Double-layer near-infrared light response liquid crystal elastomer and preparation method thereof

Abstract

According to the double-layer near-infrared light response liquid crystal elastomer and the preparation method thereof, the liquid crystal elastomer is prepared from a liquid crystal monomer, a dynamic acetal monomer, a cross-linking agent and a chain extender through a Michael addition reaction by a one-pot method, and the material is endowed with light response performance by adding a photo-thermal dye. The double-layer near-infrared light response liquid crystal elastomer is obtained by combining a liquid crystal elastomer film containing photo-thermal dye and a liquid crystal elastomer film not containing photo-thermal dye through dynamic exchange action of hot-pressing trigger dynamic covalent bonds and then mechanically stretching and orienting. According to the invention, the preparation is simple, the raw materials are easy to obtain, the reprogramming and self-repairing functions of the LCE-SCA material are realized through the dynamic exchange effect of the dynamic acetal bond under external stimulation, and a basis is provided for preparing a double-layer liquid crystal elastomer material; the multi-stimulus response is realized by adding photo-thermal dye and the like; and the formed dynamic cross-linked network can generate network structure recombination when being heated to a dynamic exchange temperature, so that the cross-linked liquid crystal elastomer can be reprocessed and recycled.

Description

Technical Field

[0001] The present invention relates to the field of functional liquid crystal elastomers, and in particular to the preparation and application of a class of dynamic bonded near-infrared light-responsive liquid crystal elastomers. Background Art

[0002] Liquid crystal elastomers are attracting increasing attention in the development and application of smart materials due to their flexible programming and large reversible deformation. Among them, photoresponsive liquid crystal elastomers have the advantages of precise control, remote regulation, and clean stimulation conditions, providing an option for the preparation of bionic intelligent soft machines.

[0003] Currently, UV-responsive liquid crystal elastomers (LCEs) account for the majority of photoresponsive liquid crystal elastomers. The reversible deformation of these materials is primarily achieved through the photoisomerization of azobenzene. However, UV light is harmful to biological tissues and has low penetrating power, significantly limiting the application of these materials. Compared to UV light, near-infrared (NIR) light offers excellent biocompatibility and higher penetrating power, making NIR-responsive LCEs a key research and development focus in the field. Furthermore, to better align with the concept of sustainable development, developing reprocessable and recyclable LCEs is a practical requirement.

[0004] Near-infrared light-responsive liquid crystal elastomers are typically prepared by adding photothermal converters such as carbon nanotubes, gold nanoparticles, and graphene to the liquid crystal elastomer, utilizing the photothermal effect to convert light energy into heat energy to achieve near-infrared light response. The response of such liquid crystal elastomers is generally achieved by using the photothermal effect to reach the clearing point of the liquid crystal elastomer, thereby causing the liquid crystal elastomer to transition from a single-domain state to a multi-domain state for actuation. However, this actuation is generally performed in the orientation direction of the liquid crystal elastomer, which greatly limits the application scenarios of liquid crystal elastomer materials.

[0005] Therefore, a near-infrared light-responsive liquid crystal elastomer with high photothermal conversion efficiency, dynamic exchange, and multi-dimensional drive is needed. Summary of the Invention

[0006] To address the shortcomings of the prior art, the present invention provides a bilayer near-infrared light-responsive liquid crystal elastomer and its preparation method. The liquid crystal elastomer is prepared in a one-pot process using a Michael addition reaction to combine a liquid crystal monomer, a dynamic acetal monomer, a crosslinker, and a chain extender. The addition of a photothermal dye imparts photoresponsive properties to the material. This bilayer near-infrared light-responsive liquid crystal elastomer is composed of a liquid crystal elastomer film containing a photothermal dye and a liquid crystal elastomer film without the dye, joined together by dynamic covalent bond exchange triggered by heat pressing, followed by mechanical stretching and orientation.

[0007] The double-layer near-infrared light-responsive liquid crystal elastomer described in the present invention is composed of a layer of liquid crystal elastomer film containing a photothermal dye and a layer of liquid crystal elastomer film not containing a photothermal dye, which are combined into a double-layer liquid crystal elastomer film through hot pressing and then oriented by mechanical stretching.

[0008] The method for preparing a double-layer near-infrared light-responsive liquid crystal elastomer according to the present invention comprises the following steps:

[0009] (1) Synthesis of acetal (SCA) fragment of liquid crystal elastomer material: The raw materials 3,9-divinyl-2,4,8,10-tetraoxaspiro[5.5]undecane and 2-aminoethanethiol hydrochloride were dissolved in methanol solution, and 2,2-dimethoxy-2-phenylacetophenone was added as a catalyst to obtain a spiral acetal fragment (SCA) with amino groups at both ends.

[0010] (2) Synthesis of photothermal dye YHD-796 in near-infrared light-responsive liquid crystal elastomer material: The raw materials thiophene-2-thiol and methyl isopipecolate were added to a flask containing a toluene solution and refluxed, and the obtained product was added with n-decanol, 2,2-dihydroxymethylpropionic acid (DMPA) and anhydrous dichloromethane. N,N'-dicyclohexylcarbodiimide (DCC) was added to the flask and stirred at 0°C under a nitrogen atmosphere. Then, the product was filtered, rotary evaporated, and purified by silica gel chromatography to obtain a preliminary product. The keto acid and the preliminary product were further added to a toluene / n-butanol solution, and the reaction mixture was heated to reflux. The mixture was transferred and diluted with dichloromethane, the solution was concentrated by rotary evaporation, and the black solid obtained by column chromatography was purified by column chromatography. The reaction mixture was purified by silica gel column chromatography to obtain photothermal dye YHD-796.

[0011] (3) Preparation of dynamic cross-linked near-infrared light-responsive liquid crystal elastomer film: 1,4-bis-[4-(6-acryloyloxyhexyloxy)benzoyloxy]-2-methylbenzene (RM82), 3,6-dioxa-1,8-octanedithiol (EDDET), the acetal fragment obtained in step (1), and YHD-796 obtained in step (2) were added to N,N-dimethylformamide (DMF) solution, dissolved and poured into a polytetrafluoroethylene mold, and heated to obtain a uniform LCE-SCA796 film. The structure is as follows:

[0012]

[0013] in, EDDET is a chain extender. It is the liquid crystal monomer RM82. is an acetal fragment (acting as a cross-linking agent in the cross-linked network), It is photothermal dye YHD-796.

[0014] (4) Preparation of a dynamic cross-linked liquid crystal elastomer film: The raw materials RM82, EDDET, and the acetal fragment obtained in step (1) were added to a DMF solution, dissolved, poured into a polytetrafluoroethylene mold, and heated to obtain a uniform LCE-SCA film. The structure is as follows:

[0015]

[0016] in, EDDET is a chain extender. It is the liquid crystal monomer RM82. It is an acetal segment (acting as a cross-linking agent in the cross-linked network).

[0017] (5) Preparation of a near-infrared light-responsive double-layer liquid crystal elastomer: The dynamically cross-linked near-infrared light-responsive liquid crystal elastomer film obtained in step (3) and the dynamically cross-linked liquid crystal elastomer film obtained in step (4) are stacked on top of each other and placed in a hot press to be hot-pressed, and the two are bonded by dynamic exchange of dynamic covalent bonds.

[0018] (6) Preparation of near-infrared light-responsive double-layer liquid crystal elastomer film: The double-layer liquid crystal elastomer film in step (5) is stretched by 40% or programmed into a desired shape, and heated in an oven at 140°C for 30 minutes to obtain a double-layer near-infrared light-responsive liquid crystal elastomer with dynamic bonds.

[0019] Furthermore, the spiroacetal in step (1) of the present invention is a class of molecules containing spiroacetal and its derivatives, preferably 3,9-divinyl-2,4,8,10-tetraoxaspiro[5.5]undecane.

[0020] Compared with the materials reported so far, the LCE-SCA material of the present invention has the following characteristics: (1) It is simple to prepare and can be successfully prepared by a simple one-pot method. (2) The dynamic exchange of dynamic acetal bonds under external stimulation realizes the reprogramming and self-repair functions of the LCE-SCA material and provides a basis for the preparation of double-layer liquid crystal elastomer materials. (3) It has a stable cross-linking system in the absence of external stimulation and can achieve multi-stimulus response by adding photothermal dyes. (4) The synthetic raw materials are easy to obtain. Therefore, this type of material is a class of polymer materials with great development prospects and application potential.

[0021] The present invention provides a simpler and more convenient method for synthesizing liquid crystal elastomers by combining aza-Michael addition and thiol-ene Michael addition reactions. The resulting dynamic crosslinked network undergoes structural reorganization when heated to the dynamic exchange temperature, allowing the crosslinked liquid crystal elastomer to be reprocessed and recycled. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] Figure 1Schematic diagram of the structure of the double-layer near-infrared light-responsive liquid crystal elastomer of the present invention.

[0023] Figure 2 This is the infrared absorption spectrum of the raw materials used in the present invention and the liquid crystal elastomer film obtained in Example 4. The disappearance of the amino, thiol and carbon-carbon double bond peaks in the liquid crystal elastomer indicates that the reaction is basically completed.

[0024] Figure 3 The following are pictures of the oriented and non-oriented liquid crystal elastomer solid films of the present invention under a polarizing microscope. Figure I is unoriented, Figure II For orientation. Figure I and Figure II Polarized optical microscopy images at angles of 0° and 45° between the molecular director and the polarizer / analyzer, respectively, demonstrate that the molecular arrangement in the liquid crystal elastomer after orientation is a monodomain molecular arrangement.

[0025] Figure 4 3 is the stress-strain curve of the liquid crystal elastomer solid film of the present invention before and after recycling. By comparing the stress-strain curves before and after recycling, it can be seen that the liquid crystal elastomer solid film still retains most of its mechanical properties after recycling.

[0026] Figure 5 is the DSC curve recorded by the differential scanning calorimeter of the liquid crystal elastomer solid film of the present invention, in which T g is the glass transition temperature, T i is the single-domain to multi-domain transition temperature.

[0027] Figure 6 This is the temperature rise curve of the liquid crystal elastomer solid film of the present invention under different power light. This figure shows that the doped photothermal dye has good photothermal conversion performance under near-infrared light. DETAILED DESCRIPTION

[0028] The present invention will be further described below by way of specific examples, but these specific embodiments do not limit the scope of protection of the present invention in any way.

[0029] Example 1

[0030] Synthesis of the spiroacetal (SCA) fragment of the LCE-SCA material: The raw materials 3,9-divinyl-2,4,8,10-tetraoxaspiro[5.5]undecane and 2-aminoethanethiol hydrochloride were dissolved in methanol. 2,2-dimethoxy-2-phenylacetophenone was added as a catalyst. The solution was stirred under ultraviolet light for 4 hours, and after stirring, it was cooled at -5°C for 24 hours. The obtained product was washed with methanol and filtered, and then the obtained product was placed in a vacuum oven at 60°C and dried for 12 hours. The dried product was placed in a 0.1 mol / L NaOH solution and stirred for 2 hours, then extracted with chloroform, rotary evaporated, and dried to obtain the final product. The preparation route is as follows:

[0031]

[0032] Example 2

[0033] Synthesis of the photothermal dye YHD-796 in LCE-SCA materials: Thiophene-2-thiol (639 mg, 5.5 mmol) and methyl isopipecolate (1.181 g, 8.25 mmol) were dissolved in toluene, sealed, protected by inert gas, and stirred at reflux for 2-10 hours. After the reaction was completed, the reaction mixture was cooled to room temperature and filtered through a short silica gel column using ethyl acetate as the eluent. The solvent was removed to obtain a pale yellow solid crude product. Subsequently, the crude product was purified by silica gel column chromatography using a silica gel solid phase and an eluent consisting of a mixture of n-hexane and ethyl acetate. After concentration, the pale yellow solid product 1-(thiophen-2-yl)piperidine-4-carboxylic acid methyl ester was obtained. 1-(thiophen-2-yl)piperidine-4-carboxylic acid methyl ester (1.00 g, 4.45 mmol) and 25 mL of 0.5 mol / L sodium hydroxide solution were added to a 100 mL round-bottom flask. The reaction mixture was heated at reflux for 3 hours. After cooling to room temperature, the mixture was washed with 10 mL of aqueous acetic acid (10% concentration). The precipitate was collected by back filtration and dried under vacuum to obtain 1-thiophene-2-yl-piperidine-4-carboxylic acid as a white solid. 1-thiophene-2-yl-piperidine-4-carboxylic acid (0.76 g, 3.60 mmol), n-decanol (0.68 g, 4.34 mmol), DMAP (0.02 g, 0.18 mmol) and anhydrous CH2Cl2 (25 mL) were added to a 50 mL Schlenk flask. Under a nitrogen atmosphere of 0°C, DCC (0.88 g, 4.27 mmol) was added to the flask at one time, and the reaction solution was stirred at room temperature for 24 hours. The solution was concentrated by rotary evaporation and column chromatography (petroleum ether: ethyl acetate 15:1) to obtain 1-thiophene-2-yl-piperidine-4-carboxylic acid decyl ester. Ketone acid 7 (0.14 g, 0.98 mmol) and 1-thiophene-2-yl-piperidine-4-carboxylic acid decyl ester (0.69 g, 1.96 mmol) were added to a solution of 30 mL of toluene / n-butanol, and the reaction mixture was heated to reflux for 1 hour. The mixture was transferred to a 100 mL round-bottom flask, diluted with 15 mL of dichloromethane, and the solution was concentrated by rotary evaporation. The black crude product solid obtained by column chromatography was purified by silica gel column chromatography (CH2Cl2: methanol 50: 1) to obtain the desired product YHD 796 as a black solid. The preparation route is as follows (the conditions of each step are as described in the above text of this paragraph):

[0034]

[0035] Example 3

[0036] Preparation of LCE-SCA796 film: 2-Methyl-1,4-phenylenebis-(4-((6-(acryloyloxy)hexyl)oxy)benzoate (RM82) (0.44 mmol, 302.06 mg), 3,6-dioxa-1,8-octanedithiol (EDDET) (0.22 mmol, 42.22 mg) and acetal fragment (0.11 mmol, 40.32 mg), and YHD-796 (1 wt %, 4 mg) were added to a DMF solution, dissolved, poured into a polytetrafluoroethylene mold, and heated at 100°C for 3 h to obtain a uniform photoresponsive liquid crystal elastomer film. The preparation route is as follows:

[0037]

[0038] in, EDDET is a chain extender. It is the liquid crystal monomer RM82. is an acetal fragment (acting as a cross-linking agent in the cross-linked network), It is photothermal dye YHD-796.

[0039] Example 4

[0040] Preparation of LCE-SCA film: 2-Methyl-1,4-phenylenebis-(4-((6-(acryloyloxy)hexyl)oxy)benzoate (RM82) (0.44 mmol, 302.06 mg), 3,6-dioxa-1,8-octanedithiol (EDDET) (0.22 mmol, 42.22 mg), and acetal fragment (0.11 mmol, 40.32 mg) were dissolved in DMF solution, poured into a polytetrafluoroethylene mold, and heated at 100°C for 3 h to obtain a liquid crystal elastomer film containing acetal fragments. The preparation route is as follows:

[0041]

[0042] in, EDDET is a chain extender. It is the liquid crystal monomer RM82. It is an acetal segment (acting as a cross-linking agent in the cross-linked network).

[0043] Example 5

[0044] Preparation of a double-layer liquid crystal elastomer film: An LCE-SCA film and an LCE-SCA796 film of the same shape are stacked on top of each other and hot-pressed at 140 degrees Celsius for 1 hour to obtain a double-layer liquid crystal elastomer film.

[0045] Example 6

[0046] Preparation of a near-infrared light-responsive double-layer liquid crystal elastomer film: The double-layer liquid crystal elastomer film in Example 5 is stretched 40% or programmed into a desired shape, and heated in an oven at 140°C for 30 minutes to obtain a near-infrared light-responsive double-layer liquid crystal elastomer.

Claims

1. A double-layer near-infrared light-responsive liquid crystal elastomer, characterized in that A layer of liquid crystal elastomer film containing a photothermal dye and a layer of liquid crystal elastomer film not containing a photothermal dye are combined into a double-layer liquid crystal elastomer film through heat pressing; and then oriented by mechanical stretching to obtain the film.

2. The method for preparing a double-layer near-infrared light-responsive liquid crystal elastomer according to claim 1, characterized in that The following steps are involved: (1) Synthesis of acetal fragments of liquid crystal elastomer materials: The raw materials 3,9-divinyl-2,4,8,10-tetraoxaspiro[5.5]undecane and 2-aminoethanethiol hydrochloride were dissolved in a methanol solution and 2,2-dimethoxy-2-phenylacetophenone was added as a catalyst to obtain a spiral acetal fragment with amino groups at both ends; (2) Synthesis of photothermal dye YHD-796 in near-infrared light-responsive liquid crystal elastomer material: raw materials thiophene-2-thiol and methyl isopipecolate were added to a flask containing a toluene solution and refluxed, the obtained product was added with n-decyl alcohol, 2,2-dihydroxymethylpropionic acid and anhydrous dichloromethane, N,N'-dicyclohexylcarbodiimide was added to the flask and stirred at 0°C under a nitrogen atmosphere; then filtered, rotary evaporated, and purified by silica gel chromatography to obtain a preliminary product; further, ketoic acid and the preliminary product were added to a toluene / n-butanol solution, and the reaction mixture was heated to reflux; the mixture was transferred and diluted with dichloromethane, the solution was concentrated by rotary evaporation, and the black solid obtained by column chromatography was purified by silica gel column chromatography to obtain photothermal dye YHD-796; (3) Preparation of dynamic bond cross-linked near-infrared light-responsive liquid crystal elastomer film: 1,4-bis-[4-(6-acryloyloxyhexyloxy)benzoyloxy]-2-methylbenzene, 3,6-dioxa-1,8-octanedithiol, the acetal fragment obtained in step (1) and YHD-796 obtained in step (2) were added to N,N-dimethylformamide solution, dissolved and poured into a polytetrafluoroethylene mold and heated to obtain a uniform LCE-SCA796 film; (4) Preparation of dynamic bond cross-linked liquid crystal elastomer film: raw materials 1,4-bis-[4-(6-acryloyloxyhexyloxy)benzoyloxy]-2-methylbenzene, 3,6-dioxa-1,8-octanedithiol, and the acetal fragment obtained in step (1) are added to N,N-dimethylformamide solution, dissolved and poured into a polytetrafluoroethylene mold and heated to obtain a uniform LCE-SCA film; (5) Preparation of a near-infrared light-responsive double-layer liquid crystal elastomer: stacking the dynamically cross-linked near-infrared light-responsive liquid crystal elastomer film obtained in step (3) and the dynamically cross-linked liquid crystal elastomer film obtained in step (4) and placing them in a hot press to bond them together through dynamic exchange of dynamic covalent bonds; (6) Preparation of near-infrared light-responsive double-layer liquid crystal elastomer film: The double-layer liquid crystal elastomer film in step (5) is stretched by 40% or programmed into a desired shape, and heated in an oven at 140°C for 30 minutes to obtain a double-layer near-infrared light-responsive liquid crystal elastomer with dynamic bonds.

3. The method for preparing a double-layer near-infrared light-responsive liquid crystal elastomer according to claim 1, characterized in that The spiroacetal described in step (1) is a class of molecules containing spiroacetal and its derivatives.

4. The method for preparing a double-layer near-infrared light-responsive liquid crystal elastomer according to claim 1, characterized in that The spiroacetal described in step (1) is 3,9-divinyl-2,4,8,10-tetraoxaspiro[5.5]undecane.

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