A thermochromic liquid with online controllable phase change temperature and preparation method thereof

Abstract

The present invention discloses a thermochromic liquid with an online controllable phase change temperature and a preparation method thereof, and belongs to the technical field of thermochromic materials. The preparation steps of the thermochromic liquid with an online controllable phase change temperature include: mixing polyvinyl alcohol, sodium bromide, water, an acid catalyst and n-propionaldehyde, polycondensing at a reaction temperature, terminating the reaction, and obtaining a polyvinyl alcohol propionaldehyde solution; the polyvinyl alcohol propionaldehyde solution is the thermochromic liquid, and the phase change temperature is the reaction temperature. The present invention proposes for the first time that a liquid with a target phase change temperature is obtained by controlling the reaction temperature during the synthesis process, and the target phase change temperature is the set reaction temperature, so that a series of liquids with the required target phase change temperature can be simply obtained, and their phase change temperatures are "obvious".

Description

Technical Field

[0001] The invention belongs to the technical field of thermochromic materials, and in particular relates to a thermochromic liquid with an online controllable phase change temperature and a preparation method thereof. Background Art

[0002] Thermochromic materials have the property of thermochromic change, which usually involves a reversible change in transparency due to heating and cooling cycles. Thermochromic materials can generally be simply divided into inorganic phase change materials and organic thermochromic materials. Commonly used inorganic materials include vanadium dioxide, liquid crystals, ionic liquids, etc. Among them, vanadium dioxide is widely used. The essence of its phase change is that due to the change of internal crystal form, a large number of free electrons are generated in the vanadium dioxide material. For organic thermochromic materials, the organic materials selected are mainly thermosensitive hydrogels, such as poly (N-isopropylacrylamide), hydroxypropyl cellulose, hydroxypropyl methylcellulose, etc. Hydrogel is a polymer chain network containing a water-rich environment and a hydrophilic structure. Among them, poly (N-isopropylacrylamide) hydrogel has been widely reported by researchers and is widely used in different fields such as biomedicine and smart windows.

[0003] The reason why thermochromic phenomena occur in thermosensitive gel materials is that they have the characteristics of low critical solution temperature (LCST). When the temperature is lower than LCST, a large number of hydrogen bonds are formed between the polymer chains and water molecules, making the polymer uniformly dispersed in a transparent state. On the contrary, when the temperature is higher than LCST, the hydrogen bonds between the polymer and water molecules are broken, and the polymer dissolves from the solution and undergoes a phase change, causing the material to change from transparent to opaque (ACS Applied Materials & Interfaces 2023 15 (3), 4385-4397), so LCST is also called phase transition temperature. The LCST of thermosensitive materials that have been reported so far can be changed by adding inorganic salts to change the affinity between the polymer and the solvent, or by changing the ratio of hydrophilic and hydrophobic monomers in the early stage of synthesis to achieve different LCST effects. Summary of the invention

[0004] The purpose of the present invention is to provide a thermochromic liquid with an online controllable phase transition temperature and a preparation method thereof. The purpose is to provide a method for preparing a thermosensitive material with different phase transition temperatures by only changing the synthesis temperature. The prepared thermosensitive material is different from most organic thermosensitive materials. Its state is liquid. When applied to the field of smart windows, it is not limited by the shape of glass and has good fluidity and antifreeze properties. The preparation process provided by the present invention is simple and does not require complicated impurity removal and purification steps.

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

[0006] One of the technical solutions of the present invention is to provide a method for preparing a thermochromic liquid with an online controllable phase change temperature, comprising the following steps:

[0007] Polyvinyl alcohol (PVA), sodium bromide, water, an acid catalyst and n-propionaldehyde are mixed, polycondensed at a reaction temperature, and the reaction is terminated to obtain a polyvinyl alcohol propionaldehyde (PVP) solution; the polyvinyl alcohol propionaldehyde solution is a thermochromic liquid, and the phase change temperature is the reaction temperature.

[0008] The present invention can be carried out at different reaction temperatures as long as the conditions for the polycondensation reaction to occur are met. For example, when the reaction is carried out at 20°C, 30°C or 50°C, the solution will eventually become turbid. Different reaction temperatures only affect the rate at which the solution changes from clear to turbid. After the reaction is terminated, the solution is placed in an environment lower than the reaction temperature, and the solution changes from turbid to clear, that is, the LCST of the obtained polyvinyl alcohol propionaldehyde solution is 20°C, 30°C or 50°C.

[0009] The sodium bromide added in the present invention utilizes its salting-out effect to change the solubility of the polymer in water, so that the solution can change from clear to turbid. At the same time, the sodium bromide in the solution can also lower the freezing point of the solution, playing an antifreeze role.

[0010] Preferably, the polycondensation step of polyvinyl alcohol and n-propionaldehyde is: adjusting the aqueous solution containing polyvinyl alcohol and sodium bromide to the reaction temperature, then adding an acid catalyst and n-propionaldehyde while stirring, and polycondensing at the reaction temperature.

[0011] The thermochromic liquid can also be prepared by mixing the raw materials used in the polycondensation reaction of the present invention together, but the preferred order of adding the raw materials can ensure the uniformity of the polycondensation.

[0012] Preferably, the degree of polymerization of the polyvinyl alcohol is 300-500, and the degree of alcoholysis is 88%-99%.

[0013] Preferably, the dosage ratio of the polyvinyl alcohol to water is 1 g:100 mL; the mass ratio of the polyvinyl alcohol to the sodium bromide is 30-35:1.

[0014] Preferably, the mass ratio of the polyvinyl alcohol to the n-propionaldehyde is 100:15-25.

[0015] Preferably, the acid catalyst is hydrochloric acid, and the amount added is such that the concentration of HCl in the polycondensation reaction system is controlled to be 0.01-0.02 mol / L.

[0016] Preferably, the termination reaction is to adjust the pH value of the reaction system to 6.5-7.5.

[0017] More preferably, the substance used to adjust the pH value is 0.3-0.4 mol / L sodium hydroxide solution.

[0018] Preferably, the method further comprises a purification step of removing n-propanal after the termination reaction.

[0019] More preferably, the specific operation of the purification step includes: removing propionaldehyde by vacuum rotary evaporation of the reaction system after the reaction is terminated.

[0020] Since the vapor pressure of n-propionaldehyde is significantly higher than that of water, unreacted n-propionaldehyde can be removed by rotary evaporation. The parameters of the reduced pressure rotary evaporation can be set to a temperature of 20° C., a pressure of 10 KPa, and a time of 3 h.

[0021] The second technical solution of the present invention is to provide a thermochromic liquid with an online controllable phase change temperature prepared according to the above-mentioned method for preparing a thermochromic liquid with an online controllable phase change temperature.

[0022] The beneficial technical effects of the present invention are as follows:

[0023] The present invention proposes for the first time that during the synthesis process, a liquid with a target phase change temperature is obtained by controlling the reaction temperature, and the target phase change temperature is the set reaction temperature, so that a series of liquids with the desired target phase change temperatures can be simply obtained, and their phase change temperatures are "obvious".

[0024] The synthesis process proposed by the present invention is simple and environmentally friendly. Compared with the synthesis of other organic thermosensitive phase change materials, it does not require the use of a large amount of water to wash and purify the product, and no toxic gas is generated during the process. The propanaldehyde obtained after rotary evaporation can also be reused.

[0025] The thermochromic liquid with online controllable phase change temperature provided by the present invention has good fluidity and can be poured in a special tank, for example, in a laminated glass. It can be poured into various glass interlayers as required without being limited to the shape of the glass.

[0026] The thermosensitive phase change material obtained by the invention has good antifreeze property, and the added sodium bromide is retained in the solution, which can disrupt the hydrogen bonds in the water clusters and reduce the freezing point. BRIEF DESCRIPTION OF THE DRAWINGS

[0027] Figure 1 This is the process of acetalizing polyvinyl alcohol to generate polyvinyl propionaldehyde in an embodiment of the present invention.

[0028] Figure 2 The PVP liquid prepared in Examples 1-4 of the present invention 1 H NMR spectrum.

[0029] Figure 3 This is a graph showing the relationship between the reaction time and the change in light transmittance of the reaction system before terminating the reaction when preparing the PVP liquid in Examples 1-3 of the present invention.

[0030] Figure 4 This is a graph showing the relationship between the light transmittance of the PVP liquid prepared in Example 2 and the change in temperature.

[0031] Figure 5 This is a graph showing the relationship between the light transmittance of the PVP liquid prepared in Example 3 and temperature.

[0032] Figure 6 This is a graph showing the relationship between the light transmittance of the PVP liquid prepared in Example 4 and the change in temperature.

[0033] Figure 7 This is a graph showing the relationship between the heat and temperature of the PVP liquid prepared in Example 2 and Example 5. DETAILED DESCRIPTION

[0034] Now, various exemplary embodiments of the present invention are described in detail, and this detailed description should not be considered as a limitation of the present invention, but should be understood as a more detailed description of certain aspects, characteristics and embodiments of the present invention. It should be understood that the terms described in the present invention are only for describing specific embodiments and are not used to limit the present invention.

[0035] In addition, for the numerical range in the present invention, it is understood that each intermediate value between the upper and lower limits of the range is also specifically disclosed. The intermediate value in any stated value or stated range, and each smaller range between any other stated value or intermediate value in the range is also included in the present invention. The upper and lower limits of these smaller ranges can be independently included or excluded in the scope.

[0036] Unless otherwise specified, all technical and scientific terms used herein have the same meaning as commonly understood by those skilled in the art to which the invention pertains. Although only preferred methods and materials are described herein, any methods and materials similar or equivalent to those described herein may also be used in the practice or testing of the present invention.

[0037] The words “include,” “including,” “have,” “contain,” etc. used in this document are open-ended terms, meaning including but not limited to.

[0038] Examples 1-5

[0039] Preparation steps of PVP liquid:

[0040] (1) Take 1 g of PVA particles in a 250 mL round-bottom flask, add 100 mL of deionized water, heat and stir at 90 °C for 4 h to dissolve the PVA particles; cool the resulting PVA aqueous solution to room temperature (28 ± 2 °C), add sodium bromide particles, stir until the sodium bromide particles are completely dissolved, then perform high-temperature degassing at 70 °C for 1 h, and cool to room temperature (28 ± 2 °C) to obtain a PVA / sodium bromide solution;

[0041] (2) The temperature of the PVA / sodium bromide solution obtained in step (1) was controlled to the target temperature (i.e., the reaction temperature in Table 1), and 84 μL of 37% hydrochloric acid (acid catalyst, the concentration of hydrochloric acid in the reaction solution was controlled to be 0.01 mol / L) was added, while stirring at a speed of 250 r / min, and stirred for 30 min to obtain an activated PVA / sodium bromide solution, and then 0.245 mL of n-propionaldehyde was added, and stirring was continued at a speed of 250 r / min until the solution changed from clear to turbid, and then 4 mL of sodium hydroxide aqueous solution (prepared by dissolving 0.5 g of sodium hydroxide particles in 40 mL of deionized water) was added to adjust the pH value of the reaction system to a range of 6.5-7.5, and the acetalization reaction was stopped to obtain an unpurified PVP liquid;

[0042] (3) The unpurified PVP liquid was subjected to rotary evaporation at 20°C and 10 KPa for 3 h to purify the PVP liquid.

[0043] The amount of each raw material used and the reaction temperature of Examples 1-5 are shown in Table 1.

[0044] Table 1 Preparation parameters of Examples 1-5

[0045] Example PVA(g) Water (mL) n-Propionaldehyde(μL) Acid catalyst (μL) Sodium bromide(g) Reaction temperature (℃) Example 1 1 100 245 84 32 20 Example 2 1 100 245 84 32 32 Example 3 1 100 245 84 32 52 Example 4 1 100 490 84 32 32 Example 5 1 100 245 84 0 32

[0046] Note: The PVA model in Table 1 is 0399, where 03 represents 300 degree of polymerization and 99 represents 99% degree of alcoholysis.

[0047] The parameter performance of the PVP liquid prepared in each embodiment was measured

[0048] (1) Determination of acetalization rate

[0049] The PVP liquid was characterized using a Bruker 400 MHz NMR spectrometer to determine the purity and acetalization rate of the product; the process of acetalization of polyvinyl alcohol to form polyvinyl alcohol propionaldehyde is as follows Figure 1 As shown, the acetal rate , wherein x is the molar number of the hydroxyl-containing unit, and y is the molar number of the propionaldehyde-containing unit.

[0050] The specific operations are as follows:

[0051] First, the PVP liquid was precipitated in a large amount of a mixture of ethyl acetate and ethanol (volume ratio of 4:1) to obtain a flocculent precipitate of PVP. The precipitate was filtered out and dried in a 60°C forced air oven for 2 days to obtain a dry PVP product. 0.60 mL of -DMSO was used as solvent, 5 mg of dry PVP product was added, and the solution was dissolved at 60 °C. 1H NMR spectral analysis, the results of Examples 1-4 are as follows Figure 2 As shown (the curves from top to bottom are Example 4, Example 1, Example 2, and Example 3), 1 In the HNMR spectrum, the peaks at δ = 4.7 ppm and δ = 4.4 ppm represent hydrogen on the aldehyde group, the peak at δ = 3.7 ppm represents hydrogen on the methyl group on the polymer chain, and the peak at δ = 0.8 ppm represents methyl hydrogen. a is the peak area of ​​methyl hydrogen, S b The peak area of ​​the methine hydrogen on the main chain, ignoring the influence of the alcoholysis degree, the acetal yield is calculated according to the following method: .

[0052] according to Figure 2 It can be calculated from the displayed results that the acetalization rates of PVP in the PVP liquids prepared in Examples 1-4 are 20.3 mol%, 14.2 mol%, 12.1 mol% and 23.3 mol%, respectively.

[0053] (2) Changes in the transmittance of PVP liquid at different reaction temperatures with reaction time

[0054] The reaction system after adding propionaldehyde was tested by ultraviolet-visible (UV) spectrum using an ultraviolet-visible spectrophotometer (PerKinElmer Lambda 950). The spectral range was set to 280-780nm, and the cuvette was filled with deionized water as the spectrum scanning background. The wide spectrum bandwidth was 2nm, and the points were taken at intervals of 1nm. The changes in the transmittance of the reaction system during the reaction process at different reaction temperatures in Examples 1-3 were detected. The transmittance at 550 nm was used as the basis. The results are as follows: Figure 3 shown.

[0055] from Figure 3 It can be seen that with the increase of reaction temperature, the time for the transmittance of the reaction system to drop to 50% gradually increases.

[0056] (3) Changes in transmittance at 550nm

[0057] The poly PVP liquid prepared in Example 1-4 was tested for UV-visible light (UV) spectrum using a UV-visible spectrophotometer (PerKinElmer Lambda 950), the spectral range was set to 280-780nm, the cuvette was filled with deionized water as the spectral scanning background, the wide spectrum bandwidth was 2nm, the points were taken at intervals of 1nm, and the light transmission (Transmission, T%) was measured for every 1°C rise. The transmittance at 550 nm was used as the basis for judging the phase change. When the transmittance at 550 nm was less than 50%, it was considered that thermochromism occurred, and an optical curve of the relationship between temperature and transmittance was plotted.

[0058] Figure 4 This is a graph showing the relationship between the light transmittance of the PVP liquid prepared in Example 2 and the change in temperature.

[0059] Figure 4 It shows that the transmittance of the PVP liquid prepared in Example 2 drops below 50% at around 32°C, so the phase transition temperature of Example 2 is 32°C.

[0060] Figure 5 This is a graph showing the relationship between the light transmittance of the PVP liquid prepared in Example 3 and temperature.

[0061] Figure 5 It shows that the transmittance of the PVP liquid prepared in Example 3 drops below 50% at about 52°C, so the phase transition temperature of Example 3 is 52°C.

[0062] Figure 6 This is a graph showing the relationship between the light transmittance of the PVP liquid prepared in Example 4 and the change in temperature.

[0063] Figure 6 It shows that the transmittance of the PVP liquid prepared in Example 4 drops below 50% at around 32°C, so the phase transition temperature of Example 4 is 32°C.

[0064] (5) Antifreeze test

[0065] The PVP liquids prepared in Example 2 and Example 5 were tested using a differential scanning calorimeter (METTLER TOLEDO DSC 3). The test method was as follows: stabilize at -40°C for 10 min, then heat from -40°C to 15°C at a heating rate of 5 K / min, stabilize for 10 min, and then cool from 15°C to -40°C at a cooling rate of 5 K / min, and plot a graph of the relationship between heat and temperature.

[0066] Figure 7 This is a relationship diagram between the heat and temperature of the PVP liquid prepared in Examples 2 and 5 of the present invention.

[0067] Figure 7 It shows that the crystallization peak and melting peak of the PVP liquid with the addition of sodium bromide obviously move to a lower temperature, indicating that sodium bromide makes the PVP liquid have good antifreeze properties.

[0068] The embodiments described above are only descriptions of the preferred modes of the present invention, and are not intended to limit the scope of the present invention. Without departing from the design spirit of the present invention, various modifications and improvements made to the technical solutions of the present invention by ordinary technicians in this field should all fall within the protection scope determined by the claims of the present invention.

Claims

1. A method for preparing a thermochromic liquid with an online controllable phase transition temperature, characterized in that: The following steps are involved: Mixing polyvinyl alcohol, sodium bromide, water, an acid catalyst and n-propionaldehyde, polycondensing at a reaction temperature until the solution changes from clear to turbid, terminating the reaction, and obtaining a polyvinyl alcohol propionaldehyde solution; the polyvinyl alcohol propionaldehyde solution is a thermochromic liquid, and the phase change temperature is the reaction temperature; The polycondensation step of polyvinyl alcohol and n-propionaldehyde is as follows: adjusting the aqueous solution containing polyvinyl alcohol and sodium bromide to the reaction temperature, then adding an acid catalyst and n-propionaldehyde while stirring, and polycondensing at the reaction temperature; The degree of polymerization of the polyvinyl alcohol is 300-500, and the degree of alcoholysis is 88%-99%; The dosage ratio of the polyvinyl alcohol to water is 1 g:100 mL; the mass ratio of the polyvinyl alcohol to the sodium bromide is 1:32; The mass ratio of the polyvinyl alcohol to the n-propionaldehyde is 100:15-25; The acid catalyst is hydrochloric acid, and the amount added is to control the HCl concentration in the polycondensation reaction system to be 0.01 mol / L.

2. The method for preparing a thermochromic liquid with an online controllable phase transition temperature according to claim 1, characterized in that: The termination reaction is to adjust the pH value of the reaction system to 6.5-7.

5.

3. The method for preparing a thermochromic liquid with an online controllable phase transition temperature according to claim 2, characterized in that: The substance used to adjust the pH value is 0.3~0.4mol / L sodium hydroxide solution.

4. The method for preparing a thermochromic liquid with an online controllable phase transition temperature according to claim 1, characterized in that: After the reaction is terminated, a purification step for removing n-propanal is also included.

5. A thermochromic liquid with an online controllable phase transition temperature prepared according to the method for preparing a thermochromic liquid with an online controllable phase transition temperature according to any one of claims 1 to 4.

Images