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 carrying out a polymerization reaction on a liquid crystal monomer, 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 dimercapto 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 base-catalyzed thiol-epoxy click chemistry. After mechanically stretching the sample of the first step, the second step of crosslinking is carried out through free-radical-initiated thiol-ene click chemistry to obtain a single-domain liquid crystal elastomer. The present invention realizes the fixation of the crosslinked network through thiol-ene 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.

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 capabilities that undergo reversible deformation when subjected to external stimuli (such as light, heat, magnetic fields, humidity), and thus 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. Currently, there are various techniques to induce macroscopic LC orientation, such as mechanically stretching partially cured LCEs and then performing photopolymerization for 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 remains 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-epoxy and thiol-ene click chemistry. In this method, a preliminary crosslinked product is first formed through a thiol-epoxy reaction, and after mechanical stretching, the final crosslinked network is fixed through a thiol-ene reaction.

[0005] The first aspect of the present invention provides a liquid crystal elastomer obtained by a polymerization reaction of a liquid crystal monomer, 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, chain extender, and crosslinking agent. Melt and mix the mixture evenly at 85°C to 95°C, and then add a catalyst to catalyze the polymerization reaction of the bis-terminal epoxy liquid crystal monomer, chain extender, and crosslinking agent at 120°C to 160°C 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 initiate the reaction of the bis-terminal vinyl liquid crystal monomer with the remaining chain extender and crosslinking agent to obtain a single-domain liquid crystal elastomer. The reaction temperature is room temperature, 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 suitable liquid crystal monomers, chain extenders, and crosslinking agents, which is different from the common two-step method. This method realizes the fixation of the crosslinked network through thiol-ene 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, using the liquid crystal elastomer of the present invention can make the liquid crystal driving element have higher 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 the single-domain liquid crystal elastomer in Embodiment 1 of the present invention.

[0017] Figure 3 It is the Fourier transform infrared spectrum of Embodiment 1 of the present invention. Among them Figure 3 (a) is the Fourier transform infrared spectrum of the reaction mixture under different conditions, Figure 3 (b) is the spectrum of C═C under the corresponding conditions,Figure 3 (c) is the spectrogram of C-O 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 a schematic diagram of the liquid crystal elastomer ring actuator in Example 2 of the present invention, as well as an example of the ring actuator grasping an object in hot water. Among them Figure 4 (a) is the form of the ring actuator at room temperature, Figure 4 (b) is its form at 80 °C, Figure 4 (c) is an example of the ring actuator shrinking and grasping an object in hot water, Figure 4 (d) is the size comparison between the ring actuator and the grasped object at room temperature.

[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 in conjunction with 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 recorded; and any lower limit can be combined with other lower limits to form a range not explicitly recorded, and similarly any upper limit can be combined with any other upper limit to form a range not explicitly recorded. In addition, although not explicitly recorded, each point or single value between the range endpoints is included in this 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 recorded.

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

[0023] The above-described invention content of the present invention does not intend to describe each disclosed embodiment or each 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 enumeration is only as a representative group 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) Reacting a liquid crystal monomer, a chain extender and a crosslinking agent through a polymerization reaction to obtain a liquid crystal elastomer;

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

[0027] (c) Exposing the stretched preliminary cross-linked product to a light source with a suitable wavelength to initiate the reaction of the bis-terminal vinyl liquid crystal monomer with the remaining chain extender and crosslinking agent 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 crosslinking agent is selected from one or more of trithiol monomers and tetrathiol monomers. As an example, the crosslinking 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 monomers in step (a) to the total molar amount of the chain extender and the crosslinking agent is 1:1. The total molar amount of epoxy groups and vinyl groups in the liquid crystal monomers 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 monomers 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 epoxy liquid crystal monomer to the bis-terminal vinyl liquid crystal monomer is 1:1 to 9:1. More preferably, the molar ratio of the bis-terminal epoxy liquid crystal monomer to the bis-terminal vinyl liquid crystal monomer is 1:1 to 6:1, and further preferably 2:1 to 4:1. The epoxy group and the vinyl group are reactive groups and participate in the thiol-epoxy and thiol-ene 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 for the second-step reaction.

[0036] In some embodiments, in step (a), the 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 crosslinking agents. For example, one or more of 1,8-diazabicyclo[5.4.0]undec-7-ene (BBU), 1,5-diazabicyclo[4.3.0]-5-nonene (DBN), and 1,5,7-triazabicyclo-[4.4.0]dec-5-ene (TBD). In some embodiments, based on the total mass of the liquid crystal monomers, the chain extender, and the crosslinking agent in the mixed solution, the usage amount of the catalyst is 0.1 wt% to 2 wt%, preferably 0.5 wt% to 1.5 wt%, such as 1 wt%.

[0037] Through the polymerization reaction in step (a), the epoxy 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 to 160°C, or 130°C to 150°C, such as 140°C. The polymerization reaction time can be selected as 15 - 35 min, or 20 - 30 min, such as 25 min.

[0038] In some embodiments, the polymerization reaction of step (a) can be carried out in a mold so that the resulting liquid crystal elastomer obtains a desired shape. The liquid crystal elastomer can be of any shape and can be selected according to actual needs. As an example, the liquid crystal elastomer can be in the form of a thin film, a polyhedron (such as a cuboid, a cube, a cross-shaped shape, etc.), a cylinder, etc. Of course, the desired shape can also be obtained by subsequent processing such as cutting the liquid crystal elastomer.

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

[0040] In some embodiments, the photoinitiator required in step (c) has been added to the system in advance in step (a). The photoinitiator can be selected from free radical photoinitiators known in the art. For example, photoinitiator 651, photoinitiator 369, 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 , more preferably 80 - 120 mW / cm 2 , such as 100 mW / cm 2 .

[0041] Through the polymerization reaction of step (c), the vinyl liquid crystal monomer reacts with the remaining chain extender and crosslinking agent to form the final liquid crystal elastomer. The temperature of the polymerization reaction is room temperature. The polymerization reaction time can be selected from 30 - 60 min, or 40 - 50 min, such as 45 min.

[0042] The present invention prepares a single-domain liquid crystal elastomer by using a suitable liquid crystal monomer, chain extender and crosslinking agent to carry out a polymerization reaction and adopting a two-step mechanical stretching method. The currently commonly used two-step method 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 uses 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 uses the liquid crystal elastomer of the present invention, it can have high driving stability.

[0045] Example

[0046] The following examples describe more specifically the disclosure of the present invention. These examples are for illustrative purposes only, as various modifications and variations within the scope of the present invention will be apparent to those skilled in the art. Unless otherwise stated, all parts, percentages, and ratios reported in the following examples are by weight, and all reagents used in the examples are commercially available or synthesized by 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, 5 mmol of the liquid crystal monomer 2-methyl-1,4-phenylene bis(4-((6-(oxiran-2-yl)hexyl)oxy)benzoate), 1 mmol of the liquid crystal monomer 2-methyl-1,4-phenylene bis(4-(oct-7-en-1-yloxy)benzoate), 4 mmol of the chain extender 3,6-dioxaoctane-1,8-dithiol, and 1 mmol of the 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 monomers, chain extender, and crosslinker in the mixture, 1 wt% of the catalyst DBU and 1 wt% of the photoinitiator 651 were added dropwise to the mixture and mixed evenly again. The catalyst-containing mixture 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 140 °C for 30 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 tensile deformation rate of 120%. The stretched sample was fixed on a polytetrafluoroethylene hollow 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 monodomain liquid crystal elastomer.

[0050] Figure 1 It is a schematic diagram of the preparation process of the monodomain liquid crystal elastomer provided by the embodiment of the present invention, which is specifically described as follows: After the reactants are melted and mixed, they are added into the mold. At a certain temperature, the bis-terminal epoxy liquid crystal monomer undergoes thermal polymerization under the action of the chain extender and crosslinker. After the reaction, it is cooled and demolded to obtain a preliminary crosslinked product. The preliminary crosslinked product is cut and then uniaxially stretched. Under ultraviolet light irradiation, the bis-terminal vinyl liquid crystal monomer undergoes photopolymerization with the remaining chain extender and crosslinker to obtain a monodomain liquid crystal elastomer.

[0051] Figure 2It is a test diagram of the driving deformation rate of the single-domain liquid crystal elastomer of Example 1. A THMS600 (Linkam Scientific) hot and cold stage was used to control the sample temperature. The single-domain liquid crystal elastomer sample was placed on the sample holder of the hot and cold stage, and a thermal cycle was performed between 30 - 80 °C. The shape change rate (driving strain) along the orientation direction was measured, and the driving strain was calculated using the following formula:

[0052]

[0053] where L0 is the length of the liquid crystal elastomer in the isotropic state, L is the maximum length after stabilization in the single-domain state, and the length of the liquid crystal elastomer was determined from the photos taken by a polarizing microscope (OLYMPUS BX35) equipped with a CCD. The length was measured using the measurement tool built into the software. The test results illustrate that the liquid crystal elastomer of the example has stable driving performance during multiple heating and cooling cycles.

[0054] Figure 3 This is the infrared spectrum diagram of Example 1 of the present invention, where Figure 3 (a) shows the polymerization degree of the liquid crystal monomer and reactants under different conditions. Before the reaction, characteristic peaks of C=C, C-O, and -SH could be observed at around 1640 cm -1 , 790 cm -1 , and 2570 cm -1 respectively. After reacting at 120 °C for 15 min, the characteristic peaks of -SH and C-O weakened, and characteristic peaks of -OH were generated at around 1650 cm -1 and 3450 cm -1 . After reacting at 120 °C for 30 min, the C-O characteristic peak basically disappeared, proving the progress of the thiol-epoxy reaction. At the same time, although the intensity of the C=C characteristic peak remained basically unchanged, it proved that no thiol-ene reaction occurred at this time. After reacting under ultraviolet light irradiation for 30 min, the characteristic peaks of C=C and -SH basically disappeared, proving the progress of the thiol-ene reaction. At this time, the monomer, chain extender, and crosslinker all had a high conversion rate. Figure 3 (b) to (e) are 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, reducing the uniformity of the polymer crosslinking network. However, the better orthogonality of 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.

[0056] Example 2

[0057] At 25 °C, 5 mmol of epoxy liquid crystal monomer 2-methyl-1,4-phenylene bis(4-((6-(oxiran-2-yl)hexyl)oxy)benzoate), 1 mmol of double bond liquid crystal monomer 2-methyl-1,4-phenylene bis(4-((6-(oxiran-2-ylhexyl)oxy)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 catalyst DBU and 1 wt% of photoinitiator 651 were added dropwise to the mixture and mixed evenly again. The mixture containing the catalyst was poured into a polytetrafluoroethylene mold with a circular ring groove. The outer diameter of the circular ring was 1.4 mm, the inner diameter was 1.0 mm, and the depth was 3 mm. Polymerization reaction was carried out at 140 °C for 30 min. After the reaction, the polymer was cooled and demolded to obtain a preliminary crosslinked product.

[0058] The preliminary crosslinked product was stretched and sleeved on a polytetrafluoroethylene rod with a diameter of 2 mm, and irradiated with ultraviolet light with a wavelength of 365 nm for 30 min on each side. The deformation was permanently fixed to obtain a circular liquid crystal elastomer actuator.

[0059] Figure 4 It is a schematic diagram of the liquid crystal elastomer circular actuator in Example 2 of the present invention, and an example of the circular actuator grasping an object in hot water. Figure 4 (a) shows the morphology of the circular actuator at room temperature. Figure 4 (b) shows its morphology at 80 °C, indicating that Figure 3 compared with (a), a shrinkage deformation occurred. Figure 4 (c) is an example of the circular actuator shrinking and grasping an object in hot water, Figure 4 (d) shows the size comparison between the circular actuator and the grasped object at room temperature.

[0060] Comparative example

[0061] 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 catalyst DPA and 1 wt% of photoinitiator 651 were added dropwise to the mixture and mixed evenly again. The catalyst-containing mixture 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 cross-linked product.

[0062] 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 polytetrafluoroethylene hollow support and irradiated with ultraviolet light with a wavelength of 365 nm for 30 min on each side, and the deformation was permanently fixed to obtain a single-domain liquid crystal elastomer.

[0063] Fourier transform infrared spectroscopy test

[0064] 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, the C=C characteristic peak of acrylate at 1640 cm -1 basically disappeared, but the -SH characteristic peak at about 2570 cm -1 still had obvious signals. This was 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 crosslinker.

[0065] The comparative example and Example 1 were prepared with the same material ratio, and the difference was the liquid crystal monomer and reaction mechanism 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 in the present invention has better orthogonality and greatly shortens the preparation time of liquid crystal elastomers.

[0066] 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 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 chain extender and a crosslinking 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 crosslinking 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 chain extender and a crosslinking agent, melt and mix the mixture evenly at 85°C to 95°C, and then add a catalyst to catalyze the polymerization reaction of the double-terminal epoxy liquid crystal monomer, the chain extender and the crosslinking agent at 120°C to 160°C to generate a preliminary crosslinked product. (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. (c) Expose the stretched preliminary crosslinked product to a light source with a suitable wavelength to initiate the reaction of the double-terminal vinyl liquid crystal monomer with the remaining chain extender and crosslinking agent to obtain a single-domain liquid crystal elastomer. The reaction temperature is room temperature, and the irradiation time is 0.5 to 2 h.

2. The liquid crystal elastomer according to claim 1, wherein The ratio of the total molar amount of the epoxy group and vinyl group of the liquid crystal monomer to the total molar amount of the chain extender and the crosslinking agent is 1:

1.

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

1.

4. The liquid crystal elastomer according to claim 1, wherein The molar ratio of the crosslinking agent to the chain extender is 1:1 to 1:

8.

5. The liquid crystal elastomer according to claim 1, wherein 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 double-terminal epoxy 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).

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, which uses the liquid crystal elastomer according to any one of claims 1-8.

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