A liquid crystal elastomer based on orthogonal click chemistry, its preparation method and a liquid crystal driving element

Abstract

The present invention discloses a liquid crystal elastomer, a preparation method thereof, and a liquid crystal driving element. The liquid crystal elastomer is obtained by polymerizing a liquid crystal monomer, a photo-base, a chain extender, and a crosslinking agent. The liquid crystal monomer is selected from bis-terminal vinyl liquid crystal monomers and bis-terminal epoxy liquid crystal monomers. The chain extender is selected from dithiol monomers. The crosslinking agent is selected from one or more of trithiol monomers and tetrathiol monomers. The liquid crystal elastomer of the present invention is based on orthogonal click chemistry, and the polymerization process is divided into two steps. The first step is to generate a lightly crosslinked network through photo-initiated thiol-ene click chemistry. After mechanically stretching the sample of the first step, the second step of crosslinking is carried out through base-catalyzed thiol-epoxy click chemistry to obtain a single-domain liquid crystal elastomer. The present invention prepares a liquid crystal elastomer through orthogonal thiol-ene and thiol-epoxy click chemistry. The relatively fast reaction shortens the preparation time of the liquid crystal elastomer, and at the same time, the regular main chain structure endows it with a large driving strain.

Description

Technical Field

[0001] The present invention belongs to the technical field of liquid crystal materials, and particularly relates to a liquid crystal elastomer, a preparation method thereof, and a liquid crystal driving element. Background Art

[0002] Liquid crystal elastomers (LCEs) are materials with intelligent deformation ability, which can produce reversible deformation when subjected to external stimuli (such as light, heat, magnetic field, humidity), so they have broad application prospects in the fields of soft robots and driving devices.

[0003] The key to the reversible deformation of LCEs is to control the overall liquid crystal (LC) orientation in the crosslinked network to form a single-domain liquid crystal elastomer. At present, there are various techniques to induce macroscopic LC orientation, such as mechanically stretching partially cured LCEs and then performing photopolymerization secondary crosslinking (two-step method); surface orientation or field-induced orientation of active LC monomers, etc. Although surface and field-induced orientation techniques allow the preparation of LCEs with complex LC orientations, they are not suitable for manufacturing large and thick samples. In contrast, the mechanical stretching two-step method has no such limitations, so the two-step method is still the most widely used strategy for preparing single-domain LCE actuators. However, the current two-step method mainly relies on the homopolymerization of acrylates to form the final crosslinked network. Problems such as oxygen inhibition of polymerization and the uniformity of the crosslinked network pose great challenges to material processing. Summary of the Invention

[0004] To solve the above technical problems, the present invention provides a preparation method of a liquid crystal elastomer based on thiol-ene and thiol-epoxy click chemistry. In this method, a preliminary crosslinked product is first formed by thiol-ene reaction, and after mechanical stretching, the final crosslinked network is fixed by thiol-epoxy reaction.

[0005] The first aspect of the present invention provides a liquid crystal elastomer, which is obtained by a polymerization reaction of a liquid crystal monomer, a photo-base, a chain extender, and a crosslinking agent, wherein the liquid crystal monomer is selected from the compounds represented by formula (I) and formula (II), the chain extender is selected from dithiol monomers, and the crosslinking agent is selected from one or more of trithiol monomers and tetrathiol monomers.

[0006]

[0007] Among them, formula (I) is a double-terminal vinyl liquid crystal monomer, and formula (II) is a double-terminal epoxy liquid crystal monomer; in formula (I) and formula (II), R1, R2, R3, and R4 each independently represent hydrogen or methyl.

[0008] The second aspect of the present invention provides a preparation method of the above liquid crystal elastomer, comprising the following steps:

[0009] (a) Provide a mixture containing the liquid crystal monomer, photoinitiator, photo-base, chain extender, and crosslinking agent. Melt and mix the mixture evenly at 85 °C to 95 °C, and then initiate the polymerization reaction of the double-ended vinyl liquid crystal monomer, chain extender, and crosslinking agent at 120 °C to 160 °C with a light source of the corresponding wavelength of the photoinitiator to generate a preliminary crosslinked product;

[0010] (b) Stretch the preliminary crosslinked product at room temperature with a deformation rate of 100% to 150%, where the deformation rate is the percentage of the increase in the size of the liquid crystal elastomer in the stretching direction to the original size of the liquid crystal elastomer in the stretching direction;

[0011] (c) Expose the stretched preliminary crosslinked product to a light source with a suitable wavelength to generate an alkali catalyst from the photo-base, and react the double-ended epoxy liquid crystal monomer with the remaining chain extender and crosslinking agent by heating to obtain a single-domain liquid crystal elastomer. The reaction temperature is 70 °C to 120 °C, and the irradiation time is 0.5 to 2 h.

[0012] The third aspect of the present invention provides a liquid crystal driving device, which uses the liquid crystal elastomer according to the present invention.

[0013] The present invention prepares a single-domain liquid crystal elastomer by a two-step mechanical stretching method using a suitable liquid crystal monomer, chain extender, and crosslinking agent, which is different from the common two-step method. This method realizes the fixation of the crosslinked network through thiol-epoxy click chemistry, and can effectively solve problems such as oxygen inhibition of polymerization and large volume shrinkage rate. Moreover, the single-domain liquid crystal elastomer of the present invention has excellent stability. Therefore, by using the liquid crystal elastomer of the present invention, the liquid crystal driving element can have high driving stability. Description of the Drawings

[0014] In order to more clearly illustrate the technical solutions in the embodiments of the present invention, the following will briefly introduce the drawings required for the description of the embodiments. Obviously, the following drawings are only some embodiments of the present invention. For those of ordinary skill in the art, without creative efforts, other drawings can also be obtained based on these drawings.

[0015] Figure 1 It is a schematic diagram of the preparation process of the single-domain liquid crystal elastomer provided by an embodiment of the present invention.

[0016] Figure 2 It is a test chart of the driving deformation rate of Embodiment 1 of the present invention.

[0017] Figure 3 It is the Fourier transform infrared spectrum diagram of Embodiment 1 of the present invention. Among them Figure 3 (a) is the Fourier transform infrared spectrum diagram of the liquid crystal monomer and reactants under different conditions,Figure 3 (b) is the spectrogram of C-O under corresponding conditions, Figure 3 (c) is the spectrogram of C═C under corresponding conditions, Figure 3 (d) is the spectrogram of -SH under corresponding conditions, Figure 3 (e) is the spectrogram of -OH under corresponding conditions.

[0018] Figure 4 is the comparison of the preparation time and driving strain of Example 1 of the present invention with those of the other two-step methods in terms of preparation time and driving strain.

[0019] Figure 5 is the Fourier transform infrared spectrogram of the comparative example of the present invention. Detailed implementation manners

[0020] In order to make the invention object, technical solution and beneficial technical effects of the present invention clearer, the present invention will be described in detail below with reference to specific embodiments. It should be understood that the embodiments described in this specification are only for explaining the present invention and not for limiting the present invention.

[0021] For simplicity, only some numerical ranges are explicitly disclosed herein. However, any lower limit can be combined with any upper limit to form a range not explicitly recited; and any lower limit can be combined with other lower limits to form a range not explicitly recited, and similarly any upper limit can be combined with any other upper limit to form a range not explicitly recited. In addition, although not explicitly recited, each point or single value between the range endpoints is included in the range. Thus, each point or single value can be used as its own lower or upper limit and combined with any other point or single value or combined with other lower or upper limits to form a range not explicitly recited.

[0022] In the description herein, it should be noted that unless otherwise specified, "above" and "below" include the recited number, and in "one or more", the meaning of "more" is two or more.

[0023] The above invention content of the present invention does not intend to describe every disclosed embodiment or every implementation manner of the present invention. The following description more specifically illustrates exemplary embodiments. Throughout the application, guidance is provided through a series of embodiments, which can be used in various combinations. In each embodiment, the listings are only representative groups and should not be construed as exhaustive.

[0024] The present invention provides a method for preparing a liquid crystal elastomer, the method comprising the following steps:

[0025] (a) subjecting a liquid crystal monomer, a photo-base, a chain extender and a crosslinking agent to a polymerization reaction to obtain a liquid crystal elastomer;

[0026] (b) Stretch the preliminary cross-linked product at a deformation rate of 100% to 150% at room temperature;

[0027] (c) Expose the stretched preliminary cross-linked product to a light source with an appropriate wavelength to generate an alkali catalyst from the photo-base, and react the bis-terminal epoxy liquid crystal monomer with the remaining chain extender and cross-linking agent under alkali catalysis by heating to obtain a single-domain liquid crystal elastomer.

[0028] In step (a), the liquid crystal monomer is selected from bis-terminal acrylate liquid crystal monomers. In some embodiments, both ends of the main chain of the bis-terminal acrylate liquid crystal monomer are acrylate groups. In some embodiments, preferably, the liquid crystal monomer is selected from the compounds represented by formula (I) and formula (II).

[0029]

[0030] In formula (I) and formula (II), the R 1 , R 2 , R 3 , R 4 each independently represents hydrogen or methyl,

[0031] In some embodiments, the bis-terminal epoxy liquid crystal monomer can be selected from one or more of 2-methyl-1,4-phenylene bis(4-(oct-7-en-1-yloxy)benzoate) and 1,4-phenylene bis(4-(oct-7-en-1-yloxy)benzoate), and the bis-terminal vinyl liquid crystal monomer is selected from one or more of 2-methyl-1,4-phenylene bis(4-((6-(oxiran-2-ylhexyl)oxy)benzoate) and 1,4-phenylene bis(4-((6-(oxiran-2-yl)hexyl)oxy)benzoate).

[0032] In step (a), the chain extender is selected from dithiol monomers. In some embodiments, both ends of the main chain of the dithiol monomer are thiol groups. As an example, the chain extender can be selected from one or more of ethylene glycol bis(3-mercaptopropionate) (EGBM), 3,6-dioxa-1,8-octanedithiol (EDDET), and 1,6-hexanedithiol.

[0033] In step (a), the cross-linking agent is selected from one or more of trithiol monomers and tetrathiol monomers. As an example, the cross-linking agent can be selected from one or more of 2-ethyl-2-[(3-mercapto-1-oxopropoxy)methyl]-1,3-propanediyl bis(3-mercaptopropionate) and pentaerythritol tetrakis(3-mercaptopropionate).

[0034] In some preferred embodiments, the ratio of the total molar amount of epoxy groups and vinyl groups in the liquid crystal monomer to the total molar amount of the chain extender and the crosslinking agent in step (a) is 1:1. The total molar amount of epoxy groups and vinyl groups in the liquid crystal monomer refers to the sum of the amounts of epoxy groups and vinyl groups contained in all liquid crystal monomers in terms of moles. The total molar amount of mercapto groups in the chain extender and the crosslinking agent refers to the sum of the amounts of mercapto groups contained in all chain extenders and the amounts of mercapto groups contained in all crosslinking agents in terms of moles. The ratio of the total molar amount of acryloyloxy groups in the liquid crystal monomer to the total molar amount of mercapto groups in the chain extender and the crosslinking agent is within an appropriate range, which is beneficial to making the reaction more complete and generating fewer impurities, thereby facilitating the obtaining of a monodomain liquid crystal elastomer with more excellent and stable driving performance.

[0035] In some preferred embodiments, in step (a), the molar ratio of the bis-terminal vinyl liquid crystal monomer to the bis-terminal epoxy liquid crystal monomer is 1:1 to 9:1. More preferably, the molar ratio of the bis-terminal vinyl liquid crystal monomer to the bis-terminal epoxy liquid crystal monomer is 1:1 to 6:1, and further preferably 2:1 to 4:1. The vinyl group and the epoxy group are reactive groups and participate in the thiol-ene and thiol-epoxy click chemical reactions respectively. The reason for maintaining the above ratio is that in the two-step method for preparing liquid crystal elastomers, the first step requires the preparation of a lightly crosslinked network, and the above ratio can make the lightly crosslinked network generated in the first step have appropriate mechanical properties and liquid crystallinity, so as to carry out the second step reaction.

[0036] In some embodiments, the photoinitiator required in step (a) can be selected from free radical photoinitiators known in the art. For example, photoinitiator 651, photoinitiator 369, photoinitiator 784, etc. The wavelength of the irradiation light source is determined according to the selected initiator. For example, if photoinitiator 651 is selected, a UV light source with a wavelength of 365 nm is selected. The light intensity is 10 - 150 mW / cm 2 , preferably, the light intensity is 50 - 140 mW / cm 2 , and further preferably 80 - 120 mW / cm 2 , such as 100 mW / cm 2 .

[0037] Through the polymerization reaction in step (a), the vinyl liquid crystal monomer reacts with part of the chain extender and the crosslinking agent to generate a preliminary crosslinked product. The temperature of the polymerization reaction can be selected as 120°C - 160°C, or 130°C - 150°C, such as 140°C. The polymerization reaction time can be selected as 1 - 5 min, or 2 - 4 min, such as 3 min.

[0038] In some embodiments, the liquid crystal elastomer can be subjected to subsequent treatments such as cutting and folding to obtain the desired shape (such as a cross shape, a star shape, etc.).

[0039] In step (b), the deformation rate can be selected from 100% to 150%, such as 120%.

[0040] In some embodiments, the photo-base required in step (c) has been added in step (a). The photo-base can be synthesized by a general method from an alkali catalyst known in the art and a corresponding complex. The alkali catalyst can be selected from the catalysts known in the art for catalyzing the polymerization reaction of epoxy monomers with mercapto chain extenders and mercapto crosslinkers. For example, one or more of 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU), 1,5-diazabicyclo[4.3.0]-5-nonene (DBN), and 1,5,7-triazabicyclo-[4.4.0]dec-5-ene (TBD), and the corresponding complex can be selected from one or more of ketoprofen and sodium tetraphenylborate. For example, if ketoprofen is selected to be combined with the corresponding alkali catalyst, the irradiation wavelength is selected to be 365 nm. It should be noted that when selecting the photo-base, the wavelength for exciting the photo-base to produce the base cannot coincide with the wavelength for initiating the photoinitiator used in step (a). For example, when photoinitiator 651 is selected, the photo-base should be selected as the complex of sodium tetraphenylborate and the alkali catalyst. In some embodiments, based on the total mass of the liquid crystal monomer, chain extender, and crosslinker in the mixed solution, the usage amount of the photo-base is 0.1 wt% to 2 wt%, preferably 0.5 wt% to 1.5 wt%, such as 1 wt%.

[0041] Through the polymerization reaction in step (c), the epoxy liquid crystal monomer reacts with the remaining chain extender and crosslinker to form the final liquid crystal elastomer. The temperature of the polymerization reaction can be selected from 60 to 90 °C, or 70 to 80 °C, such as 75 °C. The polymerization reaction time can be selected from 10 - 40 min, or 20 - 30 min, such as 25 min.

[0042] The present invention prepares a single-domain liquid crystal elastomer by adopting a mechanical stretching two-step method through the polymerization reaction of appropriate liquid crystal monomers, chain extenders, and crosslinkers. The commonly used two-step method at present mainly forms the final crosslinked network through the homopolymerization reaction of acrylic acid. The polymer network in the present invention is mainly formed by thiol-epoxy click chemistry, and the final crosslinked network is fixed by thiol-ene click chemistry, solving the problems of oxygen inhibition of polymerization and network homogeneity. At the same time, the system of the present invention has strong adjustability and fast preparation speed, providing a new idea for the preparation of single-domain liquid crystal elastomers.

[0043] The present invention also provides a liquid crystal elastomer obtained according to the above preparation method.

[0044] The present invention also provides a liquid crystal driving device, which adopts the liquid crystal elastomer according to the present invention. The liquid crystal elastomer of the present invention can be applied to any field or device that requires a flexible driver, such as driving devices, soft robots, artificial muscles, bionic devices, etc. Further, since the liquid crystal driving element adopts the liquid crystal elastomer of the present invention, it can have high driving stability.

[0045] Examples

[0046] The following examples more specifically describe the content disclosed in the present invention. These examples are only for illustrative purposes, because various modifications and variations within the scope of the present invention are obvious to those skilled in the art. Unless otherwise stated, all parts, percentages, and ratios reported in the following examples are based on weight, and all reagents used in the examples are commercially available or synthesized according to conventional methods and can be used directly without further treatment, and all instruments used in the examples are commercially available.

[0047] Example 1

[0048] At 25 °C, 1 mmol of liquid crystal monomer 2-methyl-1,4-phenylene bis(4-((6-(oxiran-2-yl)hexyl)oxy)benzoate), 5 mmol of liquid crystal monomer 2-methyl-1,4-phenylene bis(4-(oct-7-en-1-yloxy)benzoate), 4 mmol of chain extender 3,6-dioxa-1,8-octanedithiol and 1 mmol of crosslinker pentaerythritol tetra(3-mercaptopropionate) were added to a glass bottle. The mixture was heated and melted at 90 °C and stirred evenly. Based on the total mass of the liquid crystal monomer, chain extender and crosslinker in the mixture, 1 wt% of a photo-base (ketoprofen and DBU complex) and 1 wt% of a photoinitiator 784 were added dropwise to the mixture and mixed evenly again. The mixture containing the base catalyst was poured into a polytetrafluoroethylene mold with an inner groove size of 30 mm in length, 30 mm in width and 1 mm in depth, irradiated with a light source with a wavelength of 420 nm and polymerized at 120 °C for 3 min. After the reaction, the polymer was cooled and demolded to obtain a preliminary crosslinked product.

[0049] The preliminary crosslinked product was cut into strips with a length of 25 mm, a width of 3 mm and a thickness of 0.1 mm, and uniaxially stretched along the length direction, with a stretching deformation rate of 120%. After the stretched sample was fixed on a hollow polytetrafluoroethylene support and placed on a 90 °C heating table, it was irradiated with ultraviolet light with a wavelength of 365 nm for 15 min on each side, and the deformation was permanently fixed to obtain a single-domain liquid crystal elastomer.

[0050] Figure 1The present invention provides a schematic diagram of the preparation process of a monodomain liquid crystal elastomer provided by an embodiment of the present invention, which is specifically described as follows: After the reactants are melted and mixed, they are added into a mold, and a free radical-induced polymerization reaction of a bivalent vinyl liquid crystal monomer with a chain extender and a cross-linking agent is carried out under the initiation of 420nm visible light. After the reaction is completed, the product is cooled and demolded to obtain a preliminary cross-linked product. The preliminary cross-linked product is cut and uniaxially stretched, and photobase decomposed under ultraviolet light to produce a base catalyst, which catalyzes the thermal polymerization of the bivalent epoxy liquid crystal monomer with the remaining chain extender and cross-linking agent to obtain a monodomain liquid crystal elastomer.

[0051] Figure 2 This is a test diagram of the drive deformation rate of the monodomain liquid crystal elastomer of Example 1. A THMS600 (LinkamScientific) hot and cold stage was used to control the sample temperature. The monodomain liquid crystal elastomer sample was placed on the sample holder of the hot and cold stage and thermally cycled between 30-150°C to measure the shape change rate (drive strain) along the orientation direction. The drive strain was calculated using the following formula:

[0052]

[0053] Wherein L0 is the length of the liquid crystal elastomer in the isotropic state, L is the maximum length after being stabilized in the single domain state, and the length of the liquid crystal elastomer is determined by a photograph taken by a polarizing microscope (OLYMPUS BX35) equipped with a CCD, and the length is measured using the measurement tool provided by the software. The test results show that the liquid crystal elastomer of the embodiment has stable driving performance in multiple temperature rise and fall cycles.

[0054] Figure 3 is the infrared spectrum of Example 1 of the present invention, wherein Figure 3 (a) shows the reaction degree of liquid crystal monomers and reactants under different conditions. Before the reaction, they can be respectively at 1640cm -1 、790cm -1 、2570cm -1 The characteristic peaks of C=C, CO, and -SH were observed on the left and right. After 3 minutes of polymerization initiated by 420nm visible light, the characteristic peak of -SH weakened, the characteristic peak of C=C basically disappeared, and the characteristic peak of CO did not change, which proved that the thiol-ene reaction proceeded. At the same time, no thiol epoxidation reaction occurred at this time. After 30 minutes of reaction under ultraviolet light irradiation, the characteristic peaks of CO and -SH basically disappeared, and at the same time, the peak at 3450cm -1 The formation of the left and right hydroxyl characteristic peaks proves the progress of the thiol-epoxy reaction, at which time the monomer, chain extender and cross-linker all have high conversion rates. Figure 3 (b) to (e) show the intensity changes of the characteristic peaks of different groups during the reaction.

[0055] In the existing thiol-acrylate reaction, the homopolymerization problem of acrylate will affect the orthogonality of the reaction, resulting in a competitive reaction between thiol-acrylate and acrylate homopolymerization during the polymerization process, which reduces the uniformity of the polymer crosslinking network. However, the better orthogonality between thiol-ene and thiol-epoxy in this application is an inherent property from the reaction mechanism, and the above infrared results also prove the good orthogonality and feasibility of the two-step method of this design. At the same time, compared with other mechanical stretching two-step methods, this method has a shorter preparation time and a higher driving strain.

[0056] Figure 4 This is a comparison of the preparation time and driving strain of Example 1 of the present invention with other two-step methods. Compared with some methods reported in the literature, the preparation time of Example 1 is shortened to about 30 min, and the driving strain is increased to 100%. Figure 4 The relevant literature involved is: [1] ACS Macro Lett., 2023, 12, 248 - 254; [2] Angew. Chem. Int. Ed., 2023, 62, e202214339; [3] Adv. Mater., 2021, 33, 2101955; [4] Sci. Adv., 2020, 6, eaay8606; [5] Polym. Chem., 2017, 8, 1364 - 1370; [6] Polym. Chem., 2015, 6, 4835 - 4844; [7] Science, 2015, 347, 982 - 984. The asterisk represents the preparation time corresponding to Example 1 of the present invention.

[0057] Comparative Example

[0058] At 25 °C, 7 mmol of liquid crystal monomer 2-methyl-1,4-phenylene-bis(4-((6-(acryloyloxy)hexyl)oxy)benzoate) (RM82), 4 mmol of chain extender 3,6-dioxa-1,8-octanedithiol, and 1 mmol of crosslinker pentaerythritol tetrakis(3-mercaptopropionate) were added to a glass bottle. The mixture was heated and melted at 90 °C and stirred evenly. Based on the total mass of the liquid crystal monomer, chain extender, and crosslinker in the mixture, 1 wt% of base catalyst DPA and 1 wt% of photoinitiator 651 were added dropwise to the mixed solution and mixed evenly again. The catalyst-containing mixed solution was poured into a polytetrafluoroethylene mold with an inner groove size of 30 mm in length, 30 mm in width, and 1 mm in depth, and polymerized at 80 °C for 24 min. After the reaction, the polymer was cooled and demolded to obtain a preliminary crosslinked product.

[0059] The preliminary cross-linked product was cut into strips with a length of 25 mm, a width of 3 mm, and a thickness of 0.1 mm, and uniaxially stretched along the length direction, with a tensile deformation rate of 120%. The stretched sample was fixed on a hollow tetrafluoroethylene support and irradiated on each side with ultraviolet light with a wavelength of 365 nm for 30 min, and the deformation was permanently fixed to obtain a single-domain liquid crystal elastomer.

[0060] Fourier transform infrared spectroscopy test

[0061] The reaction process of the comparative example was detected by Fourier transform infrared spectroscopy, and the results are as Figure 5 shown. After the reaction ended, the C═C characteristic peak of acrylate at 1640 cm -1 basically disappeared, but there was still an obvious signal of the -SH characteristic peak at about 2570 cm -1 This is mainly because RM82 not only participated in the thiol-acrylate addition reaction, but also a large amount of acrylate reacted through its own homopolymerization, resulting in a low reaction conversion rate of the thiol chain extender and the cross-linking agent.

[0062] The comparative example and Example 1 were prepared with the same material ratio, and the difference lies in the different liquid crystal monomers and reaction mechanisms used. From Figure 3 and Figure 5 it can be seen that in Example 1, the reaction was basically complete after 30 min of ultraviolet light irradiation. In the comparative example, a large amount of thiol remained after 24 h of reaction and the reaction was not complete. Therefore, the two-step method designed by the present invention has better orthogonality and greatly shortens the preparation time of the liquid crystal elastomer.

[0063] As described above, it is only the specific implementation manner of the present invention, but the protection scope of the present invention is not limited thereto. Any person skilled in the art within the technical scope disclosed by the present invention can easily think of various equivalent modifications or substitutions, and these modifications or substitutions should all be covered within the protection scope of the present invention. Therefore, the protection scope of the present invention shall be subject to the protection scope determined by the claims.

Claims

1. A liquid crystal elastomer based on orthogonal click chemistry, characterized in that, It is obtained by the reaction of a liquid crystal monomer, a photo-base, a chain extender and a cross-linking agent. Among them, the liquid crystal monomer is selected from the compounds shown in formula (I) and (II), the chain extender is selected from dithiol monomers, and the cross-linking agent is selected from one or more of trithiol monomers and tetrathiol monomers. Among them, formula (I) is a double-terminal vinyl liquid crystal monomer, and formula (II) is a double-terminal epoxy liquid crystal monomer; in formula (I) and formula (II), R1, R2, R3, and R4 each independently represent hydrogen or methyl. The preparation method of the liquid crystal elastomer includes the following steps: (a) Provide a mixture containing the liquid crystal monomer, a photoinitiator, a photo-base, a chain extender and a cross-linking agent, melt and mix the mixture evenly at 85 °C to 95 °C, and then use a light source with a corresponding wavelength of the photoinitiator to initiate the polymerization reaction of the double-terminal vinyl liquid crystal monomer, the chain extender and the cross-linking agent at 120 °C to 160 °C to generate a preliminary cross-linked product. (b) Stretch the preliminary cross-linked product at room temperature with a deformation rate of 100% to 150%, where the deformation rate is the percentage of the increase in the size of the liquid crystal elastomer in the stretching direction to the original size of the liquid crystal elastomer in the stretching direction. (c) Expose the stretched preliminary cross-linked product to a light source with a suitable wavelength to generate a base catalyst from the photo-base, and react the double-terminal epoxy liquid crystal monomer with the remaining chain extender and cross-linking agent by heating to obtain a single-domain liquid crystal elastomer. The reaction temperature is 70 °C to 120 °C, and the irradiation time is 0.5 to 2 h.

2. The liquid crystal elastomer according to claim 1, characterized in that, The ratio of the total molar amount of the vinyl group and epoxy group of the liquid crystal monomer to the total molar amount of the chain extender and the cross-linking agent is 1:

1.

3. The liquid crystal elastomer according to claim 1, wherein The molar ratio of the double-terminal vinyl liquid crystal monomer to the double-terminal epoxy liquid crystal monomer is 1:1 to 9:

1.

4. The liquid crystal elastomer according to claim 1, characterized in that, The molar ratio of the cross-linking agent to the chain extender is 1:1 to 1:

8.

5. The liquid crystal elastomer according to claim 1, wherein, The double-terminal epoxy liquid crystal monomer is selected from one or more of 2-methyl-1,4-phenylene bis(4-((6-(oxiran-2-yl)hexyl)oxy)benzoate) and 1,4-phenylene bis(4-((6-(oxiran-2-yl)hexyl)oxy)benzoate). The double-terminal vinyl liquid crystal monomer is selected from one or more of 2-methyl-1,4-phenylene bis(4-(oct-7-en-1-yloxy)benzoate) and 1,4-phenylene bis(4-(oct-7-en-1-yloxy)benzoate).

6. The liquid crystal elastomer according to claim 1, wherein The photo-base is synthesized from a base catalyst and a corresponding complex. The base catalyst is selected from one or more of 1,8-diazabicyclo[5.4.0]undec-7-ene, 1,5-diazabicyclo[4.3.0]-5-nonene, 1,5,7-triazabicyclo-[4.4.0]dec-5-ene, and the corresponding complex is selected from one or more of ketoprofen and sodium tetraphenylborate.

7. The liquid crystal elastomer according to claim 1, wherein The chain extender is selected from one or more of 3,6-dioxa-1,8-octanedithiol, ethylene glycol bis(3-mercaptopropionate), and 1,6-hexanedithiol.

8. The liquid crystal elastomer according to claim 1, wherein The crosslinking agent is selected from one or more of 2-ethyl-2-[(3-mercapto-1-oxopropoxy)methyl]-1,3-propanediyl bis(3-mercaptopropionate) and pentaerythritol tetrakis(3-mercaptopropionate).

9. A liquid crystal driving element, wherein the liquid crystal elastomer according to any one of claims 1-8 is used.

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