Preparation method of high-toughness and high-transparency PNIPAm thermochromic composite hydrogel

Abstract

The application relates to the technical field of building energy saving, in particular to a preparation method of high-toughness and high-transparency PNIPAm thermochromic composite hydrogel, which comprises the following steps: adding a surfactant and N-isopropyl acrylamide into deionized water, carrying out a heating reaction under the condition of oxygen removal by nitrogen, and adding acrylamide and a thermal initiator to prepare a poly-N-isopropyl acrylamide microgel suspension; taking the suspension as a substrate, adding ionic liquid, acrylamide, N,N-dimethyl acrylamide, hydroxyethyl acrylate and a crosslinking agent to form a polymerization precursor liquid; after ultrasonic defoaming and ice water precooling, potassium persulfate and tetramethyl ethylenediamine are added to initiate polymerization, and solidification and molding are carried out at room temperature. DMAA improves network flexibility and transparency, HEA improves the toughness of the hydrogel and the adhesion capacity of the hydrogel to a glass substrate through hydrogen bonding, and the ultrasonic defoaming and precooling process inhibits whitening caused by large-volume polymerization heat release.

Description

Technical Field

[0001] This invention relates to the field of building energy conservation technology, specifically to a method for preparing a high-toughness, high-transparency PNIPAm thermochromic composite hydrogel. Background Technology

[0002] Thermochromic smart windows are architectural window materials that automatically adjust their light transmission according to changes in ambient temperature. Below their phase transition temperature, these materials are typically transparent or highly transparent to meet the needs of natural indoor lighting. When the temperature rises above the phase transition temperature, phase separation or conformational change occurs within the material, resulting in light scattering and a translucent or milky-white appearance. This reduces solar radiation and lowers indoor temperatures caused by solar radiation. Poly(N-isopropylacrylamide), or PNIPAm for short, has a low critical solution temperature of approximately 32°C. Below this temperature, it exhibits a hydrophilic swelling state; above this temperature, it undergoes dehydration and shrinkage. Therefore, it is a commonly used temperature-sensitive polymer for preparing thermochromic hydrogels and passive temperature-controlled window materials.

[0003] Existing PNIPAm thermochromic hydrogels mainly include homogeneous hydrogels formed by direct crosslinking and polymerization of NIPAm monomers, and composite hydrogels in which PNIPAm microgels are dispersed in a polyacrylamide matrix. Directly using high-concentration NIPAm monomers for monolithic polymerization results in a large amount of NIPAm, leading to high raw material costs. Furthermore, large-sized products are prone to optical inhomogeneity during polymerization due to exothermic reactions, concentration gradients, and localized phase separation. The two-step method of combining PNIPAm microgels with a polyacrylamide matrix can reduce the amount of NIPAm used. However, traditional matrices typically use a high proportion of acrylamide as the sole main monomer, resulting in insufficient flexibility in the crosslinked network. This leads to problems such as brittleness, low elongation at break, and cracking or edge detachment after long-term use. For large-volume casting systems, mechanical stirring introduces microbubbles, and the heat generated during polymerization can cause localized whitening of the system, affecting room-temperature transparency and appearance uniformity.

[0004] Therefore, existing technologies still require a method for preparing PNIPAm thermochromic composite hydrogels suitable for large-volume molding. This method should improve the hydrogel's room-temperature light transmittance, tensile toughness, glass interface adhesion, and thermochromic cycle stability while controlling the amount of NIPAm monomer used. It should also reduce the risk of whitening caused by microbubbles and exothermic polymerization during polymerization curing to meet the requirements of transparency, deformation resistance, and service reliability for applications such as smart temperature-controlled windows. Summary of the Invention

[0005] To address the shortcomings of existing technologies, this invention provides a method for preparing a high-toughness, high-transparency PNIPAm thermochromic composite hydrogel, which solves the problems of existing PNIPAm thermochromic hydrogels, such as high NIPAm dosage, brittle bulk framework, insufficient light transmittance at room temperature, easy whitening during large-volume curing, and insufficient bonding stability with glass substrates.

[0006] To achieve the above objectives, the present invention provides the following technical solution: a method for preparing a high-toughness, high-transparency PNIPAm thermochromic composite hydrogel, comprising the following steps: S1. Preparation of microgel suspension A surfactant and N-isopropylacrylamide monomer were added to deionized water. After heating to the reaction temperature under deoxygenation conditions, acrylamide monomer and thermal initiator were added to carry out a isothermal polymerization reaction to obtain a suspension containing poly(N-isopropylacrylamide) microgels. The suspension was then cooled to room temperature. S2, ternary synergistic matrix construction Using a microgel suspension as a dispersion medium, ionic liquid, acrylamide monomer, N,N-dimethylacrylamide monomer, hydroxyethyl acrylate monomer and crosslinking agent were added and stirred until all components were fully dissolved to obtain a polymerization precursor solution. S3, Defoaming and Precooling The polymerization precursor solution is subjected to ultrasonic treatment to remove air bubbles, and then the ultrasonically treated polymerization precursor solution is pre-cooled in an ice-water bath. S4, Initiate curing An oxidation initiator and a reduction catalyst were added to the pre-cooled polymerization precursor solution, mixed evenly, and then injected into a mold. The mixture was allowed to stand at room temperature for polymerization and curing to obtain the high-toughness and high-transparency PNIPAm thermochromic composite hydrogel.

[0007] Preferably, in step S1, based on 500 mL of deionized water, the surfactant is 0.1 g of sodium dodecyl sulfate, the amount of N-isopropylacrylamide monomer added is 7.0 g, the amount of acrylamide monomer added is 0.6 g, and the thermal initiator is 0.16 g of ammonium persulfate.

[0008] Preferably, the deoxygenation conditions in step S1 are: continuously bubbling nitrogen gas into the reaction system; the polymerization reaction temperature is 80°C; the stirring speed is 1000 r / min; and the isothermal polymerization reaction time is 60 min.

[0009] Preferably, in step S2, based on 500 mL of the microgel suspension, the ionic liquid is 5.0 mL of 1-butyl-3-methylimidazolium tetrafluoroborate, the amount of acrylamide monomer added is 85.0 g, the amount of N,N-dimethylacrylamide monomer added is 30.0 g, the amount of hydroxyethyl acrylate monomer added is 10.0 g, and the crosslinking agent is N,N'-methylenebisacrylamide, which is added in an amount of 0.2 g.

[0010] Preferably, in step S2, the ionic liquid, acrylamide monomer, N,N-dimethylacrylamide monomer and hydroxyethyl acrylate monomer are added sequentially to the microgel suspension under stirring at 500 r / min. After the monomers are completely dissolved, the crosslinking agent is added, and stirring continues for 10 min.

[0011] Preferably, in step S3, the ultrasonic treatment time is 20 min and the ice-water bath precooling time is 10 min; the ultrasonic treatment is used to eliminate microbubbles introduced by stirring in the polymerization precursor liquid, and the ice-water bath precooling is used to reduce phase separation and whitening caused by exothermic reaction during the subsequent polymerization and curing process.

[0012] Preferably, in step S4, the oxidation initiator is potassium persulfate and the reduction catalyst is tetramethylethylenediamine; based on 500 mL of the microgel suspension, the amount of potassium persulfate added is 2.5 g and the amount of tetramethylethylenediamine added is 125 μL. After adding potassium persulfate, stir for 3 min to dissolve it, add tetramethylethylenediamine and stir for 20 s, and let it stand at room temperature for 6-12 h to solidify.

[0013] Preferably, the poly(N-isopropylacrylamide) microgel formed in step S1 is dispersed in the bulk polymer network formed in step S2, wherein the bulk polymer network is formed by copolymerization of acrylamide, N,N-dimethylacrylamide and hydroxyethyl acrylate, wherein N,N-dimethylacrylamide provides flexible segments, and hydroxyethyl acrylate forms hydrogen bonds with the polymer segments through hydroxyl groups.

[0014] The high-toughness, high-transparency PNIPAm thermochromic composite hydrogel prepared by this invention comprises a bulk ternary synergistic polymer network and poly(N-isopropylacrylamide) microgels dispersed within the bulk ternary synergistic polymer network. The bulk ternary synergistic polymer network is formed by copolymerization of acrylamide, N,N-dimethylacrylamide, and hydroxyethyl acrylate, and a cross-linked structure is constructed through N,N'-methylenebisacrylamide.

[0015] The high-toughness, high-transparency PNIPAm thermochromic composite hydrogel can be placed between two transparent glass substrates to form a thermochromic intelligent temperature-controlled window. The segments formed by the polymerization of hydroxyethyl acrylate have hydroxyl groups, which can form hydrogen bonds with the silanol groups on the glass surface, ensuring stable contact between the hydrogel layer and the inner surface of the glass.

[0016] This invention provides a method for preparing a high-toughness, high-transparency PNIPAm thermochromic composite hydrogel. It possesses the following beneficial effects: 1. This invention involves first preparing a PNIPAm microgel suspension at a low concentration of poly(N-isopropylacrylamide) (PNIPAm) monomer, and then using the microgel suspension as a dispersion substrate to construct a bulk phase crosslinking network. The PNIPAm microgel, as a thermosensitive functional phase, is stably confined within the subsequently formed AM-DMAA-HEA ternary synergistic network, maintaining swelling and high light transmittance at temperatures below the lower critical dissolution temperature of PNIPAm. When the temperature rises above the phase transition temperature, the PNIPAm microgel undergoes dehydration and shrinkage, leading to an increase in the refractive index difference within the gel and resulting in light scattering, causing the hydrogel to change from a transparent state to a milky white state. This method reduces the amount of NIPAm monomer while maintaining thermochromic response capabilities. The resulting composite hydrogel exhibits a significant thermochromic light-blocking response above 35°C and retains more than 90% of its optical properties after more than 100 heating-cooling cycles.

[0017] 2. This invention constructs a ternary synergistic phase framework by introducing acrylamide (AM), N,N-dimethylacrylamide (DMAA), and hydroxyethyl acrylate (HEA) into a PNIPAm microgel suspension, and forms a three-dimensional cross-linked network using BIS. AM provides the basic mechanical support segments, DMAA improves the flexibility, hydrophilic compatibility, and gel homogeneity of the polymer network, and the hydroxyl groups of HEA form multiple hydrogen bonds with the amide groups and other groups in the network, thereby improving the network's energy dissipation capacity during tensile stress and enhancing the interfacial adhesion between the hydrogel and the glass substrate. This ternary synergistic network reduces the brittleness problem caused by traditional high-proportion acrylamide single networks. The resulting PNIPAm-DMAA / HEA composite hydrogel exhibits a tensile stress exceeding 0.12 MPa and an elongation at break of 1122%. Simultaneously, the hydroxyethyl acrylate segments can form hydrogen bonds with the glass surface, allowing the hydrogel to stably adhere to the inner surface of the glass after in-situ solidification, making it suitable for smart window interlayer structures.

[0018] 3. This invention incorporates ultrasonic degassing and ice-water bath precooling processes before polymerization and curing. Ultrasonic treatment removes microbubbles introduced during monomer dissolution and mechanical stirring, reducing the scattering effect of these bubbles on incident light. Ice-water bath precooling lowers the initial temperature of the polymerization precursor solution, buffering the reaction heat generated during the large-volume polymerization of the potassium persulfate / tetramethylethylenediamine redox initiation system, and suppressing phase separation whitening, structural inhomogeneity, and decreased optical performance caused by excessively high local temperatures. After 20 minutes of ultrasonic treatment and 10 minutes of ice-water bath precooling, the resulting composite hydrogel achieves a visible light transmittance (Tlum) of over 85% at 20°C, below the lower critical dissolution temperature of PNIPAm. Under continuous simulated sunlight irradiation, the hydrogel smart window transitions to a light-scattering and shading state as the temperature rises, reducing the heating rate and peak temperature of the simulated room. Attached Figure Description

[0019] Figure 1 This is a cross-sectional scanning electron microscope image of the PNIPAm-DMAA / HEA composite hydrogel prepared in Example 1 of the present invention after freeze-drying. Figure 2 Fourier transform infrared spectra of NIPAm monomer, AM monomer, DMAA monomer, HEA monomer, and the PNIPAm-DMAA / HEA composite hydrogel prepared in Example 1 of this invention; Figure 3 The tensile stress-strain curves of the PNIPAm-DMAA / HEA composite hydrogel prepared in Example 1 of this invention are compared with those of the pure PNIPAm hydrogel prepared in Comparative Example 1. Figure 4 A bar chart comparing the elongation at break of the PNIPAm-DMAA / HEA composite hydrogel prepared in Example 1 of this invention with that of the pure PNIPAm hydrogel prepared in Comparative Example 1. Figure 5 The UV-Vis-NIR transmittance spectra of the PNIPAm-DMAA / HEA composite hydrogel prepared in Example 1 of this invention are shown below and above the lower critical dissolution temperature, as well as the transmittance change during the heating-cooling cycle. Figure 6 The diagram shows a smart window simulated room testing device constructed using the composite hydrogel prepared in Example 1 of this invention, along with a corresponding infrared thermal imaging comparison image. Detailed Implementation

[0020] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. 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 skilled in the art without creative effort are within the scope of protection of the present invention.

[0021] Example 1: This invention provides a method for preparing a high-toughness, high-transparency PNIPAm thermochromic composite hydrogel, comprising the following steps: (1) Preparation of PNIPAm microgel suspension Take a 1000mL three-necked flask, wash it, and dry it for later use. Add 500mL of deionized water to the three-necked flask and add a magnetic stir bar. Place the flask in a constant temperature water bath, connect the nitrogen inlet pipe and the exhaust pipe, and ensure that the outlet end of the nitrogen inlet pipe is below the liquid surface so that nitrogen can form continuous bubbles in the liquid.

[0022] Add 0.1 g of sodium dodecyl sulfate to deionized water. After the sodium dodecyl sulfate dissolves, add 7.0 g of N-isopropylacrylamide monomer. Turn on the magnetic stirrer and set the stirring speed to 1000 r / min to ensure that the sodium dodecyl sulfate and N-isopropylacrylamide are fully dissolved in the water. Purge with nitrogen gas to remove oxygen and reduce the inhibitory effect of dissolved oxygen on the free radical polymerization reaction.

[0023] The reaction system was heated to 80°C. After the system temperature stabilized at 80°C, 0.6 g of acrylamide monomer and 0.16 g of ammonium persulfate were quickly added to the three-necked flask. After addition, nitrogen bubbling and stirring at 1000 rpm were continued, and the reaction was maintained at 80°C for 60 min. During the reaction, the system gradually changed from an initially clear state to a translucent state, indicating that stable and dispersed poly(N-isopropylacrylamide) microgel particles were formed in the system.

[0024] After the isothermal reaction was completed, heating was stopped and nitrogen gas was removed. The reaction system was then allowed to cool naturally at room temperature. Once the system temperature dropped to 20℃~25℃, a PNIPAm microgel suspension was obtained. This suspension was used as a dispersion substrate for the subsequent ternary synergistic polymerization network.

[0025] (2) Construction of ternary synergistic polymerization precursor liquid Transfer the cooled 500 mL PNIPAm microgel suspension to a beaker with a volume of not less than 1000 mL. Place a cleaned and dried magnetic stir bar inside the beaker, place the beaker on a magnetic stirrer, and adjust the stirring speed to 500 r / min.

[0026] Under continuous stirring, 5.0 mL of 1-butyl-3-methylimidazolium tetrafluoroborate (BMIMBF4) was added to the microgel suspension. Stirring continued after the addition of BMIMBF4 to ensure uniform dispersion in the system. Subsequently, 85.0 g of acrylamide monomer was added, and stirring was maintained until the solid monomer was completely dissolved.

[0027] After the acrylamide monomer has dissolved, add 30.0 g of N,N-dimethylacrylamide monomer to the system. N,N-dimethylacrylamide is a liquid or easily soluble monomer; it should be added slowly along the inner wall of the beaker to reduce unevenness of the polymerization precursor solution caused by sudden local concentration changes. Continue stirring to ensure that the N,N-dimethylacrylamide is evenly mixed with the suspension.

[0028] Add 10.0 g of hydroxyethyl acrylate monomer to the above mixture and continue stirring at 500 r / min for 20-30 min. Observe the state of the system. After confirming that there are no visible solid particles, no obvious stratification, and no large agglomerates in the liquid, add 0.2 g of N,N'-methylenebisacrylamide. After adding N,N'-methylenebisacrylamide, continue stirring for 10 min to ensure that the crosslinking agent is uniformly dispersed in the precursor solution, thus obtaining the AM-DMAA-HEA ternary co-polymerization precursor solution containing PNIPAm microgel.

[0029] In this polymerization precursor solution, acrylamide provides the main chain segments for the bulk network, N,N-dimethylacrylamide is used to improve the flexibility and solution compatibility of the network segments, hydroxyethyl acrylate provides hydroxyl segments capable of forming hydrogen bonds, and N,N'-methylenebisacrylamide is used to construct a three-dimensional cross-linked network during polymerization. PNIPAm microgels remain dispersed in this precursor solution and will be immobilized within the bulk network during subsequent curing.

[0030] (3) Ultrasonic defoaming and ice water bath precooling Transfer the beaker containing the polymerization precursor solution to an ultrasonic cleaner. Pre-fill the cleaning tank of the ultrasonic cleaner with an appropriate amount of water to ensure the bottom of the beaker is stably in contact with or immersed in the ultrasonic medium. Perform ultrasonic treatment on the polymerization precursor solution for 20 minutes.

[0031] During ultrasonic treatment, tiny bubbles introduced by mechanical stirring in the polymerization precursor solution gradually rise to the surface and burst. After ultrasonic treatment, remove the beaker and observe the liquid surface; there should be no obvious bubble accumulation. To avoid reintroducing air, subsequent operations should be performed with gentle stirring.

[0032] The ultrasonically treated beaker was pre-cooled in an ice-water bath for 10 minutes. The ice-water bath consisted of a mixture of ice and deionized water, ensuring full contact between the bottom and sidewalls of the beaker. Vigorous stirring was avoided during pre-cooling to allow the overall temperature of the polymerization precursor solution to decrease. This pre-cooling treatment resulted in a better exothermic buffering capacity during subsequent redox-initiated polymerization.

[0033] (4) Redox-induced polymerization and mold curing Take 2.5g of potassium persulfate and add it to the pre-cooled polymerization precursor solution. Gently stir for 3 minutes using a glass rod or a low-speed magnetic stirrer to completely dissolve the potassium persulfate. Avoid forming vortices during stirring to prevent air from re-entering the system.

[0034] 125 μL of tetramethylethylenediamine was measured using a pipette and added to the polymerization precursor solution containing potassium persulfate. Immediately after addition, the mixture was gently stirred in the same direction for 20 seconds to ensure uniform dispersion of the redox initiation system. The potassium persulfate and tetramethylethylenediamine form a room-temperature redox initiation system, initiating the free radical polymerization reaction of AM, DMAA, and HEA monomers, as well as the crosslinking agent.

[0035] The mixed precursor solution is promptly injected into a pre-prepared glass mold. The glass mold can consist of two opposing transparent glass plates, with a cavity formed between them by a sealing strip or spacer. The thickness of the cavity can be set according to the required hydrogel layer thickness for the smart window. Before injection, the inner surface of the glass is cleaned, rinsed with deionized water, and dried. After injection, the injection port is sealed, allowing the precursor solution to stand at room temperature for 6-12 hours in a sealed state.

[0036] After curing, the polymerization precursor solution forms a composite hydrogel with a three-dimensional cross-linked network. The PNIPAm microgels within the hydrogel are confined within the AM-DMAA-HEA ternary cross-linked network. Hydrogen bonds form between the hydroxyl groups in the HEA polymer segments and the glass surface, allowing the composite hydrogel layer to adhere to the inner wall of the glass. The resulting hydrogel is transparent or nearly transparent at 20°C, gradually turning milky white when heated above 35°C.

[0037] Example 2: This invention provides a method for preparing a composite hydrogel intelligent temperature control window, as detailed below: A composite hydrogel intelligent temperature-controlled window was prepared using the polymerization precursor solution prepared in Example 1. Two identical transparent glass plates were selected, each 150 mm long, 150 mm wide, and 3 mm thick. The glass plate surfaces were cleaned with a neutral detergent, followed by rinsing with deionized water and anhydrous ethanol to remove dust, oil, and residual impurities. The cleaned glass plates were then dried at 60°C or using clean air.

[0038] A 2mm thick sealing gasket is attached to the edge of one of the glass plates, forming a hollow cavity. Another glass plate is placed over the gasket, leaving an injection port at its edge. A precursor solution, which has been ultrasonically defoamed, pre-cooled in an ice-water bath, and infused with potassium persulfate and tetramethylethylenediamine, is slowly injected into the cavity between the glass plates using a syringe through the injection port.

[0039] During injection, control the syringe thrust speed to ensure the precursor solution fills the cavity from bottom to top, minimizing air bubble retention. Once the cavity is full, seal the injection port. Place the glass mold containing the precursor solution flat at room temperature and allow it to cure for 8 hours. After curing, a thermochromic smart temperature-controlled window with a hydrogel layer thickness of approximately 2 mm is obtained.

[0040] At 20°C, the hydrogel layer between the glass plates remained transparent, allowing the outlines of background objects to pass through. When the smart window was heated to above 35°C, the hydrogel layer gradually changed from transparent to milky white, and the light transmitted through the glass plates was significantly reduced. After cooling to below the lower critical dissolution temperature of PNIPAm, the hydrogel layer regained its high light transmittance.

[0041] Comparative Example 1: N-Isopropylacrylamide monomer was dissolved in deionized water, and N,N'-methylenebisacrylamide was added as a crosslinking agent. The mixture was stirred until homogeneous. Ammonium persulfate and tetramethylethylenediamine were added to the solution as a redox initiation system. After mixing, the mixture was poured into a mold and cured at room temperature to obtain pure PNIPAm hydrogel.

[0042] Comparative Example 1 did not employ the PNIPAm microgel prepolymerization process, nor did it include N,N-dimethylacrylamide, hydroxyethyl acrylate, or ionic liquids. It also did not undergo ultrasonic defoaming or ice-water bath precooling. The resulting pure PNIPAm hydrogel was used for performance comparison with the PNIPAm-DMAA / HEA composite hydrogel prepared in Example 1.

[0043] The performance of the samples prepared in Example 1 and Comparative Example 1 was tested and the results were analyzed, as follows: (1) Scanning electron microscopy characterization The composite hydrogel prepared in Example 1 was cut into samples of suitable size. To maintain the internal network morphology, the samples were pre-frozen at low temperature and then freeze-dried to obtain dry porous gel samples. The samples were fixed on the stage of a scanning electron microscope, and the sample surface was treated with a conductive coating.

[0044] The cross-sectional morphology of the hydrogel was observed using a scanning electron microscope (SEM) of Hitachi S-4800. Figure 1As can be seen, the composite hydrogel prepared in Example 1 has a relatively dense and continuous three-dimensional porous network structure with interconnected pore walls. The PNIPAm microgel phase is relatively uniformly dispersed in the bulk network, and no obvious large-scale phase separation regions were observed. This morphology indicates that the AM-DMAA-HEA bulk network and the PNIPAm microgel have good structural compatibility.

[0045] (2) Fourier transform infrared spectroscopy characterization The composite hydrogel sample prepared in Example 1 was freeze-dried and then subjected to Fourier transform infrared spectroscopy (FTIR) using a Bruker V70 instrument.

[0046] like Figure 2 As shown, absorption peaks related to amide groups and broadened absorption peaks related to hydroxyl groups are observed in the infrared spectrum of the composite hydrogel. Compared with the characteristic absorption peaks of each monomer, the corresponding peak positions in the composite hydrogel change, indicating that AM, DMAA, and HEA participate in the polymerization reaction and form a copolymer network. The changes in the absorption peaks related to hydroxyl and amide groups reflect the existence of hydrogen bonding interactions within the network, which is beneficial to improving the toughness and structural stability of the hydrogel.

[0047] (3) Tensile property test The composite hydrogel prepared in Example 1 and the pure PNIPAm hydrogel prepared in Comparative Example 1 were cut into dumbbell-shaped or rectangular tensile specimens. The specimens were equilibrated at room temperature before testing. Tensile tests were performed using a Shimadzu ASG-X1KN universal testing machine. Load and displacement data were recorded during the test, and stress-strain curves were calculated.

[0048] like Figure 3 and Figure 4 As shown, the pure PNIPAm hydrogel of Comparative Example 1 fractured when stretched to approximately 677%, with a maximum tensile stress of approximately 0.06 MPa. The PNIPAm-DMAA / HEA composite hydrogel prepared in Example 1 could withstand greater tensile deformation, with a maximum tensile stress exceeding 0.12 MPa and an elongation at break reaching 1122%. These results indicate that the AM-DMAA-HEA ternary synergistic network can improve the brittleness of traditional pure PNIPAm hydrogels, allowing for more complete segment orientation, hydrogen bond dissociation and recombination, and crosslinked network deformation during the stretching process.

[0049] (4) Visible light transmittance and thermochromic performance test The composite hydrogel prepared in Example 1 was made into test samples with a basically uniform thickness and placed in a UV-Vis-NIR spectrophotometer with a temperature control accessory for transmittance testing. The testing equipment was a Cary 5000. The test wavelength range was 400 nm to 800 nm, and the transmittance spectra of the samples were recorded at temperatures above 20 °C and 35 °C, respectively.

[0050] At 20℃, below the low critical dissolution temperature of PNIPAm, the composite hydrogel exhibits high transmittance in the visible light region, with a Tlum value exceeding 85%, and the material is generally transparent. At temperatures above 35℃, the PNIPAm microgels undergo dehydration and shrinkage, resulting in significant light scattering within the hydrogel, a marked decrease in transmittance, and the sample's appearance transforms into a milky white color. Figure 5 As shown, the material can reversibly switch between a transparent state and a milky white state with temperature changes.

[0051] The samples were subjected to a heating-cooling cycle test. In each cycle, the samples were heated from below the lower critical melting temperature to above the lower critical melting temperature, held until the transmittance was basically stable, and then cooled back to the initial temperature. After more than 100 cycles, the transmittance change curves of the samples remained highly consistent, the optical performance retention rate was greater than 90%, and no obvious cracking, delamination, or large-scale migration of microgels was observed.

[0052] (5) Observation of glass interface adhesion performance The intelligent temperature-controlled window prepared in Example 2 was placed at room temperature, and the contact state between the hydrogel layer and the glass substrate was observed. No obvious edge curling, local lifting, or large-area detachment was observed within the area covered by the hydrogel layer. After repeated heating and cooling of the intelligent window, the hydrogel layer remained on the inner surface of the glass substrate.

[0053] This phenomenon is related to the hydrogen bonding interaction between the hydroxyl groups in the HEA segments and the silanol groups on the glass surface. When the hydrogel is cured in situ in the glass cavity, the polymer segments can adhere to the inner surface of the glass, enabling the hydrogel layer to form a stable interfacial contact structure.

[0054] (6) Simulated room temperature control performance test Two identical simulated room devices were constructed. One simulated room had a composite hydrogel smart temperature-controlled window prepared in Example 2 installed on its light-transmitting side, while the other simulated room had ordinary transparent glass installed on its light-transmitting side as a control. Xenon lamps were used to simulate sunlight, with illumination conditions set to AM 1.5G and an irradiance of 100 mW / cm². 2 The temperature field changes inside two simulated rooms were monitored using an infrared thermal imager, specifically a FLIR A655sc.

[0055] Under continuous illumination, the composite hydrogel smart window absorbs solar radiation and heats up. When the hydrogel temperature rises to near the PNIPAm phase transition temperature, the hydrogel layer inside the window changes from a transparent state to a milky white state, exhibiting stronger scattering and blocking effects on visible and near-infrared radiation. Compared to the control group with ordinary glass, the simulated room with the composite hydrogel smart window installed has a lower rate of heating and a lower peak temperature. Figure 6 Infrared thermal imaging results show that the composite hydrogel smart window has dynamic temperature control characteristics, which enhances radiation blocking ability as temperature increases.

[0056] 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 method for preparing a high-toughness, high-transparency PNIPAm thermochromic composite hydrogel, characterized in that, Includes the following steps: S1. Preparation of microgel suspension A surfactant and N-isopropylacrylamide monomer were added to deionized water. After heating to the reaction temperature under deoxygenation conditions, acrylamide monomer and thermal initiator were added to carry out a isothermal polymerization reaction to obtain a suspension containing poly(N-isopropylacrylamide) microgels. The suspension was then cooled to room temperature. S2, ternary synergistic matrix construction Using a microgel suspension as a dispersion medium, ionic liquid, acrylamide monomer, N,N-dimethylacrylamide monomer, hydroxyethyl acrylate monomer and crosslinking agent were added and stirred until all components were fully dissolved to obtain a polymerization precursor solution. S3, Defoaming and Precooling The polymerization precursor solution is subjected to ultrasonic treatment to remove air bubbles, and then the ultrasonically treated polymerization precursor solution is pre-cooled in an ice-water bath. S4, Initiate curing An oxidation initiator and a reduction catalyst were added to the pre-cooled polymerization precursor solution, mixed evenly, and then injected into a mold. The mixture was allowed to stand at room temperature for polymerization and curing to obtain the high-toughness and high-transparency PNIPAm thermochromic composite hydrogel.

2. The preparation method of the high-toughness, high-transparency PNIPAm thermochromic composite hydrogel according to claim 1, characterized in that, In step S1, based on 500 mL of deionized water, the surfactant is 0.1 g of sodium dodecyl sulfate, the amount of N-isopropylacrylamide monomer added is 7.0 g, the amount of acrylamide monomer added is 0.6 g, and the thermal initiator is 0.16 g of ammonium persulfate.

3. The preparation method of the high-toughness, high-transparency PNIPAm thermochromic composite hydrogel according to claim 1, characterized in that, The deoxygenation conditions in step S1 are: continuous bubbling of nitrogen gas into the reaction system; polymerization temperature of 80℃; stirring speed of 1000 r / min; and constant temperature polymerization time of 60 min.

4. The preparation method of the high-toughness, high-transparency PNIPAm thermochromic composite hydrogel according to claim 1, characterized in that, In step S2, based on 500 mL of the microgel suspension, the ionic liquid is 5.0 mL of 1-butyl-3-methylimidazolium tetrafluoroborate, the amount of acrylamide monomer added is 85.0 g, the amount of N,N-dimethylacrylamide monomer added is 30.0 g, the amount of hydroxyethyl acrylate monomer added is 10.0 g, and the crosslinking agent is N,N'-methylenebisacrylamide, which is added in an amount of 0.2 g.

5. The preparation method of a high-toughness, high-transparency PNIPAm thermochromic composite hydrogel according to claim 1, characterized in that, In step S2, the ionic liquid, acrylamide monomer, N,N-dimethylacrylamide monomer and hydroxyethyl acrylate monomer are added sequentially to the microgel suspension under stirring at 500 r / min. After the monomers are completely dissolved, the crosslinking agent is added and stirring continues for 10 min.

6. The preparation method of a high-toughness, high-transparency PNIPAm thermochromic composite hydrogel according to claim 1, characterized in that, In step S3, the ultrasonic treatment time is 20 min and the ice-water bath precooling time is 10 min; the ultrasonic treatment is used to eliminate microbubbles introduced by stirring in the polymerization precursor liquid, and the ice-water bath precooling is used to reduce phase separation and whitening caused by exothermic reaction during the subsequent polymerization and curing process.

7. The preparation method of a high-toughness, high-transparency PNIPAm thermochromic composite hydrogel according to claim 1, characterized in that, In step S4, the oxidation initiator is potassium persulfate, and the reduction catalyst is tetramethylethylenediamine. Based on 500 mL of the microgel suspension, the amount of potassium persulfate added is 2.5 g, and the amount of tetramethylethylenediamine added is 125 μL. After adding potassium persulfate, stir for 3 min to dissolve it. After adding tetramethylethylenediamine, stir for 20 s, and let it stand at room temperature for 6–12 h to solidify.

8. The preparation method of a high-toughness, high-transparency PNIPAm thermochromic composite hydrogel according to claim 1, characterized in that, The poly(N-isopropylacrylamide) microgel formed in step S1 is dispersed in the bulk polymer network formed in step S2. The bulk polymer network is formed by copolymerization of acrylamide, N,N-dimethylacrylamide and hydroxyethyl acrylate, wherein N,N-dimethylacrylamide provides flexible segments and hydroxyethyl acrylate forms hydrogen bonds with polymer segments through hydroxyl groups.

9. A high-toughness, high-transparency PNIPAm thermochromic composite hydrogel, characterized in that, The composite hydrogel is prepared by the preparation method according to any one of claims 1-8, comprising a bulk ternary synergistic polymer network and polyN-isopropylacrylamide microgel dispersed in the bulk ternary synergistic polymer network. The bulk ternary synergistic polymer network is a crosslinked copolymer network composed of acrylamide, N,N-dimethylacrylamide and hydroxyethyl acrylate, which is formed by crosslinking with N,N'-methylenebisacrylamide.

10. A thermochromic intelligent temperature control window, characterized in that, It includes two transparent glass substrates arranged opposite each other and a hydrogel layer disposed between the two transparent glass substrates, wherein the hydrogel layer is the high-toughness and high-transparency PNIPAm thermochromic composite hydrogel as described in claim 9; the hydrogel layer is adhered to the inner surface of the transparent glass substrate through hydrogen bonding between the hydroxyethyl acrylate segments and the surface of the glass substrate.

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