Phase inversion thermally responsive gel based on ionic liquid / water binary solvent, and preparation method and application thereof

Abstract

The application discloses a phase transition heat response gel based on an ion liquid / water binary solvent and a preparation method and application thereof, and the phase transition heat response gel system comprises N-isopropyl acrylamide (NIPAm) monomers, an initiator, an accelerator, an ion liquid and water. NIPAm is used as a heat response unit, and the ion liquid is introduced to replace a crosslinking agent commonly used in polymerization of a hydrogel, multiple hydrogen bonds and ion interaction formed between the ion liquid and a poly-N-isopropyl acrylamide (PNIPAm) chain endow the system with phase transition characteristics. The phase transition heat-induced color change intelligent window prepared by the application has the characteristics of high light transmittance, fast response, adjustable response temperature and good stability, and the phase transition heat-induced color change intelligent window overcomes the defects that a traditional heat-induced color change hydrogel is seriously shrunk in volume and the heat-induced color change performance is reduced after long-term heat-induced color change cycles, and provides certain technical support for application of the heat-induced color change intelligent window.

Description

Technical Field

[0001] This invention relates to the field of polymer materials technology, specifically to a phase transition thermally responsive gel based on an ionic liquid / water binary solvent, its preparation method, and its application. Background Technology

[0002] Windows, as building components that directly connect to the outside world, especially in the hot summer, allow sunlight to penetrate poorly insulated glass and enter the room, further aggravating the rise in indoor temperature. This forces people to turn on air conditioning and other cooling equipment to cool down, undoubtedly increasing building energy consumption. To reduce building energy consumption, smart windows have emerged. Current research on smart window technologies focuses on electrochromic (electro-responsive), photochromic (photo-responsive), and thermochromic (thermal-responsive) technologies. Among these, thermochromic technology is considered the most cost-effective and easy-to-implement solution, offering significant advantages. As a passive color-changing strategy, it can autonomously adjust its transmittance by sensing the ambient temperature. For example, when the temperature is high (exceeding its critical response temperature), the smart glass becomes opaque, scattering incident sunlight and blocking its entry. When the temperature is low, it returns to transparency, allowing sunlight to enter as much as possible. Compared to electrochromic smart windows, it does not consume any energy. Furthermore, thermochromic smart windows are easy to manufacture, requiring neither the complex manufacturing process of photochromic windows nor the expensive conductive glass used in electrochromic windows, resulting in lower production costs.

[0003] Traditional thermochromic materials, such as vanadium dioxide (VO2), suffer from low optical contrast and solar modulation rate, slow response speed, and high thermal response temperature (~68℃), severely limiting their large-scale applications. Poly(N-isopropylacrylamide) hydrogels have attracted considerable attention due to their high optical contrast and solar modulation rate; however, during long-term thermochromic cycling, the three-dimensional cross-linked network structure of PNIPAm hydrogels undergoes irreversible shrinkage, disrupting its structural uniformity and reducing its thermochromic properties. Furthermore, the high water content in the hydrogel also leads to freezing under low-temperature conditions such as winter, damaging both the hydrogel's network structure and performance.

[0004] Patent CN106188388A discloses a thermo / electrically responsive gel photochromic glass material, which is obtained by polymerizing thermoresponsive monomers, ionic liquids, crosslinking agents, initiators, and catalysts dissolved in water. By modifying the polymer formed by the thermoresponsive monomers and crosslinking agents with ionic liquids, thermo / electrically responsive gel photochromic glass materials can be formed, thereby increasing the responsiveness to changes in environmental conditions; improving the temperature controllability, stability, and service life of photochromic devices.

[0005] Patent CN113512146A discloses a self-sensing hydrogel transparent smart glass and its preparation method. The self-sensing hydrogel is composed of polymeric monomers, surfactants, reinforcing agents, corresponding initiators, crosslinking agents, (accelerators), and a salt solution. This invention utilizes the surfactant in the hydrogel as the responsive unit, which spontaneously forms micelles with cations in the salt solution within the gel network. The micelles change size under the influence of heat, electricity, and pH, macroscopically manifesting as a change in the light transmittance of the prepared gel. When the hydrogel is in solution, it is injected into laminated glass and sealed. After in-situ gelation, a self-sensing multi-response transparent smart glass is obtained. Summary of the Invention

[0006] To address a series of problems in existing thermochromic hydrogel smart windows, such as poor cycling stability, decreased network structure shrinkage performance, and poor antifreeze properties, this application proposes a phase transition thermally responsive gel based on an ionic liquid / water binary solvent, its preparation method, and its application. The prepared phase transition thermally responsive gel based on an ionic liquid / water binary solvent exhibits excellent cycling stability, thermochromic properties including high light transmittance and solar light modulation rate, and fast thermal response speed. Furthermore, the introduction of the ionic liquid replaces the crosslinking agent required by traditional gels and endows the system with phase transition characteristics, enabling it to exist in a sol state (liquid) below the critical transition temperature and a gel state (solid) at high temperatures. This allows the system to recover at low temperatures even if volume shrinkage occurs, thereby improving its cycling stability.

[0007] A phase transition thermally responsive gel based on an ionic liquid / water binary solvent is obtained by polymerizing a monomer, an ionic liquid, an initiator, and an accelerator dissolved in water.

[0008] The monomer mentioned above is N-isopropylacrylamide, and the concentration of the monomer is 8.6 wt% to 9.5 wt%. The monomer mentioned above can also be a copolymer of acrylamide or acrylic acid and N-isopropylacrylamide.

[0009] The aforementioned ionic liquid is one of 1-butyl-3-methylimidazolium tetrafluoroborate, 1-butyl-3-methylimidazolium trifluoromethanesulfonate, 1-butyl-3-methylimidazolium methanesulfonate, 1-butyl-3-methylimidazolium nitrate, and 1-hexyl-3-methylimidazolium chloride. The concentration of the aforementioned ionic liquid is 0.8 wt% to 8.5 wt%. If the ionic liquid concentration is too low, PNIPAm will shrink at high temperatures, and it will take a long time for the ionic liquid / water binary solvent to dissolve the shrunken PNIPAm, affecting the switching between transparent and opaque states, i.e., the thermal response rate. If the ionic liquid concentration is too high, it will further lower the critical transition temperature of the gel, reducing its practicality. Therefore, by limiting the concentration of the ionic liquid, the response temperature can be adjusted.

[0010] The initiator mentioned above is either ammonium persulfate or potassium persulfate. The concentration of the initiator is 0.8 wt% to 1.6 wt%.

[0011] The accelerator mentioned above is N,N,N',N'-tetramethylethylenediamine. The volume concentration of the accelerator is 0.1% to 0.3%. Using an accelerator volume concentration exceeding 0.3% will only further increase the polymerization rate.

[0012] A method for preparing a phase transition thermally responsive gel based on an ionic liquid / water binary solvent includes the following steps:

[0013] (1) Dissolve the monomer, initiator and ionic liquid in water and stir magnetically under ice bath conditions to accelerate dissolution;

[0014] (2) Perform deoxygenation on the solution obtained in step (1);

[0015] (3) Add an accelerator to the solution obtained in step (2), stir evenly and let stand to obtain a phase transition thermal response gel material of ionic liquid / water binary solvent.

[0016] In step (1) above, the ice bath temperature is 0-10℃ and the stirring time is 10-30 min. In step (2) above, the deoxygenation operation specifically involves introducing nitrogen gas into the solution; the time for introducing nitrogen gas is 10-30 min.

[0017] A phase transition thermally responsive gel based on an ionic liquid / water binary solvent can be used as a thermochromic material. The thermochromic material has an adjustable response temperature range of 32.5℃ to 25℃. It is a transparent liquid at low temperatures and a white, opaque solid at high temperatures.

[0018] Different ionic liquid contents will result in different color-changing temperatures for thermochromic smart windows, which can be adjusted according to actual needs.

[0019] Preferably, it can be used to make thermochromic smart windows. Specifically, the solution obtained in step (3) is dropped into a laminated glass mold and left to stand for a period of time to obtain a thermochromic smart window containing a phase transition thermally responsive gel material of ionic liquid / water binary solvent.

[0020] The glass mold comprises parallel stacked glass panes forming cavities between them. Sealant is applied around the glass cavities, and the thickness of the cavities is determined by spacers, ranging from 0.5 to 5 mm. If the thickness is too thin, although it is opaque at high temperatures and has a transmittance of less than 1% for sunlight, essentially achieving complete sunlight blocking, it is insufficient at high temperatures. A thickness exceeding 5 mm firstly consumes too much thermally responsive gel, and secondly, at low temperatures (in the transparent state), its transmittance in the visible light range may be below 90%, because even if it is transparent, some sunlight will be lost when passing through the smart window. Furthermore, a thinner thickness results in a faster response time, while a thicker thickness reduces the response time because temperature conduction also requires time.

[0021] The phase transition thermally responsive gel of the present invention, using an ionic liquid / water binary solvent, employs N-isopropylacrylamide as a monomer. Upon addition of an initiator, NIPAm initiates a free radical polymerization reaction. The introduction of the ionic liquid alters the solubility of NIPAm in the solvent and, through ion-dipole interactions and van der Waals forces, changes the interchain interactions of PNIPAm, thereby endowing it with phase transition properties. Above the critical response temperature, the interchain interactions of PNIPAm are enhanced, and it undergoes a conformational transformation (from chain conformation to spherical conformation, see reference). Figure 1 The PNIPAm chains form scattering centers, thus blocking incident sunlight. When the ambient temperature is below the critical temperature, the interaction between the PNIPAm chains is weak, and the system exhibits a flowing sol state that is transparent, allowing incident sunlight to enter.

[0022] This application has the following advantages over the prior art:

[0023] (1) The system uses N-isopropylacrylamide as a monomer and ionic liquid replaces the traditional crosslinking agent. Ionic liquid can interact with PNIPAm molecular chains through ion-dipole interaction and van der Waals force, thereby changing the inter-chain interaction of PNIPAm, so that it exhibits different states at high / low temperatures. When the critical response temperature is exceeded, it exhibits a gel state (white and opaque); at low temperature, it reverts to a sol state (colorless and transparent).

[0024] (2) The prepared phase transition thermally responsive gel exhibits high thermochromic properties, including high visible light transmittance and solar light modulation rate, and fast thermal response speed (the switching between transparent and opaque states can be completed in 10s).

[0025] (3) The phase transition characteristics endow thermochromic hydrogels with excellent cycling stability. After 500 thermochromic cycles, their performance does not significantly decrease. This is due to the phase transition characteristics of the system. Even if the gel network shrinks after multiple cycles, the system will return to the sol state after the temperature returns to a low temperature.

[0026] (4) The prepared phase transition thermally responsive gel precursor solution can be directly poured into the laminated glass. After standing for a period of time, a thermochromic smart window can be obtained. The phase transition gel can be recycled. When cooled to below the critical response temperature, the system can be poured out from the laminated glass and reused. Attached Figure Description

[0027] Figure 1 A schematic diagram of the color change mechanism of a phase transition thermally responsive gel in an ionic liquid / water binary solvent.

[0028] Figure 2 The graph shows the UV-Vis-NIR transmittance test results of the smart window made of the ionic liquid / water binary solvent phase transition thermal response gel in Example 2 at different temperatures.

[0029] Figure 3 Transmittance images of the phase transition thermal response gel smart window (10cm×10cm) of the ionic liquid / water binary solvent in Example 2 at low temperature 20°C and high temperature 40°C.

[0030] Figure 4 Example 2: A photograph of a large-size thermochromic window (30cm×30cm) turning white when touched outdoors;

[0031] Figure 5 Photographs of the phase transition thermally responsive gel of ionic liquid / water binary solvent in Example 2 at low temperature 20°C and high temperature 40°C;

[0032] Figure 6 Example 2 illustrates the phase transition process of an ionic liquid / water binary solvent phase transition thermally responsive gel injected into water at 35°C.

[0033] Figure 7 These are photographs of Example 2 (ionic liquid modification) and Comparative Example 1 after 200 cycles of thermochromic cycling (20 / 40°C);

[0034] Figure 8 Example 2: Thermochromic visible light transmittance over 200 cycles (tested once every 10 thermochromic cycles). Detailed Implementation

[0035] The technical solution of the present invention will be clearly and completely described below with reference to the embodiments of the present invention. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative effort are within the scope of protection of the present invention.

[0036] In the following embodiments or comparative examples, the critical response temperature was determined by the following method: prepare water at different temperatures, immerse the glass bottle containing the gel in the water, and determine the critical response temperature after it turns obviously white.

[0037] Example 1:

[0038] 1.05 g of N-isopropylacrylamide monomer, 0.1 g of ammonium persulfate, and 0.1 g of 1-butyl-3-methylimidazolium tetrafluoroborate ionic liquid were dissolved in 10 mL of deionized water. The solution was magnetically stirred for 20 min in an ice bath at 0 °C to ensure complete dissolution of all components. Nitrogen gas was purged for 10 min to remove dissolved oxygen from the mixture. 10 μL of N,N,N',N'-tetramethylethylenediamine was added, and the mixture was poured into glass cavities (glass dimensions: 10 cm × 10 cm, thickness 1 mm; cavity spacing 1 mm). After standing for 5 h, a thermochromic smart window was obtained. The critical response temperature was 32 °C, the transmittance was 96.7% (at 550 nm) at 20 °C, and the transmittance was 1.8% (at 550 nm) at 40 °C.

[0039] Example 2:

[0040] 1.05 g of N-isopropylacrylamide monomer, 0.1 g of ammonium persulfate, and 0.5 g of 1-butyl-3-methylimidazolium tetrafluoroborate ionic liquid were dissolved in 10 mL of deionized water. The solution was magnetically stirred for 20 min in an ice bath at 0 °C to ensure complete dissolution of all components. Nitrogen gas was then purged for 10 min to remove dissolved oxygen from the mixture. 10 μL of N,N,N',N'-tetramethylethylenediamine was added, and the mixture was poured into glass cavities (glass dimensions: 10 cm × 10 cm, thickness 1 mm; cavity spacing 1 mm). After standing for 5 h, a thermochromic smart window was obtained. The critical response temperature was 30 °C, the transmittance was 97.2% (at 550 nm) at 20 °C, and the transmittance was 1.2% (at 550 nm) at 40 °C. (Reference) Figures 2-6 , Figure 8 .

[0041] Example 3:

[0042] 1.05 g of N-isopropylacrylamide monomer, 0.1 g of ammonium persulfate, and 1.0 g of 1-butyl-3-methylimidazolium tetrafluoroborate ionic liquid were dissolved in 10 mL of deionized water. The solution was magnetically stirred for 20 min in an ice bath at 0 °C to ensure complete dissolution of all components. Nitrogen gas was purged for 10 min to remove dissolved oxygen from the mixture. 20 μL of N,N,N',N'-tetramethylethylenediamine was added, and the mixture was poured into glass cavities (glass dimensions: 10 cm × 10 cm, thickness 1 mm; cavity spacing 2 mm). After standing for 3 h, a thermochromic smart window was obtained. The critical response temperature was 28 °C, the transmittance was 94.8% (at 550 nm) at 20 °C, and the transmittance was 0.5% (at 550 nm) at 40 °C.

[0043] Example 4:

[0044] 1.05 g of N-isopropylacrylamide monomer, 0.1 g of ammonium persulfate, and 0.5 g of 1-butyl-3-methylimidazolium chloride were dissolved in 10 mL of deionized water. The solution was magnetically stirred for 20 min in an ice bath at 0 °C to ensure complete dissolution of all components. Nitrogen gas was purged for 10 min to remove dissolved oxygen from the mixture. 10 μL of N,N,N',N'-tetramethylethylenediamine was added, and the mixture was poured into glass cavities (glass dimensions: 10 cm × 10 cm, thickness 1 mm; cavity spacing 1 mm). After standing for 5 h, a thermochromic smart window was obtained. The critical response temperature was 28.5 °C, the transmittance was 95.3% (at 550 nm) at 20 °C, and the transmittance was 1.4% (at 550 nm) at 40 °C.

[0045] Example 5:

[0046] 1.05 g of N-isopropylacrylamide monomer, 0.2 g of ammonium persulfate, and 0.5 g of 1-butyl-3-methylimidazolium methanesulfonate were dissolved in 10 mL of deionized water. The solution was magnetically stirred for 20 min in an ice bath at 0 °C to ensure complete dissolution of all components. Nitrogen gas was purged for 10 min to remove dissolved oxygen from the mixture. 10 μL of N,N,N',N'-tetramethylethylenediamine was added, and the mixture was poured into glass cavities (glass dimensions: 10 cm × 10 cm, thickness 1 mm; cavity spacing 1 mm). After standing for 5 h, a thermochromic smart window was obtained. The critical response temperature was 25 °C, the transmittance was 82.5% (at 550 nm) at 20 °C, and the transmittance was 0.8% (at 550 nm) at 40 °C.

[0047] Comparative Example 1:

[0048] 1.05 g of N-isopropylacrylamide monomer, 0.2 g of ammonium persulfate, and 0.012 g of N,N'-methylenebisacrylamide were dissolved in 10 mL of deionized water and magnetically stirred for 20 min in an ice bath at 0 °C to ensure complete dissolution of all components. Nitrogen gas was then purged for 10 min to remove dissolved oxygen from the mixture. 10 μL of N,N,N',N'-tetramethylethylenediamine was added, and the mixture was poured into glass cavities (glass dimensions: 10 cm × 10 cm, thickness 1 mm; cavity spacing 1 mm). After standing for 5 h, a thermochromic smart window was obtained. The thermochromic PNIPAm hydrogel prepared by this method has a critical response temperature of 32.5 °C, a transmittance of 94.8% (at 550 nm) at 20 °C, and a transmittance of 1.8% (at 550 nm) at 40 °C. It is worth noting that the stability of this PNIPAm hydrogel-based smart window is poor. After prolonged thermochromic cycling, the PNIPAm hydrogel in the glass interlayer will shrink in volume and its performance will degrade. Figure 7 As shown.

[0049] Comparative Example 2:

[0050] 1.05 g of N-isopropylacrylamide monomer and 0.2 g of ammonium persulfate were dissolved in 10 mL of deionized water and magnetically stirred for 20 min in an ice bath at 0 °C to ensure complete dissolution of all components. Nitrogen gas was then introduced for 10 min to remove dissolved oxygen from the mixed solution. 10 μL of N,N,N',N'-tetramethylethylenediamine was added, and the mixed solution was poured into glass cavities (glass dimensions: 10 cm × 10 cm, thickness 1 mm; glass cavity spacing 1 mm). After standing for 5 h, a thermochromic smart window was obtained. Its initial state was a sol (flowable), with a critical response temperature of 32.5 °C and a transmittance of 93.4% (at 550 nm) at 20 °C. However, after heating at 40 °C, the obtained PNIPAm sol underwent phase separation, with free water precipitating from the PNIPAm network, but it was difficult to return to the initial sol state. That is, without the assistance of ionic liquid, it could not transform from a high-temperature gel state back to a sol state, thus losing its thermochromic function.

[0051] Comparative Example 3

[0052] 1.05 g of N-isopropylacrylamide monomer, 0.1 g of ammonium persulfate, and 0.5 g of 1-butyl-3-methylimidazolium tetrafluoroborate ionic liquid were dissolved in 10 mL of deionized water and magnetically stirred for 20 min in an ice bath at 0 °C to ensure complete dissolution of all components. Nitrogen gas was purged for 10 min to remove dissolved oxygen from the mixed solution. The mixed solution was then poured into glass cavities (glass dimensions: 10 cm × 10 cm, thickness 1 mm; glass cavity spacing 1 mm). After standing for 5 h, a thermochromic smart window could not be obtained, meaning that without the assistance of an accelerator, the precursor solution could not transform into a sol state and remained transparent at 40 °C.

[0053] This invention uses NIPAm as the thermally responsive unit. The introduction of ionic liquid replaces the crosslinking agent commonly used in hydrogel polymerization. The multiple hydrogen bonds and ionic interactions formed between NIPAm and PNIPAm (poly-N-isopropylacrylamide (PNIPAm)) chains endow the system with phase transition characteristics. That is, it exhibits a sol state at low temperatures and a gel state above the critical response temperature. Furthermore, the transmittance of the gel changes significantly during the phase transition. By injecting the thermochromic gel precursor solution into laminated glass and sealing it, a thermochromic smart window can be obtained. At low temperatures, the system is in a sol state (transparent), allowing incident light to enter; while when the environment exceeds the critical gel response temperature, the system is in a gel state (opaque). The ionic liquid / water binary solvent phase transition thermochromic smart window prepared by this invention has the characteristics of high transmittance, fast response, adjustable response temperature, and good stability. Moreover, it overcomes the defects of traditional thermochromic hydrogels, such as severe volume shrinkage and decreased thermochromic performance after long-term thermochromic cycling, and has good application prospects in thermochromic smart windows.

[0054] Although embodiments of the invention have been shown and described, it will be understood by those skilled in the art that various changes, modifications, substitutions and alterations can be made to these embodiments without departing from the principles and spirit of the invention, the scope of which is defined by the appended claims and their equivalents.

Claims

1. A phase transition thermally responsive gel based on an ionic liquid / water binary solvent, characterized in that: It is obtained by polymerizing monomers, ionic liquids, initiators, and accelerators in water; The concentration of the polymeric monomer is 8.6 wt% to 9.5 wt%. The concentration of the ionic liquid is 0.8 wt% to 8.5 wt%. The initiator concentration is 0.8 wt% to 1.6 wt%. The accelerator volume concentration is 0.1% to 0.3%; The polymer monomer is N-isopropylacrylamide; The ionic liquid is one of 1-butyl-3-methylimidazolium tetrafluoroborate, 1-butyl-3-methylimidazolium trifluoromethanesulfonate, 1-butyl-3-methylimidazolium methanesulfonate, 1-butyl-3-methylimidazolium nitrate, and 1-hexyl-3-methylimidazolium chloride; The accelerator is N,N,N',N'-tetramethylethylenediamine.

2. The phase transition thermally responsive gel based on an ionic liquid / water binary solvent according to claim 1, characterized in that: The initiator is either ammonium persulfate or potassium persulfate.

3. A method for preparing a phase transition thermally responsive gel based on an ionic liquid / water binary solvent as described in any one of claims 1-2, characterized in that, Includes the following steps: (1) Dissolve the monomer, initiator and ionic liquid in water and stir magnetically under ice bath conditions to accelerate dissolution; (2) Perform deoxygenation on the solution obtained in step (1); (3) Add an accelerator to the solution obtained in step (2), stir evenly and let stand to obtain a phase transition thermal response gel material of ionic liquid / water binary solvent.

4. The preparation method according to claim 3, characterized in that: In step (1), the ice bath temperature is 0-10℃ and the stirring time is 10-30min.

5. The preparation method according to claim 3, characterized in that: The deoxygenation operation in step (2) specifically involves introducing nitrogen gas into the solution; the time for introducing nitrogen gas is 10 to 30 minutes.

6. The application of the phase transition thermally responsive gel based on an ionic liquid / water binary solvent as described in any one of claims 1-2 as a thermochromic material.

7. A thermochromic smart window, characterized in that: A thermochromic material is dropped into a laminated glass mold and left to stand for a period of time to obtain a thermochromic smart window; the thermochromic material is the thermochromic material described in claim 6.

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