A polymer network liquid crystal material and its applications

Abstract

The present invention discloses a polymer network liquid crystal material, a preparation method thereof, and an application thereof in a microwave phase shifter. The polymer network liquid crystal material comprises a liquid crystal composition for microwave use and a polymer network. The polymer network is formed by an in-situ thermal polymerization crosslinking reaction of a terphenyl-based diacrylate monomer containing a flexible alkyl chain with a molecular structure as shown in Formula I and a thermal initiator in the liquid crystal composition for microwave use. By forming the polymer network, the liquid crystal layer is dispersed into a multi-domain state, reducing the thickness of the liquid crystal layer. Moreover, the anchoring effect provided by the polymer network enables the liquid crystal molecules to return to the initial state faster during the relaxation process, shortening the response time, thereby improving the response speed of the microwave liquid crystal. The preparation method of the polymer network liquid crystal material of the present invention is applicable to a microwave phase shifter, and has the characteristics of simple operation, safety and controllability, etc. The polymer network liquid crystal material can be prepared only by an in-situ thermal polymerization crosslinking reaction in the liquid crystal composition for microwave use to form a polymer network, which can solve the problem of slow response speed of the existing liquid crystal materials for microwave use.

Description

Technical Field

[0001] The present invention belongs to the field of liquid crystal materials for microwaves, and relates to a polymer network liquid crystal material, a preparation method thereof, and an application thereof in a microwave phase shifter component. Technical Background

[0002] Developing advanced space information network systems is a major strategic requirement in China. Liquid crystal microwave phase shifters are key components in devices such as phased array radars, satellite communications, and mobile communications. They have advantages such as low operating voltage, high phase shift degree, low cost, and small size, and have been widely used in space information network systems. In particular, they have emerged in fields such as microwave communication, radar antennas and phase modulation, precision guidance, and wireless broadband communication, becoming the new favorites of military, vehicle-mounted, ship-mounted, airborne, and aerospace communication terminal platforms, with a rapid development momentum.

[0003] Liquid crystal microwave phase shifters require liquid crystal materials to meet the following conditions: ① large dielectric anisotropy, wide nematic phase temperature range, low melting point, and good low-temperature miscibility; ② stable molecular structure, small polarizability, small or no absorption of microwave absorption coefficient, and small dielectric loss; ③ low intermolecular force, low viscosity, good viscoelastic coefficient, and fast response speed; ④ the dielectric properties of the liquid crystal should be stable to temperature, especially the temperature dependence of the dielectric properties at low temperatures is small. For easy understanding, the relevant performance parameters of the liquid crystal material are introduced as follows: Δε represents dielectric anisotropy; △n represents optical anisotropy, that is, refractive index (589 nm, 25 °C); Iso. is the clearing point temperature (°C) of the phase state of the liquid crystal composition for microwaves; the dielectric anisotropy in the microwave range is defined as: Δε r ≡(ε r∥ -ε r⊥ ); the tunability (τ) is defined as: τ≡(Δε r / ε r∥ ); the material quality (η) is defined as: η≡(τ / tanδε r max ); the maximum dielectric loss is: tanδε r max ≡max{tanδε r⊥ , tanδε r||}. Among them, dielectric loss refers to the microwave frequency loss caused by the wave frequency absorption generated when microwaves irradiate or pass through the liquid crystal material, usually called microwave insertion loss; in the liquid crystal material, it is manifested as the dielectric constant "Δε r ", and the dielectric constant is divided into the component "ε r∥ " parallel to the long axis of the liquid crystal and the perpendicular component "ε r⊥ ", and the dielectric constant value is Δε r =ε r∥ -ε r⊥; The physical quantitative expression of microwave "dielectric loss" is: the tangent value of dielectric loss (tanδε r⊥ , or tanδε r max ), which is the main performance index parameter reflecting the liquid crystal material in the microwave field. Generally, it is required that the value of tanδε r⊥ (or tanδε r max ) ≤ 0.01, and the value of tanδε r|| ≤ 0.005. Birefringence is the expression method of optical anisotropy of liquid crystal compounds and mixed liquid crystal materials. It means that after light passes through the liquid crystal material, through liquid crystal refraction and scattering, ordinary light and extraordinary light are formed. The refractive index of ordinary light is expressed as "n o ", and the refractive index of extraordinary light is expressed as "n e ". Birefringence is expressed by "Δn", and "Δn = n o - n e ". The higher the value of Δn, the more beneficial it is to improve the microwave phase shift amount or phase tunability. Viscosity is an inherent property of liquid crystal materials, which is determined by the intermolecular interaction force and directly determines the response speed or response time (τ) of the microwave phase shifter; there are usually two expressions for liquid crystal viscosity: rotational viscosity and volume viscosity; the rotational viscosity coefficient (γ1) is the viscosity coefficient caused by the rotation of the liquid crystal molecular director, and the volume viscosity coefficient (μ) is the viscosity coefficient of the liquid crystal molecular director with the flow velocity direction and the flow velocity gradient direction; both reflect the magnitude of the viscous force of the liquid crystal material. "High dielectric and low loss" liquid crystal materials refer to liquid crystal materials with high dielectric anisotropy, high optical anisotropy, and low dielectric loss. Under the microwave conditions of 3.5 - 45 GHz, the performance requirements of the liquid crystal material need to reach a dielectric anisotropy Δε between 0.95 and 1.35, a response time τ less than 0.25 s, an optical anisotropy (birefringence) Δn greater than 0.35, a maximum dielectric loss requirement below 0.01, and a viscosity γ1 ≤ 800 cps. Therefore, the design, synthesis, and preparation of new liquid crystal materials for microwave are the key factors determining the performance of liquid crystal microwave phase shifters and are scientific issues that scientific workers focus on and study.

[0004] With the development needs of high-end microwave communication, the requirement for the response speed of microwave liquid crystal materials is getting higher and higher. At present, most of the microwave liquid crystal materials used at home and abroad are polybiphenyl and poly(phenylethynylene) liquid crystal compounds. Due to the carbon-carbon triple bond in poly(phenylethynylene) liquid crystals, the entire liquid crystal molecule is coplanar, the π-electron conjugation degree of the molecule is relatively high, and the optical anisotropy is relatively large. However, at the same time, the stability of the triple bond to microwaves, heat, and light is slightly weaker than that of the benzene ring, resulting in relatively large dielectric losses in the high-frequency electromagnetic field and microwave band. Polybiphenyl liquid crystal compounds are relatively stable under microwaves and have low dielectric losses. However, due to their strong molecular rigidity, the melting points and viscosities of compounds with a certain molecular weight or structure may increase, resulting in too long response times and thus limiting their practical applications. At the same time, since the research on microwave liquid crystal phase shifter technology has only just developed in recent years, both at home and abroad are still in the stage of technology verification and testing. To ensure high phase tuning amount and low insertion loss of the microwave liquid crystal phase shifter, the liquid crystal layer of the phase shifter is relatively thick, about 50-100 μm, and its cell thickness is about 8-10 times that of the liquid crystal display screen for display, which also makes the response time of the phase shifter relatively long. In order to overcome the application limitations of existing microwave liquid crystal materials, such as polybiphenyl and poly(phenylethynylene) liquid crystal compounds, it is urgent to develop a new liquid crystal material that can simultaneously meet the requirements of high dielectric constant and low loss and fast response speed for microwave liquid crystal materials, break through the technical bottleneck of the practical application of microwave liquid crystal materials, and overcome the defects of existing technologies. Summary of the Invention

[0005] The object of the present invention is to overcome the defects of the prior art. Aiming at the technical problems of the relatively thick liquid crystal cell of the existing microwave liquid crystal phase shifter, relatively large viscosity of the liquid crystal material, and slow response speed, the present invention provides a polymer network liquid crystal material and a preparation method thereof, so that the prepared polymer network liquid crystal material can be used as a liquid crystal material with high dielectric constant and low loss and fast response speed that meets the requirements of microwave phase shifters.

[0006] To achieve the above object, in the first aspect of the present invention, a polymer network liquid crystal material is provided. The polymer network liquid crystal material includes a polymer network and a liquid crystal composition for microwaves. The polymer network is formed by in-situ thermal polymerization cross-linking reaction of a monomer and a thermal initiator in the liquid crystal composition for microwaves. The molecular structure of the monomer is shown in Formula I:

[0007]

[0008] Wherein, each X is independently selected from -H, -CH3; each n is independently selected from any integer within the range of 3 to 10; each of A1 and A2 is independently selected from -OCH2-, -OCF2-, -OCO-, -CO2-, -CH2O-, -CF2O-, -CF═CF-, -CH2CH2-; each of Z1 and Z2 is independently selected from -CH2-, -O-, -S-; L is a substituent on the benzene ring and is independently selected from -H, -Cl, -F, -CH3, -C2H5; each r is independently selected from any integer within the range of 0 to 2.

[0009] The response speed of the liquid crystal is closely related to the performance of the liquid crystal device. Formulas 1 and 2 represent the relationship between the response time and the liquid crystal layer thickness (d), the liquid crystal rotational viscosity (γ1), and the elastic constant (k ii ).

[0010] t on =γ1d 2 / π 2 k ii (V on 2 / V th 2 -1)(1)

[0011] t off =γ1d 2 / π 2 k ii (2)

[0012] Wherein, t on , t off respectively represent the "on" and "off" response times in the voltage switching state of the microwave phase shifter, d represents the thickness of the liquid crystal layer; V th is the threshold voltage; V on represents the applied voltage or the operating voltage. The so-called response time of the liquid crystal refers to the sum of the "on" and "off" response times. The shorter the response time, the faster the response speed. It can be seen from Formulas (1) and (2) that the response speed of the liquid crystal is mainly related to the liquid crystal layer thickness (d), the liquid crystal rotational viscosity (γ1), and the elastic constant (k ii ). When the liquid crystal rotational viscosity (γ1) and the elastic constant (k ii ) remain unchanged, the liquid crystal layer thickness (d) has the greatest influence. The greater the liquid crystal layer thickness (d), the longer the response time.

[0013] In the technical solution of the present invention, the polymer network liquid crystal material includes a liquid crystal composition for microwave and a polymer network. An in-situ thermal polymerization cross-linking reaction occurs between a monomer with a liquid crystal structure and a thermal initiator in the liquid crystal composition for microwave to form the polymer network. The liquid crystal layer is dispersed into a multi-domain state by the polymer network, that is, the polymer network disperses the liquid crystal layer into multiple small liquid crystal domain units, reducing the thickness of the liquid crystal layer. At the same time, the anchoring effect provided by the polymer network enables the liquid crystal molecules to return to the initial state faster during the relaxation process, shortening the response time, and thus improving the response speed of the microwave liquid crystal.

[0014] In the technical solution of the present invention, thermal polymerization is adopted because in practical applications, the liquid crystal cell in the microwave phase shifter is very thick and is a copper cell. Since light cannot penetrate the copper cell, photo-polymerization cannot be achieved, so only thermal polymerization can be used; and thermal polymerization is easier to implement, the operation is simpler, and the formed polymer network can enable the liquid crystal to form a multi-domain structure, shortening the response time of the liquid crystal.

[0015] In the technical solution of the present invention, the molecular structure of the monomer is shown in Formula I. The monomer is a triphenyl-based diacrylate compound connected by a flexible alkyl chain. Selecting this type of monomer has the following advantages:

[0016] (1) Since the monomer molecule contains a flexible alkyl spacer chain, it has a smaller rigidity, which is more conducive to the thermal polymerization reaction to form a polymer network, enabling the cross-linking density of the formed polymer network to increase. Thus, the polymer network can penetrate well into the liquid crystal composition for microwave, and the liquid crystal forms a multi-domain state, reducing the thickness (d) of the liquid crystal layer. At the same time, the anchoring effect provided by the polymer network enables the liquid crystal molecules to return to the initial state faster during the relaxation process, more effectively reducing the response time of the liquid crystal and improving the response speed of the liquid crystal.

[0017] (2) Since this type of monomer contains three benzene rings, which are connected by polar groups, it has a structure and properties similar to liquid crystal molecules and has a certain polarity, enabling it to dissolve well and uniformly in the liquid crystal composition for microwave. This is the premise for realizing the in-situ thermal polymerization cross-linking reaction in the liquid crystal composition for microwave to form a polymer network.

[0018] (3) This type of monomer belongs to a bifunctional monomer and has two polymerization reaction sites, which can accelerate the polymerization reaction and increase the cross-linking density of the polymer network. Only in this way can the polymer network penetrate through the entire liquid crystal layer, enabling the liquid crystal layer to form a multi-domain structure. The polymer network disperses the liquid crystal layer into multiple small liquid crystal domain units, thereby reducing the "liquid crystal layer thickness" to achieve the purpose of shortening the response time.

[0019] (4) The thermal polymerization reaction of this type of monomer features "fast initiation and slow growth", mild reaction conditions, and is more likely to form a polymer network through thermal polymerization, which is conducive to the formation of a multi-domain structure in the liquid crystal layer and improves the response speed.

[0020] Therefore, after filling the liquid crystal cell with this type of diacrylate monomer and the liquid crystal composition for microwave use, an in-situ polymerization reaction is initiated by heating to form a polymer network structure. The liquid crystal composition for microwave use is dispersed in these polymer networks, and the liquid crystal layer forms a multi-domain structure, reducing the thickness of the liquid crystal layer. At the same time, the anchoring effect provided by the polymer network enables the liquid crystal molecules to return to the initial state more quickly during the relaxation process, shortening the response time, thereby preparing a liquid crystal cell and its microwave phase shifter that can meet the requirements of fast response for microwave phase tunability performance.

[0021] Preferably, based on the total weight of the liquid crystal composition for microwave use, the liquid crystal composition for microwave use is composed of component A with a mass fraction of 10 - 40%, component B with a mass fraction of 8 - 35%, component C with a mass fraction of 10 - 35%, and component D with a mass fraction of 5 - 20%, where:

[0022] Component A is at least one of the compounds having the structure shown in Structural Formula II below:

[0023]

[0024]

[0025] Component B is at least one of the compounds having the structure shown in Structural Formula III below:

[0026]

[0027] Component C is at least one of the compounds having the structure shown in Structural Formula IV below:

[0028]

[0029] Component D is at least one of the compounds having the structure shown in Structural Formula V below:

[0030]

[0031] Among them, R1, R2, R3, R4, R5, and R6 are each independently selected from -C m H 2m+1 , m is each independently selected from any integer within the range of 2 - 10; X1, X2, X5, X6, and X9 are ortho- or meta-substituents on the benzene ring and are each independently selected from -H, -F, -CH3, -CH2CH3; X3, X4, X7, and X8 are each independently selected from -H, -F.

[0032] When specifically selecting the liquid crystal components in the liquid crystal composition for microwaves, the larger the conjugated system of the molecules, the higher the birefringence; the higher the birefringence, the lower the dielectric loss of the formed liquid crystal composition for microwaves, and the higher the dielectric anisotropy.

[0033] In the technical solution of the present invention, the liquid crystal composition for microwaves is composed of component A with a mass fraction of 10 - 40%, component B with a mass fraction of 8 - 35%, component C with a mass fraction of 10 - 35%, and component D with a mass fraction of 5 - 20%. Tetraphenyl, terphenyl liquid crystal compounds, triphenyl diyne and triphenyl acetylene liquid crystal compounds are selected as the components of the high dielectric and low loss liquid crystal composition for microwaves. The molecular structures of such liquid crystal compounds are stable, with a small microwave absorption coefficient and small dielectric loss. The molecular structure of the liquid crystal composition for microwaves has a tetraphenyl structure, alkyl substituents, F atoms, and -NCS and other groups. The large conjugated system structure of tetraphenyl makes the liquid crystal compound have the advantage of large optical anisotropy. The alkyl substituents can increase the flexibility of its molecules and reduce the melting point of the liquid crystal compound. -F can increase the dielectric constant of such liquid crystal compounds. The -NCS group extends the length of the π-electron conjugated system of biphenyl and can increase the optical anisotropy (i.e., the birefringence Δn value) of the liquid crystal compound.

[0034] At the same time, the nematic liquid crystal composition for microwaves formed by combining tetraphenyl, terphenyl, triphenyl acetylene, and triphenyl diyne liquid crystal compounds through a specific ratio can meet the requirements of high dielectric and low loss of microwave phase shifters, but the viscosity is relatively large, resulting in a relatively slow response speed. Using this liquid crystal composition for microwaves alone cannot meet the requirements of fast response.

[0035] Preferably, based on the total weight of the polymer network liquid crystal material, the content of the liquid crystal composition for microwaves is 89.5 - 95.8 wt%, the content of the monomer is 4 - 10 wt%, and the content of the thermal initiator is 0.2 - 0.5 wt%.

[0036] In the technical solution of the present invention, based on the total weight of the polymer network liquid crystal material, the content of the liquid crystal composition for microwaves is 89.5 - 95.8 wt%, the content of the monomer is 4 - 10 wt%, and the content of the thermal initiator is 0.2 - 0.5 wt%. When the content of the polymer in the polymer network liquid crystal is 4 - 10 wt%, the monomer can maintain the morphology of a specific liquid crystal structure while forming the polymer network, and the liquid crystal composition for microwaves can be dispersed into a multi-domain state.

[0037] Meanwhile, by means of a specific ratio, the in-situ thermal polymerization reaction of the monomer and the thermal initiator in the liquid crystal composition for microwaves has the optimal effect, the morphology of the formed polymer network is better, and the prepared polymer network can penetrate well through the liquid crystal, dispersing the liquid crystal composition for microwaves into a multi-domain state. At the same time, the anchoring effect provided by the polymer network enables the liquid crystal molecules to return to the initial state more quickly during the relaxation process, thereby reducing the response time of the thick liquid crystal cell as a whole and achieving fast response.

[0038] Preferably, the monomer of formula I is selected from one or more of formula I-1 to formula I-4:

[0039]

[0040]

[0041] Wherein, each X independently is selected from -H, -CH3; each n independently is selected from any integer within the range of 3 to 10; L is a substituent on the benzene ring, and each is independently selected from -H, -Cl, -F, -CH3, -C2H5; each r independently is selected from any integer within the range of 0 to 2.

[0042] In the technical solution of the present invention, the monomer is selected from one or more of formula I-1 to formula I-4. In the molecular structural formula of this type, the bond bridging with the benzene ring is -CO2-, -CH2O-, -CF2O-, -CF=CF-. Selecting this type of monomer has a greater polarity, can make the benzene ring orient and arrange, has a similar structure and properties to the liquid crystal, has better miscibility with the liquid crystal composition for microwaves, is more conducive to the thermal polymerization reaction of this type of monomer in the liquid crystal composition for microwaves to form a polymer network, can increase the crosslinking density of the formed polymer network, so that the polymer network can penetrate well through the liquid crystal composition for microwaves, the liquid crystal forms a multi-domain state, reduces the thickness of the liquid crystal layer. At the same time, the anchoring effect provided by the polymer network enables the liquid crystal molecules to return to the initial state more quickly during the relaxation process, more effectively reduces the response time of the liquid crystal, and improves the response speed of the liquid crystal.

[0043] More preferably, in the technical solution of the present invention, the monomer is selected from one or more of formula I-2 and formula I-4. Selecting the monomer of tribenzene bisacrylate connected by a fluorine-containing flexible alkyl chain can reduce the elastic force between the polymer network and the liquid crystal, thereby reducing the response time of the liquid crystal. At the same time, it can increase the crosslinking density of the formed polymer network, the polymer network can penetrate well through the liquid crystal composition for microwaves, the liquid crystal forms a multi-domain state, the anchoring effect provided by the polymer network is enhanced, enabling the liquid crystal molecules to return to the initial state more quickly during the relaxation process, and then reducing the response time of the thick liquid crystal cell as a whole, more effectively achieving fast response.

[0044] More preferably, the monomer of Formula I-1 is selected from one or more of Formulas I-1-1 to I-1-6:

[0045]

[0046] The monomer of Formula I-2 is selected from one or more of Formulas I-2-1 to I-2-6:

[0047]

[0048]

[0049] The monomer of Formula I-3 is selected from one or more of Formulas I-3-1 to I-3-6:

[0050]

[0051]

[0052] The monomer of Formula I-4 is selected from one or more of Formulas I-4-1 to I-4-6:

[0053]

[0054]

[0055] In the technical solution of the present invention, specifically selecting the above monomers can increase the crosslinking density of the formed polymer network. The polymer network can well penetrate the liquid crystal composition for microwave use, making the liquid crystal form a multi-domain state, and the anchoring effect provided by the polymer network is enhanced, which will enable the liquid crystal molecules to return to the initial state faster during the relaxation process, thereby reducing the response time and more effectively achieving fast response.

[0056] Preferably, the thermal initiator is selected from one of benzoyl peroxide, lauroyl peroxide, diisopropylbenzene peroxide, di-tert-butyl peroxide, tert-butyl peroxybenzoate, and tert-butyl peroxypivalate.

[0057] In the technical solution of the present invention, the thermal initiator is one of benzoyl peroxide, lauroyl peroxide, diisopropylbenzene peroxide, di-tert-butyl peroxide, tert-butyl peroxybenzoate, and tert-butyl peroxypivalate. These thermal initiators have good initiation effects, will not generate bubbles during the thermal polymerization process, will not have an adverse impact on the liquid crystal material for microwave use, and can well initiate the monomers to undergo polymerization reaction to form a polymer network. The temperature and time of the polymerization reaction are appropriate, and it is easy to operate and control.

[0058] The second aspect of the present invention provides a preparation method of the polymer network liquid crystal material, including the following steps:

[0059] Step 1: Heat the liquid crystal component compound to 80 - 100 °C and stir for 20 - 30 min to dissolve and mix to prepare a liquid crystal composition for microwave use; Step 2: Add the monomer to the prepared liquid crystal composition for microwave use, heat to 70 - 100 °C and stir for 10 - 20 min to mix and dissolve, then add the thermal initiator, heat to 60 - 90 °C and stir for 5 - 15 min to obtain a uniformly mixed mixture; Step 3: Inject the mixture into a liquid crystal cell and carry out in-situ thermal polymerization at 40 - 110 °C for 3 - 12 h to obtain the polymer network liquid crystal material.

[0060] In the technical solution of the present invention, the in-situ preparation method of the polymer network liquid crystal material is divided into three steps. The first two steps are the processes of preparing a liquid crystal composition for microwave use and uniformly dispersing the monomer and the thermal initiator in the liquid crystal composition for microwave use, so that the monomer and the thermal initiator are well miscible with the liquid crystal composition for microwave use, which is the key for the monomer to undergo in-situ thermal polymerization cross-linking reaction in the liquid crystal composition for microwave use to form a polymer network; at the same time, the monomer and the thermal initiator are added to the liquid crystal composition for microwave use separately in sequence. During the heating and stirring process, they can be mixed evenly, and after adding the thermal initiator, pre-polymerization can also be carried out, which is beneficial to subsequent in-situ thermal polymerization in the liquid crystal cell to form a polymer network; the latter step is to inject the mixture into the liquid crystal cell, and the monomer and the initiator undergo in-situ thermal polymerization in the liquid crystal composition for microwave use to obtain the polymer network liquid crystal material.

[0061] Therefore, this preparation method has the characteristics of short route, simple operation, safety and controllability. The polymer network liquid crystal material can be prepared only by undergoing in-situ thermal polymerization cross-linking reaction in the liquid crystal composition for microwave use to form a polymer network, solving the problem of slow response speed of existing microwave liquid crystals. The polymer network liquid crystal material is obtained by carrying out in-situ thermal polymerization at 40 - 110 °C for 3 - 12 h. The polymerization conditions are mild, simple and easy to implement, highly operable, safe and controllable; the prepared polymer network liquid crystal material can meet the requirement of fast response speed.

[0062] Preferably, the temperature of the in-situ polymerization is 60 - 90 °C, and the time of the in-situ polymerization is 6 - 10 h.

[0063] In the technical solution of the present invention, the temperature of the in-situ polymerization is 60 - 90 °C, and the time of the in-situ polymerization is 6 - 10 h, further optimizing the temperature and time of the polymerization reaction, making the response time of the polymer network liquid crystal material shorter and the response speed faster.

[0064] Based on the polymer network liquid crystal material of the present invention can meet the requirements of high permittivity and low loss, fast response speed of the microwave phase shifter, and can be used as a liquid crystal material for microwave phase shifter components, and is applicable to microwave phase shifter components. The third aspect of the present invention provides the application of the polymer network liquid crystal material as a liquid crystal material for microwave phase shifter components.

[0065] Compared with the prior art, the technical solution of the present invention has the following beneficial effects:

[0066] (1) The polymer network liquid crystal material of the present invention includes a polymer network and a liquid crystal composition for microwave use. The polymer network is formed by in-situ thermal polymerization cross-linking reaction of monomers and a thermal initiator in the liquid crystal composition for microwave use. The liquid crystal is dispersed into a multi-domain state through the polymer network, reducing the thickness of the liquid crystal layer. Moreover, the anchoring effect provided by the polymer network enables the liquid crystal molecules to return to the initial state faster during the relaxation process, shortening the response time, and thus improving the response speed of the microwave liquid crystal.

[0067] (2) The monomer of the present invention is a terphenyl-based diacrylate compound connected by a flexible alkyl chain. Due to the presence of the flexible alkyl chain in the monomer molecule, its rigidity is smaller, which is more conducive to the formation of a polymer network by thermal polymerization reaction, enabling the cross-linking density of the formed polymer network to increase. Thus, the polymer network can penetrate well through the liquid crystal composition for microwave use, and the liquid crystal forms a multi-domain state, reducing the thickness of the liquid crystal layer. At the same time, the anchoring effect provided by the polymer network enables the liquid crystal molecules to return to the initial state faster during the relaxation process, more effectively reducing the response time of the liquid crystal and improving the response speed of the liquid crystal. Since this type of monomer contains three benzene rings connected by polar groups, it has a structure and properties similar to those of liquid crystal molecules and has a certain polarity, which can be well and uniformly dissolved in the liquid crystal composition for microwave use. This is the prerequisite for realizing the in-situ thermal polymerization cross-linking reaction in the liquid crystal composition for microwave use to form a polymer network. This type of diacrylate polymerization monomer belongs to a bifunctional monomer with two polymerization reaction sites, which can accelerate the polymerization reaction and increase the cross-linking density of the polymer network, enabling the polymer network to penetrate through the entire liquid crystal layer, forming a multi-domain structure in the liquid crystal layer. The polymer network disperses the liquid crystal layer into multiple small liquid crystal domain units, thereby reducing the thickness of the liquid crystal layer to achieve the purpose of shortening the response time. The thermal polymerization reaction of this type of diacrylate polymerization monomer has fast initiation and slow growth, mild reaction conditions, and is more likely to form a polymer network by thermal polymerization, which is beneficial to the formation of a multi-domain structure in the liquid crystal layer and improves the response speed.

[0068] (3) The structure of the liquid crystal composition for microwaves of the present invention simultaneously has a terphenyl structure, an alkyl substituent, an F atom, and a -NCS group, etc. The terphenyl structure makes the liquid crystal compound have the advantage of large optical anisotropy. The alkyl substituent can increase the flexibility of its molecules and lower the melting point of the liquid crystal compound. -F can increase the dielectric constant of this type of liquid crystal compound. The -NCS group extends the length of the terphenyl π-electron conjugation system and can increase the optical anisotropy (i.e., the birefringence Δn value) of the liquid crystal compound. At the same time, the liquid crystal composition for microwaves of the present invention is a nematic liquid crystal composition formed by combining terphenyl-based, terphenyl-based liquid crystal compounds with other liquid crystal compositions for microwaves such as triphenylacetylene-based and triphenyldiyne-based through a specific ratio combination, which can meet the requirements of high dielectric constant and low loss of microwave phase shifters, but the viscosity is relatively large.

[0069] (4) The reason why thermal polymerization is adopted in the present invention is that in practical applications, the liquid crystal cell in the microwave phase shifter is very thick and is a copper cell. Since light cannot penetrate the copper cell, it is impossible to achieve photo-polymerization. Therefore, only the method of thermal polymerization can be adopted. And adopting thermal polymerization is easier to achieve, the operation is simpler, and the formed polymer network can make the liquid crystal form a multi-domain structure and shorten the response time of the liquid crystal.

[0070] (5) The in-situ preparation method of the polymer network liquid crystal material of the present invention is divided into three steps. The first two steps are the processes of preparing the liquid crystal composition for microwaves and uniformly dispersing monomers and thermal initiators in the liquid crystal composition for microwaves and pre-polymerizing the monomers and thermal initiators, which is beneficial to the subsequent in-situ thermal polymerization in the liquid crystal cell to form a polymer network. The last step is to fill the liquid crystal cell to undergo a thermal polymerization reaction to obtain the polymer network liquid crystal material. This preparation method has the characteristics of short route, simple operation, safety and controllability. Only by undergoing an in-situ thermal polymerization cross-linking reaction in the liquid crystal composition for microwaves to form a polymer network can the polymer network liquid crystal material be prepared, which improves the response speed of the liquid crystal and solves the problem of slow response speed of the liquid crystal composition for microwaves.

[0071] (6) The polymer network liquid crystal material of the present invention is a liquid crystal material with high dielectric constant, low loss and fast response speed that meets the requirements of microwave phase shifters, and can be used as a liquid crystal material for microwave phase shifter components and is applicable to microwave phase shifter components.

[0072] Other features and advantages of the present invention will be described in detail in the subsequent specific implementation part. Brief Description of the Drawings

[0073] Figure 1 It is a schematic diagram of the molecular structural formula of the monomer provided by the present invention. Specific Embodiments

[0074] The preferred embodiments of the present invention will be described in more detail below. Although the preferred embodiments of the present invention are described below, it should be understood that the present invention can be implemented in various forms and should not be limited by the embodiments set forth herein. On the contrary, these embodiments are provided to make the present invention more thorough and complete, and to fully convey the scope of the present invention to those skilled in the art. For those conditions not specified in the examples, they are carried out according to conventional conditions or the conditions recommended by the manufacturer. For reagents or instruments whose manufacturers are not specified, they are all conventional products that can be obtained through commercial purchase or synthesized by known chemical methods.

[0075] For ease of understanding, the symbols and explanations of the microwave dielectric property parameters involved are as follows:

[0076] Δε represents the dielectric anisotropy of the liquid crystal; Δε ≡ (ε ∥ -ε ⊥ ) at low frequencies; where ε ∥ is the dielectric constant in the direction parallel to the molecular axis; ε ⊥ is the dielectric constant in the direction perpendicular to the molecular axis;

[0077] △n is the optical anisotropy, i.e., the birefringence (589nm, 20 °C); △n = n o -n e

[0078] Iso. is the clearing point temperature (°C) of the liquid crystal phase state of the composition; Cr represents the anisotropic crystal transition point temperature or melting point; S or Sm represents the smectic phase state and its phase state temperature transition point; N represents the nematic phase state and its phase state temperature transition point;

[0079] The dielectric anisotropy in the microwave range is defined as: Δε r ≡ (ε r∥ -ε r⊥ ).

[0080] The maximum dielectric loss in the microwave K band is: tanδε r max . ≡ max.{tanδε r⊥ : tanδε r∥}; in rod-shaped liquid crystal molecules, the maximum dielectric loss in the nematic phase state is the perpendicular dielectric loss: tanδε r max ≡ tanδε r⊥ .

[0081] The microwave phase tunability (τ) is defined as: τ ≡ Δε r / ε r∥ .

[0082] The quality (η) of the microwave dielectric material is defined as: η ≡ τ / tanδε r max

[0083] For the sake of convenience of expression, in the following embodiments, the group structures of the liquid crystal compositions for microwaves are represented by the codes listed in Table 1.

[0084] Table 1 Group Structure Codes of Liquid Crystal Compounds

[0085]

[0086] Preparation of Liquid Crystal Composition M-1 for Microwaves and Testing of Its Microwave Dielectric Properties

[0087] Components A, B, C, and D are used as raw materials for formulating the liquid crystal composition for microwaves. After weighing the raw materials according to the mass fractions in Table 2 with an electronic balance, the liquid crystal composition for microwaves is formulated by heating and stirring at 100 °C for 30 min. The dielectric properties of the formulated liquid crystal composition for microwaves at high frequencies are all tested by Chengdu Enchi Microwave Technology Co., Ltd. using the rectangular resonant cavity perturbation method. The test results of the dielectric properties are shown in Table 2.

[0088] Table 2 Mass Fractions and Performance Parameters of Each Component in Liquid Crystal Composition M-1 for Microwaves

[0089]

[0090]

[0091] Preparation of Liquid Crystal Composition M-2 for Microwaves and Testing of Its Microwave Dielectric Properties

[0092] Components A, B, C, and D are used as raw materials for formulating the liquid crystal composition for microwaves. After weighing the raw materials according to the mass fractions in Table 3 with an electronic balance, the liquid crystal composition for microwaves is formulated by heating and stirring at 100 °C for 30 min. The dielectric properties of the formulated liquid crystal composition for microwaves at high frequencies are all tested by Chengdu Enchi Microwave Technology Co., Ltd. using the rectangular resonant cavity perturbation method. The test results of the dielectric properties are shown in Table 3.

[0093] Table 3 Mass Fractions and Performance Parameters of Each Component in Liquid Crystal Composition M-2 for Microwaves

[0094]

[0095]

[0096] Preparation of Polymer Network Liquid Crystal Material M-1-1 in Example 3

[0097] The liquid crystal composition for microwaves used in this example is the liquid crystal composition M-1 prepared in Example 1. The molecular structural formula of monomer Ⅰ-1-1 used is as follows:

[0098]

[0099] The thermal initiator used is benzoyl peroxide.

[0100] According to the following mass fractions: 89.5% of the liquid crystal composition for microwave, 10% of the monomer, and 0.5% of the thermal initiator, the corresponding weights were weighed respectively to prepare the polymer network liquid crystal material.

[0101] Preparation of the polymer network liquid crystal material: Weigh 1 g of the liquid crystal composition for microwave prepared in Example 1, add 111.7 mg of the monomer to the prepared liquid crystal composition for microwave, heat and stir at 90 °C for 20 min for mixing and dissolution, then add 5.6 mg of the thermal initiator, heat and stir at 80 °C for 10 min to obtain a uniformly mixed mixture; inject the mixture into a liquid crystal cell, and carry out in-situ thermal polymerization at 70 °C for 6 h to obtain the polymer network liquid crystal material M-1-1.

[0102] Preparation of the polymer network liquid crystal material M-1-2 in Example 4

[0103] The liquid crystal composition for microwave used in this example is the liquid crystal composition M-1 prepared in Example 1, and the molecular structural formula of the monomer I-1-1 used is as follows:

[0104]

[0105] The thermal initiator used is benzoyl peroxide.

[0106] According to the following mass fractions: 92.6% of the liquid crystal composition for microwave, 7% of the monomer, and 0.4% of the thermal initiator, the corresponding weights were weighed respectively to prepare the polymer network liquid crystal material.

[0107] Preparation of the polymer network liquid crystal material: Weigh 1 g of the liquid crystal composition for microwave prepared in Example 1, add 75.6 mg of the monomer to the prepared liquid crystal composition for microwave, heat and stir at 90 °C for 20 min for mixing and dissolution, then add 4.3 mg of the thermal initiator, heat and stir at 80 °C for 10 min to obtain a uniformly mixed mixture; inject the mixture into a liquid crystal cell, and carry out in-situ thermal polymerization at 70 °C for 6 h to obtain the polymer network liquid crystal material M-1-2.

[0108] Preparation of the polymer network liquid crystal material M-1-3 in Example 5

[0109] The liquid crystal composition for microwave used in this example is the liquid crystal composition M-1 prepared in Example 1, and the molecular structural formula of the monomer I-1-1 used is as follows:

[0110]

[0111] The thermal initiator used is benzoyl peroxide.

[0112] According to the following mass fractions: 95.8% of the liquid crystal composition for microwave, 4% of the monomer, and 0.2% of the thermal initiator, the corresponding weights are weighed respectively to prepare the polymer network liquid crystal material.

[0113] Preparation of the polymer network liquid crystal material: Weigh 1 g of the liquid crystal composition for microwave prepared in Example 1, add 41.8 mg of the monomer to the prepared liquid crystal composition for microwave, heat and stir at 90 °C for 20 min for mixing and dissolving, then add 2.1 mg of the thermal initiator, heat and stir at 80 °C for 10 min to prepare a uniformly mixed mixture; inject the mixture into the liquid crystal cell, and carry out in-situ thermal polymerization at 70 °C for 6 h to obtain the polymer network liquid crystal material M-1-3.

[0114] Preparation of the polymer network liquid crystal material M-1-4 in Example 6

[0115] The liquid crystal composition for microwave used in this example is the liquid crystal composition for microwave M-1 prepared in Example 1, and the molecular structural formula of the monomer I-2-1 used is as follows:

[0116]

[0117] The thermal initiator used is benzoyl peroxide.

[0118] According to the following mass fractions: 92.6% of the liquid crystal composition for microwave, 7% of the monomer, and 0.4% of the thermal initiator, the corresponding weights are weighed respectively to prepare the polymer network liquid crystal material.

[0119] Preparation of the polymer network liquid crystal material: Weigh 1 g of the liquid crystal composition for microwave prepared in Example 1, add 75.6 mg of the monomer to the prepared liquid crystal composition for microwave, heat and stir at 90 °C for 20 min for mixing and dissolving, then add 4.3 mg of the thermal initiator, heat and stir at 80 °C for 10 min to prepare a uniformly mixed mixture; inject the mixture into the liquid crystal cell, and carry out in-situ thermal polymerization at 70 °C for 6 h to obtain the polymer network liquid crystal material M-1-4.

[0120] Preparation of the polymer network liquid crystal material M-1-5 in Example 7

[0121] The liquid crystal composition for microwave used in this example is the liquid crystal composition for microwave M-1 prepared in Example 1, and the molecular structural formula of the monomer I-3-1 used is as follows:

[0122]

[0123] The thermal initiator used is benzoyl peroxide.

[0124] According to the following mass fractions: 92.6% of the liquid crystal composition for microwave, 7% of the monomer, and 0.4% of the thermal initiator, the corresponding weights are weighed respectively to prepare the polymer network liquid crystal material.

[0125] Preparation of the polymer network liquid crystal material: Weigh 1 g of the liquid crystal composition for microwave prepared in Example 1, add 75.6 mg of the monomer to the prepared liquid crystal composition for microwave, heat and stir at 90 °C for 20 min to dissolve them uniformly, then add 4.3 mg of the thermal initiator, heat and stir at 80 °C for 10 min to obtain a uniformly mixed mixture; inject the mixture into a liquid crystal cell, and carry out in-situ thermal polymerization at 70 °C for 6 h to obtain the polymer network liquid crystal material M-1-5.

[0126] Preparation of the polymer network liquid crystal material M-1-6 in Example 8

[0127] The liquid crystal composition for microwave used in this example is the liquid crystal composition M-1 prepared in Example 1, and the molecular structural formula of the monomer I-4-1 used is as follows:

[0128]

[0129] The thermal initiator used is benzoyl peroxide.

[0130] According to the following mass fractions: 92.6% of the liquid crystal composition for microwave, 7% of the monomer, and 0.4% of the thermal initiator, the corresponding weights are weighed respectively to prepare the polymer network liquid crystal material.

[0131] Preparation of the polymer network liquid crystal material: Weigh 1 g of the liquid crystal composition for microwave prepared in Example 1, add 75.6 mg of the monomer to the prepared liquid crystal composition for microwave, heat and stir at 90 °C for 20 min to dissolve them uniformly, then add 4.3 mg of the thermal initiator, heat and stir at 80 °C for 10 min to obtain a uniformly mixed mixture; inject the mixture into a liquid crystal cell, and carry out in-situ thermal polymerization at 70 °C for 6 h to obtain the polymer network liquid crystal material M-1-6.

[0132] Preparation of the polymer network liquid crystal material M-2-1 in Example 9

[0133] The liquid crystal composition for microwave used in this example is the liquid crystal composition M-2 prepared in Example 2, and the molecular structural formula of the monomer I-1-1 used is as follows:

[0134]

[0135] The thermal initiator used is benzoyl peroxide.

[0136] Prepare the polymer network liquid crystal material by weighing the corresponding weights according to the following mass fractions: 89.5% of the liquid crystal composition for microwave, 10% of the monomer, and 0.5% of the thermal initiator.

[0137] Preparation of the polymer network liquid crystal material: Weigh 1 g of the liquid crystal composition for microwave prepared in Example 2, add 111.7 mg of the monomer to the prepared liquid crystal composition for microwave, heat and stir at 90 °C for 20 min to dissolve them, then add 5.6 mg of the thermal initiator, heat and stir at 80 °C for 10 min to obtain a uniformly mixed mixture; inject the mixture into a liquid crystal cell and conduct in-situ thermal polymerization at 70 °C for 6 h to obtain the polymer network liquid crystal material M-2-1.

[0138] Preparation of the polymer network liquid crystal material M-2-2 in Example 10

[0139] The liquid crystal composition for microwave used in this example is the liquid crystal composition M-2 prepared in Example 2. The molecular structural formula of the monomer I-1-1 used is as follows:

[0140]

[0141] The thermal initiator used is benzoyl peroxide.

[0142] Prepare the polymer network liquid crystal material by weighing the corresponding weights according to the following mass fractions: 92.6% of the liquid crystal composition for microwave, 7% of the monomer, and 0.4% of the thermal initiator.

[0143] Preparation of the polymer network liquid crystal material: Weigh 1 g of the liquid crystal composition for microwave prepared in Example 2, add 75.6 mg of the monomer to the prepared liquid crystal composition for microwave, heat and stir at 90 °C for 20 min to dissolve them, then add 4.3 mg of the thermal initiator, heat and stir at 80 °C for 10 min to obtain a uniformly mixed mixture; inject the mixture into a liquid crystal cell and conduct in-situ thermal polymerization at 70 °C for 6 h to obtain the polymer network liquid crystal material M-2-2.

[0144] Preparation of the polymer network liquid crystal material M-2-3 in Example 11

[0145] The liquid crystal composition for microwave used in this example is the liquid crystal composition M-2 prepared in Example 2. The molecular structural formula of the monomer I-1-1 used is as follows:

[0146]

[0147] The thermal initiator used is benzoyl peroxide.

[0148] Prepare the polymer network liquid crystal material by weighing the corresponding weights according to the following mass fractions: 95.8% of the liquid crystal composition for microwave, 4% of the monomer, and 0.2% of the thermal initiator.

[0149] Preparation of the polymer network liquid crystal material: Weigh 1 g of the liquid crystal composition for microwave prepared in Example 2, add 41.8 mg of the monomer to the prepared liquid crystal composition for microwave, heat and stir at 90 °C for 20 min for mixing and dissolving, then add 2.1 mg of the thermal initiator, heat and stir at 80 °C for 10 min to obtain a uniformly mixed mixture; inject the mixture into a liquid crystal cell, and carry out in-situ thermal polymerization at 70 °C for 6 h to obtain the polymer network liquid crystal material M-2-3.

[0150] Preparation of the polymer network liquid crystal material M-2-4 in Example 12

[0151] The liquid crystal composition for microwave used in this example is the liquid crystal composition for microwave M-2 prepared in Example 2. The molecular structural formula of the monomer I-2-1 used is as follows:

[0152]

[0153] The thermal initiator used is benzoyl peroxide.

[0154] Prepare the polymer network liquid crystal material by weighing the corresponding weights according to the following mass fractions: 92.6% of the liquid crystal composition for microwave, 7% of the monomer, and 0.4% of the thermal initiator.

[0155] Preparation of the polymer network liquid crystal material: Weigh 1 g of the liquid crystal composition for microwave prepared in Example 2, add 75.6 mg of the monomer to the prepared liquid crystal composition for microwave, heat and stir at 90 °C for 20 min for mixing and dissolving, then add 4.3 mg of the thermal initiator, heat and stir at 80 °C for 10 min to obtain a uniformly mixed mixture; inject the mixture into a liquid crystal cell, and carry out in-situ thermal polymerization at 70 °C for 6 h to obtain the polymer network liquid crystal material M-2-4.

[0156] Preparation of the polymer network liquid crystal material M-2-5 in Example 13

[0157] The liquid crystal composition for microwave used in this example is the liquid crystal composition for microwave M-2 prepared in Example 2. The molecular structural formula of the monomer I-3-1 used is as follows:

[0158]

[0159] The thermal initiator used is benzoyl peroxide.

[0160] According to the following mass fractions: 92.6% of the liquid crystal composition for microwave, 7% of the monomer, and 0.4% of the thermal initiator, the corresponding weights were weighed respectively to prepare the polymer network liquid crystal material.

[0161] Preparation of the polymer network liquid crystal material: Weigh 1 g of the liquid crystal composition for microwave prepared in Example 2, add 75.6 mg of the monomer to the prepared liquid crystal composition for microwave, heat and stir at 90 °C for 20 min for mixing and dissolving, then add 4.3 mg of the thermal initiator, heat and stir at 80 °C for 10 min to prepare a uniformly mixed mixture; inject the mixture into a liquid crystal cell, and carry out in-situ thermal polymerization at 70 °C for 6 h to obtain the polymer network liquid crystal material M-2-5.

[0162] Preparation of the polymer network liquid crystal material M-2-6 in Example 14

[0163] The liquid crystal composition for microwave used in this example is the liquid crystal composition for microwave M-2 prepared in Example 2. The molecular structural formula of the monomer I-4-1 used is as follows:

[0164]

[0165] The thermal initiator used is benzoyl peroxide.

[0166] According to the following mass fractions: 92.6% of the liquid crystal composition for microwave, 7% of the monomer, and 0.4% of the thermal initiator, the corresponding weights were weighed respectively to prepare the polymer network liquid crystal material.

[0167] Preparation of the polymer network liquid crystal material: Weigh 1 g of the liquid crystal composition for microwave prepared in Example 2, add 75.6 mg of the monomer to the prepared liquid crystal composition for microwave, heat and stir at 90 °C for 20 min for mixing and dissolving, then add 4.3 mg of the thermal initiator, heat and stir at 80 °C for 10 min to prepare a uniformly mixed mixture; inject the mixture into a liquid crystal cell, and carry out in-situ thermal polymerization at 70 °C for 6 h to obtain the polymer network liquid crystal material M-2-6.

[0168] Measurement of the response time in Example 15

[0169] The response times of the liquid crystal composition M-1 for microwaves in Example 1 (as Comparative Example 1), the liquid crystal composition M-2 for microwaves in Example 2 (as Comparative Example 2), the polymer network liquid crystal materials M-1-1 to M-1-6 in Examples 3 to 8, and the polymer network liquid crystal materials M-2-1 to M-2-6 in Examples 9 to 14 were tested. The response time was tested by the optical path system of the microwave phase shifter carried by the University of Electronic Science and Technology. The test conditions were as follows: The prepared 10-μm liquid crystal cell sample was placed in the optical path system of the microwave phase shifter, and the applied voltage amplitude was 10 Vpp - 20 Vpp. The test results are shown in Table 4:

[0170] Table 4 Response times of different samples

[0171]

[0172] The test results of the response time show that, compared with the original liquid crystal composition for microwaves, the response time of the polymer network liquid crystal material provided in the embodiments of the present invention is shortened, and the response speed is greatly improved, proving that the polymer network liquid crystal material of the present invention can solve the problem of slow response speed of the existing microwave liquid crystal materials.

[0173] Test of dielectric loss in Example 16

[0174] Other dielectric property parameters of the polymer network liquid crystal material except for the dielectric loss will not change. Therefore, the dielectric loss of the liquid crystal composition M-1 for microwaves in Example 1 (as a comparative example) and the polymer network liquid crystal material M-1-2 in Example 4 was tested. The dielectric loss was tested by Chengdu Enchi Microwave Technology Co., Ltd. using the rectangular resonant cavity perturbation method. The test temperature was 20°C. The test results are shown in Table 5:

[0175] Table 5 Dielectric losses of different samples

[0176] Frequency (GHz) <![CDATA[M-1(tanδε ⊥ )]]> <![CDATA[M-1-2(tanδε ⊥ )]]> 18.1 0.0087 0.0093 29.5 0.0079 0.0087

[0177] The test results of the dielectric loss show that, compared with the original liquid crystal composition for microwaves, the dielectric loss of the polymer network liquid crystal material provided in the embodiments of the present invention has increased but is less than 0.01, proving that the dielectric loss of the polymer network liquid crystal material of the present invention can meet the requirements of the microwave phase shifter.

[0178] In summary, the polymer network liquid crystal material of the present invention is a liquid crystal material with high permittivity, low loss, and fast response speed that meets the requirements of the microwave phase shifter, can be used as the liquid crystal material for the microwave phase shifter, and is applicable to the microwave phase shifter.

[0179] The above are only the preferred embodiments of the present invention, and do not limit the patent scope of the present invention. For those skilled in the art, the present invention can have various changes and modifications. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present invention shall be included within the patent protection scope of the present invention.

Claims

1. A polymer network liquid crystal material, characterized in that, The polymer network liquid crystal material includes a polymer network and a liquid crystal composition for microwave. The polymer network is formed by in-situ thermal polymerization crosslinking reaction of a monomer and a thermal initiator in the liquid crystal composition for microwave. The molecular structure of the monomer is shown in Formula I: Wherein, each X independently selects from -H, -CH3; each n independently selects any integer within the range of 3 to 10; each of A1 and A2 independently selects from -OCH2-, -OCF2-, -OCO-, -CO2-, -CH2O-, -CF2O-, -CF=CF-, -CH2CH2-; each of Z1 and Z2 independently selects from -CH2-, -O-, -S-; L is a substituent on the benzene ring and independently selects from -H, -Cl, -F, -CH3, -C2H5; each r independently selects any integer within the range of 0 to 2; Based on the total weight of the liquid crystal composition for microwave, the liquid crystal composition for microwave consists of component A with a mass fraction of 10 to 40%, component B with a mass fraction of 8 to 35%, component C with a mass fraction of 10 to 35%, and component D with a mass fraction of 5 to 20%, wherein: Component A is at least one of the compounds with the structure shown in Formula II below: Component B is at least one of the compounds with the structure shown in Formula III below: Component C is at least one of the compounds with the structure shown in Formula IV below: Component D is at least one of the compounds with the structure shown in Formula V below: Among them, R1, R2, R3, R4, R5, and R6 are each independently selected from -C m H 2m+1 , m is each independently selected from any integer within the range of 2 to 10; X1, X2, X5, X6, and X9 are ortho- or meta-substituents on the benzene ring and are each independently selected from -H, -F, -CH3, -CH2CH3; X3, X4, X7, and X8 are each independently selected from -H, -F.

2. The polymer network liquid crystal material according to claim 1, wherein: Based on the total weight of the polymer network liquid crystal material, the content of the liquid crystal composition for microwave is 89.5 - 95.8 wt%, the content of the monomer is 4 - 10 wt%, and the content of the thermal initiator is 0.2 - 0.5 wt%.

3. The polymer network liquid crystal material according to claim 1, wherein: The monomer of Formula I is selected from one or more of Formula I-1 to Formula I-4: Wherein, each X independently selects from -H, -CH3; each n independently selects any integer within the range of 3 to 10; L is a substituent on the benzene ring and independently selects from -H, -Cl, -F, -CH3, -C2H5; each r independently selects any integer within the range of 0 to 2.

4. The polymer network liquid crystal material according to claim 3, characterized in that: The monomer of Formula I is selected from one or more of Formula I-2 and Formula I-4.

5. The polymer network liquid crystal material according to claim 1, wherein: The thermal initiator is selected from one of benzoyl peroxide, lauroyl peroxide, diisopropylbenzene peroxide, di-tert-butyl peroxide, tert-butyl peroxybenzoate, tert-butyl peroxy pivalate.

6. A method for preparing a polymer network liquid crystal material according to any one of claims 1 to 5, characterized in that, Including the following steps: Step 1: Heat the liquid crystal component compound to 80 - 100 °C and stir for 20 - 30 min to dissolve and mix to prepare the liquid crystal composition for microwave; Step 2: Add the monomer to the prepared liquid crystal composition for microwave, heat to 70 - 100 °C and stir for 10 - 20 min to mix and dissolve, then add the thermal initiator, heat to 60 - 90 °C and stir for 5 - 15 min to prepare a uniformly mixed mixture; Step 3: Inject the mixture into a liquid crystal cell and perform in-situ thermal polymerization at 40 - 110 °C for 3 - 12 h to obtain the polymer network liquid crystal material.

7. The preparation method of the polymer network liquid crystal material according to claim 6, wherein: The temperature of the in-situ polymerization is 60-90 °C, and the time of the in-situ polymerization is 6-10 h.

8. Application of the polymer network liquid crystal material according to any one of claims 1 to 5 in a microwave phase shifter assembly.

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