An azobenzene polyurea-polyacrylate based double network liquid crystal elastomer and a method of making the same

Abstract

The application belongs to the technical field of high polymer materials, and particularly relates to an azobenzene polyurea-polyacrylate-based double-network liquid crystal elastomer and a preparation method thereof. The preparation method comprises the following steps: performing a polymerization reaction on polyols and diisocyanate and azobenzene monomers in an organic solvent under the action of a catalyst to obtain azobenzene polyurea; dissolving liquid crystal monomers, mercaptan, a crosslinking agent containing mercapto, a photoinitiator and a catalyst in an organic solvent to obtain a mixed solution; performing a polymerization reaction on the azobenzene polyurea and the mixed solution after blending to obtain a multi-domain liquid crystal elastomer material; performing uniaxial stretching on the multi-domain liquid crystal elastomer material, and performing light curing to obtain a single-domain azobenzene polyurea-polyacrylate-based double-network liquid crystal elastomer. The azobenzene liquid crystal element network and the polyacrylate network are interpenetrated to improve the driving strain under the action of a small load, and the preparation of a photo-thermal driving double-response liquid crystal elastomer is realized.

Description

Technical Field

[0001] The invention belongs to the technical field of high molecular polymer materials, and particularly relates to an azobenzene polyurea-polyacrylate-based double-network liquid crystal elastomer and a preparation method thereof. Background Art

[0002] Liquid crystal elastomers (LCEs) organically combine liquid crystals and polymer networks, possessing the dual properties of both liquid crystals and elastomers. Mesogens are attached to the polymer backbone, which is weakly cross-linked to form a moderately cross-linked polymer network, ultimately forming the LCE. The cross-links maintain its shape, while the liquid state imparts fluidity. LCEs possess the properties of both elastomers and liquid crystals, such as elasticity, order, and fluidity. LCEs are extremely sensitive to external fields, such as heat, electricity, and light. They can rapidly deform under these conditions and return to their original state after the external field disappears, potentially achieving fully reversible deformation. Under the influence of an external field (heat, light, or electricity), the mesogens rotate, altering the arrangement of the mesogens. This rotation stretches the molecular chains and induces macroscopic, reversible elastic deformation of the elastomer, resulting in a change in the shape of the LCE. This shape memory property makes them suitable for use as actuating materials, leading to widespread applications in soft robotics, artificial muscles, sensors, and biomimetic systems.

[0003] In recent years, research on the performance of thermoresponsive LCEs has primarily focused on their multifunctionality. Currently, the response mode of polyacrylate-based LCEs is relatively simple, primarily thermally driven. Multifunctional LCEs combine different driving modes within the same material. For example, by introducing conductive materials into LCEs, a coupling effect of temperature and electrical responses can be achieved, thereby realizing temperature-sensitive electronic devices. Yang Yang et al. utilized the photothermal effect of carbon nanotubes to convert light energy into thermal energy for thermal actuation, and to achieve local controllable responsive deformation using illumination. When Li Zhen et al. dispersed polydopamine (PDA) nanoparticles into xLCE, PDA, as a photosensitive material, exhibited photoresponsiveness, absorbing light energy and converting it into thermal energy, replacing carbon nanotubes as a highly efficient photothermal agent in the system and further enhancing the photothermal conversion effect. Chambers et al. adsorbed a carbon nanoconductive layer on the surface of an LCE film and utilized the resistive Joule heating effect to drive the LCE film. Ford et al. embedded LCE into liquid metal droplets, utilizing the electrothermal effect to convert electrical energy into thermal energy for thermal actuation. These reports combine different drive methods within the same material, significantly expanding the scope and application of LCEs compared to single-drive LCEs. However, research on the load-output work performance of dual-drive LCEs is limited, and further research is needed to determine their performance as actuators capable of achieving dual responses under different loads. Summary of the Invention

[0004] In order to solve the above problems, the present invention realizes the preparation of light-heat driven dual-responsive liquid crystal elastomer by interpenetrating azobenzene liquid crystal unit network and polyacrylate-based network to increase the driving strain under small load.

[0005] Specifically, in one aspect, the present invention provides a method for preparing an azobenzene polyurea-polyacrylate-based double-network liquid crystal elastomer, which comprises the following steps:

[0006] (1) Polyol, diisocyanate and azobenzene monomer are polymerized in an organic solvent 1 under the action of a catalyst 1 to obtain azobenzene polyurea;

[0007] (2) dissolving a liquid crystal monomer, a thiol, a thiol-containing crosslinking agent, a photoinitiator, and a catalyst 2 in an organic solvent 2 to obtain a mixed solution, wherein the liquid crystal monomer is a mixture of 2-methyl-1,4-phenyl-4(3-acryloxypropoxy)benzoate, 1,4-bis-[4-(6-acryloxyhexyloxy)benzoyloxy]-2-methylbenzene, or 2-methyl-1,4-phenyl-4(3-acryloxypropoxy)benzoate and / or 1,4-bis-[4-(6-acryloxyhexyloxy)benzoyloxy]-2-methylbenzene and one or more compounds selected from 4'-(6-(acryloxy)hexyloxy)biphenylnitrile, 3-(4-(dodecyloxy)phenyl)-2-acryloyl acrylate, and 1,6-hexanediol diacrylate;

[0008] (3) blending the azobenzene polyurea obtained in step (1) with the mixed solution obtained in step (2) and performing a polymerization reaction to obtain a multi-domain azobenzene polyurea-polyacrylate-based double network liquid crystal elastomer;

[0009] (4) The multi-domain azobenzene polyurea-polyacrylate-based double network liquid crystal elastomer is uniaxially stretched and photocured to obtain a single-domain azobenzene polyurea-polyacrylate-based double network liquid crystal elastomer.

[0010] Furthermore, the azobenzene monomer is one or more of 4,4'-azodiphenylamine, 2,2'-(azo-1,2-diyl)diphenylamine, and 2,4-diaminoazobenzene.

[0011] Furthermore, the polyol is one or more of trimethylolpropane, glycerol, polyether polyol, polyester polyol, and 1,2,6-hexanetriol.

[0012] Furthermore, the diisocyanate is one or more of hexamethylene diisocyanate, diphenylmethane diisocyanate, toluene diisocyanate, and isophorone diisocyanate.

[0013] Furthermore, the organic solvent 1 is one or more of dimethyl sulfoxide, N,N-dimethylformamide, N-methylpyrrolidone, acetone, and toluene.

[0014] Furthermore, the catalyst 1 is one or more of dibutyltin dilaurate, dimethylaminoimidazole, and triethylamine.

[0015] Furthermore, in the reaction system of step (1), the molar ratio of amino group to hydroxyl group is 90:10 to 30:10.

[0016] Furthermore, in the reaction system of step (1), the molar ratio between the total molar amount of amino groups and hydroxyl groups and the molar amount of isocyanate groups is 1:0.8-1.2.

[0017] Furthermore, in the reaction system of step (1), the molar amount of catalyst 1 is 0.5-1% of the molar amount of hydroxyl groups.

[0018] Furthermore, the polymerization reaction in step (1) is carried out at 25-50° C. for 1-5 hours.

[0019] Furthermore, step (1) comprises first reacting the polyol and diisocyanate at 25-50° C. for 10-60 min under the action of catalyst 1, and then adding azobenzene monomer and reacting at 25-50° C. for 1-3 h.

[0020] Furthermore, the thiol is one or more of 2,2'-(1,2-ethanediyldioxy)bis(ethanedithiol), 1,2-ethanedithiol, 1,4-butanedithiol, and bis(2-mercaptoethoxy)ethane.

[0021] Furthermore, the mercapto-containing cross-linking agent is one or more of pentaerythritol tetrakis(3-mercaptopropionate), pentaerythritol tetrakismercaptoacetate, pentaerythritol tetrakismercaptohexanoate, glycerol trimercaptopropionate, and 1,2,3-trimercaptopropane.

[0022] Furthermore, the catalyst 2 is one or more of N,N-dimethyl-N'N'-di(2-hydroxypropyl)-1,3-propylenediamine, triethylamine, and 1,5-diazabicyclo[4.3.0]nonane.

[0023] Furthermore, the photoinitiator is one or more of bis-2,6-(2-fluoro-3-pyrrolylphenyl) titanocene (Igacure 784), benzophenone, 1-hydroxycyclohexylphenyl ketone, and phenylbis(2,4,6-trimethylbenzoyl)phosphine oxide. Igacure 784 is a visible light initiator that triggers the self-polymerization of acrylate groups in the visible light band, making it suitable for visible light curing of thicker coatings. It also has good thermal stability and can be used in the self-polymerization reaction of azobenzene polyurea-polyacrylate groups, preventing the azobenzene from absorbing light and preventing the acrylate from self-polymerizing.

[0024] Furthermore, the organic solvent 2 is one or more of toluene, dichloromethane, tetrahydrofuran, and cyclohexane.

[0025] Furthermore, in the mixed solution of step (2), the molar ratio of acrylate groups to mercapto groups is 1.15:1 to 1.3:1.

[0026] Furthermore, in the mixed solution of step (2), the thiol molar ratio between the thiol and the thiol-containing cross-linking agent is 50:50 to 90:10.

[0027] Furthermore, in the mixed solution of step (2), the added amount of catalyst 2 is 0.5-2% of the total molar amount of mercapto groups.

[0028] Furthermore, in the mixed solution of step (2), the amount of the photoinitiator added is 0.1-1% of the mass of the liquid crystal monomer.

[0029] Furthermore, in step (3), the molar amount of azobenzene polyurea is 5-20% of the molar amount of the liquid crystal monomer.

[0030] Furthermore, the polymerization reaction in step (3) is carried out at room temperature for 12-48 hours.

[0031] Furthermore, the polymerization reaction is carried out under vacuum and dry conditions.

[0032] Furthermore, the preparation method further comprises post-processing the solidified material to remove the solvent.

[0033] Furthermore, the post-treatment comprises heating at 60-90° C. for 6-48 hours.

[0034] In other aspects, the present invention provides an azobenzene polyurea-polyacrylate-based double-network liquid crystal elastomer obtained by the preparation method described herein.

[0035] Advantageous Effects of the Invention

[0036] The present invention constructs an azobenzene polyurea-polyacrylate dual-network LCE, which exhibits dual responses to UV light (bending toward the light) and heat (shrinkage strain). The authors found that the introduction of the azobenzene polyurea network improves the LCE's fracture toughness and fracture strain, while reducing its fracture stress. Compared to pure LCE, the 15 mol% Azo-LCE exhibits a 55.3% reduction in fracture stress and a 72.2% increase in fracture strain. Increasing the content of the azobenzene secondary network decreases the overall crosslink density of the polymer network and reduces the threshold stress under applied load. Due to its low crosslinking density, the 15 mol% Azo-LCE exhibits the lowest driving strain and threshold stress under applied load. When the applied load is less than 0.33 MPa, the actuation strain of 10 mol% Azo-LCE increases by 87.8%, from 21.3% to 40.0%, compared to pure LCE. The actuation strain of 5 mol% Azo-LCE increases by 70.4%, from 21.3% to 36.3%, compared to pure LCE. Therefore, within a small load range, 5 mol% and 10 mol% Azo-LCE can improve the actuation strain. The maximum output work density of 5 mol% and 10 mol% Azo-LCE is similar to that of pure LCE, while the maximum output work stress decreases from 0.7 MPa to 0.27 MPa. However, within a small stress range (0.4 MPa), the output work density of 5 mol% and 10 mol% Azo-LCE is greater than that of pure LCE. Therefore, 5 mol% and 10 mol% Azo-LCE can provide greater actuation strain and output work density within this small load range. BRIEF DESCRIPTION OF THE DRAWINGS

[0037] Figure 1 The molecular formulas of the following compounds are shown: (a) 2-methyl-1,4-phenyl-4-(3-acryloyloxypropoxy)benzoate (RM257); (b) pentaerythritol tetrakis(3-mercaptopropionate) (PETMP); (c) 2,2'-(1,2-ethylenedioxy)bis(ethanediol) (EDDET); (d) N,N-dimethyl-N'N'-bis(2-hydroxypropyl)-1,3-propanediamine (DPB); and (e) 2-hydroxy-4'-(2-hydroxyethoxy)-2-methylpropiophenone (HHMP).

[0038] Figure 2 The molecular structures of the azobenzene polyurea reaction monomers are shown: (a) 4,4'-azodianiline; (b) hexamethylene diisocyanate (HDI); (c) trimethylolpropane (TMP); (d) dibutyltin dilaurate (DBTL); and (e) bis(2,6-difluoro-3-pyrrolylphenyl) titanocene (Igacure 784).

[0039] Figure 3The infrared spectra of the reactive monomer of azobenzene polyurea and azobenzene polyurea-polyacrylate-based LCE are shown.

[0040] Figure 4 Shown are the fracture stress-strain curves of: (a) azobenzene polyurea-polyacrylate-based multidomain LCE and pure 30%-65:35 multidomain LCE and (b) azobenzene polyurea-polyacrylate-based multidomain LCE and 30%-85:15-multidomain pure LCE, 30%-75:25 multidomain pure LCE.

[0041] Figure 5 shows the POM image of azobenzene polyacrylate-based LCE film.

[0042] Figure 6 The 2D-WAXS spectra of the prepared azobenzene polyurea-polyacrylate-based LCE films are shown: (a) multi-domain Azo-LCE film; (b) single-domain Azo-LCE film; (c) azimuth angles and absorption peaks of single-domain and multi-domain Azo-LCE and (d) scattering vectors and intensities of single-domain and multi-domain Azo-LCE.

[0043] Figure 7 Shown are the XRD patterns of azobenzene polyurea-polyacrylate-based LCE and pure LCE.

[0044] Figure 8 Dynamic thermomechanical curves of LCE based on azobenzene polyurea-polyacrylate with different contents (5 mol%, 10 mol% and 15 mol%) are shown.

[0045] Figure 9 Schematic diagram showing the thermal stimulus response of azobenzene polyurea-polyacrylate-based single-domain LCE.

[0046] Figure 10 The external load-thermal response deformation fitting lines are shown: (a) 30%-65:35LCE with different azobenzene contents (5 mol%, 10 mol%, 15 mol%) and pure 30%-65:35LCE; (b) 30%-65:35LCE with different azobenzene contents (5 mol%, 10 mol% and 15 mol%) and pure 30%-85:15, 30%-75:25LCE.

[0047] Figure 11The external load-thermal response output work density fitting curves are shown: (a) different azobenzene content second networks (5 mol%, 10 mol% and 15 mol%)-30%-65:35LCE and pure 30%-65:35LCE; (b) different azobenzene content second networks (5 mol%, 10 mol% and 15 mol%)-30%-65:35LCE and pure 30%-85:15, 30%-75:25LCE.

[0048] Figure 12 Schematic diagram of the thermal response driven weight lifting of 5 mol% Azo-65:35 single-domain LCE.

[0049] Figure 13 Shown is a schematic diagram of the response driving of azobenzene polyurea-polyacrylate-based LCE film under 365nm ultraviolet light. DETAILED DESCRIPTION

[0050] The present invention is further described below with reference to specific examples, which, however, are not intended to limit the present invention in any way. Unless otherwise specified, the reagents, methods, and equipment used in the present invention are conventional reagents, methods, and equipment in the art.

[0051] Experimental materials:

[0052] 4-(3-acryloyloxypropoxy)benzoic acid 2-methyl-1,4-phenyl ester (RM257, 97%), average relative molecular mass (Mn) 588.6 g / mol, purchased from Zhengzhou Alpha Chemical Co., Ltd. 2,2'-(1,2-ethanediylbis(oxy))bisethanethiol (EDDET, 95%), average relative molecular mass (Mn) 488.6 g / mol, purchased from Sigma-Aldrich. Pentaerythritol tetra(3-mercaptopropionate) (PETMP, 95%), average relative molecular mass (Mn) 182.3 g / mol, purchased from Sigma-Aldrich. Catalyst N,N-dimethyl-N'N'-di(2-hydroxypropyl)-1,3-propanediamine (DPB, 98%), average relative molecular mass (Mn) 218.3 g / mol. Solvent toluene, purchased from Shanghai Aladdin Biochemical Technology Co., Ltd. 4,4'-azobisphenylamine (97%), average relative molecular mass (Mn) 212.25 g / mol, purchased from Zhengzhou Alpha Chemical Co., Ltd.; hexamethylene diisocyanate (HDI), average relative molecular mass (Mn) 168.19 g / mol, purchased from Shanghai Aladdin Biochemical Technology Co., Ltd.; trimethylolpropane (TMP, 98%), average relative molecular mass (Mn) 134.17 g / mol; catalyst dibutyltin dilaurate (DBTL, 95%), average relative molecular mass (Mn) 631.56 g / mol, purchased from Shanghai Aladdin Biochemical Technology Co., Ltd.; 520 nm visible light initiator bis 2,6-2-fluoro-3-pyrrolylphenyl titanocene (Igacure 784, 95%), purchased from Shanghai McLean Biochemical Technology Co., Ltd.; dimethyl sulfoxide (DMSO, ), purchased from Shanghai Aladdin Biochemical Technology Co., Ltd. The above raw materials do not need to be further purified and are used directly. Their molecular structural formulas are shown in Figure 1 and 2

[0053] Example: Preparation of azobenzene polyurea-polyacrylate-based double network liquid crystal elastomer (abbreviated as Azo-LCE)

[0054] ​Synthesis of the second network of azobenzene polyurea: 4,4'-azodiphenylamine (4,4-azodianiline) was pre-heated in a vacuum oven at 60°C for 24 hours to completely remove moisture from the monomer. The amino:hydroxyl molar ratio was 90:10, and the total molar ratio of amino and hydroxyl groups to isocyanate groups was 1:1. Dibutyltin laurate (DBTL) was present at a molar ratio of 0.66% of the total hydroxyl group ratio. Trimethylolpropane (TMP) was placed in a three-necked flask and wrapped in tin foil to protect from light. Dimethyl sulfoxide (DMSO, 2g) was added and dissolved completely. Hexamethylene diisocyanate (HDI) was then added and stirred at 40°C for 5 minutes. Dibutyltin dilaurate (DBTL) was thoroughly diluted in DMSO (DMSO, 0.5g) and added to the flask, reacting at 40°C for 30 minutes. Then, 4,4'-azodianiline was added and the mixture was reacted at 30°C for 1.5 h.

[0055] Synthesis of the first polyacrylate-based LCE network and its composite with the second network: 4 g (7.06 mmol) of 2-methyl-1,4-phenyl-4-(3-acryloyloxypropoxy)benzoate (RM257) was added to a brown seed bottle. 40 wt% (1.6 g) of toluene was then added. The mixture was heated to 80°C in an oil bath and stirred until completely dissolved. After RM257 was completely dissolved, the brown seed bottle was cooled in an ice-water bath until it returned to room temperature. 2,2'-(1,2-ethylenedioxy)bis(ethanediol)thiol (EDDET) and pentaerythritol tetrakis(3-mercaptopropionate) (PETMP) were then added to the solution and stirred to dissolve. The molar ratio of thiol groups of EDDET to PETMP was 65:35, and the molar ratio of acrylate to thiol groups was 1.3:1. If RM257 precipitated during the process, the mixture was heated to 80°C until completely dissolved, then cooled again in an ice-water bath to room temperature. If precipitation occurred during the process, the above steps were repeated. Once completely dissolved, add 0.5 wt% of the total monomer weight of the visible light initiator, bis(2,6-difluoro-3-pyrrolylphenyl)titanocene (Igacure 784), to the solution and stir to dissolve. Prepare a diluted catalyst solution of 1% mol of N,N-dimethyl-N'N'-bis(2-hydroxypropyl)-1,3-propanediamine (DPB) with a 50:1 wt% ratio of toluene to DPB. Slowly add the diluted catalyst solution dropwise to the brown inoculum bottle and stir for 2 minutes. After mixing, add the azobenzene polyurea secondary network and blend for 30 minutes. Immediately after casting, place the sample in a vacuum dish to evacuate the sample, ensuring it is airtight and free of water. Allow it to react at room temperature for 24 hours. Remove the sample from the mold and heat it on an 80°C hot plate for 24 hours to remove the solvent. Azobenzene polyurea second network was added at 5 mol%, 10 mol%, and 15 mol% of the molar amount of the monomer (i.e., RM257), respectively. The prepared samples were subsequently abbreviated as 30%-65:35-5 mol% Azo-LCE, 30%-65:35-10 mol% Azo-LCE, and 30%-65:35-15 mol% Azo-LCE, where 30% indicates that the molar ratio of acrylate groups to thiol groups is 1.3:1, i.e., the acrylate groups in the system are 30 mol% excess over the thiol groups, and 65:35 indicates that the molar ratio of the thiol groups of EDDET and PETMP is 65:35.

[0056] Tensile specimens were prepared according to ASTM D638-2003. Each set of specimens was tested three times. Endpoints were marked on both sides of the specimen, and the original lengths at both endpoints were measured. A universal testing machine was used to clamp the specimen at both ends. A constant tensile rate and visible light wavelength of 520 nm were applied to the stretched portion of the specimen, irradiating the front and back surfaces for 15 minutes each. After irradiation, the film was removed and the end point lengths were observed to confirm their correspondence with the set tensile strain. This fixes the stretched length of the film, indicating a monodomain LCE film with fully aligned liquid crystal elements.

[0057] Comparative Example: Preparation of Polyacrylate-Based Single-Network Liquid Crystalline Elastomer (abbreviated as Pure Azo-LCE)

[0058] Add 4g (7.06mmol) of 2-methyl-1,4-phenyl-4-(3-acryloyloxypropoxy)benzoate (RM257) to a brown spawn bottle. Then, add 40wt% (1.6g) of toluene. Heat to 80°C in an oil bath and stir until completely dissolved. Once RM257 is completely dissolved, cool the bottle in an ice-water bath until it returns to room temperature. Then, add 2,2'-(1,2-ethylenedioxy)bis(ethylenethiol) (EDDET) and pentaerythritol tetrakis(3-mercaptopropionate) (PETMP) to the solution and stir to dissolve. If RM257 precipitates during this process, heat to 80°C until completely dissolved, then cool to room temperature in an ice-water bath again. If precipitation occurs, repeat the above steps. Once completely dissolved, add the photoinitiator 2-hydroxy-4'-(2-hydroxyethoxy)-2-methylpropiophenone (HHMP) at 0.5wt% of the total monomer weight and stir to dissolve. A diluted catalyst solution of N,N-dimethyl-N'N'-di(2-hydroxypropyl)-1,3-propanediamine (DPB) with a thiol molar content of 1% mol was prepared with toluene:DPB in a ratio of 50:1 (wt%). This diluted catalyst solution was slowly added dropwise to the brown inoculum bottle and stirred for 2 minutes. Immediately after mixing, the solution was poured into a mold and allowed to stand at room temperature for at least 18 hours. The film was then carefully removed from the mold, laid flat on a Teflon plate, and placed in an 80°C oven for 24 hours to evaporate the remaining toluene solvent. This resulted in a milky white, opaque film.

[0059] The thiol molar ratio of EDDET to PETMP was adjusted between 50:50 and 90:10, and the molar ratio of acrylate to thiol was set to 1.15:1 and 1.3:1, respectively, resulting in a 15 mol% and 30 mol% excess of acrylate over thiol in the system. When the thiol molar ratio of EDDET to PETMP was 50:50 and 60:40, the high crosslink density of the LCE made it difficult to stretch and align the liquid crystal, resulting in a low degree of orientation and very little response deformation. When the thiol molar ratio of EDDET to PETMP was 100:0 and 90:10, the initial shape of the film was difficult to fix, resulting in an inability to maintain the orientation after the second stretching and orientation step. Furthermore, films with a 5 mol% excess of acrylate groups had a low degree of orientation; films with a 40 mol% excess of acrylate groups had a high number of unreacted acrylate groups, resulting in poor film-forming properties and failure to form. In summary, multidomain LCEs with varying acrylate excesses and different EDDET / PETMP thiol ratios were prepared, as described in Table 2-1. These samples are subsequently referred to as 15%-85:15 LCE, 15%-75:25 LCE, 15%-65:35 LCE, 30%-85:15 LCE, 30%-75:25 LCE, and 30%-65:35 LCE. For example, 15%-85:15 indicates a 15 mol% excess of acrylate groups relative to thiol groups and an EDDET / PETMP thiol molar ratio of 85:15.

[0060] Test Example 1: Structural Characterization of Azo-LCE

[0061] Figure 3 This is the Fourier infrared spectrum of the reactive monomer of the azobenzene second network and the synthesized double network multi-domain Azo-LCE. The absorption peak of the isocyanate group is 2270 cm -1 The absorption peak of hydroxyl -OH is 3500 cm -1 The absorption peak of primary amine -NH2 is a double peak, and the absorption peak of azobenzene N=N is 1500cm -1 The absorption peak of acrylate C=C is 1604 cm -1 .Depend on Figure 3 Compared with the synthesized azobenzene polyurea-polyacrylate-based multi-domain LCE, it was found that the absorption peaks of hydroxyl, primary amine and isocyanate groups disappeared, while the absorption peaks at 1500 cm -1 and 1604cm -1 The absorption peaks of azobenzene N=N and acrylate C=C appear at the positions respectively, indicating that azobenzene polyurea-polyacrylate based double network LCE is prepared.

[0062] Test Example 2: Performance Characterization of Azo-LCE

[0063] 1. Composition and orientation behavior of Azo-LCE

[0064] Figure 4 The stress-strain curves of Azo-LCE with different contents were obtained by tensile strain until the spline fractured. The fracture stress and fracture strain were recorded in Table 1, respectively. Figure 4 In (a), the fracture stress of the 30%-65:35-5 mol% Azo-LCE was 1.32 MPa, a 12% decrease compared to the 30%-65:35-pure LCE fracture stress of 1.50 MPa. The fracture stress of the 30%-65:35-10 mol% Azo-LCE was 1.19 MPa, a 20.6% decrease. The fracture stress of the 30%-65:35-15 mol% Azo-LCE was 0.67 MPa, a 55.3% decrease. It was found that the fracture stress decreased with increasing azobenzene secondary network content. Figure 4 (b) shows that the fracture stress of 30%-65:35-15mol% Azo-LCE is similar to that of 30%-85:15-pure LCE, which is 0.66 MPa. The fracture stress of 30%-65:35-5mol% and 10mol% Azo-LCE are higher than that of pure 30%-75:25 and 85:15-LCE, while that of 30%-65:35-15mol% Azo-LCE is lower than that of pure 30%-65:35-LCE.

[0065] Furthermore, the fracture strains of 30%-65:35-5 mol% Azo-LCE were 280%, 294%, and 30%-65:35-10 mol% Azo-LCE, respectively, increasing to 303%. These increases were 59.1%, 67.0%, and 72.2% higher than those of pure LCE, respectively. Increasing the azobenzene secondary network content increases the fracture strain and toughness of Azo-LCE. Table 1 shows the fracture stress and fracture strain of Azo-LCE with varying Azo-LCE content, with a fixed tensile strain of 200% selected for comprehensive comparison.

[0066] Table 1: Fracture stress and fracture strain of azobenzene polyurea-polyacrylate-based multidomain LCE

[0067] sample Fracture stress (MPa) Fracture strain (%) 30％-65:35 1.50 176 30%-65:35-5mol%Azo 1.32 280 30%-65:35-10mol%Azo 1.19 294 30%-65:35-15mol%Azo 0.67 303 30％-85:15 0.66 250 30％-75:25 0.98 210

[0068] Polarizing microscopy and two-dimensional wide-angle X-ray diffraction were used to characterize the orientation of Azo-LCE. As shown in the figure, when the two orthogonal polarizers are at 45° to the molecular orientation direction of the LCE film ( Figure 5 a), the field of view is brightest. When the film is rotated 45°, that is, the two orthogonal polarizers are perpendicular or parallel to the stretching direction of the Azo-LCE film, that is, the orientation direction of the liquid crystal unit ( Figure 5b) shows the lowest light transmittance, indicating that the liquid crystal elements have been fully aligned, resulting in a single-domain Azo-LCE. Rotating the film at 45° intervals reveals periodic changes in brightness and darkness.

[0069] Two-dimensional wide-angle X-ray diffraction was used to characterize the orientation of the Azo-LCE film. Figure 6 As shown in (ab), the two-dimensional wide-angle X-ray diffraction pattern (2D WAXS) shows that a single-domain Azo-LCE film was successfully prepared. The liquid crystal units in the film sample were oriented, and the diffraction rings in the 2D WAXS pattern split to form diffraction arcs. Figure 6 (c) shows the azimuth angles and corresponding intensities of single-domain and multi-domain Azo-LCE. It is found that the intensities of multi-domain LCE at all azimuth angles are basically the same, while the single-domain LCE shows two strong peaks at azimuth angles of 100° and 275°, indicating that the internal liquid crystal units are oriented, and single-domain Azo-LCE is successfully prepared. Figure 6 (d) Scattering vectors and corresponding intensities of single-domain and multi-domain Azo-LCE.

[0070] Figure 7 The XRD spectra of multi-domain Azo-LCE and multi-domain pure LCE show that the peaks of the multi-domain pure LCE after adding 10 mol% azobenzene second network are wider and sharper than those of the multi-domain pure LCE. The analysis shows that the azobenzene liquid crystal unit promotes the crystallization of the overall liquid crystal unit within the network, resulting in higher crystallinity and a larger crystal phase size than that of the multi-domain pure LCE.

[0071] 2. Thermomechanical properties of Azo-LCE

[0072] Figure 8 The dynamic thermomechanical characterization curves of Azo-LCE with different contents of 30%-65:35-(5mol%, 10mol% and 15mol%) are shown. The glass transition temperatures (T g ), clear point (T i ), T g to T i The temperature range between is the liquid crystal phase temperature range (T g -T i ) is shown in Table 2. It is found that with the increase of the content of the second network of azobenzene, T g As the content of the second azobenzene network increases from 5 mol% to 10 mol%, T g -T i Increase.

[0073] Table 2: Glass transition temperature, clearing point, and liquid crystal phase temperature range of multi-domain LCEs with different metal ion liquid contents

[0074]

[0075] 3. Thermal response performance of Azo-LCE

[0076] Figure 9 (ab) are schematic diagrams of the thermal stimulus response of single-domain Azo-LCE films. During the heating and cooling process, the film produces reciprocating contraction deformation in the stress-stretching orientation direction, that is, a reversible driving strain is generated in response to heat.

[0077] Figure 10 The figure shows the applied load-thermal response deformation fitting lines for different Azo-LCE contents. It was found that the threshold stress under applied load decreased with increasing azobenzene secondary network content. Compared to pure LCE, the threshold stress of 5 mol% Azo-LCE decreased by 60.0%, 10 mol% Azo-LCE decreased by 66.2%, and 10 mol% Azo-LCE decreased by 89.2%.

[0078] Figure 11 (ab) show the external load-thermal response output power density fitting curves of different Azo-LCE contents. It is found that the maximum output power density of 5 mol% Azo-LCE is 50.63 kJ / m 3 The maximum output power density of 10 mol% Azo-LCE is 48.70 kJ / m 3 The maximum output power density of purer 30%-65:35-LCE is 49.26kJ / m 3 The difference is not significant, and the output work density of 5 mol% and 10 mol% Azo-LCE is greater than that of pure LCE in a small stress range (0.4 MPa).

[0079] Table 3 shows the threshold maximum output work density and maximum output work stress for 30%-65:35 LCE with different azobenzene contents (5 mol%, 10 mol%, and 15 mol%) and pure LCE with different ratios. It was found that pure 30%-65:35-LCE exhibited a lower actuation strain and a higher maximum load for work. While Azo-LCE exhibited a lower maximum load for work, it exhibited a higher actuation strain at low loads. Therefore, Azo-LCE provided higher output work density at low loads than pure LCE. Furthermore, the maximum output work stress of 5 mol% Azo-LCE decreased from 0.7 MPa to 0.27 MPa, while that of 10 mol% Azo-LCE decreased to 0.22 MPa.

[0080] Table 3: Threshold maximum output power density and maximum output power stress of different azobenzene contents (5 mol%, 10 mol% and 15 mol%)-30%-65:35 LCE and pure LCE at different ratios

[0081]

[0082] Figure 12 (ae) Schematic diagram of the thermal response-driven lifting of different gram weights by 5mol%Azo-30%-65:35 single-domain LCE. Figure 12 (a) It can be seen that the actuation strain of the weight of 7g is about 40.0%, which is higher than the actuation strain of pure 30%-65:35LCE (27.0%). When the weight is increased to 15g and 20g ( Figure 12 bc), the driving strain is reduced to 33.3% and 18.8%. When the weight is increased to 30g ( Figure 12 d), the actuation strain is only 6.3%, compared with pure LCE with an 80g weight actuation strain of 8.7%.

[0083] 4. Photoresponsive properties of Azo-LCE

[0084] Figure 13 The following diagrams illustrate the response of an azobenzene polyurea-polyacrylate-based LCE film to 365nm UV light. The illumination direction is perpendicular to the surface of the table and the film. (A to F) illustrate the film's response during the switch from visible light to UV light. (A) shows the film under visible light irradiation, where the front end of the film is parallel to the tabletop, indicating no response. (B) shows the film at the beginning of UV irradiation, where an upward bending tendency is observed, but the degree of bending is minimal. As the irradiation time increases, the degree of film bending gradually increases (C to F), until (G) reaches its maximum bending under UV light.

[0085] It should be noted that the preferred embodiments of the present invention are given in the specification and drawings of the present invention. However, the present invention can be implemented in many different forms and is not limited to the embodiments described in this specification. These embodiments are not intended to be additional limitations on the content of the present invention. The purpose of providing these embodiments is to make the understanding of the disclosure of the present invention more thorough and comprehensive. In addition, the above-mentioned technical features can be combined with each other to form various embodiments not listed above, which are all considered to be within the scope of the description of the present invention. Furthermore, it is obvious to those skilled in the art that improvements or changes can be made based on the above description, and all such improvements and changes should fall within the scope of protection of the claims attached to the present invention.

Claims

1. A method for preparing an azobenzene polyurea-polyacrylate-based double-network liquid crystal elastomer, characterized in that: The following steps are involved: (1) Polyol, diisocyanate and azobenzene monomer are polymerized in an organic solvent 1 under the action of a catalyst 1 to obtain azobenzene polyurea; (2) dissolving a liquid crystal monomer, a thiol, a thiol-containing crosslinking agent, a photoinitiator, and a catalyst 2 in an organic solvent 2 to obtain a mixed solution, wherein the liquid crystal monomer is a mixture of 2-methyl-1,4-phenyl-4(3-acryloxypropoxy)benzoate, 1,4-bis-[4-(6-acryloxyhexyloxy)benzoyloxy]-2-methylbenzene, or 2-methyl-1,4-phenyl-4(3-acryloxypropoxy)benzoate and / or 1,4-bis-[4-(6-acryloxyhexyloxy)benzoyloxy]-2-methylbenzene and one or more compounds selected from 4'-(6-(acryloxy)hexyloxy)biphenylnitrile, 3-(4-(dodecyloxy)phenyl)-2-acryloyl acrylate, and 1,6-hexanediol diacrylate; (3) blending the azobenzene polyurea obtained in step (1) with the mixed solution obtained in step (2) and performing a polymerization reaction to obtain a multi-domain azobenzene polyurea-polyacrylate-based double network liquid crystal elastomer; (4) The multi-domain azobenzene polyurea-polyacrylate-based double network liquid crystal elastomer is uniaxially stretched and photocured to obtain a single-domain azobenzene polyurea-polyacrylate-based double network liquid crystal elastomer.

2. The preparation method according to claim 1, characterized in that The azobenzene monomer is one or more of 4,4'-azodiphenylamine, 2,2'-(azo-1,2-diyl)diphenylamine, and 2,4-diamineazobenzene; Furthermore, the polyol is one or more of trimethylolpropane, glycerol, polyether polyol, polyester polyol, and 1,2,6-hexanetriol; Furthermore, the diisocyanate is one or more of hexamethylene diisocyanate, diphenylmethane diisocyanate, toluene diisocyanate, and isophorone diisocyanate; Furthermore, the organic solvent 1 is one or more of dimethyl sulfoxide, N,N-dimethylformamide, N-methylpyrrolidone, acetone, and toluene; Furthermore, the catalyst 1 is one or more of dibutyltin dilaurate, dimethylaminoimidazole, and triethylamine.

3. The preparation method according to claim 1, characterized in that In the reaction system of step (1), the molar ratio of amino group to hydroxyl group is 90:10 to 30:10; Furthermore, in the reaction system of step (1), the molar ratio between the total molar amount of amino groups and hydroxyl groups and the molar amount of isocyanate groups is 1:0.8-1.2; Furthermore, in the reaction system of step (1), the molar amount of catalyst 1 is 0.5-1% of the molar amount of hydroxyl groups.

4. The preparation method according to claim 1, characterized in that The polymerization reaction in step (1) is carried out at 25-50° C. for 1-5 hours.

5. The preparation method according to claim 1, characterized in that The thiol is one or more of 2,2'-(1,2-ethylenedioxy)bis(ethanediol), 1,2-ethanedithiol, 1,4-butanedithiol, and bis(2-mercaptoethoxy)ethane; Furthermore, the thiol-containing cross-linking agent is one or more of pentaerythritol tetrakis(3-mercaptopropionate), pentaerythritol tetrakismercaptoacetate, pentaerythritol tetrakismercaptohexanoate, glycerol trimercaptopropionate, and 1,2,3-trimercaptopropane; Further, the catalyst 2 is one or more of N,N-dimethyl-N'N'-di(2-hydroxypropyl)-1,3-propylenediamine, triethylamine, and 1,5-diazabicyclo[4.3.0]nonane; Furthermore, the photoinitiator is one or more of bis-2,6-2-fluoro-3-pyrrolphenyl titanocene, benzophenone, 1-hydroxycyclohexyl phenyl ketone, and phenyl bis(2,4,6-trimethylbenzoyl)phosphine oxide; Furthermore, the organic solvent 2 is one or more of toluene, dichloromethane, tetrahydrofuran, and cyclohexane.

6. The preparation method according to claim 1, characterized in that In the mixed solution of step (2), the molar ratio of acrylate groups to mercapto groups is 1.15:1 to 1.3:1; Furthermore, in the mixed solution of step (2), the thiol molar ratio between the thiol and the thiol-containing cross-linking agent is 50:50 to 90:10; Furthermore, in the mixed solution of step (2), the amount of catalyst 2 added is 0.5-2% of the total molar amount of mercapto groups; Furthermore, in the mixed solution of step (2), the amount of the photoinitiator added is 0.1-1% of the mass of the liquid crystal monomer.

7. The preparation method according to claim 1, characterized in that In step (3), the molar amount of azobenzene polyurea is 5-20% of the molar amount of the liquid crystal monomer.

8. The preparation method according to claim 1, characterized in that The polymerization reaction in step (3) is carried out at room temperature for 12-48 hours; Furthermore, the polymerization reaction is carried out under vacuum and dry conditions.

9. The preparation method according to claim 1, characterized in that The preparation method further comprises post-processing the solidified material to remove the solvent; Furthermore, the post-treatment comprises heating at 60-90° C. for 6-48 hours.

10. An azobenzene polyurea-polyacrylate-based double-network liquid crystal elastomer obtained by the preparation method according to any one of claims 1 to 9.

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