Polysiloxane cross-linking agent, liquid crystal elastomer and preparation methods of polysiloxane cross-linking agent and liquid crystal elastomer

Abstract

The invention discloses a polysiloxane cross-linking agent, a liquid crystal elastomer and a preparation method of the liquid crystal elastomer, and the polysiloxane cross-linking agent comprises a) a polysiloxane main chain; the first side chain and the second side chain are connected to the polysiloxane main chain, the first side chain contains sulfydryl, and the second side chain contains a catechol structure protected by silyl. The invention provides a polysiloxane cross-linking agent with covalent cross-linking sites and coordinatable cross-linking sites at the same time, and the polysiloxane cross-linking agent is used for constructing a liquid crystal elastomer with a covalent cross-linking network structure or a bi-cross-linking network structure with covalent cross-linking and metal-coordination cross-linking at the same time. Therefore, the problems of poor flexibility or difficulty in consideration of toughness and flexibility and the like in the existing liquid crystal elastomer are solved, and the defects in the prior art are overcome.

Description

TECHNICAL FIELD

[0001] The present application belongs to the field of liquid crystal elastomers, and particularly relates to a polysiloxane crosslinking agent, a liquid crystal elastomer and a preparation method thereof. BACKGROUND

[0002] Liquid crystal elastomers (LCEs) are a class of smart responsive materials with both liquid crystal ordered structure and polymer elastic properties. They are usually composed of liquid crystal monomers, crosslinking agents and chain extenders to form a polymer system. Under the action of a catalyst, the system is crosslinked to form a three-dimensional network structure, which can produce large reversible deformation under external stimuli (such as heat, light, electricity, magnetism, etc.). Due to its unique deformation mechanism and response performance, liquid crystal elastomers have broad application prospects in the fields of artificial muscles, flexible sensors, soft robots, etc.

[0003] At present, the softness of existing liquid crystal elastomers needs to be improved, or the toughness and softness are difficult to balance, which limits their practical application in flexible devices and human wearable systems. SUMMARY

[0004] The present application aims to at least partially solve one of the technical problems in the related art. To this end, the present application proposes a polysiloxane crosslinking agent, a liquid crystal elastomer and a preparation method thereof. The present application provides a polysiloxane crosslinking agent with both covalent crosslinking sites and coordination crosslinking sites, which is used to construct a liquid crystal elastomer with a covalent crosslinking network structure or a dual crosslinking network structure with both covalent crosslinking and metal-coordination crosslinking, so that it can realize high toughness (up to more than 67 MJ / m 3 ) while maintaining low modulus (<5 MPa) flexibility.

[0005] The first aspect of the present application proposes a polysiloxane crosslinking agent. According to the embodiments of the present application, the polysiloxane crosslinking agent comprises:

[0006] a) a polysiloxane main chain; and

[0007] b) a first side chain and a second side chain connected to the polysiloxane main chain, the first side chain containing a mercapto group, and the second side chain containing a silane-protected o-diphenol structure;

[0008] The silane-protected o-diphenol structure is as follows:

[0009] Si is a silicon atom, and R is an alkyl group.

[0010] The polysiloxane crosslinking agent of the above-mentioned embodiments of the present application selects a flexible polysiloxane with a thiol group as a basic crosslinking agent, and introduces a coordination unit on this basis to form a macromolecular crosslinking agent-polysiloxane crosslinking agent with both covalent crosslinking sites (i.e. thiol groups) and coordination crosslinking sites (i.e. ortho-phenol structures protected by silane groups), which is used to construct a liquid crystal elastomer with a covalent crosslinking network structure or a dual crosslinking network structure with both covalent crosslinking and metal-coordination crosslinking. In the covalent crosslinking network structure, the polysiloxane chain and the liquid crystal unit endow the network with flexibility and low modulus, thereby obtaining a liquid crystal elastomer with better softness; on the basis of this covalent crosslinking network structure, a liquid crystal elastomer with a dual crosslinking network structure with both covalent crosslinking and metal-coordination crosslinking can be further constructed through the coordination crosslinking sites, which endows it with high tensile toughness, and the balance between toughness and softness is achieved through the synergistic effect of sacrificial bonds and permanent networks, so that it has excellent anti-fracture toughness while maintaining softness.

[0011] In addition, the polysiloxane crosslinking agent according to the above-mentioned embodiments of the present application can also have the following additional technical features:

[0012] In some embodiments of the present application, the first side chain comprises a first alkyl chain, and the thiol group is connected to the polysiloxane main chain through the first alkyl chain, and the first alkyl chain comprises a linear alkyl group with a carbon atom number of 3-6;

[0013] And / or, the ortho-phenol structure is connected to the polysiloxane main chain through a structure as shown in Formula I, the Formula I is -R1-S-R2-, R1 is a second alkyl chain, S is a sulfur atom, and R2 is a third alkyl chain, the second alkyl chain comprises a linear alkyl group with a carbon atom number of 3-6, and the third alkyl chain comprises a linear alkyl group with a carbon atom number of 3-6;

[0014] And / or, the number of repeating units in the polysiloxane main chain is 30-60.

[0015] In some embodiments of the present application, the polysiloxane crosslinking agent has a general structure as shown below:

[0016] In the formula, X is any natural number between 1 and 39, R1 is the second alkyl chain, R2 is the third alkyl chain, and R3 is the first alkyl chain.

[0017] Alternatively, the polysiloxane crosslinking agent has a structure as shown in Formula II:

[0018] The Formula II is: x is any natural number between 1 and 39, and Et is an ethyl group.

[0019] The second aspect of the present application provides a liquid crystal elastomer. According to an embodiment of the present application, the liquid crystal elastomer comprises a covalently cross-linked network structure;

[0020] The covalently cross-linked network structure comprises liquid crystal monomers, a cross-linking agent and a chain extender connected to each other, and the cross-linking agent comprises the polysiloxane cross-linking agent of any one of the first aspect.

[0021] The liquid crystal elastomer of the above embodiment of the present application, the polysiloxane chain and the liquid crystal unit in the covalently cross-linked network structure of the liquid crystal elastomer impart flexibility and low modulus to the network, so that a liquid crystal elastomer with better softness is obtained.

[0022] In addition, the liquid crystal elastomer according to the above embodiment of the present application can also have the following additional technical features:

[0023] In some embodiments of the present application, the liquid crystal elastomer further comprises a coordination cross-linked network structure;

[0024] The coordination cross-linked network structure comprises a coordination unit, and the structural formula of the coordination unit is as follows:

[0025] M n+ is a metal ion.

[0026] In some embodiments of the present application, the molar content of the coordination unit in the coordination cross-linked network structure is 7.7-25%;

[0027] And / or, the metal ion comprises at least one of Fe 3+ , Cu 2+ or Zn 2+ .

[0028] In some embodiments of the present application, the physicochemical property parameters of the liquid crystal elastomer include: Young's modulus of 2.6-4.8 MPa; and / or, toughness of 16.1-67.0 MJ / m 3 .

[0029] The third aspect of the present application provides a preparation method of the liquid crystal elastomer of the second aspect. According to an embodiment of the present application, the preparation method of the liquid crystal elastomer comprises the following steps:

[0030] The liquid crystal monomer, the cross-linking agent and the chain extender are added into a solvent for dissolution to obtain a mixture;

[0031] The catalyst is added into the mixture for reaction to obtain the liquid crystal elastomer.

[0032] The method for preparing the liquid crystal elastomer of the above-mentioned embodiments of the present application. The method forms a covalent crosslinked network structure by the obtained polysiloxane crosslinking agent, liquid crystal monomer and chain extender under the Michael addition reaction condition. Thus, the polysiloxane chain and liquid crystal unit in the covalent crosslinked network structure of the liquid crystal elastomer obtained by the above-mentioned method endow the network with flexibility and low modulus, thereby obtaining a liquid crystal elastomer with better softness.

[0033] In addition, the method for preparing the liquid crystal elastomer of the above-mentioned embodiments of the present application can also have the following additional technical features:

[0034] In some embodiments of the present application, the molar ratio of the liquid crystal monomer, the crosslinking agent and the chain extender is (1-2):(0.1-0.5):(0.8-1.2);

[0035] And / or, the weight addition amount of the catalyst accounts for 0.8-1.5wt% of the total weight of the liquid crystal monomer, the crosslinking agent and the chain extender;

[0036] And / or, the working condition parameters of the reaction include: temperature 20-30℃, time 12-24h;

[0037] And / or, the liquid crystal monomer includes at least one of 1,4-bis-[4-(3-acryloyloxypropoxy)benzoyloxy]-2-methylbenzene, 1,4-bis-[4-(6-acryloyloxyhexyloxy)benzoyloxy]-2-methylbenzene, 2-methyl-1,4-diphenol 4-(((4-(acryloyloxy)butyloxy)carbonyl)oxy)benzoate, 2-methyl-1,4-phenylene bis(4-(4-(acryloyloxy)butyloxy)benzoate), (3R,3AR,6S,6AR)-hexahydrofuran[3,2-B]furan-3,6-diyl bis(4-((acryloyloxy)butyloxy)carbonyl)oxy)benzoyl)oxy)benzoate), 4-cyanophenyl 4'-(6-acryloyloxyhexyloxy)benzoate, 4-[[6-[(1-oxo-2-propenyl)oxy]hexyl]oxy]benzoic acid 4-methoxyphenyl ester, 4-[4-[(1-oxo-2-propenyl)oxy]butoxy]-benzoic acid biphenyl-4-yl ester and 4-[[6-[(1-oxo-2-propenyl)oxy]hexyl]oxy]-benzoic acid 4-ethylphenyl ester;

[0038] and / or, the chain extender comprises at least one of 2,2'-(1,2-ethanediylbis oxy)-bis-ethanethiol, 2,4-bis(hydrothiomethyl)-1,3,5-trimethylbenzene, mercapto-polyethylene glycol-mercapto, ethylene glycol bis-mercaptoacetate, di(mercaptoacetate)-1,4-butanediol, bis(3-mercaptopropionic acid) ethylene glycol, bis(2-mercaptoethyl)ether, 1,4-butanedithiol, 1,5-pentanedithiol, 1,6-hexanedithiol, 1,8-octanedithiol, 1,9-nonanedithiol, and 1,10-decanedithiol;

[0039] and / or, the catalyst comprises at least one of dipropylamine, triethylamine, and tetramethylguanidine;

[0040] and / or, the solvent comprises at least one of tetrahydrofuran, toluene, and dichloromethane.

[0041] In some embodiments of the present application, the method for preparing the liquid crystal elastomer further comprises:

[0042] immersing the liquid crystal elastomer into an acidic solution for stirring to obtain a deprotection-treated liquid crystal elastomer;

[0043] immersing the deprotection-treated liquid crystal elastomer into a solution containing metal ions for a coordination reaction.

[0044] Additional aspects and advantages of the present application will be in part apparent and in part pointed out hereinafter. BRIEF DESCRIPTION OF DRAWINGS

[0045] The above and / or additional aspects and advantages of the present application will become apparent and be readily appreciated from the following description, including the appended drawings.

[0046] Figure 1 is a chemical reaction equation for preparing a polysiloxane crosslinking agent in Example 1 of the present application;

[0047] Figure 2 is a chemical reaction equation for preparing a liquid crystal monomer in Example 8 of the present application Figure 1 ;

[0048] Figure 3 is a chemical reaction equation for preparing a liquid crystal monomer in Example 8 of the present application Figure 2 ;

[0049] Figure 4 is a nuclear magnetic resonance hydrogen spectrum obtained in Test Example 1 of the present application;

[0050] Figure 5 is a small angle X-ray scattering characterization diagram obtained in Test Example 2 of the present application;

[0051] Figure 6Figure for Raman spectrum characterization of the obtained in Test Example 2 of the present application;

[0052] Figure 7 Figure for mechanical property test results of the metal coordination bond enhancing the toughness of the liquid crystal elastomer in Test Example 3 of the present application;

[0053] Figure 8 Figure for test results of the toughening effect of different metal coordination bond contents on the liquid crystal elastomer material in Test Example 4 of the present application;

[0054] Figure 9 Figure for high-temperature mechanical property test results in Test Example 5 of the present application. DETAILED DESCRIPTION

[0055] Embodiments of the present application are described in detail below, examples of which are shown in the accompanying drawings, wherein the same or similar reference signs represent the same or similar elements or elements having the same or similar functions throughout. The embodiments described below by reference to the accompanying drawings are exemplary and are intended to explain the present application, and cannot be understood as a limitation of the present application.

[0056] The first aspect of the present application proposes a polysiloxane crosslinking agent. According to embodiments of the present application, the polysiloxane crosslinking agent comprises:

[0057] a) a polysiloxane main chain; and

[0058] b) a first side chain and a second side chain connected to the polysiloxane main chain, the first side chain containing a mercapto group, and the second side chain containing a silane-protected catechol structure;

[0059] The silane-protected catechol structure is as follows:

[0060] Si is a silicon atom, and R is an alkyl group.

[0061] The liquid crystal elastomer widely used at present is usually constructed by a chemical network with low crosslinking density to achieve good softness and responsiveness. However, such a low crosslinking structure often leads to insufficient mechanical properties of the material, especially prone to fracture in the tensile state, resulting in low overall toughness of the material. The existing literature reports that the tensile toughness of a typical LCE is generally less than 3 MJ / m 3 , which is difficult to meet the application requirements in high-stress environments or complex deformation scenarios.

[0062] To improve the mechanical properties of liquid crystal elastomers, some studies propose introducing double network structure, nano-filler or physical crosslinking point into the material. For example, by forming an interpenetrating network (IPN) or adding rigid fillers such as carbon nanotubes, graphene, etc., the strength and toughness of the material can be enhanced to some extent. However, these methods are often accompanied by a large increase in Young's modulus, resulting in a decrease in material flexibility, which limits its practical application in flexible devices and human wearable systems.

[0063] The polysiloxane crosslinking agent of the above-mentioned embodiments of the present application selects a flexible polysiloxane with a thiol group as a basic crosslinking agent, and introduces a coordination unit on this basis to form a macromolecular crosslinking agent-polysiloxane crosslinking agent with both covalent crosslinking sites (i.e. thiol groups) and coordination crosslinking sites (i.e. ortho-phenol structures protected by silane groups), which is used to construct liquid crystal elastomers with covalent crosslinking network structure or double crosslinking network structure with both covalent crosslinking and metal-coordination crosslinking. Among them, the polysiloxane chain and liquid crystal units in the covalent crosslinking network structure endow the network with flexibility and low modulus, thereby obtaining a liquid crystal elastomer with better softness; on the basis of this covalent crosslinking network structure, a liquid crystal elastomer with double crosslinking network structure with both covalent crosslinking and metal-coordination crosslinking can be further constructed through the coordination crosslinking sites, which endows it with high tensile toughness, and the balance between toughness and softness is achieved through the synergistic effect of sacrificial bonds and permanent network, so that it has excellent anti-fracture toughness while maintaining softness.

[0064] In the embodiments of the present application, the above-mentioned polysiloxane main chain refers to a flexible backbone composed of repeating siloxane units (-Si-O- or -Si-O-Si-).

[0065] In the embodiments of the present application, the above-mentioned ortho-phenol structure protected by a silane group refers to a compound structure with a stable silane ether bond formed by protecting the hydroxyl group in the ortho-phenol (also known as 1,2-dihydroxybenzene) molecule with a silane group such as -SiEt3, and the hydroxyl group in the structure can be restored to a free phenolic hydroxyl group under acidic conditions.

[0066] According to still some specific embodiments of the present application, the first side chain comprises a first alkyl chain, and the thiol group is connected to the polysiloxane main chain through the first alkyl chain, and the first alkyl chain comprises a straight-chain alkyl group with a carbon atom number of 3-6;

[0067] And / or, the ortho-phenol structure is connected to the polysiloxane main chain through a structure as shown in formula I to form the second side chain, and the formula I is: -R1-S-R2-, R1 is a second alkyl chain, S is sulfur, and R2 is a third alkyl chain;

[0068] and / or, the catechol structure is connected to the polysiloxane main chain through a structure as shown in Formula I: -R1-S-R2-, R1 is a second alkyl chain, S is a sulfur atom, R2 is a third alkyl chain, the second alkyl chain comprises a linear alkyl group with a carbon atom number of 3-6, and the third alkyl chain comprises a linear alkyl group with a carbon atom number of 3-6; the inventors have found that by limiting the chain length of the first alkyl chain, the second alkyl chain and the third alkyl chain within the above range, it is beneficial to promote the formation of a liquid crystal elastomer network; if the chain length of the first side chain (such as the chain length of the first alkyl chain being 10) or the second side chain (such as the chain length of the second alkyl chain being 10 and the chain length of the third alkyl chain being 8) is too long, it will affect the liquid crystal phase state, thereby causing performance degradation.

[0069] and / or, the number of repeating units in the polysiloxane main chain is 30-60, for example, the number of repeating units can be 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, 52, 53, 54, 55, 56, 57, 58, 59, 60. The inventors have found that by limiting the number of repeating units of the polysiloxane main chain within the above range, it is beneficial to form a network with moderate crosslinking density; if the number of repeating units is too large, it will make the network crosslinking density too large, thereby causing performance degradation.

[0070] According to still some specific embodiments of the present application, the polysiloxane crosslinking agent has a general structure as shown below:

[0071] wherein X is any natural number between 1 and 39, R1 is the second alkyl chain, R2 is the third alkyl chain, and R3 is the first alkyl chain;

[0072] Alternatively, the polysiloxane crosslinking agent has a structure as shown in Formula II:

[0073] the Formula II: x is any natural number between 1 and 39, for example, it can be 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39; and Et is an ethyl group.

[0074] According to still some specific embodiments of the present application, the polysiloxane crosslinking agent as shown in Formula II is exemplified (the remaining polysiloxane crosslinking agents can be prepared by adaptively replacing the raw materials and referring to the following method), and the preparation method of the polysiloxane crosslinking agent comprises the following steps:

[0075] Eugenol (4.98 g, 30.0 mmol) and triethylsilane (10.68 g, 90 mmol) were added into a round bottom flask with a reflux condenser, after stirring to mix evenly, a catalyst tris(pentafluorophenyl)boron (TPFPB, 0.03 g, 0.06 mmol) was added. The reaction mixture was stirred at room temperature for 6 hours, and the reaction process was monitored by thin layer chromatography. After the reaction was completed, the catalyst was removed by neutral alumina column chromatography (with dichloromethane as eluent), followed by removal of unreacted triethylsilane and solvent using a rotary evaporator, and finally a colorless liquid product was obtained.

[0076] The thiol-hanging polysiloxane chain PMMS, the above-mentioned complexable unit added in the required molar ratio, and the photoinitiator (DMPA, 1wt% of the total mass) were added into a vial, about 1 mL of chloroform solvent was added to dissolve, and stirred to form a uniform solution. Then, under stirring, the reaction bottle was irradiated under ultraviolet light (365 nm, 0.2 W·cm -2 ) for about 5 minutes to promote the reaction between the thiol and the double bond, thereby preparing a polysiloxane crosslinking agent with different x, respectively.

[0077] The second aspect of the present application proposes a liquid crystal elastomer. According to embodiments of the present application, the liquid crystal elastomer comprises a covalently cross-linked network structure;

[0078] The covalently cross-linked network structure comprises liquid crystal monomers, cross-linking agents and chain extenders connected to each other, and the cross-linking agent comprises the polysiloxane cross-linking agent of any one of the first aspect.

[0079] The liquid crystal elastomer of the above-mentioned embodiments of the present application, the polysiloxane chain and the liquid crystal unit in the covalently cross-linked network structure of the liquid crystal elastomer impart flexibility and low modulus to the network, thereby obtaining a liquid crystal elastomer with better softness.

[0080] According to still other specific embodiments of the present application, the liquid crystal elastomer further comprises a coordination cross-linked network structure;

[0081] The coordination cross-linked network structure comprises a coordination unit, and the structural formula of the coordination unit is as follows:

[0082] M n+ is a metal ion. The inventors found that by introducing a coordination cross-linked network structure on the basis of the above-mentioned covalently cross-linked network structure, a liquid crystal elastomer with a double cross-linked network structure of covalent cross-linking and metal-coordination cross-linking is constructed, which imparts high tensile toughness, and the balance between toughness and softness is realized through the cooperation of sacrificial bonds and permanent networks, so that it has excellent anti-fracture toughness while maintaining softness.

[0083] According to still another specific embodiment of the present application, the coordination crosslinking network structure has a coordination unit molar content of 7.7-25%, for example, 7.7, 14.3, 20, or 25%.

[0084] And / or, the metal ions include at least one of Fe 3+ , Cu 2+ , or Zn 2+ , preferably Fe 3+ .

[0085] The inventors have found that the mechanical properties of the liquid crystal elastomer described above in the present application are related to the type of metal ions and the content of metal coordination units, and by adjusting the type, concentration and coordination treatment method of metal ions, the mechanical properties of the whole or part of the material can be controlled. For example, by introducing a higher content of coordination bonds, higher toughness can be achieved, but in terms of metal ion selection, Fe 3+ is the best.

[0086] It should be noted that the above-mentioned coordination unit molar content specifically refers to the proportion of coordination units relative to the polysiloxane chain repeating unit; its calculation method can be obtained by calculating the ratio of "x" to "40" in the structural formula "x / 40".

[0087] According to still another specific embodiment of the present application, the physicochemical property parameters of the liquid crystal elastomer include: Young's modulus of 2.6-4.8 MPa; and / or, toughness of 16.1-67.0 MJ / m 3 .

[0088] The liquid crystal elastomer material prepared by the present application has a Young's modulus maintained within the flexible range (<5 MPa), and achieves a high performance of a fracture toughness of up to 67 MJ / m 3 , and has good flexibility, deformability and mechanical stability, and is suitable for application in various high-performance soft material fields such as flexible structure, buffer energy absorption, intelligent response, etc.

[0089] The third aspect of the present application provides a preparation method of the liquid crystal elastomer described above in the second aspect. According to an embodiment of the present application, the preparation method of the liquid crystal elastomer includes the following steps:

[0090] The liquid crystal monomer, the crosslinking agent and the chain extender are added to the solvent for dissolution to obtain a mixture;

[0091] The catalyst is added to the mixture for reaction to obtain the liquid crystal elastomer.

[0092] The preparation method of the liquid crystal elastomer of the above-mentioned embodiments of the present application. The method forms a covalent cross-linked network structure by the obtained polysiloxane cross-linking agent and liquid crystal monomers and chain extenders under the conditions of Michael addition reaction. Thus, the polysiloxane chains and liquid crystal units in the covalent cross-linked network structure of the liquid crystal elastomer obtained by the above-mentioned method endow the network with flexibility and low modulus, thereby obtaining a liquid crystal elastomer with better softness.

[0093] In the embodiments of the present application, the above-mentioned dissolving manner is not particularly limited, for example, the liquid crystal monomers, the cross-linking agent and the chain extender can be dissolved in the solvent to form a uniform solution by mechanical stirring or ultrasonic vibration and the like.

[0094] In the embodiments of the present application, the container for the above-mentioned reaction is not particularly limited, for example, the mixture formed after adding the catalyst can be poured into a polytetrafluoroethylene mold for reaction; at the same time, in order to accelerate the removal of residual solvent after the reaction is completed, the obtained product can be dried, and the drying manner is not particularly limited, for example, the obtained product can be dried in a vacuum environment at 35°C for 48 hours.

[0095] According to still some specific embodiments of the present application, the molar ratio of the liquid crystal monomers, the cross-linking agent and the chain extender is (1-2):(0.1-0.5):(0.8-1.2), for example, it can be 1.2:0.3:1.05, 1.2:0.3:0.9, 1.2:0.3:1.0, 1.2:0.3:1.1, 1.1:0.3:1.05, 1.3:0.3:1.05, 1.5:0.3:1.05, 1.2:0.2:1.05, 1.2:0.4:1.05, etc.;

[0096] And / or, the weight addition amount of the catalyst accounts for 0.8-1.5wt% of the total weight of the liquid crystal monomers, the cross-linking agent and the chain extender, for example, it can be 0.8wt%, 0.9wt%, 1.0wt%, 1.1wt%, 1.2wt%, 1.3wt%, 1.4wt%, 1.5wt% and the like;

[0097] And / or, the working condition parameters of the reaction include: temperature 20-30°C, time 12-24h;

[0098] and / or the liquid crystal monomer comprises at least one of 1,4-bis-[4-(3-acryloyloxypropyloxy)benzoyloxy]-2-methylbenzene, 1,4-bis-[4-(6-acryloyloxyhexyloxy)benzoyloxy]-2-methylbenzene, 2-methyl-1,4-phenylene bis(4-(4-(acryloyloxy)butoxy)benzoate), (3R,3AR,6S,6AR)-hexahydrofuro[3,2-B]furan-3,6-diyl bis(4-((acryloyloxy)butoxy)carbonyl)oxy)benzoyl)oxy)benzoate), 4-cyanophenyl 4'-(6-acryloyloxyhexyloxy)benzoate, 4-[[6-[(1-oxo-2-propenyl)oxy]hexyl]oxy]benzoic acid 4-methoxyphenyl ester, 4-[4-[(1-oxo-2-propenyl)oxy]butoxy]-benzoic acid biphenyl-4-yl ester, and 4-[[6-[(1-oxo-2-propenyl)oxy]hexyl]oxy]-benzoic acid 4-ethylphenyl ester;

[0099] and / or the chain extender comprises at least one of 2,2'-(1,2-ethanediylbis(oxy))bis(ethanethiol), 2,4-bis(hydrothiomethyl)-1,3,5-trimethylbenzene, mercapto-polyethylene glycol-mercapto, ethylene glycol bis-mercaptoacetate, di(mercaptoacetic acid)-1,4-butanediyl bisester, bis(3-mercaptopropionic acid) ethylene glycol, bis(2-mercaptoethyl)ether, 1,4-butanediol, 1,5-pentanediol, 1,6-hexanediol, 1,8-octanediol, 1,9-nonanediol, and 1,10-decanediol;

[0100] and / or the catalyst comprises at least one of dipropylamine, triethylamine, and dichloromethane;

[0101] and / or the solvent comprises at least one of tetrahydrofuran, toluene, and dichloromethane.

[0102] According to still other specific embodiments of the present application, the method of preparing the liquid crystal elastomer further comprises:

[0103] immersing the liquid crystal elastomer into an acidic solution and stirring to obtain a deprotected liquid crystal elastomer;

[0104] immersing the deprotected liquid crystal elastomer into a solution containing metal ions and performing a coordination reaction. Thereby, coordination sites are exposed by the deprotection treatment and further stabilized metal-coordination crosslinking structures are formed by the metal ion solution treatment, resulting in a liquid crystal elastomer with a dual crosslinking network structure of covalent crosslinking and metal-coordination crosslinking.

[0105] In the embodiments of the present application, the method of removing the silane group by immersing in an acidic solution is not particularly limited, for example, the liquid crystal elastomer can be immersed in a 1:1 (volume ratio) hydrochloric acid / tetrahydrofuran solution, gently stirred for 12 hours to remove the protecting group, and then washed three times (15 minutes each time) in a 1:1 (volume ratio) water / tetrahydrofuran mixture. At the same time, if storage is required, drying treatment can be performed, for example, drying at 35°C under vacuum for 48 hours.

[0106] In the embodiments of the present application, the method of performing coordination reaction by immersing in a solution containing metal ions is not particularly limited, for example, the liquid crystal elastomer after removal of the silane group can be immersed in a metal ion / tetrahydrofuran solution of a suitable concentration, and left to stand at room temperature for 24 hours to perform coordination reaction; at the same time, after completion of the coordination reaction, the product can be washed three times with pure tetrahydrofuran to remove unbound metal ions, and then dried at 35°C under vacuum for 48 hours, etc.

[0107] The embodiments of the present application are described in detail below. It should be noted that the embodiments described below are exemplary and are used to explain the present application, and cannot be understood as a limitation of the present application. In addition, if not specifically stated, all reagents used in the following embodiments are commercially available or can be synthesized according to the methods described herein or known methods, and the reaction conditions not listed are also readily available to those skilled in the art.

[0108] Example 1

[0109] This embodiment provides a polysiloxane crosslinking agent, which comprises:

[0110] a) a polysiloxane main chain; and

[0111] b) a first side chain and a second side chain connected to the polysiloxane main chain, the first side chain containing a mercapto group, and the second side chain containing a catechol structure protected by a -SiEt3 silane group; in particular:

[0112] the mercapto group is connected to the polysiloxane main chain through a first alkyl chain to form the first side chain;

[0113] the catechol structure is connected to the polysiloxane main chain through a structure as shown in Formula I to form the second side chain, the Formula I being -R1-S-R2-, R1 being a second alkyl chain, S being sulfur, and R2 being a third alkyl chain;

[0114] the number of repeating units in the polysiloxane main chain is 40, and the first alkyl chain, the second alkyl chain, and the third alkyl chain are each independently selected from a linear alkyl group having 3 carbon atoms (i.e., "-CH2-CH2-CH2-");

[0115] The structural formula of the polysiloxane crosslinking agent is shown in Formula II below:

[0116] The formula II: x is 3.

[0117] like Figure 1 As shown, the preparation method of the above-mentioned polysiloxane crosslinking agent comprises the following steps:

[0118] Eugenol (4.98 g, 30.0 mmol) and triethylsilane (10.68 g, 90 mmol) were added to a round-bottom flask equipped with a reflux condenser and stirred until uniformly mixed. The catalyst tris(pentafluorophenyl)boron (TPFPB, 0.03 g, 0.06 mmol) was then added. The reaction mixture was stirred at room temperature for 6 hours, and the reaction process was monitored by thin-layer chromatography. After the reaction was completed, the catalyst was removed by neutral alumina column chromatography (using dichloromethane as the eluent), and then the unreacted triethylsilane and solvent were removed by a rotary evaporator to finally obtain a colorless liquid product with a coordination unit, which was recorded as SPCAT (i.e. Figure 1 in (1));

[0119] The polysiloxane chain PMMS with thiol groups hanging from it (PMMS was purchased from Gelest, product model GEL-SMS-992), the coordination unit SPCAT added in the required molar ratio (the specific molar ratio of the thiol groups of PMMS and SPCAT is 0.325:0.025) and the photoinitiator 2,2-dimethoxy-2-phenylacetophenone (DMPA, 1 wt% of the total mass of PMMS and SPCAT) were added to a vial, and about 1 mL of chloroform solvent was added to dissolve it and stirred until a uniform solution was formed. Then, under stirring, the reaction bottle was placed under ultraviolet light (365 nm, 0.2 W·cm -2 ) for 5 minutes to promote the reaction between the thiol group and the double bond, thereby preparing a macromolecular crosslinker grafted with a coordination unit - a polysiloxane crosslinker, which is recorded as PMMS-CATx (x=3).

[0120] Example 2

[0121] This embodiment provides a polysiloxane crosslinking agent and a preparation method thereof, which differs from Example 1 only in that:

[0122] (1) In the structural formula of the polysiloxane crosslinker, x=6;

[0123] (2) In the preparation method of the polysiloxane crosslinker, the molar ratio of the thiol group of PMMS and SPCAT is adjusted to 0.35:0.05.

[0124] The polysiloxane crosslinking agent obtained in this example is recorded as PMMS-CATx (x=6).

[0125] Example 3

[0126] This embodiment provides a polysiloxane crosslinking agent and a preparation method thereof, which differs from Example 1 only in that:

[0127] (1) In the structural formula of the polysiloxane crosslinker, x=8;

[0128] (2) In the preparation method of the polysiloxane crosslinker, the molar ratio of the thiol group of PMMS and SPCAT is adjusted to 0.075:0.375.

[0129] The polysiloxane crosslinking agent obtained in this example is recorded as PMMS-CATx (x=8).

[0130] Example 4

[0131] This embodiment provides a polysiloxane crosslinking agent and a preparation method thereof, which differs from Example 1 only in that:

[0132] (1) The structural formula of the polysiloxane crosslinker wherein x=10;

[0133] (2) In the preparation method of the polysiloxane crosslinker, the molar ratio of the thiol group of PMMS and SPCAT is adjusted to 0.1:0.4.

[0134] The polysiloxane crosslinking agent obtained in this example is recorded as PMMS-CATx (x=10).

[0135] Example 5

[0136] This embodiment provides a liquid crystal elastomer, which includes a covalently cross-linked network structure. The covalently cross-linked network structure is formed by covalently connecting a liquid crystal monomer, a cross-linking agent, and a chain extender. The cross-linking agent is the polysiloxane cross-linking agent obtained in Example 1, the liquid crystal monomer is 1,4-bis-[4-(3-acryloyloxypropoxy)benzoyloxy]-2-methylbenzene (RM257), and the chain extender is 2,2'-(1,2-ethylenedioxy)bis(ethylenethiol) (EDDET).

[0137] like Figure 2 As shown, the preparation method of the liquid crystal elastomer comprises the following steps:

[0138] To a vial, 0.1504 g of the polysiloxane crosslinker PMMS-CATx(x = 3) (mercapto group content: 0.3 mmol) obtained in Example 1, 0.6062 g of the liquid crystal monomer (1.2 mmol), and 0.1504 g of the chain extender (1.05 mmol) were added, and about 3 mL of tetrahydrofuran was added to dissolve, and stirred until a uniform solution was formed. Subsequently, a diluted catalyst dipropylamine (DPA, 1 wt% of the total mass of the polysiloxane crosslinker, the liquid crystal monomer, and the chain extender) was added as a catalyst for the thiol-acrylate Michael addition reaction, and the mixture was again shaken to obtain a uniform mixture. The resulting mixture was poured into a polytetrafluoroethylene mold, and left to cure overnight. After the reaction was completed, the obtained thin film was dried in a vacuum environment at 35°C for 48 hours to sufficiently remove the residual solvent, and a liquid crystal elastomer containing coordination units was obtained (i.e., LCE represented as a liquid crystal elastomer), denoted as CAT3-ori. Figure 2 The liquid crystal elastomer containing coordination units is immersed in a 1:1 (volume ratio) hydrochloric acid / tetrahydrofuran solution, and stirred for 12 hours to remove the protecting group; then washed three times (15 minutes each time) in a 1:1 (volume ratio) water / tetrahydrofuran mixture, and if storage is required, dried in a vacuum at 35°C for 48 hours; if the subsequent metal coordination is to be performed immediately, the wet film can be used directly without additional drying; the resulting deprotected liquid crystal elastomer is denoted as CAT3-dep.

[0139] On the basis of the liquid crystal elastomer containing coordination units described above, the present example further constructs a coordination crosslinking network structure on the covalent crosslinking network structure, the coordination crosslinking network structure being formed by removing the silane group from the catechol structure in the polysiloxane crosslinker and connecting the metal ion with a coordination bond, the content of the coordination unit in the coordination crosslinking network structure being 7.7%, and the metal ion being Fe 3+ ; as shown in Formula (3), LCE represents a liquid crystal elastomer), denoted as CAT3-Fe. Figure 3 The preparation method includes the following steps:

[0140] The CAT3-ori is immersed in a 1:1 (volume ratio) hydrochloric acid / tetrahydrofuran solution, and stirred for 12 hours to remove the protecting group; then washed three times (15 minutes each time) in a 1:1 (volume ratio) water / tetrahydrofuran mixture, and if storage is required, dried in a vacuum at 35°C for 48 hours; if the subsequent metal coordination is to be performed immediately, the wet film can be used directly without additional drying; the resulting deprotected liquid crystal elastomer is denoted as CAT3-dep.

[0141] The deprotected CAT3-dep is immersed in a 0.01 mol·L -1 -1 iron nitrate nonahydrate / tetrahydrofuran solution, and left to stand at room temperature for 24 hours for the coordination reaction; after the coordination is completed, the thin film is washed three times with pure tetrahydrofuran to remove the unbound trivalent iron ions, and then dried in a vacuum at 35°C for 48 hours to obtain a liquid crystal elastomer having a double crosslinking network structure of covalent crosslinking and metal-coordination crosslinking, denoted as CAT3-Fe.

[0142] Example 6

[0143] The present example provides a liquid crystal elastomer and a preparation method thereof, and the difference from Example 5 is only that:

[0144] (1) the crosslinking agent is the polysiloxane crosslinking agent obtained in Example 2, and the content of the coordination unit in the coordination crosslinking network structure is 14.3%.

[0145] Example 7

[0146] This example provides a liquid crystal elastomer and a preparation method thereof, which is only different from Example 5 in that:

[0147] (1) the crosslinking agent is the polysiloxane crosslinking agent obtained in Example 3, and the content of the coordination unit in the coordination crosslinking network structure is 20%.

[0148] Example 8

[0149] This example provides a liquid crystal elastomer and a preparation method thereof, which is only different from Example 5 in that:

[0150] (1) the crosslinking agent is the polysiloxane crosslinking agent obtained in Example 4, and the content of the coordination unit in the coordination crosslinking network structure is 25%.

[0151] Test Example 1

[0152] This test example characterizes the coordination unit SPCAT, the mercapto-hung polysiloxane chain PMMS and the macromolecular crosslinking agent grafted with the coordination unit in Example 2, and the obtained nuclear magnetic resonance hydrogen spectrum is as shown in Figure 4 .

[0153] It can be known from Figure 4 that the nuclear magnetic resonance hydrogen spectrum shows that the prepared macromolecular crosslinking agent has both covalent crosslinking sites and coordination crosslinking sites.

[0154] Test Example 2

[0155] This test example characterizes CAT6-ori, CAT6-dep and CAT6-Fe in Example 6, and the obtained small-angle X-ray scattering characterization graph and Raman spectrum characterization graph are as shown in Figure 5 and Figure 6 respectively;

[0156] It can be known from Figure 5 and Figure 6 that the small-angle X-ray scattering spectrum of Fe 2p of CAT6-Fe shows two characteristic peaks, respectively at 710.8eV and 724.9eV, corresponding to Fe 2p1 / 2and Fe 2p3 / 2( Figure 5 ). In contrast, no corresponding characteristic peaks are observed in the Fe 2p spectrum of CAT6-ori and CAT6-dep, indicating that there is an Fe-O coordination bond interaction in CAT6-Fe. In the Raman spectrum, there are peaks at about 490 to 620cm -1 and 1250 to 1500cm -1The characteristic peaks in the two regions are believed to be related to the coordination between iron ions and catechol groups ( Figure 6 ), indicating that coordination bonds exist as additional cross-linking points in CAT6-Fe.

[0157] Test Example 3

[0158] This test example tests the mechanical properties of CAT6-ori, CAT6-dep and CAT6-Fe in Example 6, and compares the tensile stress-strain curves to illustrate the toughening effect of metal coordination bonds on liquid crystal elastomer materials. Figure 7 As shown ( Figure 7 In the figure, CAT6-ori is CAT6-ori (x=6), CAT6-dep is CAT6-dep (x=6), and CAT6-Fe is CAT6-Fe (x=6); the test method conditions include: tensile rate: 10 mm / min.

[0159] Depend on Figure 7 It can be seen that after deprotection, the toughness of CAT6 dep is slightly improved from 4.5MJ·m -3 Increased to 5.3MJ·m -3 This improvement is attributed to the weak hydrogen bond interaction between the coordination units. After the coordination treatment, the coordination cross-linking significantly enhanced the interaction between the polymer chains. During the stretching process, the coordination bonds exchanged, which greatly improved the toughness of the material (compared to 4.5MJ·m -3 Compared with the previous study, it has increased by about 6 times, reaching 28.5MJ·m -3 ).

[0160] Test Example 4

[0161] This test example tests the mechanical properties of CATx-Fe in Examples 5, 6, 7, and 8 above, and compares the toughening effect of different metal coordination bond contents on liquid crystal elastomer materials through tensile stress-strain curves. Figure 8 As shown ( Figure 8 CAT3-Fe is the CAT3-Fe (x=3) in Example 5, CAT6-Fe is the CAT6-Fe (x=6) in Example 6, CAT8-Fe is the CAT8-Fe (x=8) in Example 7, and CAT10-Fe is the CAT10-Fe (x=10) in Example 8); the test method conditions are the same as those in Test Example 3.

[0162] Depend on Figure 8 It can be seen that when the coordination bond content increases, the toughness enhancement effect becomes more significant. Specifically, when the coordination unit content increases from CAT3-Fe to CAT10-Fe, the toughness increases from 16.2MJ·m -3Significantly increased to 67.0 MJ·m -3 , which is 22 times higher than that of CAT10-ori. Although the coordination crosslinking greatly improves the toughness of the material, the Young's modulus of all CATx-Fe networks remains below 5 MPa due to the use of flexible macromolecular crosslinking agents, which have both toughness and softness. The reversible coordination bond exchange further improves the elongation at break and tensile strength of the CATx-Fe network during stretching.

[0163] Test Example 5

[0164] This test example tests the high temperature mechanical properties of CAT6-ori and CAT6-Fe in Example 6, and compares the toughening effect of metal coordination bonds on liquid crystal elastomer materials under high temperature conditions through the tensile stress-strain curve. The test results are shown in Figure 9 Figure 9 : CAT6-ori is CAT6-ori (x = 6), and CAT6-Fe is CAT6-Fe (x = 6)); the test method conditions include: test temperature: 120℃, tensile rate: 10mm / min.

[0165] It can be seen from Figure 9 : At 120℃ (55℃ higher than the phase transition temperature), the toughness of the material is still considerable, reaching 5.5 MJ·m -3 , which is 6 times higher than that of CAT6 ori, which is 0.98 MJ·m -3 .

[0166] Test Example 6

[0167] This test example tests the Young's modulus of the CATx-ori and CATx-Fe liquid crystal elastomers obtained in Examples 5-8, and the test results are shown in Table 1; the test method conditions include: tensile rate: 10mm / min.

[0168] Table 1

[0169]

[0170]

[0171] This test example also tests the toughness of the CATx-Fe liquid crystal elastomers obtained in Examples 5-8 based on the above Test Example 4 using the same test method, and the test results are shown in Table 2.

[0172] Table 2

[0173]

[0174] ​From table 1 and table 2, it can be seen that after coordination treatment, the toughness of the network is greatly improved, and the modulus of each material is kept below 5MPa. The present application effectively solves the performance contradiction between toughness and softness of the existing liquid crystal elastomer, especially the technical bottleneck of realizing high toughness under low modulus conditions.

[0175] In the description of the present specification, the description referring to the terms "one embodiment", "some embodiments", "an example", "a specific example", or "some examples" and the like means that the specific features, structures, materials or characteristics described in connection with the embodiment or example are included in at least one embodiment or example of the present application. In the present specification, the illustrative description of the above terms does not necessarily refer to the same embodiment or example. Moreover, the specific features, structures, materials or characteristics described can be combined in any appropriate manner in any one or more embodiments or examples. Furthermore, different embodiments or examples described in the present specification and the features of different embodiments or examples can be combined and combined by those skilled in the art without contradiction.

[0176] Although the embodiments of the present application have been shown and described above, it should be understood that the above-described embodiments are exemplary and should not be construed as limiting the present application, and those skilled in the art can make changes, modifications, replacements and variations to the above-described embodiments within the scope of the present application.

Claims

1. A polysiloxane crosslinking agent, characterized in that include: a) a polysiloxane backbone; as well as b) a first side chain and a second side chain connected to the polysiloxane main chain, wherein the first side chain contains a thiol group and the second side chain contains a catechol structure protected by a silyl group; The structure of the silyl-protected catechol is shown below: Si is a silicon atom, and R is an alkyl group.

2. The polysiloxane crosslinking agent according to claim 1, characterized in that The first side chain includes a first alkyl chain, the thiol group is connected to the polysiloxane main chain through the first alkyl chain, and the first alkyl chain includes a linear alkyl group with 3 to 6 carbon atoms; And / or, the catechol structure is connected to the polysiloxane main chain through a structure as shown in Formula I, wherein the formula I is: -R1-S-R2-, wherein R1 is a second alkyl chain, S is a sulfur atom, and R2 is a third alkyl chain, wherein the second alkyl chain includes a straight-chain alkyl group having 3 to 6 carbon atoms, and the third alkyl chain includes a straight-chain alkyl group having 3 to 6 carbon atoms; And / or, the number of repeating units in the polysiloxane main chain is 30 to 60.

3. The polysiloxane crosslinking agent according to claim 2, characterized in that The general structural formula of the polysiloxane crosslinking agent is shown below: wherein X is any natural number from 1 to 39, R1 is the second alkyl chain, R2 is the third alkyl chain, and R3 is the first alkyl chain; Alternatively, the structural formula of the polysiloxane crosslinking agent is shown in Formula II below: The formula II: x is any natural number from 1 to 39, and Et is an ethyl group.

4. A liquid crystal elastomer, characterized in that: The liquid crystal elastomer comprises a covalently cross-linked network structure; The covalently cross-linked network structure includes interconnected liquid crystal monomers, a cross-linking agent, and a chain extender, wherein the cross-linking agent includes the polysiloxane cross-linking agent according to any one of claims 1 to 3.

5. The liquid crystal elastomer according to claim 4, wherein The liquid crystal elastomer also includes a coordinated cross-linked network structure; The coordination cross-linked network structure includes a coordination unit, and the structural formula of the coordination unit is as follows: M n+ For metal ions.

6. The liquid crystal elastomer according to claim 5, wherein The molar content of coordination units in the coordination cross-linked network structure is 7.7% to 25%; And / or, the metal ions include Fe 3+ 、Cu 2+ or Zn 2+ At least one of .

7. The liquid crystal elastomer according to claim 6, wherein The physical and chemical properties of the liquid crystal elastomer include: a Young's modulus of 2.6 to 4.8 MPa; and / or a toughness of 16.1 to 67.0 MJ / m 3 .

8. A method for preparing a liquid crystal elastomer according to any one of claims 4 to 7, characterized in that: The preparation method of the liquid crystal elastomer comprises: adding a liquid crystal monomer, a cross-linking agent, and a chain extender into a solvent and dissolving them to obtain a mixture; A catalyst is added to the mixture to carry out a reaction to obtain the liquid crystal elastomer.

9. The method for preparing a liquid crystal elastomer according to claim 8, wherein: The molar ratio of the liquid crystal monomer, the cross-linking agent and the chain extender is (1-2):(0.1-0.5):(0.8-1.2); and / or, the weight added amount of the catalyst accounts for 0.8 to 1.5 wt% of the total weight of the liquid crystal monomer, the cross-linking agent and the chain extender; And / or, the reaction working condition parameters include: temperature of 20 to 30° C., time of 12 to 24 h; And / or, the liquid crystal monomers include 1,4-bis-[4-(3-acryloyloxypropoxy)benzoyloxy]-2-methylbenzene, 1,4-bis-[4-(6-acryloyloxyhexyloxy)benzoyloxy]-2-methylbenzene, (4-(((4-(acryloyloxy)butoxy)carbonyl)oxy)benzoic acid 2-methyl-1,4-diphenolate, 2-methyl-1,4-phenylene bis(4-(4-(acryloyloxy)butoxy)benzoate), (3R,3AR,6S,6AR)-hexahydrofuran-3,2-diphenyl-2-ol at least one of 4-((acryloyloxy)butoxy)carbonyl)oxy)benzoyl)oxy)benzoate, 4-cyanophenyl 4'-(6-acryloyloxyhexyloxy)benzoate, 4-methoxyphenyl 4-[[6-[(1-oxo-2-propenyl)oxy]hexyl]oxy]benzoate, 4-biphenyl-4-yl 4-[4-[(1-oxo-2-propenyl)oxy]butoxy]benzoate, and 4-ethylphenyl 4-[[6-[(1-oxo-2-propenyl)oxy]hexyl]oxy]benzoate; and / or the chain extender comprises at least one of 2,2'-(1,2-ethanediyldioxy)bisethanethiol, 2,4-bis(mercaptomethyl)-1,3,5-trimethylbenzene, mercapto-polyethylene glycol-mercapto, ethylene glycol bisthioglycolate, bis(thioglycolate)-1,4-butylene glycol, bis(3-mercaptopropionic acid) glycol, bis(2-mercaptoethyl) ether, 1,4-butanediol, 1,5-pentanedithiol, 1,6-hexanedithiol, 1,8-octanedithiol, 1,9-nonanedithiol, and 1,10-decanedithiol; and / or, the catalyst comprises at least one of dipropylamine, triethylamine and tetramethylguanidine; And / or, the solvent includes at least one of tetrahydrofuran, toluene and dichloromethane.

10. The method for preparing a liquid crystal elastomer according to any one of claims 8 to 9, characterized in that: The preparation method of the liquid crystal elastomer further comprises: immersing the liquid crystal elastomer in an acidic solution and stirring the solution to obtain a deprotected liquid crystal elastomer; The deprotected liquid crystal elastomer is immersed in a solution containing metal ions to carry out a coordination reaction.

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