A method for preparing a simple, reversible thermochromic cholesteric liquid crystal film

Abstract

The present application relates to the field of reversible temperature-sensitive ink, and discloses a method for preparing a simple, reversible temperature-sensitive color-changing spherical cholesteric liquid crystal film, comprising the following steps: using nematic liquid crystal and chiral agent liquid crystal as a liquid crystal mixture, dissolving polylactic acid and the liquid crystal mixture through a benign solution of both to form a homogeneous solution; under 10-40 DEG C, using a spin coating rotation speed from 50-100 rpm to gradiently increase to 7000-8000 rpm to spin coat the homogeneous solution of step S1 on a substrate; after spin coating is completed, adopting a step-by-step heating method to dry, and after natural cooling, a reversible temperature-sensitive color-changing film is obtained. The present application uses homogeneous solution phase separation technology, adopts a suitable process, realizes that the phase separation cholesteric liquid crystal can not only change color, but also avoids liquid crystal encapsulation failure, and avoids the influence of surfactants, shear force and core wall ratio. The reversible temperature-sensitive color-changing film has the characteristics of high color-changing sensitivity, fast response and good stability.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of reversible temperature-sensitive ink, in particular to a method for preparing a simple reversible temperature-sensitive color-changing spherical cholesteric liquid crystal color-changing film. BACKGROUND

[0002] There are two common methods for preparing liquid crystal reversible temperature-sensitive materials at present:

[0003] Emulsion method. Through shear force, a structure similar to "O / W" is formed between the liquid crystal and the hydrophilic polymer, that is, the structure in which the water phase wraps the oil phase. At this time, the system is in a two-phase coexistence state, which is a metastable structure in thermodynamics. At this time, the liquid crystal exists in the form of droplets in the polymer, and if necessary, a surfactant needs to be added to maintain the normal form of the droplets. The commonly used polymer is polyvinyl alcohol. The surfactant has an influence on the orientation of the liquid crystal, which affects the color change and the color change interval. At the same time, the action time of the shear force needs to be strictly controlled: if the action time is too short, the droplets will be too large, the stability will be poor, and the emulsion will be easily broken. If the action time is too long, the droplets will be too small, and the light scattering effect will be strong. Secondly, the range of hydrophilic polymer systems is limited, and it is difficult to effectively broaden the range of available polymers.

[0004] Liquid crystal capsule method. First, the cholesteric liquid crystal needs to be oriented in a specific polymer aqueous solution, and then the liquid crystal droplets dispersed in the polymer are polymerized to form a capsule wall through emulsion polymerization. After that, multiple water washing is carried out to remove other polymer impurities. Then, the polymer matrix is blended, and the liquid crystal capsules are uniformly dispersed through shear force. This method is complex and has a complicated process. The corresponding disadvantages are: during polymerization, the amount of liquid crystal and polymer wall needs to be matched one by one, otherwise the core-wall ratio will be too high or too low, which will affect the light scattering and thus affect the color display; the blending of the polymer matrix changes the viscosity of the system, which makes the dispersion of the liquid crystal capsules uneven, and thus affects the color display. SUMMARY

[0005] In order to solve the problem of complex preparation process of traditional temperature-sensitive color-changing film, the present application provides a method for preparing a simple reversible temperature-sensitive color-changing spherical cholesteric liquid crystal film. Through homogeneous solution phase separation technology, the liquid crystal is uniformly dispersed in the polymer, and the spherical cholesteric liquid crystal with uniform particle size is formed in the polymer, so that the color can be changed reversibly within a certain temperature range.

[0006] To achieve the above purpose, the present application provides a method for preparing a simple reversible temperature-sensitive color-changing spherical cholesteric liquid crystal film, which comprises the following steps:

[0007] S1, dissolving polylactic acid and liquid crystal mixture by good solution of both to form transparent homogeneous solution with nematic liquid crystal and chiral liquid crystal as liquid crystal mixture;

[0008] S2, spin coating homogeneous solution of step S1 on substrate with spin coating speed gradient increasing from 50-200 rpm to 7000-8000 rpm, keeping 30-90 s for each spin coating gradient, and spin coating thickness of each spin coating gradient is 5-25 μm, wherein the substrate can be glass or plastic sheet, and the liquid crystal mixture forms spherical cholesteric phase liquid crystal after coating, with uniform particle size;

[0009] S3, after spin coating, gradually increasing the temperature of the spin-coated homogeneous solution to completely evaporate the solvent, and naturally cooling to obtain reversible temperature-sensitive color-changing film.

[0010] As a further preferred technical solution of the present application, in step S1, polylactic acid accounts for 70-99% of the total mass of polylactic acid and liquid crystal mixture, and liquid crystal mixture accounts for 1-30% of the total mass of polylactic acid and liquid crystal mixture; nematic liquid crystal accounts for 50-99% of the total mass of liquid crystal mixture, and chiral agent accounts for 1-50% of the total mass of liquid crystal mixture.

[0011] As a further preferred technical solution of the present application, in step S1, the good solution is one or a mixture of two or more of methanol, ethanol, isopropanol, acetone, dimethyl sulfoxide, dichloromethane, chloroform, acetonitrile, tetrahydrofuran, ethyl acetate, n-hexane, toluene, xylene, n-butanol, acetic acid, and pyridine.

[0012] As a further preferred technical solution of the present application, the nematic liquid crystal is at least one of 4-cyano-4'-nonyloxy biphenyl, 4-cyano-4'-hexyloxy biphenyl, 4-cyano-4'-heptyloxy biphenyl, 4-cyano-4'-octyloxy biphenyl, 4-(4-cyanophenoxy)-4'-octyl biphenyl, and 4-cyano-4'-dodecyloxy biphenyl; and the chiral liquid crystal is at least one of R5011, S5011, R811, and S811.

[0013] As a further preferred technical solution of the present invention, in step S2, the spin coating speed is increased from 50-100 rpm to 7000-8000 rpm in at least three gradients. More preferably, the spin coating speed is increased in five stages, specifically: the first stage spin coating speed is 50-200 rpm, and the holding time is 50-60 s; the second stage spin coating speed is 200-500 rpm, and the holding time is 50-60 s; the third stage spin coating speed is 500-1000 rpm, and the holding time is 50-60 s; the fourth stage speed is 1500-3000 rpm, and the holding time is 50-60 s; the fifth stage is 7000 rpm, and the holding time is 50-60 s.

[0014] As a further preferred technical solution of the present invention, in step S1, a homogeneous solution is formed by adding a stir bar and stirring at a speed of 500-1500 rpm for 1-6 hours; or by shaking on a shaker for 2-3 hours; or by using an ultrasonic cleaner at 40 kHz for 5 seconds to 1 hour.

[0015] As a further preferred technical solution of the present invention, in step S3, the drying can be carried out by one of ultraviolet drying, infrared drying, hot air heating drying, and hot table heating drying.

[0016] As a further preferred technical solution of the present invention, in step S3, drying is carried out by increasing the temperature from room temperature at a rate of 5-10°C every 5 minutes, and maintaining the temperature for 1-5 minutes after each 5-10°C increase, with the maximum temperature not exceeding 200°C.

[0017] According to another aspect of the present invention, the present invention also provides a reversible thermochromic thin film prepared by the above method.

[0018] This invention employs homogeneous solution phase separation technology and a phase-adaptive process to achieve phase separation of cholesteric liquid crystals, which not only change color but also form spherical cholesteric liquid crystals with uniform particle size. This avoids liquid crystal encapsulation failure and the influence of surfactants, shear force, and core-to-wall ratio.

[0019] This invention utilizes homogeneous solution phase separation technology to prepare reversible, thermochromic spherical cholesteric liquid crystal films, which feature high color-changing sensitivity, fast response, and good stability.

[0020] Compared to traditional techniques, the method of this invention is simpler to operate and can efficiently prepare reversible thermochromic spherical cholesteric liquid crystal materials. This reversible thermochromic film can change color according to temperature changes and can be applied to color-changing labels, anti-counterfeiting encryption, smart materials, and other fields. Attached Figure Description

[0021] The present invention will now be described in further detail with reference to the accompanying drawings and specific embodiments.

[0022] Figure 1 This is a schematic diagram illustrating the preparation of reversible thermochromic ink.

[0023] Figure 2 The absorption wavelength is the color-changing temperature of the thin film sample prepared in Example 1.

[0024] Figure 3 The images shown are from an observation of the film sample under a POM (polarizing microscope) during the drying process of Example 1: the left image was taken when the drying temperature was maintained at 80 °C; the right image was taken when the film was cooled to room temperature (20 °C).

[0025] Figure 4 The particle size distribution of the thin film sample prepared in Example 1 was observed under a POM (polarizing microscope) after a three-day interval (20°C).

[0026] The objectives, features, and advantages of this invention will be further explained in conjunction with the embodiments and with reference to the accompanying drawings. Detailed Implementation

[0027] The specific embodiments of the present invention will be described in detail below with reference to the accompanying drawings. It should be understood that the specific embodiments described herein are for illustration and explanation only and are not intended to limit the present invention.

[0028] Unless otherwise defined, the technical terms used in the following embodiments have the same meanings as commonly understood by those skilled in the art to which this invention pertains. Unless otherwise specified, the experimental reagents used in the following embodiments are conventional biochemical reagents; and the experimental methods described are conventional methods.

[0029] Example 1

[0030] like Figure 1 As shown in this embodiment, the method for preparing the reversible thermochromic spherical cholesteric liquid crystal thin film specifically includes the following steps:

[0031] S1. Prepare a homogeneous solution for liquid crystal:

[0032] Polylactic acid (PLA) was used as the polymer base material, and R5011 and 8CB were used as a liquid crystal mixture. 47 mg of PLA, 1.05 mg of R5011 and 1.95 mg of 8CB were dissolved in 1 ml of chloroform. The dissolution process was carried out by stirring with a stir bar (stirring speed of 1000 rpm for 6 h). During this process, the multiple components were uniformly dispersed in the solvent to form a homogeneous solution, which is a uniform and transparent liquid.

[0033] S2, gradient spin coating

[0034] The transparent liquid is coated on the glass substrate by dynamic spin coating, in which: the first stage of spin coating speed is 200 rpm, time is 60 s, the second stage of spin coating speed is 500 rpm, time is 60 s, the third stage of spin coating speed is 1000 rpm, time is 60 s, the fourth stage speed is 3000 rpm, time is 60 s, the fifth stage is 7000 rpm, time is 60 s, the amount of each spin coating is 250 μL, the total thickness of coating is 80 μm. This process, using gradient spin coating, provides favorable conditions for the stable separation of two phases of polymer and liquid crystal mixture and the self-assembly of liquid crystal polymer into cholesteric liquid crystal.

[0035] S3, drying into a film

[0036] The transparent liquid coated on the glass substrate is dried by hot stage heating, starting from 20 ℃ to 100 ℃ at 10 ℃ / 5 min, in which every 10 ℃ is raised and kept for 5 min, and then naturally cooled, to obtain a reversible thermochromic film. By gradient heating to control the evaporation rate, the stable separation of two phases of polymer and liquid crystal mixture is realized, and the liquid crystal mixture is successfully self-assembled into cholesteric liquid crystal. Only the formation of cholesteric liquid crystal can have temperature response color change, and by Figure 3 As shown in FIG. 6, it can be confirmed that there is a phase separation process and spherical cholesteric liquid crystal is formed. Figure 4 As shown in FIG. 7, the number of spherical cholesteric liquid crystals in each particle size interval is observed and counted under POM, and it can be seen that the particle size mainly distributes between 80-130 um, with ~109 um being the most, indicating that the particle size distribution is uniform, no agglomeration, and the size conforms to the normal distribution.

[0037] It is tested that the reversible thermochromic film sample prepared in Example 1 can realize reversible color change of 27-37 ℃, and show no color when the temperature is lower than 27 ℃ or higher than 37 ℃. Figure 2 For the heating process, the absorption wavelength of the film is measured at three temperature points (27 ℃, 29 ℃, 36 ℃) in the color change interval, which corresponds to the color visible to the naked eye at the temperature.

[0038] Example 2:

[0039] The preparation method is basically the same as that in Example 1, except that the rotation speed of the five spin-coating gradients in step 2 is different, and the holding time of each stage is different, which is specifically as follows: the spin-coating speed of the first stage is 100 rpm, the time is 50 s, the spin-coating speed of the second stage is 200 rpm, the time is 50 s, the spin-coating speed of the third stage is 500 rpm, the time is 50 s, the spin-coating speed of the fourth stage is 15000 rpm, the time is 50 s, the spin-coating speed of the fifth stage is 7000 rpm, the time is 50 s, the amount of each spin-coating is 250 μL, and the total thickness of coating is 80 μm.

[0040] It is tested that the response performance of the reversible thermochromic film sample prepared in Example 2 is equivalent to that in Example 1, which can realize reversible color change of 27 ℃-37 ℃, and shows no color when the temperature is lower than 27 ℃ or higher than 37 ℃.

[0041] Comparative Example 1:

[0042] As a comparative experiment of Example 1, the preparation method is basically the same as that in Example 1, except that the stirring parameters in the dissolution process of step S1 are different, that is, the stirring speed is 1000 rpm, and the time is 0.5 h, to form a transparent homogeneous solution.

[0043] The POM (polarizing microscope) is used to observe whether there is liquid crystal in the finally formed film, and the size, number and morphology of the liquid crystal. Although there is liquid crystal under the observation of POM in Comparative Example 1, the size of the liquid crystal is too small to form droplets (spheres), the color change is uneven, and the particle size is not uniform. By comparison with Example 1, it is proved that the suitable stirring speed and time for configuring the liquid crystal to form a homogeneous solution is one of the keys to form a reversible color-changing, spherical cholesteric liquid crystal film.

[0044] Comparative Example 2:

[0045] As a comparative experiment of Example 1, the preparation method is basically the same as that in Example 1, except that the spin-coating process is used in step S2, and the spin-coating thickness is fixed at 80 μm.

[0046] The film formed in Comparative Example 2 is light blue, and has no temperature response color change. It is observed under POM that there is liquid crystal, but only a small part of the liquid crystal forms cholesteric liquid crystal, and the naked eye cannot distinguish the color change, so there is no temperature response color change. By comparison with Example 1, it is proved that the gradient coating process in the present application is one of the keys to form a reversible color-changing, spherical cholesteric liquid crystal film.

[0047] Comparative Example 3:

[0048] As a comparative experiment of Example 1, the preparation method is basically the same as that of Example 1, the only difference is that the drying temperature in step S2 is fixed at 50℃, and there is no gradient change in temperature.

[0049] In Comparative Example 3, the drying temperature is fixed, which causes the polymer to fail to wrap the liquid crystal, so that the liquid crystal floats on the surface. By observing under POM, when focusing on the surface of the film, the liquid crystal appears in clusters, and after wiping with a paper towel, the film surface is observed under POM, and there is no liquid crystal; when focusing on the inside of the film, only a small amount of liquid crystal exists. By comparison with Example 1, it is proved that the step-by-step temperature rising drying of the solvent in the present application is one of the keys to form the reversible color-changing, spherical cholesteric liquid crystal film.

[0050] Comparative Example 4:

[0051] As a comparative experiment of Example 1, the preparation method is basically the same as that of Example 1, the only difference is that in step S2, the fourth and fifth stages of spin coating are missing, and the total coating thickness is 80 μm by adjusting the spin coating amount of the first, second and third stages.

[0052] The film formed in Comparative Example 4 is not uniform on the surface and cannot uniformly cover the entire substrate. Because the liquid crystal is not uniformly distributed, some areas of the film have temperature-responsive color change, while some areas do not have temperature-responsive color change.

[0053] Comparative Example 5:

[0054] As a comparative experiment of Example 1, the preparation method is basically the same as that of Example 1, the only difference is that in step S2, the spin coating speed is gradually increased from the initial speed of 200 rpm to 7000 rpm at a fixed acceleration, the total time is 300 s, and the total coating thickness is 80 μm.

[0055] The spin coating speed of Comparative Example 5 has no step change, which causes the film surface to be whitish, a large number of bubbles exist under POM observation, and the color change cannot be distinguished by the naked eye, so there is no temperature-responsive color change.

[0056] Although the specific embodiments of the present application are described above, those skilled in the art should understand that these are only examples, and various changes or modifications can be made to the present embodiments without departing from the principles and essence of the present application, and the protection scope of the present application is only limited by the appended claims.

Claims

1. A method for preparing simple, reversibly thermochromic spherical cholesteric liquid crystal thin films, characterized in that, Includes the following steps: S1. Using nematic liquid crystal and chiral liquid crystal as a liquid crystal mixture, polylactic acid and the liquid crystal mixture are dissolved in a good solution of both to form a transparent homogeneous solution. S2. Spin-coat the homogeneous solution from step S1 onto the substrate by increasing the spin-coating speed from 50-200 rpm to 7000-8000 rpm, wherein each spin-coating gradient is maintained for 30-90 s and the spin-coating thickness of each spin-coating gradient is 5-25 μm. S3. After spin coating, the homogeneous solution spin coated in step S2 is dried by gradually increasing the temperature and then naturally cooled to obtain a reversible thermochromic film. In step S1, polylactic acid accounts for 70-99% of the total mass of the polylactic acid and liquid crystal mixture, and the liquid crystal mixture accounts for 1-30% of the total mass of the polylactic acid and liquid crystal mixture; the nematic liquid crystal accounts for 50-99% of the total mass of the liquid crystal mixture, and the chiral agent accounts for 1-50% of the total mass of the liquid crystal mixture. In step S1, a homogeneous solution is formed by adding a stir bar and stirring at 500-1500 rpm for 1-6 hours; or by shaking on a shaker for 2-3 hours; or by using an ultrasonic cleaner at 40 kHz for 5 seconds to 1 hour. In step S2, the spin coating speed is increased from 50-100 rpm to 7000-8000 rpm in at least three gradients.

2. The method for preparing simple, reversibly thermochromic spherical cholesteric liquid crystal thin films according to claim 1, characterized in that, In step S1, the benign solution is one or a mixture of two or more of the following: methanol, ethanol, isopropanol, acetone, dimethyl sulfoxide, dichloromethane, chloroform, acetonitrile, tetrahydrofuran, ethyl acetate, n-hexane, toluene, xylene, n-butanol, acetic acid, and pyridine.

3. The method for preparing simple, reversibly thermochromic spherical cholesteric liquid crystal thin films according to claim 1, characterized in that, The nematic liquid crystal is at least one of 4-cyano-4'-nonoxybiphenyl, 4-cyano-4'-hexyloxybiphenyl, 4-cyano-4'-heptyloxybiphenyl, 4-cyano-4'-octyloxybiphenyl, 4-(4-cyanophenoxy)-4'-octylbiphenyl, and 4-cyano-4'-dodecyloxybiphenyl; the chiral liquid crystal is at least one of R5011, S5011, R811, and S811.

4. The method for preparing simple, reversibly thermochromic spherical cholesteric liquid crystal thin films according to claim 1, characterized in that, In step S2, the spin coating speed increases in five stages with a gradual increase, as follows: The first stage spin coating speed is 50-200 rpm, and the holding time is 50-60 s; the second stage spin coating speed is 200-500 rpm, and the holding time is 50-60 s; the third stage spin coating speed is 500-1000 rpm, and the holding time is 50-60 s; the fourth stage speed is 1500-3000 rpm, and the holding time is 50-60 s; the fifth stage is 7000 rpm, and the holding time is 50-60 s.

5. The method for preparing simple, reversibly thermochromic spherical cholesteric liquid crystal thin films according to claim 1, characterized in that, In step S3, the drying process can be carried out using one of the following methods: ultraviolet drying, infrared drying, hot air drying, or hot table drying.

6. The method for preparing simple, reversibly thermochromic spherical cholesteric liquid crystal thin films according to claim 1, characterized in that, In step S3, drying is carried out by increasing the temperature from room temperature by 5-10 °C every 5 minutes, and holding the temperature at each 5-10 °C increase for 1-5 minutes.

7. The spherical cholesteric liquid crystal film prepared by the method according to any one of claims 1-6.

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