857g / g, and it still has temperature sensitivity and pH-induced reversible color change. It can be seen from the above examples that the temperature-sensitive alkali-induced color-changing double-layer hydrogel prepared by the present invention has both temperature sensitivity, high water absorption, salt resistance and pH-responsive induced reversible color change, good performance, and broad application prospects.

[0099] It can be seen from the above examples that the thermosensitive alkali-chromic double-layer hydrogel prepared by the present invention has temperature sensitivity, high water absorption, salt resistance and reversible color change induced by pH response, has good performance and has broad application prospects.

[0100] The above description is only a preferred specific implementation manner of the present invention, and the protection scope of the present invention is not limited thereto. Any technician familiar with the technical field can make equivalent replacements or changes according to the technical scheme and inventive concept of the present invention within the technical scope disclosed by the present invention, and they should be covered by the protection scope of the present invention.

Claims

1. A thermosensitive alkali-chromic double-layer hydrogel, characterized in that: The double-layer hydrogel comprises a color-changing layer gel and a water-absorbing layer gel; the monomers for preparing the color-changing layer gel comprise acrylamide and 2-(1-(4-vinylbenzyl)pyridin-1-yl)-ethane-1-sulfonate, and the raw materials for the water-absorbing layer gel comprise methyl cellulose, acrylic acid and a neutralizer; The water-absorbing layer gel uses methyl cellulose as a temperature-sensitive raw material and acrylic acid as a monomer and is obtained by aqueous solution polymerization; the color-changing layer gel uses acrylamide and 2-(1-(4-vinylbenzyl)pyridin-1-yl)-ethane-1-sulfonate as monomers and is obtained by copolymerization of two monomers; the preparation of the double-layer hydrogel is obtained by adding a color-changing layer polymer precursor solution after the water-absorbing layer is completely polymerized and then polymerizing it.

2. The thermosensitive alkali-chromic double-layer hydrogel according to claim 1, characterized in that: The monomer molar ratio of acrylamide and 2-(1-(4-vinylbenzyl)pyridin-1-yl)-ethane-1-sulfonate in the color-changing layer gel is 1:(0.5-1.5); the amount of methyl cellulose used is 4%-8% of the mass of acrylic acid; the neutralizer is added according to the neutralization degree of acrylic acid being 45%-75%; the neutralizer includes sodium hydroxide and / or potassium hydroxide.

3. A method for preparing the thermosensitive alkali-chromic double-layer hydrogel according to claim 1 or 2, characterized in that: The color-changing layer gel and the water-absorbing layer gel are both obtained by free radical polymerization under the action of a crosslinking agent and an initiator, which specifically includes the following steps: (1) dispersing methylcellulose with water to obtain a methylcellulose dispersion; mixing a neutralizer with acrylic acid for neutralization reaction to obtain an acrylic acid solution; adding a crosslinking agent, an initiator and the acrylic acid solution to the methylcellulose dispersion for polymerization reaction to obtain a water-absorbing layer gel; (2) dissolving acrylamide, 2-(1-(4-vinylbenzyl)pyridin-1-yl)-ethane-1-sulfonate, a crosslinking agent and an initiator in water to obtain a color-changing layer gel precursor solution, which is then placed on the surface of the water-absorbing layer gel obtained in step (1), and the polymerization reaction is continued to obtain the temperature-sensitive alkali-induced color-changing double-layer hydrogel.

4. The method for preparing the thermosensitive alkali-chromic double-layer hydrogel according to claim 3, characterized in that: In step (1) and step (2), the cross-linking agent is N,N-methylenebisacrylamide, and the initiator is potassium persulfate and / or ammonium persulfate; In step (1), the amount of the cross-linking agent is 0.03% to 0.1% of the mass of acrylic acid; the amount of the initiator is 0.2% to 1% of the mass of acrylic acid; in step (2), the amount of the cross-linking agent is 0.05% to 0.09% of the total mass of acrylamide and 2-(1-(4-vinylbenzyl)pyridin-1-yl)-ethane-1-sulfonate, and the amount of the initiator is 0.2% to 0.8% of the total mass of acrylamide and 2-(1-(4-vinylbenzyl)pyridin-1-yl)-ethane-1-sulfonate.

5. The method for preparing the thermosensitive alkali-chromic double-layer hydrogel according to claim 3, characterized in that: In step (1) and step (2), the polymerization reaction temperature is 65-80° C.; the polymerization reaction time in step (1) is 2.5-3.5 hours, and the polymerization reaction time in step (2) is 3-4 hours.

6. Use of the thermosensitive alkali-chromic double-layer hydrogel according to any one of claims 1 to 2 in soil salinization detection.

7. A soil salinization detection indicator, characterized in that: The invention comprises the temperature-sensitive alkali-chromic double-layer hydrogel as described in any one of claims 1 to 2.

8. Use of the thermosensitive alkali-chromic double-layer hydrogel according to any one of claims 1 to 2 in improving saline-alkali land.

9. A saline-alkali land improver, characterized in that: The invention comprises the temperature-sensitive alkali-chromic double-layer hydrogel as described in any one of claims 1 to 2.

Images