Cholesteric liquid crystal polymer film responding to pH value, preparation method and application

Abstract

The invention discloses a cholesteric liquid crystal polymer film responding to a pH value, and a preparation method and application thereof, and belongs to the field of liquid crystal materials and chemical detection. The invention provides a recyclable pH value measuring film, a liquid crystal monomer, a non-polymerizable liquid crystal, a hydrogen bond molecule triggering agent, a chiral agent, a photoinitiator and a cross-linking agent are added into the film and treated by an organic solvent, and the film shows different film colors through high-sensitivity reaction to pH values, so that the purpose of testing different pH values is achieved. Different solutions contain different hydrogen ion concentrations, different film colors can be obtained, the pH value can be read out, after the test solution volatilizes, the film color disappears, and cyclic test is achieved. According to the invention, the problems of low pH value measurement sensitivity, environmental pollution and the like of the traditional pH value test paper are solved. And meanwhile, the device has the advantages of high response speed, recyclability, low cost, simple manufacturing process and the like.

Description

TECHNICAL FIELD

[0001] The present application belongs to the field of liquid crystal materials and chemical detection, and particularly relates to a cholesteric liquid crystal polymer film responsive to pH value, a preparation method and application. BACKGROUND

[0002] pH measurement has a wide range of applications in many fields. In environmental science, it is used to assess water quality, such as acid rain detection, ground water, industrial wastewater, and drinking water. In agriculture, monitoring the pH of the soil is related to crop yield. In aquaculture, the pH of the water is crucial for maintaining a suitable environment for aquatic organisms to grow. In the food industry, the pH of food needs to be measured and monitored throughout the processing to ensure food safety and quality. There are many other fields that often measure pH, and pH measurement is the basis of many experiments and analyses in scientific research. In medical care, pH measurement is used to detect the acidity and alkalinity of body fluids and tissues. In pharmaceutical engineering, pH needs to be measured at all times to ensure the growth environment of microorganisms. These applications highlight the importance and diversity of pH measurement. There are many ways and instruments for pH measurement on the market, the most common being pH test paper and pH meter.

[0003] pH test paper measurement is simple to use. After the test solution is dropped on the center of the test paper, the acid-base indicator is sensitive to the concentration of hydrogen ions, causing the molecular structure to change, affecting the absorption and reflection of light by the molecules, and then compared with the color chart to obtain the pH of the solution. During the measurement, it is necessary to ensure the cleanliness of the test paper. Whether it is caused by hand picking or improper storage, the dirtiness of the test paper will affect the measurement results. With the development of science and technology, the disadvantages of traditional pH testing methods are becoming more and more prominent. pH test paper is used once, and new test paper is needed for each test, which will increase the cost. At the same time, mass production of test paper and improper disposal of test paper containing solution after use are not conducive to environmental protection. In addition, different brands of pH test paper indicators may cause differences in measurement results. Although pH meter can achieve more accurate measurement and recycling, it requires certain energy consumption and has high manufacturing cost, and is not suitable for routine pH measurement. SUMMARY

[0004] To address the aforementioned problems, this invention proposes a pH-responsive cholesteric liquid crystal polymer film and a method for preparing a recyclable pH testing film. This cholesteric liquid crystal polymer film is obtained by functionalizing a liquid crystal cell in the organic solvent polyvinyl alcohol (PVA), forming a hydrogen-bonded polymer network through hydrogen bonding interactions using various liquid crystal monomer mixtures such as hydrogen-bonding molecular triggers, and adding non-polymerizable liquid crystal 5CB to increase response sensitivity. The film is then injected into the functionalized liquid crystal cell and oriented for curing. After the hydrogen bonds are washed away by an alkaline solution, cholesteric liquid crystal carboxylates are formed. Based on the characteristic of cholesteric liquid crystals reflecting circularly polarized light with a single rotation, changes in the solution pH cause the polymer network to expand or compress, thereby altering the pitch of the cholesteric liquid crystal and thus adjusting the reflective band of the cholesteric liquid crystal, resulting in a change in the color of the polymer film. Different pH solutions have different hydrogen ion concentrations, leading to different film colors, thus enabling pH measurement. The color change of the film reflects the variation of the polymer network at different pH values. After the measurement is completed, the membrane will return to its colorless state due to the evaporation of absorbed moisture; or if the next measurement needs to be performed immediately, it can be cleaned again with an alkaline solution of a specific concentration to return it to its colorless state, thus achieving recycling.

[0005] This patent is designed based on the optical properties of cholesteric liquid crystals. Experimental manipulation of the cholesteric liquid crystals yields a special polymer network, which is then used to measure pH values ​​through its rapid response to pH and the corresponding color change of the film. Compared to traditional pH test strips, this method is reusable, has a faster response, and offers higher measurement accuracy. This invention provides a more energy-efficient, environmentally friendly, and lower-cost pH testing method.

[0006] This invention provides a method for preparing a pH-responsive cholesteric phase liquid crystal polymer film, comprising:

[0007] Step 1: Spin-coat the pretreated glass substrate with PVA solution and dry it to obtain a functionalized glass substrate;

[0008] Step 2: Place the glass substrates together with the sides coated with PVA solution facing each other, and use PET film as spacers on both sides; apply solid UV adhesive to the PET film and cure it to form a liquid crystal cell;

[0009] Step 3: Dissolve the liquid crystal monomer, non-polymerizable liquid crystal, hydrogen bond molecule trigger, chiral agent, photoinitiator, and crosslinking agent in a solvent, ultrasonically vibrate and mix them evenly, then heat to obtain a uniformly mixed liquid crystal polymer solution;

[0010] Step 4: Use a capillary tube to draw the liquid crystal polymer solution into the edge of the liquid crystal cell. After the liquid crystal polymer solution slowly penetrates into the liquid crystal cell, cool the liquid crystal cell to a certain temperature and keep it for a period of time to make the liquid crystal mixture exhibit a focal conic state.

[0011] Step 5: Apply pressure to the liquid crystal cell to transform the focal conic state into a planar state, and then cure it to obtain a cholesteric liquid crystal polymer network film;

[0012] Step 6: The cholesteric liquid crystal polymer film is first soaked in a solvent to remove the non-polymerizable liquid crystal and then dried, followed by alkali treatment and drying to obtain a cholesteric liquid crystal polymer film that is sensitive to pH value.

[0013] Furthermore, the pretreatment of the glass substrate is as follows: nitrogen gas is used to blow away surface dust and impurities, followed by immersion in anhydrous ethanol and ultrasonic cleaning; the concentration of the PVA solution is 8-10 wt%; the drying temperature of the PVA solution is 70℃-80℃ and the time is 10-15 min; the PVA solution is evenly coated on the dried glass substrate and placed in a spin coater at 500 r / min for 2 min.

[0014] Furthermore, in step 2, the thickness of the PET film is 10–20 μm; the curing is performed by irradiation with a 365 nm UV lamp for 3–5 minutes.

[0015] Further, in step 3, the liquid crystal monomer is RM105; the crosslinking agent is C6M; the non-polymerizable liquid crystal is 5CB; the hydrogen bond triggering agent is two of 6OBA, POBA, and 4OBA; the chiral agent is R5011; the photoinitiator is Ig819; the solvent is an acrylic acid solution; the mass ratio of the liquid crystal monomer, hydrogen bond triggering agent, crosslinking agent, non-polymerizable liquid crystal, photoinitiator, and chiral agent is 16.8:23.6:6.8:32.1:0.81:1.44; the heating temperature is 60–80°C, and the time is 3–5 min.

[0016] Furthermore, in step 4, the temperature of the liquid crystal cell is reduced to 40°C for 25–30 minutes.

[0017] Furthermore, in step 5, the curing process involves irradiating the surface with a 365nm UV lamp for 3–5 minutes.

[0018] Furthermore, in step 6, the solvent for removing the non-polymerizable liquid crystal is acetone or tetrahydrofuran, and the soaking time is 10 min; the alkali treatment uses a 1-1.2 mol / L KOH solution.

[0019] The present invention also provides a pH-responsive cholesteric liquid crystal polymer thin film material, which is obtained by the preparation method as described in claims 1 to 7.

[0020] This invention also provides an application of a cholesteric liquid crystal polymer film material that responds to pH values. The cholesteric liquid crystal polymer film material is used for pH value testing. When the test solution is titrated onto the cholesteric liquid crystal polymer film material, different colors of film layers will appear when the test solution is tested at different pH values. The pH value of the test solution can be determined by color comparison.

[0021] Furthermore, by testing the expansion or compression of the internal network of the thin film due to different hydrogen ion concentrations in the solution, the pitch of the cholesteric liquid crystal filled in it changes, causing the reflective band to shift, thus exhibiting different film colors. The pH value of the test solution can be obtained by color comparison.

[0022] The beneficial effects of this invention are as follows:

[0023] This invention is based on the hydrogen ion concentration response of a cholesteric liquid crystal polymer network. The absorption of hydrogen ions by the broken hydrogen bonds in the thin-film polymer network causes the polymer network to expand or contract, changing its liquid crystal pitch and thus shifting the reflective band to reflect different film colors. The reflective band basically covers the visible light range, enabling measurements from strong acids to strong bases. After the test solution evaporates, the thin film can be recycled. It has advantages such as recyclability, high measurement accuracy, and low cost. Attached Figure Description

[0024] Figure 1 This is a flowchart illustrating the preparation process of the cholesteric phase liquid crystal polymer film of the present invention.

[0025] In the figure, (1) is the preparation of functionalized liquid crystal cell, (2) is the preparation of original cholesteric phase liquid crystal film, (3) is the liquid crystal injection state transition, (4) UV curing, (5) cleaning liquid crystal 5CB, and (6) KOH alkali treatment.

[0026] Figure 2 This is a schematic diagram of the pH cycling test of the cholesteric liquid crystal polymer film of the present invention;

[0027] The reference numerals in the figure are as follows: (a) colorless test film, (b) solution titration, (c) color development and comparative reading, (d) evaporation of titration solution, (e) repeated test, (f) color development and comparative reading, and (g) pH value color comparison card. Detailed Implementation

[0028] The present invention will now be further described with reference to the accompanying drawings.

[0029] This invention provides a recyclable pH measuring film, proposing a feasible scheme to measure pH by utilizing the different colors generated by the structural expansion and contraction of a cholesteric liquid crystal network due to different pH values, resulting in different reflective bands. It features recyclability, zero energy consumption, bright colors, fast response, and low cost.

[0030] A pH-responsive cholesteric liquid crystal polymer film is disclosed. Different hydrogen ion concentrations in the test solutions cause the internal network of the film to expand or compress, essentially altering the pitch of the cholesteric liquid crystals within the film. This results in a shift in the reflective bands, manifesting as different film colors. The pH value of the test solution can be obtained by color comparison, and the color automatically disappears after the solution evaporates, enabling cyclic testing.

[0031] A method for preparing a pH-responsive cholesteric phase liquid crystal polymer film, comprising:

[0032] First, a functionalized liquid crystal cell is prepared using glass slides. Surface dust and impurities are removed by blowing away with nitrogen gas, and the slides are thoroughly cleaned with anhydrous ethanol to remove surface contaminants. Then, an 8-10 wt% PVA (polyvinyl alcohol) solution is spin-coated onto the surface of the glass slide, and the slide is heated in an oven at 70-80°C for 10-15 minutes to complete the functionalization of the glass substrate. Two treated functionalized glass slides are used as substrates, and a 10-20 μm thick PET film is used as a spacer. Solid UV adhesive is then applied to the edges, and the slide is cured by irradiation with a 365nm UV lamp for 3-5 minutes to obtain the functionalized liquid crystal cell.

[0033] 2. A liquid crystal mixture is formed by selecting a monomeric liquid crystal, a non-polymerizable monomer, a crosslinking agent, a chiral dopant, and a photoinitiator in an adjustable ratio, and is mixed uniformly by ultrasonic vibration. The mixture is then heated to 60–80°C for 3–5 minutes to achieve isotropy. The mass ratio of the liquid crystal monomer, hydrogen bond molecule trigger, crosslinking agent, non-polymerizable liquid crystal, photoinitiator, and chiral agent is 16.8:23.6:6.8:32.1:0.81:1.44.

[0034] 3. Apply the mixture to the edge of the functionalized liquid crystal cell using a capillary tube. Allow the liquid crystal mixture to slowly penetrate into the liquid crystal cell. Then, cool the liquid crystal cell to 40°C and maintain the temperature for 25–30 minutes. The liquid crystal mixture will exhibit a focal conic state. Apply a certain pressure to convert the focal conic state to obtain a uniformly oriented cholesteric phase (planar state). Finally, perform UV curing. The curing time is 5–10 minutes.

[0035] 4. Peel the cured liquid crystal film off the liquid crystal cell, remove non-polymerizable liquid crystal impurities, activate the liquid crystal film by alkaline treatment, and then take it out and dry it. The preparation of the cholesteric phase polymer film that responds to pH value is completed. The alkaline treatment breaks hydrogen bonds to form carboxylate, which will respond to solutions with different pH values.

[0036] 5. Expand and fix the membrane into a strip of suitable size that is easy to store. Drop the solution to be tested onto the test membrane. Different membrane colors will appear for solutions with different pH values, thus determining the specific pH value. When the solution evaporates, the membrane color disappears, thereby enabling cyclic testing.

[0037] Example 1

[0038] A method for preparing a pH-responsive cholesteric phase liquid crystal polymer film, comprising:

[0039] 1. Cut the glass slides into 2cm × 4cm dimensions, blow away surface dust and impurities with nitrogen gas, and thoroughly clean them with anhydrous ethanol to remove surface contaminants. Subsequently, spin-coat a 10wt% PVA (polyvinyl alcohol) solution onto the surface of the glass slides and heat them in an 80℃ oven for 10 minutes to complete the functionalization of the glass substrate.

[0040] 2. Using two processed functionalized glass slides as substrates, assemble them with a 20μm thick PET film as spacers. Then, apply solid UV adhesive to the edges and cure them by irradiating with a 365nm UV lamp for 3 minutes. This yields a functionalized liquid crystal cell.

[0041] 3. The selected liquid crystal monomers RM105, 6OBA, POBA, C6M, 5CB, Ig819, and R5011 were mixed in a mass ratio of 16.8:16.8:6.8:6.8:32.1:0.81:1.44. The mixture was then subjected to ultrasonic vibration to ensure homogeneity, and finally heated to 80°C to achieve isotropy.

[0042] 4. After heating the mixture to 80°C to achieve isotropy, the liquid crystal mixture is drawn into the edge of the functionalized liquid crystal cell through a capillary tube. The liquid crystal mixture slowly penetrates into the liquid crystal cell. Then, the liquid crystal cell is cooled to 40°C and maintained for 30 minutes. The liquid crystal mixture exhibits a focal conic state.

[0043] 5. Apply 0.5N pressure to the liquid crystal cell to transform the focal conic phase into the cholesteric phase, irradiate it with a 365nm UV lamp for 3 minutes to cure it, and then use a scalpel to peel off the liquid crystal cell to obtain a cholesteric phase liquid crystal polymer network film.

[0044] VI. Preliminary Steps: Adding liquid crystal 5CB can broaden the reflection band and increase sensitivity. To remove the non-polymerizable liquid crystal 5CB, we immersed the cholesteric liquid crystal polymer network film in acetone solution for 10 min and dried it at room temperature for 20 min. We then washed it with a 1.2 mol / L KOH solution to break hydrogen bonds, forming carboxylates, which became colorless.

[0045] 7. Cut the colorless hydrogen-bonded cholesteric liquid crystal film into easily retractable strips to form a test film. Titrate the solution to be tested onto the film; solutions with different pH values ​​will show different colored film layers. By comparison, a definite pH value can be obtained. After the solution evaporates, the film returns to colorless, thus enabling cyclic testing.

[0046] In summary, this invention mixes the hydrogen-bonding molecular trigger 6OBA with POBA and liquid crystal monomers in a certain proportion, adds non-polymerizable liquid crystal 5CB to expand the porosity and increase the response range and sensitivity, and breaks hydrogen bonds through alkaline treatment to create a polymer network responsive to hydrogen ion concentration. Polymerizable cholesteric liquid crystals fill the network. Since the light reflection band of the cholesteric liquid crystal largely depends on the molecular pitch, changes in the pH of the test solution provide different concentrations of hydrogen ions. The absorption of different concentrations of hydrogen ions causes the polymer network to expand or compress, thereby changing the pitch of the cholesteric liquid crystal, which macroscopically manifests as a color change in the film. When the test solution is dropped onto the test film, the different concentrations of hydrogen ions it contains are absorbed. Due to the change in the pitch within the network, the liquid crystal reflection band shifts, and light of a specific color is reflected to the human eye. Different solutions with different pH values ​​result in different film colors. When the test solution evaporates, the film color disappears, thus enabling the recycling of the pH test film.

[0047] The above description is only a preferred embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any equivalent substitutions or modifications made by those skilled in the art within the scope of the technology disclosed in the present invention, based on the technical solution and inventive concept of the present invention, should be covered within the scope of protection of the present invention.

Claims

1. A method for producing a cholesteric liquid crystal polymer film responsive to pH, characterized by, The application relates to a preparation method of a pH value sensitive cholesteric liquid crystal polymer film. Step 1: spin coating a PVA solution on a pretreated glass substrate, drying to obtain a functionalized glass substrate; Step 2: clamping the glass substrate with the PVA solution coated side opposite, and using PET film as a spacer on the two side edges; applying solid UV glue on the PET film and curing to form a liquid crystal cell; Step 3: dissolving liquid crystal monomers, non-polymerizable liquid crystals, hydrogen bond molecule trigger, chiral agent, photoinitiator and crosslinking agent in a solvent, uniformly mixing through ultrasonic oscillation, and then heating to obtain a uniformly mixed liquid crystal polymer solution; Step 4: using a capillary to suck the liquid crystal polymer solution to contact the edge of the liquid crystal cell, slowly penetrating the liquid crystal polymer solution into the liquid crystal cell, lowering the liquid crystal cell to a certain temperature and keeping for a period of time, so that the liquid crystal mixture presents a focal conic state; Step 5: applying pressure to the liquid crystal cell to convert the focal conic state into a planar state, and curing to obtain a cholesteric liquid crystal polymer network film; Step 6: the cholesteric liquid crystal polymer film is first immersed in a solvent to remove non-polymerizable liquid crystals and then dried, and then treated with alkali and dried, to obtain a cholesteric liquid crystal polymer film sensitive to pH value.

2. The method for producing a pH-responsive cholesteric liquid crystalline polymer film according to claim 1, characterized by, The pretreatment of the glass substrate is: blowing off surface dust and impurities with nitrogen, immersing in anhydrous ethanol, and then ultrasonic cleaning; the PVA solution concentration is 8-10wt%; the PVA solution drying temperature is 70-80 DEG C, and the time is 10-15 min; uniformly applying the PVA solution on the dried glass substrate, and placing in a glue spreading machine to spread at 500 r / min for 2 min.

3. The method of claim 1, wherein the pH-responsive cholesteric liquid crystal polymer film is prepared by the steps of: (a) dissolving a cholesteric liquid crystal polymer in a solvent to form a solution; (b) coating the solution on a substrate to form a film; (c) drying the film; and (d) removing the solvent from the film. In step 2, the PET film thickness is 10-20 mu m; the curing is performed by using a 365 nm UV lamp for 3-5 min.

4. The method for preparing a pH-responsive cholesteric liquid crystal polymer film according to claim 1, characterized in that, In step 3, the liquid crystal monomer is RM105; the crosslinking agent is C6M; the non-polymerizable liquid crystal is 5CB; the hydrogen bond molecule trigger is two of 6OBA, POBA and 4OBA; the chiral agent is R5011; the photoinitiator is Ig819; the solvent is an acrylic acid solution; the mass ratio of the liquid crystal monomer, hydrogen bond molecule trigger, crosslinking agent, non-polymerizable liquid crystal, photoinitiator and chiral agent is 16.8:23.6:6.8:32.1:0.81:1.44; the heating temperature is 60-80 DEG C, and the time is 3-5 min.

5. The method for preparing a pH-responsive cholesteric phase liquid crystal polymer film according to claim 1, characterized in that, In step 4, the liquid crystal cell temperature is lowered to 40 DEG C for 25-30 min.

6. The method for preparing a pH-responsive cholesteric liquid crystal polymer film according to claim 1, characterized in that, In step 5, the curing is performed by using a 365 nm UV lamp for 3-5 min.

7. The method for preparing a pH-responsive cholesteric liquid crystal polymer film according to claim 1, characterized in that, In step 6, the solvent for removing the non-polymerizable liquid crystal is selected from acetone or tetrahydrofuran, and the immersion time is 10 min; the alkali treatment is performed by using a 1-1.2 moL / L KOH solution.

8. A pH-responsive cholesteric liquid crystal polymer film material, characterized by, The preparation method is obtained by using the preparation method in claims 1-7.

9. Use of a pH-responsive cholesteric liquid crystalline polymer film material according to claim 8, characterized in that The cholesteric liquid crystal polymer film material is applied to pH value testing; dropping a solution to be tested on the cholesteric liquid crystal polymer film material, and different pH value solutions to be tested can exhibit different color film layers, and the pH value of the solution to be tested can be determined through color comparison.

10. Use of a pH-responsive cholesteric liquid crystalline polymer film material according to claim 9, characterized in that By testing the different hydrogen ion concentration of the solution, the network inside the film is expanded or compressed, which makes the pitch of the cholesteric liquid crystal filled in it change, causing the reflection band to move, and then embodying different film layer colors. Through color contrast, the pH value of the test solution can be obtained.

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