Cholesteric liquid crystal elastomer, cholesteric liquid crystal elastomer film containing the same, and method for producing the same

Abstract

To provide a cholesteric liquid crystal elastomer that exhibits mechanochromic properties when uniaxially and biaxially stretched perpendicularly or horizontally to the printing direction, and a method for producing the cholesteric liquid crystal elastomer.SOLUTION: The method includes: a first step of mixing a liquid crystal monomer, a chain extender, a photoinitiator and a chiral dopant to produce a cholesteric liquid crystal mixture; a second step of oligomerizing the cholesteric liquid crystal mixture to produce a cholesteric liquid crystal ink; a third step of extruding the cholesteric liquid crystal ink through a 3D printer and printing it onto a substrate by a direct ink writing method; a fourth step of annealing the printed cholesteric liquid crystal ink; and a fifth step of UV-curing the annealed cholesteric liquid crystal ink to produce a cholesteric liquid crystal elastomer.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present invention relates to a cholesteric liquid crystal elastomer, and more particularly to a cholesteric liquid crystal elastomer having mechanochromic properties. [Background technology]

[0002] In recent years, mechanochromic materials, which change nanoscale structural color in response to mechanical stress-induced events such as tension, compression, and bending, have attracted considerable attention.

[0003] Mechanochromic materials based on structural color can be designed to exhibit diverse optical responses, ranging from reversible changes in color or fluorescence to more complex changes in refractive index or light scattering.

[0004] Such mechanochromism is a direct result of deformation-induced changes in the periodic lattice constant of structurally colored materials. Such structurally colored mechanochromic materials can be classified into three main groups according to their nanoscale architecture: photonic crystals, photonic glasses, and liquid crystals (LCs).

[0005] In particular, liquid crystals (LCs), including cholesteric, vaulted nematic, and blue phase liquid crystals, have shown considerable potential as structurally colored mechanochromic materials due to their unique superstructures and anisotropic properties.

[0006] Cholesteric liquid crystal elastomers (CLCEs) are unique anisotropic rubbers that can change their structural color in response to various stimuli such as heat, chemicals, electric fields, and mechanical stress. Methods such as anisotropic deswelling and surface alignment have been adopted to fabricate cholesteric liquid crystal elastomers, but there are limitations in creating spatially controlled cholesteric liquid crystal elastomer shapes.

[0007] Previously, there had been a report of a cholesteric liquid crystal elastomer (CLCE) being produced by the direct-ink-writing method, in which the helical axis in the sample was tilted at a specific angle (Professor Albert Schenning of Eindhoven University in the Netherlands). However, there had been no report on the mechanochromic property of a cholesteric liquid crystal elastomer (CLCE) with a helical axis tilted at a specific angle. [Prior art documents] [Non-patent literature]

[0008] [Non-Patent Document 1] Adv. Mater. 2021, 33, 2103309 [Non-Patent Document 2] Adv.Funct.Mater.2023,33,2304506 Summary of the Invention [Problem to be solved by the invention]

[0009] The present invention has been devised to solve the above problems, and an object of the present invention is to provide a cholesteric liquid crystal elastomer that is manufactured by blade coating and direct-ink-writing, and has mechanochromic properties in which the structural color appears different depending on the viewing direction due to the helical axis being tilted at a specific angle, and a manufacturing method thereof.

[0010] The technical problems that the present invention aims to solve are not limited to the technical problems mentioned above, and other technical problems not mentioned will be clearly understood by a person having ordinary skill in the art to which the present invention pertains from the following description. [Means for solving the problem]

[0011] In order to achieve the above object, the present invention provides a method for preparing a cholesteric liquid crystal elastomer, comprising: a first step of preparing a cholesteric liquid crystal mixture by mixing a liquid crystal monomer, a chain extender, a photoinitiator, and a chiral dopant; a second step of preparing a cholesteric liquid crystal ink by oligomerizing the cholesteric liquid crystal mixture; a third step of extruding the cholesteric liquid crystal ink through a 3D printer and printing it on a substrate by direct-ink-writing; a fourth step of annealing the printed cholesteric liquid crystal ink; and a fifth step of UV-curing the annealed cholesteric liquid crystal ink to prepare a cholesteric liquid crystal elastomer.

[0012] In the first step, the liquid crystal monomer and the chain extender may be mixed in a molar ratio of 1 to 2:1.

[0013] In the first step, the photoinitiator and the chiral dopant may be mixed in an amount of 1 to 2 parts by weight and 3 to 10 parts by weight, respectively, with respect to 100 parts by weight of the cholesteric liquid crystal mixture.

[0014] The third step is performed at 70 to 80°C for 5 to 15 mms. -1 It can be printed at speeds of 100 to 700 kPa and pressures of 100 to 700 kPa.

[0015] The fourth step may involve annealing at 30 to 50° C. for 5 to 15 minutes.

[0016] The fifth step can be UV curing on a hot plate at 30 to 50° C. for 10 to 30 minutes.

[0017] The present invention also provides a cholesteric liquid crystal elastomer produced by the above-mentioned production method.

[0018] The liquid crystal elastomer may include a liquid crystal monomer, a chain extender, a photoinitiator, and a chiral dopant.

[0019] The liquid crystal monomer and the chain extender may be contained in a molar ratio of 1 to 2:1.

[0020] The photoinitiator may be contained in an amount of 1 to 2 parts by weight, and the chiral dopant may be contained in an amount of 3 to 10 parts by weight, relative to 100 parts by weight of a mixture obtained by mixing the liquid crystal monomer, the chain extender, the photoinitiator, and the chiral dopant.

[0021] The inclination angle of the helical axis of the cholesteric liquid crystal elastomer may be 20 to 40°.

[0022] The present invention also provides a cholesteric liquid crystal elastomer film comprising the cholesteric liquid crystal elastomer produced by the above-mentioned production method.

[0023] The cholesteric liquid crystal elastomer film may have a thickness of 10 to 70 μm. Effect of the Invention

[0024] According to the present invention, it is possible to provide a cholesteric liquid crystal elastomer (CLCE) having various designs printed by a direct-ink-writing method, and in particular, a cholesteric liquid crystal elastomer that has a structural color that appears different depending on the viewing direction due to the characteristic that the helical axis is tilted at a specific angle.

[0025] Furthermore, the present invention can provide a cholesteric liquid crystal elastomer that exhibits mechanochromic properties when uniaxially and biaxially stretched perpendicularly or horizontally to the printing direction.

[0026] In addition, the present invention can be utilized for anticounterfeiting or structural health monitoring applications by utilizing the characteristic that different structural colors can be expressed when stretched perpendicularly and horizontally to the printing direction.

[0027] It can also be used in optical and display (AR / VR) applications by tilting the gradient of the helical axis of cholesteric liquid crystal elastomers.

[0028] The effects of the present invention are not limited to those mentioned above, and other effects not mentioned can be clearly understood by those skilled in the art from the description of the claims. [Brief description of the drawings]

[0029] [Figure 1] FIG. 1 is a schematic diagram of the reaction of a cholesteric liquid crystal ink and a cholesteric liquid crystal elastomer. [Diagram 2] FIG. 2 shows the 1H NMR spectrum of the cholesteric liquid crystal oligomer. [Diagram 3] FIG. 3 is an FT-IR spectrum of the cholesteric liquid crystal ink taken while cured. [Figure 4] Figure 4 is a POM image of the cholesteric liquid crystal ink at 54°C. [Diagram 5] FIG. 5 shows DSC thermograms of the cholesteric liquid crystal ink and cholesteric liquid crystal elastomer obtained during the second heating cycle at a rate of 10° C. / min−1. [Figure 6] Figure 6 shows the DMA measurement results of the planar aligned cholesteric liquid crystal elastomer. [Figure 7] FIG. 7 shows the transmittance spectra of planar aligned cholesteric liquid crystal elastomer films doped with 5.0 wt % and 6.2 wt % S1011. [Figure 8]FIG. 8(a) shows the constant shear viscosity of the cholesteric liquid crystal ink as a function of shear rate and temperature, and FIG. 8(b) shows a schematic diagram of the cholesteric liquid crystal elastomer manufacturing method (the self-organization structures of the cholesteric liquid crystal ink and cholesteric liquid crystal elastomer at each manufacturing stage (printing, thermal annealing, and UV curing)). [Figure 9] Figure 9 shows the structural colors observed in the cholesteric phase (40 °C) before and after annealing ((a) photograph, (b) UV spectrum) and (c) schematic diagrams of the formation of the cholesteric phase before and after annealing. [Figure 10] FIG. 10 is a photograph of a cholesteric liquid crystal elastomer film doped with 6.2 wt % S1011 fabricated inside a surface-aligned cell. [Figure 11] FIG. 11 shows the optical properties of the printed cholesteric liquid crystal elastomer doped with 6.2 wt % S1011 (green film). FIG. 11(a) is a photograph of the printed cholesteric liquid crystal elastomer (uniaxial line) (the arrow in the center indicates the printing direction), FIG. 11(b) is a diagram of the printed cholesteric liquid crystal elastomer with a tilted helical axis, FIG. 11(c) is the transmittance spectrum of the printed cholesteric liquid crystal elastomer acquired along the printing direction, FIG. 11(d) is the transmittance spectrum of the printed cholesteric liquid crystal elastomer acquired at an angle perpendicular to the printing direction, FIG. 11(e) (i) is a photograph of the printed cholesteric liquid crystal elastomer with concentric circles acquired by printing path of concentric circles, (ii) is a normal, and (iii) is a photograph of the printed cholesteric liquid crystal elastomer with concentric circles acquired by observing in oblique incidence mode, and FIG. 11(f) is a CIE 1931 color space diagram of the printed cholesteric liquid crystal elastomer with concentric circles displayed. [Figure 12] FIG. 12 shows the experimental setup for measuring the transmittance of printed cholesteric liquid crystal elastomer films: (A) along the printing direction and (B) along the direction perpendicular to the printing direction. [Figure 13]FIG. 13 shows the transmittance spectra of a printed cholesteric liquid crystal elastomer doped with 6.2 wt % S1011, viewed perpendicular to the printing axis while tilted (a) to the right and (b) to the left with respect to the printing direction. [Figure 14] FIG. 14 shows the mechanochromic results of uniaxially printed cholesteric liquid crystal elastomer doped with 5.0 wt % S1011 (red film). [Figure 15] FIG. 15 shows photographs and transmittance spectra of cholesteric liquid crystal elastomer films stretched "perpendicular" to the print axis with various magnitudes and various tilt angles. [Figure 16] FIG. 16 shows photographs and transmittance spectra of cholesteric liquid crystal elastomer films stretched "parallel" to the print axis with various magnitudes and various tilt angles. [Figure 17] FIG. 17 shows the stress-strain curves of cholesteric liquid crystal elastomer films stretched parallel and perpendicular to the printing direction. [Figure 18] FIG. 18 shows the percent reduction in thickness of uniaxially and biaxially stretched cholesteric liquid crystal elastomers. [Figure 19] Figure 19 shows 2D WAXS patterns of cholesteric liquid crystal elastomers doped with 5.0 wt% S1011 ((a) uniaxial stretch perpendicular to the printing direction, (b) uniaxial stretch parallel to the printing direction, and (c) biaxial stretch). [Figure 20] FIG. 20 shows POM images of printed cholesteric liquid crystal elastomers stretched (a) perpendicularly and (b) parallel to the print axis at various deformation levels (0% to 125%). [Figure 21] FIG. 21 shows the microstructural change of a printed cholesteric liquid crystal elastomer film by a stretching model. [Figure 22]Figure 22(a) shows the printing path of a cholesteric liquid crystal elastomer film with a concentric regular square structure (left) and a photograph of the cholesteric liquid crystal elastomer film exposed to various deformation rates from 0% to 150% (right). Figure 22(b) shows the printing path of a cholesteric liquid crystal elastomer film with the letters "P", "N", and "U" inserted, where the printing directions of the letters and background are perpendicular to each other (left), and a photograph of the resulting cholesteric liquid crystal elastomer film exposed to various deformation rates from 0% to 150% (right). [Diagram 23] Figure 23(a) is a photograph of a cholesteric liquid crystal elastomer film with a concentric circle-square pattern exposed to various deformation levels from 0% to 150% under unpolarized light, Figure 23(b) is a photograph of the film exposed between crossed polarizers rotated at 0 degrees, and Figure 23(c) is a photograph of the film exposed between crossed polarizers rotated at 45 degrees. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0030] The terms used in the present invention are selected from currently widely used general terms as much as possible, taking into consideration the functions in the present invention, but these may change according to the intentions of those skilled in the art, precedents, the emergence of new technologies, etc. In addition, in certain cases, the applicant may arbitrarily select terms, and in such cases, the meanings are described in detail in the description of the invention. Therefore, the terms used in the present invention are defined based on the meanings of the terms and the overall content of the present invention, rather than simply the names of the terms.

[0031] Unless otherwise defined, all terms used herein, including technical and scientific terms, have the same meaning as commonly understood by a person of ordinary skill in the art to which this invention pertains. Terms as defined in commonly used dictionaries should be interpreted to have a meaning consistent with the meaning they have in the context of the relevant art, and should not be interpreted in an idealized or overly formal sense unless expressly defined in this application.

[0032] Throughout the specification, when a part "comprises" a certain element, this does not mean excluding other elements, but rather includes other elements, unless specifically stated to the contrary.

[0033] The present invention may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein.

[0034] The present invention will be described in detail below.

[0035] The present invention provides a method for preparing a cholesteric liquid crystal elastomer, comprising: a first step of preparing a cholesteric liquid crystal mixture by mixing a liquid crystal monomer, a chain extender, a photoinitiator, and a chiral dopant; a second step of preparing a cholesteric liquid crystal ink by oligomerizing the cholesteric liquid crystal mixture; a third step of extruding the cholesteric liquid crystal ink through a 3D printer and printing it on a substrate by direct-ink-writing; a fourth step of annealing the printed cholesteric liquid crystal ink; and a fifth step of UV-curing the annealed cholesteric liquid crystal ink to prepare a cholesteric liquid crystal elastomer.

[0036] The liquid crystal monomer may be at least one selected from the group consisting of 1,4-bis-[4-(3-acryloyloxypropyloxy)benzoyloxy]-2-methylbenzene (RM257), 1,4-bis-[4-(6-acryloyloxyhexyloxy)benzoyloxy]-2-methylbenzene (RM82), and 4-[6-(acryloyloxy)hexyloxy]phenyl 4-[6-(acryloyloxy)hexyloxy]benzoate. Desirably, it may be 1,4-bis-[4-(3-acryloyloxypropyloxy)benzoyloxy]-2-methylbenzene (RM257), but is not limited thereto.

[0037] The liquid crystal monomer and the chain extender may be mixed in a molar ratio of 1 to 2:1, and preferably in a molar ratio of 1.02:1, but is not limited thereto.

[0038] The chain extender is a component that connects the liquid crystal monomers and may be n-hexylamine.

[0039] The photoinitiator, which is the configuration for the photocrosslinking carried out in the fifth step, may be 1,2-Diphenyl-2,2-dimethoxyethanone (I-651), and the chiral dopant may be Benzoic acid, 4-(Trans-4-pentylcyclohexyl)-,(1S)-1-phenyl-1,2-ethanediylester (S1011).

[0040] The photoinitiator and the chiral dopant may be mixed in an amount of 1 to 2 parts by weight and 3 to 10 parts by weight, respectively, based on 100 parts by weight of the cholesteric liquid crystal mixture. The content of the chiral dopant may be adjusted according to a target reflection wavelength.

[0041] The second step may include step 2-1 of mixing the cholesteric liquid crystal mixture by vortexing while applying heat with a heat gun, step 2-2 of oligomerizing the mixed cholesteric liquid crystal mixture at 40 to 60° C. for 15 to 25 hours to prepare a cholesteric liquid crystal ink, and step 2-3 of further heating the cholesteric liquid crystal ink at 110 to 120° C. for 1 to 5 minutes. The chiral dopant crystallized by step 2-3 can be completely dissolved in the liquid crystal matrix.

[0042] The oligomerization in the third step may be performed by an Aza-Michael addition reaction. More specifically, the Aza-Michael addition reaction may be performed by chain extension of the liquid crystal monomer and the chain extender present in the liquid crystal mixture, and the liquid crystal monomer and the chain extender may be oligomerized by the addition reaction.

[0043] The third step can be a direct-ink-writing method of the cholesteric liquid crystal ink using a 3D printer with a hot melt extruder including a 0.2 mm diameter nozzle. The cholesteric liquid crystal ink can be printed on a polyvinyl alcohol (PVA) coated glass substrate by programming G-code (NewCreatorK). The PVA coating can be used to conveniently remove the sample from the substrate after printing.

[0044] The third step is performed at 70 to 80°C for 5 to 15 mms. -1 The printing speed can be set at 9 mm / s at 75°C. -1 The printing speed can be, but is not limited to, 100 rpm.

[0045] In addition, the third step can be printed at a pressure of 100 to 700 kPa. By changing the pressure within the range, the thickness of the printed film can be controlled.

[0046] Since the optical properties of the cholesteric liquid crystal elastomer vary greatly depending on the alignment, a cholesteric liquid crystal elastomer with a uniform structural color can be manufactured by optimizing printing conditions such as thickness, temperature, printing speed, and pressure.

[0047] The fourth step may be annealed at 30 to 50° C. for 5 to 15 minutes, preferably, but not limited to, 40° C. for 10 minutes. The annealing process promotes the mesogens to self-assemble at a faster rate into a chiral nematic phase having a more uniform cholesteric liquid crystal helix, thereby reducing scattering and increasing transmittance.

[0048] The wavelength of the UV curing in the fourth step is 300 to 400 nm, and the ultraviolet intensity is 2 to 5 mW / cm -2Preferably, the wavelength is 365 nm and the ultraviolet intensity is 3.8 mW / cm. -2 It may be, but is not limited to this.

[0049] The fifth step may be UV curing for 10 to 30 minutes on a hot plate at 30 to 50° C. Preferably, UV curing may be performed for 20 minutes on a hot plate at 40° C., but is not limited thereto.

[0050] The UV curing can be performed by irradiating the annealed cholesteric liquid crystal elastomer with high-power UV light to perform photocrosslinking and cure the oligomerized cholesteric liquid crystal ink.

[0051] The present invention also provides a cholesteric liquid crystal elastomer produced by the above-mentioned production method.

[0052] The cholesteric liquid crystal elastomer may include a liquid crystal monomer, a chain extender, a photoinitiator, and a chiral dopant.

[0053] The liquid crystal monomer and the chain extender may be contained in a molar ratio of 1 to 2:1.

[0054] The photoinitiator may be contained in an amount of 1 to 2 parts by weight, and the chiral dopant may be contained in an amount of 3 to 10 parts by weight, relative to 100 parts by weight of a mixture obtained by mixing the liquid crystal monomer, the chain extender, the photoinitiator, and the chiral dopant.

[0055] The cholesteric liquid crystal elastomer may be colored by uniaxial or biaxial stretching. More specifically, when uniaxially stretched perpendicular or horizontal to the printing direction, a different structural color may be expressed, and when biaxially stretched, a different structural color may be expressed from when uniaxially stretched.

[0056] The cholesteric liquid crystal elastomer can exhibit different structural colors depending on the viewing direction.

[0057] The inclination angle of the helical axis of the cholesteric liquid crystal elastomer may be 20° to 40°, preferably 30° to 35°, and more preferably 32°, but is not limited thereto. The inclination angle φ of the printed cholesteric liquid crystal elastomer is determined by the reflection wavelength λ of the printed cholesteric liquid crystal elastomer. max1 and the reflection wavelength λ of the planar aligned cholesteric liquid crystal elastomer fabricated by surface alignment with the same chiral dopant concentration. max2 It can be calculated by comparing with.

[0058] According to one embodiment of the present invention, a planar aligned cholesteric liquid crystal elastomer doped with 6.2 wt % S1011 exhibits a λ max2 In contrast, a printed cholesteric liquid crystal elastomer doped with the same concentration of S1011 exhibited a much shorter λ of 550 nm even at normal incidence. max1 Thus, the significant reduction in the reflected wavelength of the printed cholesteric liquid crystal elastomer indicates the presence of a tilted helical axis. Since the difference in reflected wavelength is caused by the preparation method and not the chemical composition of the cholesteric liquid crystal elastomer, it is assumed that the φ value of the printed cholesteric liquid crystal elastomer is the same as the incident angle θ of the planar aligned cholesteric liquid crystal elastomer. The incident angle θ of the planar aligned cholesteric liquid crystal elastomer was calculated using the following Equations 1 and 2, and this value was assumed to be the same as the tilt angle φ of the printed cholesteric liquid crystal elastomer.

[0059]

number

[0060]

number

[0061] (where n is the average refractive index and P is the helical pitch)

[0062] The tilt angle can be calculated by substituting the value obtained by Equation 1 into Equation 2.

[0063] The present invention also provides a cholesteric liquid crystal elastomer film comprising the cholesteric liquid crystal elastomer produced by the above-mentioned production method.

[0064] The thickness of the cholesteric liquid crystal elastomer film may be 10 to 70 μm. More preferably, it may be 15 to 60 μm, but is not limited thereto. When the thickness is within the above range, a uniform structural color can be obtained with an appropriate transmittance.

[0065] Hereinafter, the present invention will be described in detail with reference to examples in order to specifically explain the present invention. However, the examples according to the present invention may be modified into various other forms, and the scope of the present invention is not to be construed as being limited to the examples described below. The examples in the present specification are provided to more completely explain the present invention to those skilled in the art.

[0066] <Example> Preparation of Cholesteric Liquid Crystal Elastomer (CLCE) Film Step 1: Manufacturing Cholesteric Liquid Crystal (CLC) ink The cholesteric liquid crystal mixture (RM257) consisted of a chain extender (n-hexylamine), a chiral dopant (S1011) and a photoinitiator (I-651).

[0067] Cholesteric liquid crystal inks were synthesized by chain-extending a diacrylate-functionalized LC monomer (RM257) with n-hexylamine via Aza-Michael addition-based step-growth polymerization. The molar ratio of RM257 / n-hexylamine was set to 1.02:1, and the weight ratio of I-651 was fixed at 1.5 wt% of the total cholesteric liquid crystal mixture. The content of chiral dopant (S1011) in the cholesteric liquid crystal mixture was adjusted according to the target reflection wavelength (i.e., 5.0 and 6.2 wt% for red and green reflection, respectively).

[0068] A cholesteric liquid crystal mixture containing RM257 (1000 mg, 1.70 mmol), n-hexylamine (168.5 mg, 1.67 mmol), S1011 (62.5 mg), and I-651 (18.8 mg) was added to a 10 mL syringe and thoroughly mixed by vortexing with the application of external heat from a heat gun. The cholesteric liquid crystal mixture was then placed in a convection oven at 50 °C (i.e., chiral nematic phase) for 20 h to oligomerize. After oligomerization, the viscous cholesteric liquid crystal ink was further heated at 115 °C for 3 min to completely dissolve the crystallized chiral dopant into the LC matrix.

[0069] Step 2: Fabrication of printed cholesteric liquid crystal elastomer (CLCE) films Direct-ink-writing (DIW) of cholesteric liquid crystal ink was performed using a 3D printer (Dr.INVIVO4D, Rocket Healthcare) equipped with a hot melt extruder with a 0.2 mm diameter nozzle. The cholesteric liquid crystal ink was printed on poly(vinyl alcohol) (PVA)-coated glass slides by programmed G-code (NewCreatorK). The PVA coating was used for convenient removal of the sample from the substrate after printing. For PVA coating, the glass slides were washed with acetone, treated with O2 plasma for 10 min, then spin-coated with 4 wt% PVA solution (aqueous), followed by thermal annealing at 100 °C for 5 min. Printing was performed at 75 °C for 9 mms. -1The thickness of the printed samples was controlled by varying the pressure, which affects the extrusion rate, from 100 to 700 kPa. After printing, the samples were annealed on a hotplate at 40 °C for 10 min to promote uniform alignment of the helical structures. Finally, the printed ink was UV-cured (OmniCure S1500, 365 nm, 3.8 mW cm) on a hotplate at 40 °C for 20 min. -2 ) to prepare a cholesteric liquid crystal elastomer film.

[0070] <Comparative Example> Preparation of Planar Alignment Cholesteric Liquid Crystal Elastomer (CLCE) Film The glass slide was washed with acetone, sonicated with isopropanol, and then treated with O2 plasma (Harrick Plasma, US / PDC-32G) for 10 minutes. The plasma-treated glass substrate was then spin-coated with a commercially available polyamic acid solution and placed on a hot plate at 70°C for 5 minutes to remove residual solvent. The substrate was then heated at 200°C for 1 hour to perform thermal imidization. The polyimide-coated surface was polished with a velvet cloth, and the glass layers were assembled non-parallel using 36 μm-sized spacers to achieve planar alignment.

[0071] The liquid crystal mixture (RM257, n-hexylamine, S1011, I-651) in the isotropic phase (80 °C) was filled into the liquid crystal cell and oligomerized in the nematic phase (50 °C) in a convection oven for 18 h. After oligomerization, the liquid crystal cell was exposed to UV light (OmniCure S1500, 365 nm, 3.8 mW cm) for 30 min for temporary crosslinking. -2 ) to produce a planar aligned cholesteric liquid crystal elastomer film.

[0072] Experimental Example 1. Synthesis and Characterization of Cholesteric Liquid Crystal Ink and Cholesteric Liquid Crystal Elastomer Experimental Example 1-1. 1 HNMR measurements Measurements were made using a Varian 500 MHz spectrometer using CDCl3 as the solvent.

[0073] Referring to FIG. 2, the average molecular weight of the C-type base cholesteric liquid crystal oligomer (or cholesteric liquid crystal ink in the case of DIW) is1 4180 g mol by HNMR spectroscopy -1 was confirmed.

[0074] Experimental Example 1-2. Attenuated Total Reflection Fourier Transform Infrared Spectrophotometer (ATR-FTIR) Measurement ATR-FTIR was performed using a Jasco FTIR-4600 spectrometer at 4000–650 cm -1 The back-ground spectrum was measured in the wavenumber range of 4 cm -1 The data were obtained by recording 64 scans at a resolution of 100x100x100.

[0075] Referring to FIG. 3, in the FT-IR spectrum, -1 The disappearance of the acrylate peak at 1000 nm indicated that the cholesteric liquid crystal elastomer had been successfully produced.

[0076] Experimental Example 1-3. Analysis of gel fraction The sample was immersed in chloroform for 48 hours to extract the non-crosslinked portion, and then dried in a vacuum oven at room temperature for 24 hours. The mass of the sample was measured before and after extraction, and the gel fraction G was calculated as follows: G(%)=m f / m i ×100(%) (where m f and m i indicates the final mass after extraction and drying and the initial mass before extraction, respectively)

[0077] The gel fraction G calculated by the above formula was 80.1%, indicating that the cholesteric liquid crystal elastomer was successfully produced.

[0078] Experimental Example 1-4. Texture and phase transition temperature of cholesteric liquid crystal oligomer The texture and phase transition temperature of the cholesteric liquid crystal oligomer were measured by temperature-controlled POM. To better distinguish the texture, the cholesteric liquid crystal oligomer was examined after being filled into a 36 μm thick glass cell.

[0079] Referring to FIG. 4, the oil striped texture of the cholesteric phase during cooling in the isotropic phase and the cholesteric-isotropic transition temperature T Ch-1 was observed.

[0080] Experimental Example 1-5. Differential Scanning Calorimetry (DSC) Measurement The thermal transition temperatures of the cholesteric liquid crystal oligomers and cholesteric liquid crystal elastomers were further investigated by differential scanning calorimetry (DSC, Figure 1c). Each sample was heated to 150°C, cooled to -50°C, and then heated for 10°C for 10 min under nitrogen flow. -1 DSC (TA Instruments, Discovery DSC25) was performed using a heat-cool-heat protocol with reheating to 150° C. at a rate of 100° C.

[0081] Referring to FIG. 5, the glass transition temperature (Tg) and T Ch-I The values ​​were found to be −10° C. and 51° C., respectively. However, the Tg of the cholesteric liquid crystal elastomer is estimated to be 5° C., which is 15° C. higher than that of the cholesteric liquid crystal oligomer due to the contracted network structure.

[0082] Experimental Example 1-6. Dynamic Mechanical Analysis (DMA) Measurement The viscoelastic properties of the samples [7.0 mm (l) × 5.0 mm (w) × 0.1 mm (t)] were measured by DMA (TA Instruments, DMA Q850) using tensile clamps under increasing temperature. The samples were tensile tested at a constant frequency of 1 Hz, amplitude of 14.0 μm, and preload of 0.01 N for 3 °C min. -1 It was heated from -50 to 150°C at a heating rate of .

[0083] The viscoelastic properties of the planar aligned cholesteric liquid crystal elastomer can be seen in Figure 6. The temperature-dependent coefficient of the cholesteric liquid crystal elastomer was examined by dynamic mechanical analysis (DMA), and it was found that the Tg was 29°C, which was ≈24°C higher than the value measured by DSC (see the tan δ peak in Figure 1d).

[0084] Experimental Example 1-7. UV-Visible Spectrum UV-visible spectra were recorded at room temperature using a deuterium-tungsten halogen light source (Ocean Insight, DH-2000-BAL) and a fiber optic spectrometer (Ocean Insight, FLAME-T-XR1-ES) in transmission mode. A thin layer of silicone oil was applied to the surface to minimize light scattering during UV measurements.

[0085] Referring to Figure 7, the reflection bands of a representative planar aligned cholesteric liquid crystal elastomer film fabricated using a surface aligned cell were clearly detected in the UV-Vis absorption spectrum. Increasing the S1011 concentration in the cholesteric liquid crystal elastomer film from 5.0 to 6.2 wt % caused a blue shift from 780 nm to 650 nm.

[0086] Experimental Example 2. Rheological properties of cholesteric liquid crystal ink To determine the rheological properties of the cholesteric liquid crystal inks, constant shear experiments were performed to study the shear viscosity with temperature.

[0087] The normal shear viscosity of the cholesteric liquid crystal ink was measured using a stress-controlled rheometer (HR20, TA Instruments) with a 25 mm parallel plate geometry. In this experiment, the sample was placed on the bottom of a Peltier plate and the viscosity was measured by applying rotational shear between the parallel plates at various temperatures. During the measurement, the shear rate was varied from 0.1 to 100 s -1 was gradually increased to

[0088] Referring to Figure 8, the cholesteric liquid crystal ink exhibited shear thinning behavior, suggesting that it may be aligned at high shear rates. The cholesteric liquid crystal ink exhibited shear thinning behavior, suggesting that it may be aligned at high shear rates (Figure 8a).

[0089] Shear thinning of cholesteric liquid crystal inks occurs due to a flow-induced disorder-order transition (i.e., a transition from a polydomain or isotropic state to an anisotropic state). As the temperature increases, the viscosity of cholesteric liquid crystal inks gradually decreases, and the shear thinning effect weakens and eventually disappears at 85 °C. In particular, the shear thinning is presumed to be due to the transition between an isotropic state and an aligned nematic state, T ch-I The print and bed temperatures were then maintained at 51°C (DCS measurement) or 55°C (POM measurement). However, cholesteric liquid crystal inks require higher shear rates to induce alignment at higher temperatures. To achieve fast extrusion and efficient self-organization to generate helical structures at the bottom viscosity state, 75°C (isotropic phase) and 40°C (cholesteric phase) were selected as the print and bed temperatures, respectively (Figure 8b). The printed circuit board was then annealed at 40°C for 10 min to efficiently form stable helical structures, which were then photocrosslinked at 40°C to permanently fix them by forming a network. Since the extrusion was performed in the isotropic phase, the cholesteric phase was achieved after annealing at 40°C for several minutes. The annealing process can promote the self-assembly of the mesogens into a chiral nematic phase with more uniform cholesteric liquid crystal helices at a faster rate, reducing scattering and increasing transmittance (Figure 9).

[0090] Experimental Example 3. Anisotropic chiroptical properties of printed cholesteric liquid crystal elastomers The microstructural changes of the printed cholesteric liquid crystal elastomers upon uniaxial and biaxial stretching were confirmed by UV spectroscopy, X-ray scattering analysis and polarized optical microscopy (POM).

[0091] Referring to FIG. 10, the position of the reflection band of the cholesteric liquid crystal elastomer doped with 6.2 wt % S1011 (650 nm) was blue-shifted at oblique viewing angles regardless of the viewing direction.

[0092] Referring to FIG. 11, the printed cholesteric liquid crystal elastomer doped with the same amount of S1011 showed a reflected light of a wavelength of 550 nm under normal incidence, but this value was approximately 100 nm lower than that of the planar aligned cholesteric liquid crystal elastomer (FIG. 11(a) and (c)).

[0093] Spectroscopic measurements were performed using the setup shown in Figure 12, where the reflected light was red or blue shifted at viewing angles tilted along the printing direction. In particular, a red or blue shift of the reflection band was observed when the sample was viewed from a forward or backward position relative to the printing direction, respectively. Such a specific anisotropic light reflection (i.e., dependence on viewing direction) strongly suggests the presence of a tilted helix along the printing direction rather than a pre-existing perpendicular alignment (Figure 11(b)). In contrast, a similar range of blue shifts was observed at tilted viewing angles perpendicular to the printing direction (Figures 11(a), (d) and 13). The tilted helical axis of the cholesteric liquid crystal elastomer was attributed to the viscous properties of the cholesteric liquid crystal ink, induced by the interaction between the shear forces generated during extrusion and the elongational forces generated during substrate deposition. Indeed, reducing the viscosity of the cholesteric liquid crystal ink by diluting it with a solvent can result in a pre-existing planar alignment.

[0094] As shown in Figure 11(e), a concentric circle pattern of cholesteric liquid crystal elastomer film doped with 6.2 wt% S1011 was printed to reflect the entire visible light spectrum. When viewed from the normal direction, the film appeared green, but at an oblique viewing angle (30°), it reflected the entire visible light range due to the combined effect of the circular print path and anisotropic light reflection. Referring to Figure 11(f), the result of CIE 1931 color space analysis demonstrated the ability of the cholesteric liquid crystal elastomer film to reflect the entire visible light spectrum by designating each reflection point of the cholesteric liquid crystal elastomer film as "A" to "H".

[0095] The inclination angle φ of the printed cholesteric liquid crystal elastomer is determined by the reflection wavelength λ of the printed cholesteric liquid crystal elastomer. max1and the reflection wavelength λ of the planar aligned cholesteric liquid crystal elastomer fabricated by surface alignment with the same chiral dopant concentration. max2 It was calculated by comparing with.

[0096] According to one embodiment of the present invention, a planar aligned cholesteric liquid crystal elastomer doped with 6.2 wt % S1011 exhibits a λ max2 In contrast, a printed cholesteric liquid crystal elastomer doped with the same concentration of S1011 exhibited a much shorter λ of 550 nm even at normal incidence. max1 Thus, the significant reduction in the reflected wavelength of the printed cholesteric liquid crystal elastomer indicates the presence of a tilted helical axis. Since the difference in reflected wavelength is caused by the preparation method and not the chemical composition of the cholesteric liquid crystal elastomer, it is assumed that the φ value of the printed cholesteric liquid crystal elastomer is the same as the incident angle θ of the planar aligned cholesteric liquid crystal elastomer. The incident angle θ of the planar aligned cholesteric liquid crystal elastomer was calculated using the following Equations 1 and 2, and this value was assumed to be the same as the tilt angle φ of the printed cholesteric liquid crystal elastomer.

[0097]

number

[0098]

number

[0099] (where n is the average refractive index and P is the helical pitch)

[0100] The inclination angle is calculated by substituting the value obtained by the formula 1 into the formula 2.

[0101] Experimental Example 4. Anisotropic mechanochromic properties of printed cholesteric liquid crystal elastomers The mechanochromic properties of printed cholesteric liquid crystal elastomers were investigated using films doped with 5.0 wt% S1011, which exhibited red reflection (660 nm). In particular, the mechanochromic properties of the samples were investigated under various deformation conditions, including uniaxial and biaxial stretching along directions parallel and perpendicular to the printing track.

[0102] The stress-strain ratio was measured at room temperature using a general-purpose testing machine (DR-100, manufactured by Dr TECH) with a specimen measuring 20 mm (l) x 20 mm (w) x 0.1 mm (t) at a rate of 25 mm / min. -1 A Canon EOS Rebel T3 camera (Canon EF 100 nm macro lens, 1:2 / 8 USM) was used to capture images and videos of the cholesteric liquid crystal elastomers.

[0103] Referring to FIG. 14, when uniaxial stretching was performed in the direction perpendicular to the printing direction, a significant blue shift in reflected light was observed from 660 nm to 500 nm at normal incidence as the deformation increased to 125% (FIG. 14(ai)).

[0104] Also, referring to Figures 14(a-ii) and 15, the reflected wavelength of the stretched sample (ε=100%) was examined at oblique angles and showed a viewing angle dependence similar to that observed for the unstretched sample (i.e., a red or blue shift occurred when viewed from the front or back of the print path). The significant blue shift (150 nm) suggests that the helical pitch gradually decreased during stretching due to a decrease in sample thickness. However, even at deformations up to 125%, the helix remained tilted as the reflected light continued to vary with viewing direction.

[0105] Referring to Figure 14(bi), the sample stretched uniaxially parallel to the printing direction exhibited a smaller blue shift in reflected light at normal incidence at the same 125% deformation (660~590nm). In particular, the middle part of the sample, which is the part with the least deformation, maintained its red color even after 125% elongation. This is in stark contrast to the blue color observed for the sample stretched perpendicularly under the same deformation.

[0106] Referring to FIG. 14(b-ii) and FIG. 16, the viewing angle dependence of the wavelength reflected along the printing direction showed a tendency to decrease as the deformation rate increased from 0 to 100%, and eventually disappeared at 125% deformation. The disappearance of the viewing angle dependence at high deformation rates means that the tilted helices of the printed cholesteric liquid crystal elastomer were realigned vertically during stretching to resemble existing planar aligned cholesteric liquid crystal elastomers. In other words, when stretching was performed parallel to the printing direction, the blue shift due to stretching (i.e., the reduction in thickness) competes with the red shift due to the reduction in the tilt angle. As a result, a relatively smaller blue shift was observed (Δλ=70 and 160 nm, respectively) than when stretching was performed perpendicular to the printing direction.

[0107] 17, although there was a significant difference in the optical properties of the cholesteric liquid crystal elastomers stretched horizontally and perpendicularly to the printing direction, similar mechanical properties were observed, suggesting that the change in optical properties was mainly due to the structural change and reorientation of the helical axis rather than the general mechanical response of the cholesteric liquid crystal elastomer.

[0108] Referring to Figure 14(ci), biaxial stretching of the printed cholesteric liquid crystal elastomer in addition to uniaxial stretching resulted in a much larger blue shift in the reflected wavelength (Δλ=290 nm at 100% strain) than observed with uniaxial stretching. In particular, the sample failed at 125% stretch because the sample thickness decreased much faster than observed with uniaxial stretching (Figure 18).

[0109] Referring to FIG. 14(c-ii), the reflected wavelengths of the cholesteric liquid crystal elastomer observed at oblique angles in the sample biaxially stretched at 50% strain followed a similar optical trend to the uniaxial stretch performed perpendicular to the print axis, indicating that the change in the tilt angle during stretching was minimized.

[0110] Experimental Example 5. Morphological changes of cholesteric liquid crystal elastomers due to stretching To investigate in more detail the microstructural changes of cholesteric liquid crystal elastomers during stretching, wide-angle X-ray scattering (WAXS) analysis was carried out in normal versus planar geometric configurations under various stretching conditions at various deformation rates.

[0111] WAXS measurements were carried out using the 9A U-SAXS beamline at Pohang Accelerator Laboratory (PAL, Korea), where the X-ray wavelength λ and sample-detector distance are 1.119 Å (E = 11.08 keV) and 0.22 m, respectively. The 2D scattering patterns were collected using a 2D charge-coupled device area detector (Rayonix, MX170-HS). The directional coefficient P2, also called the order parameter S, was calculated on the azimuthal plot using the following equation:

[0112]

number

[0113] (where I(φ) is the intensity with azimuth angle (φ) at constant scattering vector [q=(4π / λ)sin(θ / 2)].)

[0114] Referring to FIG. 19, in the case of the unstretched cholesteric liquid crystal elastomer, an isotropic ring-shaped pattern is formed at qi=1.10 Å. -1 While the vertically and horizontally stretched samples showed two clearly separated arcs that intensified with increasing deformation rate (Fig. 19(a) and (b)). Moreover, the one-dimensional azimuthal scan profile acquired at qi = 1.10 Å-1 confirmed that the distortion of the helical structure reoriented the mesogens of the cholesteric liquid crystal elastomer into a hierarchical structure.

[0115] The order parameter S was calculated for all samples stretched parallel and perpendicularly at different deformation rates (0%, 50% and 100%). 0% =0, S 50% = 0.18 and S 100% = 0.29) was found to be consistent at each deformation rate, suggesting that the helical unwinding of the microstructure of the cholesteric liquid crystal elastomer occurred during both parallel and perpendicular stretching.

[0116] To further confirm the unwinding of the helices in the cholesteric liquid crystal elastomers during uniaxial stretching, POM images were acquired while rotating the samples at 45° intervals under vertical or parallel stretching at various strain levels. Referring to FIG. 20, the undeformed cholesteric liquid crystal elastomers exhibited nearly uniform birefringence regardless of the observation angle (i.e., a circular, uniform configuration when viewed from above), whereas the birefringence of the deformed cholesteric liquid crystal elastomers gradually disappeared at 0°, 90°, 180°, and 270° as the strain increased, regardless of the stretching direction. These results suggest that all mesogens in the cholesteric liquid crystal elastomers tended to align in the applied uniaxial strain direction and lost their helical structure. In contrast, the WAXS patterns of the biaxially stretched specimens showed a consistent ring shape, albeit with reduced intensity, and maintained an order parameter of 0 up to 100% stretch (FIG. 19(c)). This means that the cholesteric liquid crystal elastomer maintained the helical structure throughout the biaxial stretching.

[0117] The stretch-induced microstructural changes of the printed cholesteric liquid crystal elastomer based on the results of UV-Vis spectroscopy, WAXS analysis, and POM are shown in Figure 21. When the cholesteric liquid crystal elastomer was stretched uniaxially, the helical structure was unraveled and gradually transformed into a layered structure, while the biaxially stretched cholesteric liquid crystal elastomer maintained the helical structure throughout the entire stretch. In addition, the initial tilt angle of the cholesteric liquid crystal elastomer obtained through direct-ink-writing (DIW)-based printing was significantly reduced when the sample was stretched uniaxially parallel to the printing axis. However, the tilt angle did not change significantly when the cholesteric liquid crystal elastomer was stretched perpendicular to the printing direction or stretched biaxially.

[0118] Experimental Example 6. Mechanochromic response programming of cholesteric liquid crystal elastomers We fabricated an optically programmable mechanochromic film by utilizing the anisotropic mechanochromic properties of printed cholesteric liquid crystal elastomer. First, a concentric rectangular pattern of retroreflective film was printed using cholesteric liquid crystal elastomer doped with 5.0 wt% S1011. When this film was stretched to 150%, a striking contrast between blue and red reflection was observed (Figure 22(a)). In particular, the blue reflection was enhanced at both the top and bottom ends of the film, which corresponds to stretching along the direction perpendicular to the printing track. In contrast, the left and right parts of the film reflected mainly red light while stretching along the printing axis. This striking color contrast is due to the large difference in the degree of blue shift that occurs when the printed cholesteric liquid crystal elastomer film is stretched parallel or perpendicular to the printing direction, as mentioned above.

[0119] Referring to Figure 23, when viewed between crossed polarizers at 0° rotation, the regions stretched parallel to the printing direction appear dark because the cholesteric helix is ​​untwisted and transformed into a quasi-monodomain structure. In contrast, the regions stretched perpendicular to the printing direction still exhibit a reflected color. This difference in optical properties is probably due to the fact that the nematic director turns easily and completely when stretched parallel to the printing direction, but when stretched perpendicular to the printing direction, the turning is restricted and incomplete due to the resistance of the adjacent domains.

[0120] We then utilized the excellent color contrast to fabricate a photon film capable of encryption. To this end, a cholesteric liquid crystal elastomer film was printed with the initials of Pusan ​​National University, "P", "N", and "U", with the printing direction perpendicular to the background (Figure 22(b)). When the film was stretched 150% in the direction perpendicular to the printing direction of the letters, the "P", "N", and "U" had significant blue reflection, and the background had significant red reflection (Figure 22(b)). The clear color contrast between the unstretched and stretched states, and the viewing direction dependence of the engineered photon film, could potentially be exploited to develop encryption or anti-counterfeiting devices.

[0121] The above is a detailed description of specific embodiments of the present invention. Those skilled in the art will understand that the present invention can be embodied in modified forms without departing from the essential characteristics of the present invention. Therefore, the embodiments of the present disclosure are considered to be illustrative rather than restrictive. The scope of the present invention is defined by the claims rather than the above description, and all differences within the scope of the equivalents are included in the present invention.

Claims

1. A first step of preparing a cholesteric liquid crystal mixture by mixing a liquid crystal monomer, a chain extender, a photoinitiator and a chiral dopant; a second step of oligomerizing the cholesteric liquid crystal mixture to produce a cholesteric liquid crystal ink; a third step of extruding the cholesteric liquid crystal ink through a 3D printer and printing it onto a substrate by direct-ink-writing; a fourth step of annealing the printed cholesteric liquid crystal ink; and a fifth step of UV curing the annealed cholesteric liquid crystal ink to produce a cholesteric liquid crystal elastomer; Includes A method for producing a cholesteric liquid crystal elastomer.

2. In the first step, the liquid crystal monomer and the chain extender are mixed in a molar ratio of 1 to 2:

1. The method for producing the cholesteric liquid crystal elastomer according to claim 1 .

3. In the first step, the photoinitiator and the chiral dopant are mixed in an amount of 1 to 2 parts by weight and 3 to 10 parts by weight, respectively, with respect to 100 parts by weight of the cholesteric liquid crystal mixture. The method for producing the cholesteric liquid crystal elastomer according to claim 1 .

4. The third step is performed at 70 to 80° C. for 5 to 15 mms. －１ and a pressure of 100-700 kPa. The method for producing the cholesteric liquid crystal elastomer according to claim 1 .

5. The fourth step is annealing at 30 to 50° C. for 5 to 15 minutes. The method for producing the cholesteric liquid crystal elastomer according to claim 1 .

6. The fifth step is UV curing on a hot plate at 30 to 50° C. for 10 to 30 minutes. The method for producing the cholesteric liquid crystal elastomer according to claim 1 .

7. Produced by the method of claim 1 A cholesteric liquid crystal elastomer characterized by:

8. Contains liquid crystal monomers, chain extenders, photoinitiators and chiral dopants A cholesteric liquid crystal elastomer characterized by:

9. The liquid crystal monomer and the chain extender are contained in a molar ratio of 1 to 2:

1. The cholesteric liquid crystal elastomer according to claim 8.

10. The photoinitiator is included in an amount of 1 to 2 parts by weight, and the chiral dopant is included in an amount of 3 to 10 parts by weight, based on 100 parts by weight of a mixture of the liquid crystal monomer, the chain extender, the photoinitiator, and the chiral dopant. The cholesteric liquid crystal elastomer according to claim 8.

11. The inclination angle of the helical axis of the cholesteric liquid crystal elastomer is 20 to 40 degrees. The cholesteric liquid crystal elastomer according to claim 8.

12. The cholesteric liquid crystal elastomer produced by the method of claim 1 is included. A cholesteric liquid crystal elastomer film.

13. The thickness of the cholesteric liquid crystal elastomer film is 10 to 70 μm. The cholesteric liquid crystal elastomer film of claim 12.

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