A vinyl acetate-n-propyl vinyl ether copolymer film for use in mesophilic smart windows and its preparation method.

Abstract

This invention belongs to the field of mechanochromic materials technology, specifically relating to a vinyl acetate-n-propyl vinyl ether copolymer film for mechanochromic smart windows and its preparation method. The invention first dissolves VAc and PVE in methanol, heats under anaerobic conditions, and then initiates a polymerization reaction by continuously adding a methanol solution of AIBN. The resulting film is then formed to obtain the vinyl acetate-n-propyl vinyl ether copolymer mechanochromic film. The method of this invention is relatively simple, has high reaction safety, and is easy to scale up production. By adjusting the raw material ratio, initiator dosage, and reaction parameters, the molecular structure, molecular weight, and physical properties of the reaction products can be effectively controlled. The reaction products can be cured and molded using various methods to obtain various copolymer products with different forms of mechanochromic effects. Mechanochromic properties can be achieved without complex molding processes, which is beneficial for large-scale production and widespread application in fields such as smart windows.

Description

Technical Field

[0001] This invention belongs to the field of mechanochromic materials technology, specifically relating to a vinyl acetate-n-propyl vinyl ether copolymer film that can be used in mechanochromic smart windows and its preparation method. Background Technology

[0002] Intelligent windows are windows that can dynamically change their color, transparency, reflectivity, and other properties as needed. They can block light in strong sunlight and increase transmittance in weak sunlight, intelligently regulating indoor lighting according to weather and lighting conditions. This, in turn, regulates indoor temperature and improves building energy efficiency. It is this potential environmental and energy-saving value that has garnered widespread attention. Furthermore, this technology can be used to protect residents' privacy and improve living comfort, and can be applied to a range of scenarios, including aircraft windows, car sunroofs, greenhouse sunroofs, and smart sunglasses. The core of intelligent window technology is functional materials that can reversibly change color in response to external stimuli, including electrochromic materials, thermochromic materials, photochromic materials, and mechanochromic materials. Among them, mechanochromic materials can change their surface morphology or internal structure when subjected to mechanical strain, thereby altering their transmittance through visible light scattering or diffraction. They are particularly noteworthy for their advantages, such as changing color without the need for an external electric or thermal field, simple structure, low cost, and easily adjustable response time.

[0003] The color-changing effect of mechanochromic materials is mainly based on the changes in the designed microstructure under stress. Common color-changing mechanisms include micro-wrinkles, micro-cracks, delamination buckling, surface periodic micro / nano structures, and internal variable interfaces. For example, Lin et al. (2017, Self-similar Hierarchical Wrinkles as a Potential Multifunctional Smart Window with Simultaneously Tunable Transparency, Structural Color, and Droplet Transport, Acs Applied Materials & Interfaces, 9(31):26510-26517) generated a rigid silica layer on the surface of a pre-strained polydimethoxysilane elastomer film through multiple plasma treatments. After the stress was removed, the silica layer relaxed and formed wrinkles, resulting in a significant decrease in the film transparency. After stress was applied again, the wrinkles disappeared and the transparency was restored. Cho et al. (2020, High-Contrast Optical Modulation from Strain-Induced Nanogaps at 3D Heterogeneous Interfaces, Advanced Science, 7(11)) incorporated rigid alumina nanoshells into a transparent PDMS matrix to form an elastomer film with a periodic three-dimensional nanostructure. During stretching, micro-gaps were generated at the interface between the nanoshells and the matrix due to modulus mismatch, resulting in a significant decrease in film transmittance. However, most existing mechanochromic materials rely on specially designed multilayer structures or periodic surface micro / nano structures to achieve the mechanochromic effect, which involves high complexity in preparation and molding, hindering large-scale production and widespread application.

[0004] Polyvinyl acetate (PVAc) is a polymer that has been mass-produced and widely used, commonly serving as a raw material for adhesives, coatings, chewing gum, or as a primary raw material for other copolymers and composites. Propyl vinyl ether (PVE) is typically not used as a homopolymer raw material, but rather as a secondary raw material in copolymers to improve the product's processing properties. However, there are currently no reports on the preparation of mechanochromic materials using PVAc or PVE. Summary of the Invention

[0005] To overcome the shortcomings of the prior art, the present invention provides a method for preparing a vinyl acetate / n-propyl vinyl ether copolymer by solution copolymerization and curing it into a film. The resulting copolymer film has high transparency and a reversible mechanochromic effect, which can be applied to the design and manufacture of smart windows.

[0006] To achieve the above objectives, the technical solution adopted by the present invention is as follows:

[0007] The first aspect of this invention provides a method for preparing a vinyl acetate-n-propyl vinyl ether copolymer film, specifically: dissolving vinyl acetate (VAc) and n-propyl vinyl ether (PVE) in methanol, then heating the mixed solution to 50-70°C under anaerobic conditions, and then initiating a polymerization reaction by continuously adding a methanol solution of azobisisobutyronitrile (AIBN). After the reaction is completed, the reaction product is formed into a film to obtain a vinyl acetate-n-propyl vinyl ether copolymer film.

[0008] The method of this invention prepares a block copolymer of vinyl acetate and n-propyl vinyl ether, wherein the proportion of vinyl acetate segments is mainly affected by the proportion of monomers in the raw materials. The resulting copolymer is soluble in methanol, forming a colorless, transparent or translucent solution. The copolymer solution can be cured by heating to form a film with high transparency under visible light, a smooth and flat surface, and good bonding with ordinary glass, plexiglass, polypropylene plastics, etc.

[0009] Preferably, the mass ratio of vinyl acetate to n-propyl vinyl ether is 25:15-35:5.

[0010] Preferably, when vinyl acetate and n-propyl vinyl ether are dissolved in methanol, the mass of the methanol is 20%-50% of the sum of the masses of vinyl acetate and n-propyl vinyl ether.

[0011] Preferably, the mass of the azobisisobutyronitrile is 0.2%-1% of the sum of the masses of vinyl acetate and n-propyl vinyl ether.

[0012] Preferably, the amount of methanol used to dissolve azobisisobutyronitrile is the same as the amount of methanol used to dissolve vinyl acetate and n-propyl vinyl ether.

[0013] Preferably, nitrogen gas is introduced to purge the air from the reaction environment to create an oxygen-free condition.

[0014] Preferably, when continuously adding azobisisobutyronitrile (AIBN) methanol solution, the initial addition should be completed by adding 2-4 mL of AIBN methanol solution per 50-60 g of mixed solution to start the polymerization reaction process, and then the solution should be continuously injected over 3-10 hours using a syringe.

[0015] Preferably, the reaction products are formed into a film in a glass petri dish, and the film formation process includes drop coating, blade coating, and spin coating.

[0016] More preferably, the glass culture dish is a glass culture dish that has undergone hydrophobic treatment.

[0017] More preferably, the glass culture dish is first treated in a plasma cleaner for 5-20 minutes, then soaked in perfluorooctyltrimethoxysilane for 1-5 hours, and then washed and dried.

[0018] The second aspect of the present invention provides a vinyl acetate-n-propyl vinyl ether copolymer film prepared by the preparation method described in the first aspect.

[0019] The vinyl acetate-n-propyl vinyl ether copolymer film prepared by the method of this invention has high transparency and a preferred thickness of 400-450 micrometers. The film exhibits whitening and a significant decrease in transparency when stretched uniaxially; however, it quickly recovers its original shape and transparency after stress is removed without causing damage.

[0020] The third aspect of this invention provides the application of the vinyl acetate-n-propyl vinyl ether copolymer film described in the second aspect in the preparation of mechanochromic smart window materials.

[0021] Compared with the prior art, the beneficial effects of the present invention are:

[0022] This invention discloses a method for preparing a transparent film with mechanochromic effect by solution copolymerization of vinyl acetate-n-propyl vinyl ether copolymer. First, VAc and PVE are dissolved in methanol, heated under anaerobic conditions, and then polymerization is initiated by continuously adding a methanol solution of AIBN. The film is then formed. This method is relatively simple, has high reaction safety, and is easy to scale up. Furthermore, by adjusting the raw material ratio, initiator dosage, and reaction parameters, the molecular structure, molecular weight, and physical properties of the reaction product can be effectively controlled. The reaction product can be cured and molded using various methods to obtain various forms of copolymer products with mechanochromic effect. Mechanochromic properties can be achieved without complex molding processes, which is beneficial for large-scale production and widespread application in fields such as smart windows. Attached Figure Description

[0023] Figure 1 The reaction equation for the synthesis of vinyl acetate-n-propyl vinyl ether copolymer via solution copolymerization is as follows: Vinyl acetate (VAc) and n-propyl vinyl ether (PVE) monomers are dissolved in methanol, and azobisisobutyronitrile (AIBN) is used as an initiator to initiate the copolymerization reaction, thereby synthesizing vinyl acetate-n-propyl vinyl ether copolymer (PVAc-co-PPVE).

[0024] Figure 2 This is a schematic diagram of the reaction apparatus used for solution copolymerization of vinyl acetate-n-propyl vinyl ether copolymer.

[0025] Figure 3 The film sample obtained in Example 1 shows that after the copolymer film is peeled off from the highly hydrophobic glass culture dish, it forms a transparent, soft, and flat film.

[0026] Figure 4 The figures show the 1H NMR spectra of the copolymer samples obtained in Examples 1, 2, and the Comparative Example; 87% PVAc, 93% PVAc, and 100% PVAc in the figures correspond to the samples in Examples 1, 2, and the Comparative Example, respectively, representing that the samples contain 87%, 93%, and 100% PVAc segments as calculated. The same applies to the subsequent figures.

[0027] Figure 5 The images show the infrared spectra of the copolymer samples obtained in Examples 1, 2, and the comparative examples.

[0028] Figure 6 The XRD diffraction patterns are those of the copolymer samples prepared in Examples 1, 2 and the comparative examples.

[0029] Figure 7 Thermogravimetric curves of the copolymer samples obtained in Examples 1, 2 and the comparative examples are shown.

[0030] Figure 8 The DSC temperature rise curves are for the copolymer samples obtained in Examples 1, 2 and the comparative examples.

[0031] Figure 9 Typical tensile curves of the copolymer samples prepared in Examples 1, 2 and the comparative examples.

[0032] Figure 10 The appearance changes of the copolymer sample prepared in Example 2 during stretching and recovery.

[0033] Figure 11 The UV / Vis / NIR spectra of the copolymer samples prepared in Examples 1, 2 and the comparative examples were measured in their original state, in the stretched discoloration state and after 2 hours of recovery.

[0034] Figure 12 The morphological images of the samples prepared in Examples 1 (1)(2)(3), Examples 2 (4)(5)(6), and Comparative Examples (7)(8)(9) are magnified 20 times under an optical microscope. Detailed Implementation

[0035] The specific embodiments of the present invention will be further described below. It should be noted that these descriptions are for the purpose of aiding understanding the present invention, but do not constitute a limitation thereof. Furthermore, the technical features involved in the various embodiments of the present invention described below can be combined with each other as long as they do not conflict with each other.

[0036] Unless otherwise specified, the experimental methods used in the following embodiments are conventional methods, and the experimental materials used in the following embodiments are all available through conventional commercial channels.

[0037] Example 1:

[0038] This embodiment provides a method for preparing a vinyl acetate-n-propyl vinyl ether copolymer film, as detailed below:

[0039] First, build such Figure 2 The reaction apparatus shown consists of a glass round-bottom four-necked flask as the main body. A polytetrafluoroethylene (PTFE) stirrer and stir plug are installed at the middle mouth of the flask, which is driven by a mechanical stirrer. The other three ports are the protective gas inlet, the feed / initiator injection port, and the serpentine condenser installation port, respectively. The gas inlet is connected to a gas flow meter and a valve to control the gas flow. The upper part of the serpentine condenser serves as the gas outlet, which is connected to a valve and a gas washing bottle containing deionized water.

[0040] Next, open all valves and circulate cooling water through the condenser. Then, mix 30g VAc, 10g PVE, and 10g methanol and pour the mixture into the flask, sealing the inlet with a rubber stopper. Start the mechanical stirrer at 250 rpm, simultaneously controlling the gas flow with a gas flow meter and blowing nitrogen into the solution at a flow rate of 20 mL / min. After 30 minutes, heat the reaction apparatus to 60°C using a preheated oil bath. Meanwhile, dissolve 0.4g AIBN in 10g methanol as the initiator solution. Inject 3mL of this solution into the reaction apparatus through the rubber stopper to initiate the polymerization process, then stop the gas flow and close all valves to ensure a seal. During the reaction, continuously inject the remaining initiator solution into the flask using a syringe pump and injection tubing for 6 hours. After the injection is complete, stop heating and allow the reaction product to cool naturally in an oil bath for 2 hours. Then, remove the solution from the reaction apparatus and pour it into a glass petri dish. Dry the solution in an oven at 60°C for 12 hours. Dissolve the resulting solid in methanol at a weight ratio of 25%, and then use an ultrasonic cleaner to disperse the solution by shaking for 30 minutes before use (the reaction process is as follows). Figure 1 (As shown).

[0041] Take a 9cm diameter glass petri dish, wash and dry it, and treat it in a plasma cleaner for 10 minutes. After removing it, drop 5mL of perfluorooctyltrimethoxysilane onto its surface to completely wet the surface. After standing for 2 hours, pour off the perfluorooctyltrimethoxysilane, wash the petri dish repeatedly with anhydrous ethanol, and dry it. Then, drop 20mL of the aforementioned 25% by weight solution onto the petri dish, dry it in an oven at 40°C for 12 hours, and then at 60°C for 1 hour. After cooling, carefully peel off the transparent and flat copolymer film from the petri dish; its thickness is approximately 420μm.

[0042] like Figure 3 As shown, by dropping a copolymer solution onto a highly hydrophobic glass culture dish and drying it in an oven at 60°C, the copolymer film can be easily peeled off the culture dish, forming a transparent, soft, and flat film.

[0043] Example 2:

[0044] This embodiment provides a method for preparing a vinyl acetate-n-propyl vinyl ether copolymer film, as detailed below:

[0045] First, build such Figure 2 The reaction apparatus shown consists of a glass round-bottom four-necked flask as the main body. A polytetrafluoroethylene stirring paddle and stirring plug are installed at the middle mouth of the flask, which is driven by a mechanical stirrer. The other three ports are the protective gas inlet, the feed / initiator injection port, and the condenser installation port, respectively. The gas inlet is connected to a gas flow meter and a valve to control the gas flow. The gas outlet above the condenser is connected to a valve and a gas washing bottle containing deionized water.

[0046] Next, open all valves and circulate cooling water through the condenser. Then, mix 34g VAc, 6g PVE, and 10g methanol and pour the mixture into the flask, sealing the inlet with a rubber stopper. Start the mechanical stirrer at 250 rpm, simultaneously controlling the gas flow with a gas flow meter and blowing nitrogen into the solution at a flow rate of 20 mL / min. After 30 minutes, heat the reaction apparatus to 60°C using a preheated oil bath. Meanwhile, dissolve 0.4g AIBN in 10g methanol as the initiator solution. Inject 3mL of this solution into the reaction apparatus through the rubber stopper to initiate the polymerization process, then stop the gas flow and close all valves to ensure a seal. During the reaction, continuously inject the remaining initiator solution into the flask using a syringe pump and injection tubing for 6 hours. After the injection is completed, stop heating and allow the reaction product to cool naturally in an oil bath for 2 hours. Then, remove the solution from the reaction apparatus and pour it into a glass petri dish. Dry it in an oven at 60°C for 12 hours. Dissolve the obtained solid in methanol at a weight ratio of 25%, and then use an ultrasonic cleaner to disperse the solution by shaking for 30 minutes before use.

[0047] Take a 9cm diameter glass petri dish, wash and dry it, and treat it in a plasma cleaner for 10 minutes. After removing it, add 5mL of perfluorooctyltrimethoxysilane to its surface to completely wet it. After standing for 2 hours, pour off the perfluorooctyltrimethoxysilane, wash the petri dish repeatedly with anhydrous ethanol, and dry it. Then, add 20mL of the newly prepared copolymer solution to the petri dish, dry it in an oven at 40℃ for 12 hours, and then at 60℃ for 1 hour. After cooling, carefully peel off the transparent and flat copolymer film from the petri dish. Its thickness is approximately 450μm.

[0048] Example 3:

[0049] This embodiment provides a method for preparing a vinyl acetate-n-propyl vinyl ether copolymer film that can be used in a mechanochromic smart window, as detailed below:

[0050] First, build such Figure 2 The reaction apparatus shown consists of a glass round-bottom four-necked flask as the main body. A polytetrafluoroethylene stirring paddle and stirring plug are installed at the middle mouth of the flask, which is driven by a mechanical stirrer. The other three ports are the protective gas inlet, the feed / initiator injection port, and the condenser installation port, respectively. The gas inlet is connected to a gas flow meter and a valve to control the gas flow. The gas outlet above the condenser is connected to a valve and a gas washing bottle containing deionized water.

[0051] Next, open all valves and circulate cooling water through the condenser. Then, mix 26g VAc, 14g PVE, and 10g methanol and pour the mixture into the flask, sealing the inlet with a rubber stopper. Start the mechanical stirrer at 250 rpm, simultaneously controlling the gas flow with a gas flow meter and blowing nitrogen into the solution at a flow rate of 20 mL / min. After 30 minutes, heat the reaction apparatus to 60°C using a preheated oil bath. Meanwhile, dissolve 0.2g AIBN in 10g methanol as the initiator solution. Inject 3mL of this solution into the reaction apparatus through the rubber stopper to initiate the polymerization process, then stop the gas flow and close all valves to ensure a seal. During the reaction, continuously inject the remaining initiator solution into the flask using a syringe pump and injection tubing for 6 hours. After the injection is completed, stop heating and allow the reaction product to cool naturally in an oil bath for 2 hours. Then, take out the solution from the reaction apparatus and drop 20 mL onto a clean glass petri dish with a diameter of 9 cm. Dry the petri dish at 60°C for 12 hours in an oven to obtain a transparent copolymer film with a thickness of about 420 μm that is attached to the surface of the petri dish.

[0052] Example 4:

[0053] This embodiment provides a method for preparing a vinyl acetate-n-propyl vinyl ether copolymer film, as detailed below:

[0054] First, build such Figure 2 The reaction apparatus shown consists of a glass round-bottom four-necked flask as the main body. A polytetrafluoroethylene stirring paddle and stirring plug are installed at the middle mouth of the flask, which is driven by a mechanical stirrer. The other three ports are the protective gas inlet, the feed / initiator injection port, and the condenser installation port, respectively. The gas inlet is connected to a gas flow meter and a valve to control the gas flow. The gas outlet above the condenser is connected to a valve and a gas washing bottle containing deionized water.

[0055] Next, open all valves and circulate cooling water through the condenser. Then, mix 30g VAc, 10g PVE, and 20g methanol and pour the mixture into the flask, sealing the inlet with a rubber stopper. Start the mechanical stirrer at 250 rpm, simultaneously controlling the gas flow with a gas flow meter and blowing nitrogen into the solution at a flow rate of 10 mL / min. After 30 minutes, heat the reaction apparatus to 60°C using a preheated oil bath. Meanwhile, dissolve 0.2g AIBN in 10g methanol as an initiator solution. Inject 3 mL of this solution into the reaction apparatus through the rubber stopper in one go to initiate the polymerization process. During the reaction, continuously inject the remaining initiator solution into the flask using a syringe pump and injection tubing for 6 hours, while maintaining a gas flow rate of 10 mL / min. After injection, stop heating and aeration, allowing the reaction product to cool naturally in the oil bath for 2 hours. The solution in the reaction apparatus was removed, and 5 mL was dropped onto a clean, circular glass slide with a diameter of 9 cm. The slide was then placed on a spin coater and spin-coated at 1000 rpm for 30 seconds. Finally, the glass slide was dried at 40°C for 2 hours to obtain a transparent copolymer film with a thickness of approximately 90 μm attached to the glass surface.

[0056] Comparative example:

[0057] This comparative example provides a method for preparing a vinyl acetate homopolymer film, as detailed below:

[0058] First, build such Figure 2 The reaction apparatus shown consists of a glass round-bottom four-necked flask as the main body. A polytetrafluoroethylene stirring paddle and stirring plug are installed at the middle mouth of the flask, which is driven by a mechanical stirrer. The other three ports are the protective gas inlet, the feed / initiator injection port, and the condenser installation port, respectively. The gas inlet is connected to a gas flow meter and a valve to control the gas flow. The gas outlet above the condenser is connected to a valve and a gas washing bottle containing deionized water.

[0059] Next, open all valves and circulate cooling water through the condenser. Then, mix 40g of VAc and 10g of methanol and pour the mixture into the flask, sealing the inlet with a rubber stopper. Start the mechanical stirrer at 250 rpm, simultaneously controlling the gas flow with a gas flow meter and blowing nitrogen into the solution at a flow rate of 20 mL / min. After 30 minutes, heat the reaction apparatus to 60°C using a preheated oil bath. Meanwhile, dissolve 0.4g of AIBN in 10g of methanol as the initiator solution. Inject 3mL of this solution into the reaction apparatus through the rubber stopper to initiate the polymerization process, then stop the gas flow and close all valves to ensure a seal. During the reaction, continuously inject the remaining initiator solution into the flask using a syringe pump and injection tubing for 6 hours. After the injection is completed, stop heating and allow the reaction product to cool naturally in an oil bath for 2 hours. Then add 30g of methanol to the solution in the flask to dilute it appropriately. Then remove it and pour it into a glass petri dish. Dry it in an oven at 60°C for 12 hours. Dissolve the obtained solid in methanol at a weight ratio of 25%. Use an ultrasonic cleaner to shake and disperse the resulting solution for 30 minutes before use.

[0060] Take a 9cm diameter glass petri dish, wash and dry it, and treat it in a plasma cleaner for 10 minutes. After removing it, add 5mL of perfluorooctyltrimethoxysilane to its surface to completely wet it. After standing for 2 hours, pour off the perfluorooctyltrimethoxysilane, wash the petri dish repeatedly with anhydrous ethanol, and dry it. Then, add 20mL of the newly prepared homopolymer solution to the petri dish, dry it in an oven at 40℃ for 12 hours, and then at 60℃ for 1 hour. After cooling, carefully peel off the transparent and flat homopolymer film from the petri dish. Its thickness is approximately 410μm.

[0061] Experimental Example: Characterization and Performance Testing of Different Thin Films

[0062] (1) From Figure 4 The proton NMR spectra show that the copolymer and homopolymer samples have significantly different spectra, which is consistent with theoretical predictions. The most distinctive characteristic peaks are those corresponding to H1 and H8. By integrating their intensities, the proportions of PVAc and PPVE segments in each sample can be determined. The samples in Example 1, Example 2, and the comparative example contain 87%, 93%, and 100% PVAc segments, respectively.

[0063] (2) From Figure 5 As can be seen from the infrared spectra, the samples of Example 1, Example 2, and the comparative example are very similar, with only 1100 cm⁻¹. -1 The intensity of the COC tensile vibration peak at the point increases slightly with the increase of the PPVE chain segment ratio.

[0064] (3) From Figure 6 The XRD diffraction patterns show that no sharp peaks appeared in the spectrum, indicating that the samples of Example 1, Example 2 and the comparative example did not show a highly oriented crystal structure at room temperature. The relative intensity at 2θ = 14° decreased significantly with the increase of the PPVE segment ratio, which may be related to the introduction of alkoxy groups further inhibiting the orientation of the copolymer molecules.

[0065] (4) From Figure 7 The thermogravimetric curves show that the thermogravimetric curves of the samples from Example 1, Example 2 and the comparative example are very similar, and the temperature at which obvious thermal decomposition occurs is around 320°C.

[0066] (5) Prior to the heating process shown in the DSC curves, the samples of Examples 1, 2, and the Comparative Example all underwent a heating-cooling cycle from room temperature to 250°C to -50°C to reduce the influence of thermal history; from Figure 8 The DSC heating curves show that no obvious melting peaks were found in the curves, but there were endothermic steps corresponding to the glass transition temperature, which further proves that there is a lack of highly oriented crystalline structure inside. As the proportion of PPVE segments decreases, the glass transition temperature of the copolymer shows a significant upward trend, which is very likely related to the enhanced molecular orientation caused by the reduction of alkoxy side groups.

[0067] (6) Figure 9 The typical tensile curves for samples from Examples 1, 2, and the comparative example are shown. The tensile speed was 50 mm / min. It can be seen that as the proportion of PPVE segments increases, the elongation at break increases significantly, while the tensile strength decreases significantly. This phenomenon is likely a result of both changes in polymer molecular weight and molecular arrangement. In addition, the yield behavior of the polymer also changed significantly. The tensile curve of the comparative example sample (100% PVAc) has a more obvious yield point, while the yield points of the two copolymers in Examples 1 and 2 become very indistinct. This indicates that the addition of alkoxy side groups transforms the mechanical behavior of the polymer towards a direction similar to that of an elastomer.

[0068] (7) Figure 10 The appearance changes of the copolymer sample of Example 2 during stretching and recovery are reflected from left to right as follows: original state, stretched to 350% strain, stretched to 450% strain, and appearance after stress is removed and it is fully recovered for 1 hour. The sample has high transparency before and after stretching, but appears white and the transparency is greatly reduced during stretching. Figure 11The results reflect the UV / Vis / NIR spectra of the copolymer samples in Examples 1, 2, and the Comparative Example in their original state, under stretched and discolored conditions (approximately 700% strain in Example 1, approximately 400% strain in Example 2, and approximately 300% strain in the Comparative Example), and after 2 hours of recovery. All three samples exhibit high transparency in their original state, particularly in the visible light band. During stretching, the samples in Examples 1 and 2 achieve higher opacity. After stress removal and recovery, the samples in Examples 1 and 2 recover their transparency, while the sample in the Comparative Example struggles to do so. Figure 12 The images show the morphological features of the samples from Examples 1, 2, and the comparative example under an optical microscope at 20x magnification. From left to right, the images show the original state, the morphology after stretching to complete discoloration, and the morphology after recovery. The stretching direction is always longitudinal. Micropores are present in the samples. During stretching, a large number of stripes along the stretching direction appear inside the samples. After stretching, the number of micropores inside the samples increases. It can be considered that the changes in light refraction and diffraction caused by the extension of micropores are the main mechanism of sample whitening. Only the comparative example sample showed cracks and silver streaks perpendicular to the stretching direction during stretching, and these cracks and silver streaks could not be eliminated after the stress was removed. This indicates that the presence and elimination of cracks and silver streaks are the decisive factors in the difference in transparency recovery between the samples in Examples 1, 2, and the comparative example.

[0069] Meanwhile, as can be seen from Table 1, the ratio of the two monomers in the raw materials not only affects the molecular structure of the product, but also the molecular weight of the product, and further affects its mechanical properties.

[0070] Table 1. Information on the tensile properties and molecular weight of the samples prepared in Examples 1, 2 and the comparative examples.

[0071]

[0072]

[0073] Note: Elongation at break and tensile strength are the average values ​​of 5 tests. Molecular weight information was obtained by DMF gel permeation chromatography, with PMMA as the calibrator.

[0074] Furthermore, the transparent copolymer films prepared in Examples 3 and 4 also exhibit similar tensile properties and mechanochromic effects as those in Examples 1 or 2. This demonstrates that by adjusting the raw material ratio, initiator dosage, and reaction parameters, transparent copolymer films / coatings with similar properties can also be obtained. This indicates that the method of the present invention can effectively control the molecular structure, molecular weight, and physical properties of the reaction products, and obtain a variety of copolymer products with different forms and mechanochromic effects.

[0075] The embodiments of the present invention have been described in detail above, but the present invention is not limited to the described embodiments. For those skilled in the art, various changes, modifications, substitutions, and variations can be made to these embodiments without departing from the principles and spirit of the present invention, and these variations still fall within the protection scope of the present invention.

Claims

1. Use of a vinyl acetate-n-propyl vinyl ether copolymer film in the preparation of a piezochromic smart window material, characterized in that, The preparation method of the vinyl acetate-n-propyl vinyl ether copolymer film comprises the following steps: dissolving vinyl acetate and n-propyl vinyl ether in methanol, then heating the mixed solution to 50-70 DEG C under oxygen-free condition, initiating the polymerization reaction by continuously adding methanol solution of azobisisobutyronitrile, and then preparing the reaction product into a film to obtain the vinyl acetate-n-propyl vinyl ether copolymer film; the mass ratio of the vinyl acetate and the n-propyl vinyl ether is 25:15-35:

5.

2. Use according to claim 1, characterized in that, When the vinyl acetate and the n-propyl vinyl ether are dissolved in the methanol, the mass of the methanol is 20%-50% of the sum of the mass of the vinyl acetate and the n-propyl vinyl ether.

3. Use according to claim 1, characterized in that, The mass of the azobisisobutyronitrile is 0.2%-1% of the sum of the mass of the vinyl acetate and the n-propyl vinyl ether.

4. Use according to claim 1, characterized in that, The amount of the methanol used for dissolving the azobisisobutyronitrile is the same as the amount of the methanol used for dissolving the vinyl acetate and the n-propyl vinyl ether.

5. The use according to claim 1, characterized in that, The oxygen-free condition is created by exhausting the air in the reaction environment by nitrogen.

6. Use according to claim 1, characterized in that, When the methanol solution of the azobisisobutyronitrile is continuously added, the first addition is completed according to the standard of adding 2-4 mL of the methanol solution of the azobisisobutyronitrile per 50-60 g of the mixed solution to start the polymerization process, and then the methanol solution of the azobisisobutyronitrile is continuously injected through a syringe within 3-10 h.

7. Use according to claim 1, characterized in that, The reaction product is prepared into a film in a glass culture dish, and the film forming process comprises drop coating, blade coating and spin coating.

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