A sandwich structure liquid crystal high thermal conductivity composite film material and preparation method thereof

Abstract

The present invention discloses a sandwich-structured liquid crystal high-thermal-conductivity composite film material and its preparation method. The composite film material comprises a boron nitride nanosheet-liquid crystal elastomer as the upper and lower surfaces, and a carbon nanotube / boron nitride nanosheet-liquid crystal elastomer as the intermediate layer. The composite film material utilizes the template effect of the liquid crystal elastomer to construct a thermally conductive channel between the boron nitride and carbon nanotubes, thus resolving the issue of interfacial thermal resistance between the filler and the matrix, and between the fillers themselves. The result is a composite film material with excellent thermal conductivity and insulation properties.

Description

Technical Field

[0001] The present invention belongs to the technical field of thermal conductive materials, and in particular relates to a sandwich structure liquid crystal high thermal conductive composite film material and a preparation method thereof. Background Art

[0002] With the rapid development of electronic devices and components towards thinness, miniaturization, integration, and high performance, the widespread use of flexible wearable devices and thermal management textiles has led to a sharp increase in power density within devices, posing new requirements and challenges for traditional heat dissipation materials. The rapid and timely transfer of heat generated by components is a top priority in the development of thermally conductive materials. Current research focuses on two main approaches to improving thermal conductivity: first, adjusting the intrinsic thermal conductivity of polymer materials by controlling the arrangement of molecular segments, enhancing intermolecular interactions, and manipulating molecular structure. However, current intrinsically conductive polymer materials are limited by their intrinsic structure, synthesis methods, and post-processing processes. Second, polymer-based thermally conductive composites are prepared by incorporating highly thermally conductive fillers into the polymer matrix. Simply adding a low content of a single thermally conductive filler to a polymer matrix is ​​difficult to significantly improve the thermal conductivity of a polymer. This is primarily because at low filler loadings, the fillers are isolated from each other by the matrix, forming an island-in-the-sea structure. Therefore, only high filler loadings (typically >50 vol%) can form effective thermal pathways. However, high filler loadings often come at the expense of mechanical properties, processability, and insulation. Therefore, combining both intrinsic and filler-based approaches to improve thermal conductivity, preparing polymers with oriented structures and improving and reinforcing the filler-polymer matrix interface to construct an ordered filler network, is an effective approach to preparing highly thermally conductive polymer composites. Summary of the Invention

[0003] In order to solve the above-mentioned problems existing in the prior art, the present invention provides a sandwich structure liquid crystal high thermal conductivity composite film material and its preparation method. Specifically, the present invention first successfully peels off and prepares boron nitride nanosheets with amino groups on the surface with the assistance of urea by ball milling peeling, then adopts a two-step method to prepare a single-domain liquid crystal elastomer, and utilizes the orderly arrangement characteristics of the liquid crystal elastomer to achieve the orientation arrangement of boron nitride in the liquid crystal elastomer by impregnation, thereby preparing a boron nitride nanosheet-liquid crystal elastomer film with an oriented structure, and then coats carbon nanotubes (CNTs) on the upper and lower surfaces of the film to prepare a thermal conductive composite film with a coordinated orientation structure. Finally, the three layers of film are hot pressed, wherein the boron nitride nanosheet-liquid crystal elastomer film is set as the upper and lower layers, and the boron nitride nanosheet / carbon nanotube-liquid crystal elastomer film is set as the middle layer, thereby preparing a sandwich structure liquid crystal elastomer composite film with high thermal conductivity and high insulation.

[0004] In order to achieve the above-mentioned purpose, the present invention provides the following technical solutions:

[0005] In one aspect, the present invention provides a method for preparing a sandwich structure liquid crystal high thermal conductivity composite film material, the preparation method comprising the following steps:

[0006] (1) Preparation of Boron Nitride Nanosheets / Carbon Nanotubes-Liquid Crystalline Elastomer Films;

[0007] (2) Preparation of a three-layer composite material with boron nitride nanosheets / carbon nanotubes-liquid crystal elastomer film as the intermediate layer.

[0008] In some embodiments, in step (1), the preparation of the boron nitride nanosheet / carbon nanotube-liquid crystal elastomer film comprises the following steps:

[0009] 1-1) Preparation of Boron Nitride Nanosheets:

[0010] Boron nitride nanosheets with amino groups on their surfaces were prepared using boron nitride powder and urea, and were labeled as BNNS-NH2;

[0011] 1-2) Preparation of Boron Nitride Nanosheet-Liquid Crystalline Elastomer Film

[0012] A. pre-crosslinking an olefinic liquid crystal monomer (M1), a crosslinking agent (J1), a carboxyl liquid crystal monomer (M2), and polymethyl hydrogen siloxane (PMHS) to obtain a single-crosslinked liquid crystal elastomer film;

[0013] B. dispersing the BNNS-NH2 prepared in step 1-1) into the liquid crystal elastomer film prepared in step A by an impregnation method to obtain a boron nitride nanosheet-liquid crystal elastomer film;

[0014] 1-3) Preparation of Boron Nitride Nanosheets / Carbon Nanotubes-Liquid Crystalline Elastomer Films

[0015] Carbon nanotubes are coated on the upper and lower surfaces of the boron nitride nanosheet-liquid crystal elastomer film prepared in step 1-2) to obtain a boron nitride nanosheet / carbon nanotube-liquid crystal elastomer composite film; preferably, the coating can be performed by any method known in the art, including but not limited to an automatic coating machine.

[0016] In some embodiments, in step (2), the preparation of the three-layer structure composite material with the boron nitride nanosheet / carbon nanotube-liquid crystal elastomer film as the middle layer is carried out as follows: the boron nitride nanosheet-liquid crystal elastomer film is prepared by steps 1-2), and then placed on the upper and lower surfaces of the boron nitride nanosheet / carbon nanotube-liquid crystal elastomer film prepared in steps 1-3), respectively, and then hot-pressed to obtain a sandwich structure liquid crystal high thermal conductivity composite film material; preferably, the hot pressing process is: temperature of 70-105°C, pressure of 2-7MPa, and hot pressing time of 20-30min.

[0017] Furthermore, in step 1-1) of the present invention, there are no special requirements for the preparation method of the boron nitride nanosheets, and the ball milling exfoliation method well known in the art can be used. Specifically, the preparation of the boron nitride nanosheets includes the following steps: mechanically exfoliating a mixture of boron nitride powder and urea by ball milling, dispersing the product in deionized water to obtain a mixed solution, ultrasonicating the mixed solution, washing with water to obtain a filter cake, dispersing the filter cake in deionized water, centrifuging, collecting the supernatant, and freeze-drying to obtain boron nitride nanosheets with amino groups on the surface, labeled as BNNS-NH2. More specifically, the preparation of the boron nitride nanosheets includes the following steps: mixing boron nitride powder and urea in a certain mass ratio, exfoliating the mixture using a star-shaped ball mill at a certain speed (e.g., 600 rpm) for a certain time (e.g., 12 hours), dispersing the product in deionized water, removing urea from the solution by ultrasonication, filtration, and deionized water washing, dispersing the filter cake in deionized water, centrifuging, collecting the upper layer of BNNS dispersion, and freeze-drying to obtain boron nitride nanosheets, labeled as BNNS-NH2.

[0018] Furthermore, in step A of step 1-2), the olefinic liquid crystal monomer (M1) is at least one selected from propyldicyclohexylbutene, vinylcyclohexylbenzonitrile and methyldiphenylbutene;

[0019] Further, in step A of step 1-2), the cross-linking agent (J1) is 1,11-dodecadiene;

[0020] Furthermore, in step A of step 1-2), the carboxyl liquid crystal monomer (M2) can be prepared by a method comprising the following steps:

[0021] A-1) Preparation of p-hydroxyazobenzoic acid:

[0022] Weigh p-aminobenzoic acid and concentrated hydrochloric acid in a molar ratio of 1:2, mix them, and place them in an ice-water bath. Weigh 1 mol of sodium nitrite and dissolve it in ice water. Control the temperature at 0-5°C and drip it into the above system. After reacting for 1 hour, slowly drip a mixed solution of 1 mol of phenol and 2 mol of sodium hydroxide. React for 0.5 hour, filter to obtain a solid product, and dry it. Recrystallize the dried solid product from ethyl acetate to obtain orange crystals, and dry it to obtain p-hydroxyazobenzoic acid.

[0023] A-2) Preparation of undecylenyl chloride:

[0024] 1 mol of undecylenic acid and 4 mol of thionyl chloride, a small amount of polymerization inhibitor benzoquinone, and acid binding agent pyridine were weighed and added to a reaction vessel. The mixture was stirred at room temperature for 2 hours, heated to 80°C, and reacted for 6 hours. The excess thionyl chloride was filtered off under reduced pressure to obtain a yellow oily liquid, which is undecylenic acid chloride.

[0025] A-3) Preparation of carboxyl liquid crystal monomer (M2):

[0026] Weigh 1 mol of p-hydroxyazobenzoic acid prepared in step A-1) and 1 mol of pyridine and dissolve them in acetone, then weigh 1.2 mol of undecenoyl chloride prepared in step A-2) and slowly add them dropwise to the above solution system. After the addition is completed, heat to 60°C and react for 8 hours. After the reaction is completed, filter the liquid and evaporate to dryness to obtain a solid. Wash it with water 3-4 times and then recrystallize it with ethanol to obtain orange crystals. After drying, you can obtain a carboxyl liquid crystal monomer (M2). The synthetic route of carboxyl liquid crystal monomer M2 can be found in Figure 3 .

[0027] Furthermore, step A of step 1-2) can be performed as follows:

[0028] A vinyl liquid crystal monomer, a crosslinker, a carboxyl liquid crystal monomer, and polymethyl hydrogen siloxane are dissolved in toluene, and the mixed solution is ultrasonically treated for 5 to 10 minutes to remove bubbles in the solvent. 0.5 mL of a tetrahydrofuran solution of hexachloroplatinic acid (5 mg / L) is then added. The mixed solution is then transferred to a square polytetrafluoroethylene mold (length × width × depth: 4 cm × 2 cm × 1 cm). The mold is then placed in an oven and reacted at 55 to 65° C. for 8 to 12 hours. After the mold is removed from the oven, a large amount of n-hexane is immediately dripped into the mold for demolding. The removed film is naturally dried to obtain a once-crosslinked liquid crystal elastomer film.

[0029] Preferably, in step A of step 1-2), the molar ratio of the polymethyl hydrogen siloxane (PMHS) to the crosslinker (J1), the olefinic liquid crystal monomer (M1), and the carboxyl liquid crystal monomer (M2) is 0.471:0.3:(2.682-2.664):(0.018-0.036); preferably 0.471:0.3:2.682:0.018, 0.471:0.3:2.676:0.024, or 0.471:0.3:2.670:0.030.

[0030] Furthermore, step B of step 1-2) can be performed as follows:

[0031] The BNNS-NH2 prepared in step 1-1) is added to a toluene solution and ultrasonicated for 30 to 60 minutes to obtain a uniformly dispersed BNNS-NH2 toluene dispersion. The once-crosslinked liquid crystal elastomer film prepared in step A is then placed in the toluene solution of BNNS-NH2 and soaked for 1 to 2 hours. The film is taken out and, after drying, uniaxially stretched to 150% of its original length by external force and kept fixed. The film is then placed in an oven and reacted at 55 to 65° C. for 12 to 24 hours to obtain a boron nitride nanosheet-liquid crystal elastomer film. Preferably, the concentration of the BNNS-NH2 toluene dispersion is 15 to 20 mg / ml.

[0032] Furthermore, in step B of step 1-2), the amount of BNNS-NH2 used is 8-20% of the mass of the once-crosslinked liquid crystal elastomer film; preferably 10% or 15%.

[0033] Furthermore, in step 1-3), the preparation of the boron nitride nanosheet / carbon nanotube-liquid crystal elastomer film includes the following steps: using the boron nitride nanosheet-liquid crystal elastomer film as a substrate, and using an automatic coating machine to coat a certain concentration of carbon nanotube chloroform dispersion on the upper and lower surfaces of the boron nitride nanosheet-liquid crystal elastomer film, and preparing a boron nitride nanosheet / carbon nanotube-liquid crystal elastomer composite film after drying; wherein, preferably, the concentration of the carbon nanotube chloroform dispersion is 20-30 mg / ml; the amount of carbon nanotubes used is 1% to 5% of the mass of the boron nitride nanosheet-liquid crystal elastomer film; preferably 2%, 3%, or 5%.

[0034] On the other hand, the present invention also provides a sandwich structure liquid crystal high thermal conductivity composite film material, which is prepared by the method provided by the present invention.

[0035] In another aspect, the present invention further provides the use of the sandwich structure liquid crystal high thermal conductivity composite film material prepared by the method of the present invention in a flexible thermal interface material. Preferably, the flexible thermal interface material includes wearable devices, intelligent robots, and 4D printing.

[0036] In recent years, numerous studies have shown that factors influencing the thermal conductivity of polymer-based composites include the polymer matrix, fillers, interfaces (filler-matrix interface and filler-filler interface), and structure. Fillers and matrices are key factors in determining the thermal conductivity of polymers, while interfaces and structure are important pathways to improving thermal conductivity.

[0037] Therefore, the first step is to improve the intrinsic thermal conductivity of polymers by regulating the arrangement and interaction of molecular segments through specialized processing techniques and synthesizing polymers with unique structures. Furthermore, based on thermal pathway theory, by improving the distribution of fillers within the composite material, the formation of thermal pathways is promoted, while simultaneously reducing the thermal resistance at the interface between the thermal pathway and the matrix, as well as at the interface between the fillers within the thermal pathway itself. This is a key technology for preparing polymers with high thermal conductivity.

[0038] The present invention is based on the theory of intrinsic and filled thermally conductive polymers. It first prepares a liquid crystal elastomer with a highly oriented molecular structure, and uses boron nitride and carbon nanotubes with excellent thermal conductivity as thermal conductive fillers to invent a sandwich structure liquid crystal high thermal conductivity composite film material with high thermal conductivity and thermal stability.

[0039] Specifically, the above solution of the present invention includes at least the following beneficial effects:

[0040] (1) The present invention first realizes the amination of boron nitride nanosheets by ball milling and exfoliation, and further introduces carboxyl groups into the liquid crystal elastomer. The interaction between the amino groups and the carboxyl groups is utilized to facilitate the self-assembly of boron nitride in the liquid crystal elastomer during the impregnation process. At the same time, during the second step of cross-linking to prepare the single-domain liquid crystal elastomer, the orientation of the liquid crystal molecules also induces the orientation of the boron nitride through electrostatic interaction.

[0041] (2) The present invention prepares a sandwich structure with an interlayer structure, organically combines the synergistic effect between BN and CNT, avoids the spacing effect of the polymer matrix on the thermal conductive filler, builds a high-density thermal conductive path, and prepares a composite film material with high in-plane thermal conductivity and excellent insulation performance. BRIEF DESCRIPTION OF THE DRAWINGS

[0042] Figure 1 The ratio of the liquid crystal monomer, polymethyl hydrogen siloxane and cross-linking agent in Example 1 is:

[0043] Figure 2 is the structural formula of vinyl cyclohexylbenzonitrile, the olefinic liquid crystal monomer used in Example 1;

[0044] Figure 3 is a synthetic route of the carboxyl liquid crystal monomer M2 in Example 1;

[0045] Figure 4is the infrared test spectrum of the carboxyl liquid crystal monomer M2 in Example 1;

[0046] Figure 5 This is a polarized image of the carboxyl liquid crystal monomer in Example 1;

[0047] Figure 6 This is a scanning electron micrograph of a cross section of the boron nitride nanosheet / carbon nanotube-liquid crystal elastomer film obtained in Example 2;

[0048] Figure 7 This is a physical picture of the sandwich structure liquid crystal high thermal conductivity composite film material prepared in Example 3;

[0049] Figure 8 The rebound value and room temperature compression permanent deformation test data of the sandwich structure liquid crystal high thermal conductivity composite film of Example 1-3;

[0050] Figure 9 A heat release rate curve of a sandwich structure liquid crystal high thermal conductivity composite film material prepared in some embodiments;

[0051] Figure 10 This is a total heat release (THR) curve of the sandwich structure liquid crystal high thermal conductivity composite film material prepared in some examples. DETAILED DESCRIPTION

[0052] In order to make the technical problems, technical solutions and advantages to be solved by the present invention clearer, a detailed description will be given below with reference to specific embodiments.

[0053] The olefinic liquid crystal monomers were purchased from Beijing Bayi Space-Time Liquid Crystal Technology Co., Ltd., and carbon nanotubes were purchased from Beijing Dekedaojin Technology Co., Ltd. Their dimensions were <8 nm in outer diameter, 2-5 nm in inner diameter, and 10-30 μm in length. The remaining raw materials, including boron nitride, p-aminobenzoic acid, and acrylic acid, were commercially available and were not particularly limited. During the preparation of the liquid crystal elastomer-boron nitride film, the feed ratios of polymethylhydrogensiloxane (PMHS), olefinic liquid crystal monomer (M1), carboxyl liquid crystal monomer (M2), and crosslinker (J1) were as follows: Figure 1 .

[0054] Example 1

[0055] The preparation of a sandwich structure liquid crystal high thermal conductivity composite film material comprises the following steps:

[0056] (1) Weigh 10 g of boron nitride powder and 400 g of urea, mix them mechanically, and then place them in a planetary ball mill for exfoliation at 600 rpm for 12 h. After the ball milling exfoliation is completed, the product is dispersed in deionized water and ultrasonicated for 60 min. After the ultrasonication, the product is filtered and washed with deionized water several times to remove excess urea. The filter cake is then dispersed in deionized water and centrifuged at 4000 rpm for 30 min. After the centrifugation, the supernatant is collected to obtain a BNNS-NH2 dispersion. Finally, the product is dried in a freeze dryer for 24 h to obtain boron nitride nanosheets, which are labeled as BNNS-NH2 for future use.

[0057] (2) Preparation of carboxyl liquid crystal monomers:

[0058] (a) Preparation of p-hydroxyazobenzoic acid:

[0059] Weigh p-aminobenzoic acid and concentrated hydrochloric acid in a 1:2 molar ratio, mix, and place in an ice-water bath. Weigh 1 mol of sodium nitrite and dissolve it in ice water. Control the temperature at 0-5°C and drip it into the above system. Let it react for 1 hour. Weigh 1 mol of phenol and 2 mol of sodium hydroxide and dissolve them in a beaker. After the reaction is complete, slowly drip the phenol and sodium hydroxide mixture. Let it react for 0.5 hours. Filter the solid product and dry it. Recrystallize the solid from ethyl acetate to form orange crystals. Dry to obtain p-hydroxyazobenzoic acid.

[0060] (b) Preparation of undecylenyl chloride:

[0061] 1 mol of undecylenic acid and 4 mol of thionyl chloride, a small amount of polymerization inhibitor benzoquinone, and acid binding agent pyridine were weighed and added to a reaction vessel. The mixture was stirred at room temperature for 2 hours, heated to 80°C, and reacted for 6 hours. The excess thionyl chloride was filtered off under reduced pressure to obtain a yellow oily liquid, which is undecylenic acid chloride.

[0062] (c) Preparation of carboxyl liquid crystal monomer (M2):

[0063] Weigh 1 mol of p-hydroxyazobenzoic acid and 1 mol of pyridine and dissolve them in acetone, then weigh 1.2 mol of undecenoyl chloride and slowly add them dropwise to the above solution system. After the addition is completed, heat to 60°C and react for 8 hours. After the reaction is completed, filter the liquid and evaporate to dryness to obtain a solid. Wash it with water 3-4 times and then recrystallize the orange crystals with ethanol. After drying, the carboxyl liquid crystal monomer M2 can be obtained.

[0064] (3) According to Figure 1The feeding ratio of P1 is adjusted to 0.5:1, and the olefinic liquid crystal monomers vinylcyclohexylbenzonitrile (M1), 1,11-dodecadiene (J1), the carboxyl liquid crystal monomer (M2) prepared in step (2) above, and polymethyl hydrogen siloxane (PMHS) are weighed and dissolved in dried toluene. The mixed solution is then ultrasonically treated for 5 min to remove bubbles. 0.5 mL of a tetrahydrofuran solution of hexachloroplatinic acid (5 mg / L) is then added. The mixed solution is then slowly poured into a square mold made of polytetrafluoroethylene (length × width × depth: 4 cm × 2 cm × 1 cm). The mold is then placed in an oven and reacted at 60°C for 8 h. The mold is then taken out of the oven, and a large amount of n-hexane is immediately dripped into the mold for demoulding. The film obtained by demoulding is naturally dried to obtain a once-crosslinked liquid crystal elastomer film.

[0065] (4) Weighing 10% of the mass of the liquid crystal elastomer film prepared in step (3) BNNS-NH2 to prepare a toluene dispersion with a concentration of 15 mg / ml, ultrasonicating for 30 minutes to obtain a uniformly dispersed BNNS-NH2 toluene dispersion, then placing the film prepared in step (3) in the toluene solution of BNNS-NH2, soaking for 2 hours to absorb the toluene dispersion, and then taking out the film. After the film is dried, it is uniaxially stretched to 150% of the original length of the film using external force and kept fixed, and then placed in an oven at 60°C for 12 hours to react to obtain a boron nitride nanosheet-liquid crystal elastomer film;

[0066] (5) First, 2% of the mass of the boron nitride nanosheet-liquid crystal elastomer film in step (4) is weighed and dispersed in chloroform. The concentration of the dispersion is 20 mg / ml. Then, the boron nitride nanosheet-liquid crystal elastomer film is placed on an automatic coating machine as a substrate. The carbon nanotube chloroform dispersion is coated on the upper and lower surfaces of the boron nitride nanosheet-liquid crystal elastomer film using the automatic coating machine. After vacuum drying, a boron nitride nanosheet / carbon nanotube-liquid crystal elastomer composite film is prepared.

[0067] (6) Placing the boron nitride nanosheet / liquid crystal elastomer film obtained in step (4) on the upper and lower surfaces of the intermediate layer film obtained in step (5), and hot pressing the film on a flat-plate vulcanizer to obtain a sandwich structure thermally conductive composite film. The hot pressing process parameters are: temperature 80°C, pressure 3 MPa, and time 20 min.

[0068] Figure 2 This is the structural formula of vinyl cyclohexylbenzonitrile, the olefinic liquid crystal monomer used in Example 1.

[0069] Figure 3 This is the synthesis route of the carboxyl liquid crystal monomer M2 in Example 1.

[0070] Figure 4 This is the infrared test spectrum of the carboxyl liquid crystal monomer M2 in Example 1. It can be seen from the figure that at 1580 cm-1 The strong absorption peak at 1600-1700cm is the peak formed by N=N. -1 There are two absorption peaks at 1605~1420cm, which are formed by C=O and C=C respectively. -1 The absorption peak at 1304~1017cm is the CC stretching vibration absorption peak of the benzene ring skeleton. -1 The peaks between them are all the stretching vibration absorption peaks of CO. -1 The absorption peak at 3500 cm indicates that this compound contains unsaturated C=C double bonds. -1 There is no obvious absorption peak at the position of the azo compound, indicating that there is no OH bond and the hydroxyl group (-OH) in the azo compound has disappeared. The infrared spectrum test results show that the target monomer has been successfully synthesized.

[0071] Figure 5 This is a polarizing microscope image of the carboxyl liquid crystal monomer in Example 1 when heated to 170°C. In the polarizing microscope test, it can be seen that when the temperature is raised to 105.3°C, the field of view of the polarizing microscope begins to brighten, and a typical nematic texture can be observed. When the temperature is further raised to 178.6°C, the field of view darkens again, and the liquid crystal phase disappears, entering the isotropic state.

[0072] The thermal conductivity of the sandwich structure liquid crystal composite film prepared in this embodiment was tested using a laser flash method. The in-plane thermal conductivity of the film was 10.27 W / m·K, and the vertical thermal conductivity was 2.74 W / m·K.

[0073] Example 2

[0074] The preparation method of the sandwich structure liquid crystal high thermal conductivity composite film material provided in this embodiment is basically the same as that in embodiment 1, except that: the synthesis basis of the liquid crystal elastomer in step (3) is Figure 1 The amount of BNNS-NH2 added in step (4) is 15wt% of the mass of the liquid crystal elastomer film, and the amount of carbon nanotubes used in step (5) is 3% of the mass of the boron nitride nanosheet-liquid crystal elastomer film.

[0075] Figure 6 The figure shows a cross-sectional scanning electron microscope image of the boron nitride nanosheet / carbon nanotube-liquid crystal elastomer film obtained in Example 2. From the figure, it can be seen that the boron nitride in the film has a good layered structure. At the same time, the CNTs exist in the gaps between the layered boron nitride, acting as a bridge and constructing an effective three-dimensional heat conduction path.

[0076] The thermal conductivity of the sandwich structure liquid crystal high thermal conductivity composite film prepared in this embodiment was tested using a laser flash method. The in-plane thermal conductivity of the film was 13.95 W / m·K, and the vertical thermal conductivity was 3.61 W / m·K.

[0077] The conductivity of the carbon nanotube / boron nitride nanosheet-liquid crystal elastomer film prepared in step (5) of this example is 0.0247 S / cm, while the conductivity of the liquid crystal high thermal conductivity composite film with a sandwich structure finally prepared in this example is 2.31×10 -12 S cm -1 , which is much higher than the application standard of insulation materials.

[0078] It can be seen that the present invention not only constructs a more effective heat conduction path after introducing carbon nanotubes, but also due to the special layered structure of the composite film, the boron nitride in the two layers of boron nitride / liquid crystal elastomer films on the upper and lower surfaces effectively suppresses the migration of charge carriers, resulting in the insulation performance of the composite film not being affected by the presence of carbon nanotubes.

[0079] Example 3

[0080] The method for preparing the sandwich structure liquid crystal high thermal conductivity composite film material provided in this embodiment is basically the same as that in embodiment 2, except that: the synthesis basis of the liquid crystal elastomer in step (3) is Figure 1 The amount of carbon nanotubes used in step (5) is 5% of the mass of the boron nitride nanosheet-liquid crystal elastomer film.

[0081] The thermal conductivity of the sandwich structure liquid crystal composite film prepared in this embodiment was tested using the laser flash method. The in-plane thermal conductivity of the film was 15.22 W / m·K, the vertical thermal conductivity was 4.03 W / m·K, and the electrical conductivity was 2.62×10 -12 S cm -1 .

[0082] Figure 7 A physical picture of the sandwich structure liquid crystal high thermal conductivity composite film material prepared in Example 3 is shown.

[0083] Example 4

[0084] The preparation of liquid crystal elastomer film material includes the following steps:

[0085] (1) Preparation of carboxyl liquid crystal monomer: The preparation process is the same as that in Example 1.

[0086] (2) According to Figure 1The feed ratio of P3 is adjusted, and the determined ratio of olefinic liquid crystal monomer propyldicyclohexylbutene (M1), 1,11-dodecadiene, carboxyl liquid crystal monomer (M2), and polymethyl hydrogen siloxane (PMHS) are weighed and dissolved in dried toluene. The mixed solution is then ultrasonically treated for 5 minutes to remove bubbles. After that, 0.5 mL of tetrahydrofuran solution of hexachloroplatinic acid (5 mg / L) is added. The mixed solution is then slowly poured into a square mold made of polytetrafluoroethylene (length × width × depth: 4 cm × 2 cm × 1 cm), and then placed in an oven to react at 60°C for 8 hours. The mold is then taken out of the oven, and a large amount of n-hexane is immediately dripped into the mold for demolding. The film obtained by demolding is naturally dried to obtain a once-crosslinked liquid crystal elastomer film.

[0087] (4) The liquid crystal elastomer film prepared in step (3) is uniaxially stretched to 150% of the original length of the film using an external force and kept fixed, and then placed in an oven for reaction at 60° C. for 12 hours to obtain a liquid crystal elastomer film.

[0088] (5) Stacking three layers of the liquid crystal elastomer film obtained in step (4) and hot pressing them on a flat-plate vulcanizer to obtain a three-layer liquid crystal elastomer film. The hot pressing process parameters are: temperature 80° C., pressure 3 MPa, and time 20 min.

[0089] The thermal conductivity of the sandwich structure liquid crystal composite film prepared in this embodiment was tested using the laser flash method. The film had an in-plane thermal conductivity of 1.72 W / m·K, a vertical thermal conductivity of 0.14 W / m·K, and an electrical conductivity of 1.82×10 -12 S cm -1 .

[0090] Comparison of Examples 1-4 shows that the introduction of boron nitride and carbon nanotubes can form a dense thermal conductivity path within the liquid crystal elastomer, significantly improving thermal conductivity. Furthermore, comparison shows that the insulation performance of the composite film still meets the requirements after the introduction of carbon nanotubes.

[0091] Example 5

[0092] The preparation of a sandwich structure liquid crystal high thermal conductivity composite film material comprises the following steps:

[0093] Steps (1) to (4) are the same as those in Example 3.

[0094] (5) The boron nitride nanosheet / liquid crystal elastomer film obtained in step (4) is stacked in three layers and hot-pressed on a flat-plate vulcanizer to obtain a three-layer thermally conductive composite film. The hot-pressing process parameters are: temperature 80° C., pressure 3 MPa, and time 20 min.

[0095] The thermal conductivity of the sandwich structure liquid crystal composite film prepared in this embodiment was tested using the laser flash method. The film had an in-plane thermal conductivity of 6.15 W / m·K, a vertical thermal conductivity of 0.56 W / m·K, and an electrical conductivity of 2.01×10 -12 S cm -1 .

[0096] The test results show that carbon nanotubes play an important role in constructing thermal conductive pathways in composite films.

[0097] Example 6

[0098] The preparation of a sandwich structure liquid crystal high thermal conductivity composite film material comprises the following steps:

[0099] Wherein step (1) and step (2) are the same as those in Example 3.

[0100] (3) According to Figure 1 The feeding ratio of P3 is adjusted, and the determined ratio of olefinic liquid crystal monomers propyldicyclohexylbutene (M1), 1,11-dodecadiene (J1), carboxyl liquid crystal monomer (M2), and polymethyl hydrogen siloxane (PMHS) are weighed and dissolved in dried toluene. The mixed solution is then ultrasonically treated for 5 minutes to remove bubbles. After that, 0.5 mL of tetrahydrofuran solution of hexachloroplatinic acid (5 mg / L) is added. The mixed solution is then slowly poured into a square mold made of polytetrafluoroethylene (length × width × depth: 4 cm × 2 cm × 1 cm), and then placed in an oven to react at 60°C for 20 hours. The mold is then taken out of the oven, and a large amount of n-hexane is immediately dripped into the mold for demolding. The film obtained by demolding is naturally dried to obtain a liquid crystal elastomer film.

[0101] (4) Weighing 15% of the mass of the liquid crystal elastomer film prepared in step (3) BNNS-NH2 to prepare a toluene dispersion with a concentration of 15 mg / ml, ultrasonicating for 30 minutes to obtain a uniformly dispersed BNNS-NH2 toluene dispersion, then placing the film prepared in step (3) in the toluene solution of BNNS-NH2, soaking for 2 hours to absorb the toluene dispersion, taking out the film, and then placing it in an oven at 60°C for 12 hours to obtain a boron nitride nanosheet-liquid crystal elastomer film;

[0102] (5) First, 5% of the mass of the boron nitride nanosheet-liquid crystal elastomer film in step (4) is weighed and dispersed in chloroform. The concentration of the dispersion is 20 mg / ml. Then, the boron nitride nanosheet-liquid crystal elastomer film is placed on an automatic coating machine as a substrate. The carbon nanotube chloroform dispersion is coated on the upper and lower surfaces of the boron nitride nanosheet-liquid crystal elastomer film using the automatic coating machine. After vacuum drying, a boron nitride nanosheet / carbon nanotube-liquid crystal elastomer composite film is prepared.

[0103] (6) Placing the boron nitride nanosheet / liquid crystal elastomer film obtained in step (4) on the upper and lower surfaces of the intermediate layer film obtained in step (5), and hot pressing the film on a flat-plate vulcanizer to obtain a sandwich structure thermally conductive composite film. The hot pressing process parameters are: temperature 80°C, pressure 3 MPa, and time 20 min.

[0104] The thermal conductivity of the sandwich structure liquid crystal composite film prepared in this embodiment was tested using the laser flash method. The film had an in-plane thermal conductivity of 6.72 W / m·K, a vertical thermal conductivity of 1.15 W / m·K, and an electrical conductivity of 1.79×10 -12 S cm -1 .

[0105] As can be seen from Example 6, the thermal conductivity of the sandwich-structured liquid crystal composite film produced therein is significantly reduced. This is because the liquid crystal elastomer preparation process does not employ the classic two-step method for preparing a single-domain liquid crystal elastomer, but instead uses a single-step synthesis to produce a multi-domain liquid crystal elastomer. At room temperature, the molecular chains of a multi-domain liquid crystal elastomer are not aligned. During the preparation of the boron nitride nanosheet-liquid crystal elastomer film, the liquid crystal elastomer does not act as an orientation template, resulting in the boron nitride failing to form an effective orientation network within the matrix, which impairs thermal conductivity.

[0106] Example 7 Performance Test

[0107] Figure 7 This is a physical picture of the sandwich structure liquid crystal high thermal conductivity composite film material prepared in Example 3. The prepared sandwich structure liquid crystal high thermal conductivity composite film material can be bent at any angle.

[0108] The rebound value of the liquid crystal composite film of Example 1-3 was measured according to ISO 4662-2017 "Rubber, vulcanized or thermoplastic - Determination of rebound value". The room temperature compression set was tested according to ISO 815-1-2014 "Rubber, vulcanized or thermoplastic - Determination of compression set - Part 1: At room temperature or higher". The test conditions were 23°C × 120h. The data are shown in Figure 8 Tests have shown that the thermal interface material obtained by the present invention has excellent flexibility and adhesion, and can be well contacted with electronic devices in practical applications, thereby achieving better heat conduction and reducing thermal resistance.

[0109] This example uses a cone calorimeter (CC) to simulate the combustion behavior of polymer materials under fire conditions. Through CC testing, combustion parameters are quantitatively analyzed, including heat release rate (HRR) and total heat release (THR) curves. Figure 9 and Figure 10 shown.

[0110]

[0111]

[0112] The ignition time of the oriented structure liquid crystal composite film prepared in Example 3 is extended by 28 seconds and 16 seconds compared with the liquid crystal elastomer film prepared in Example 4 and the disordered structure liquid crystal composite film prepared in Example 6, respectively. This is because the network structure of the liquid crystal composite film prepared in Example 3 can restrict the flow of the matrix material to a greater extent, reduce the heated area of ​​the material surface, and thus increase the difficulty of ignition; in addition, the LOI and UL-94 test results show that the introduction of carbon nanotubes and the oriented structure of boron carbide in Example 3 effectively play a flame retardant role and quench the flame.

[0113] Figure 9 and Figure 10 The HRR and THR test results indicate that the liquid crystal composite film with an oriented network structure prepared in Example 3 is more effective in promoting carbonization of the liquid crystal matrix and blocking heat and mass transfer. This is attributed to the boron nitride / carbon nanotube network structure providing structural support for the formation of the carbon layer during the combustion process. The internal structure of the carbon residue after combustion of the liquid crystal composite film is dense and continuous, with no obvious holes, forming a denser and more stable thermal and oxygen-insulating layer, effectively improving the flame retardant properties.

[0114] It should be noted that within the range of the material ratios and process parameters described in the present invention, specific values ​​can be selected at will, and the resulting materials can achieve the technical effects described in the present invention. Therefore, the present invention will not list them one by one.

[0115] The above description is merely a preferred embodiment of the present invention and does not limit the technical scope of the present invention. Therefore, any technical features that are the same or similar to those of the above embodiments of the present invention are within the scope of protection of the present invention.

Claims

1. A method for preparing a sandwich structure liquid crystal high thermal conductivity composite film material, the preparation method comprising the following steps: (1) Preparation of Boron Nitride Nanosheets / Carbon Nanotubes-Liquid Crystalline Elastomer Films; (2) Preparation of a three-layer composite material with a boron nitride nanosheet / carbon nanotube-liquid crystal elastomer film as the middle layer; In step (1), the preparation of the boron nitride nanosheet / carbon nanotube-liquid crystal elastomer film includes the following steps: 1-1) Preparation of Boron Nitride Nanosheets Boron nitride nanosheets with amino groups on their surfaces, labeled BNNS-NH2, were prepared using boron nitride powder and urea. The boron nitride powder and urea mixture was mechanically exfoliated by ball milling, and the product was dispersed in deionized water to obtain a mixed solution. The mixed solution was ultrasonicated and washed with water to obtain a filter cake. The filter cake was dispersed in deionized water, centrifuged, and the supernatant was collected and freeze-dried to obtain boron nitride nanosheets with amino groups on their surfaces, labeled BNNS-NH2. 1-2) Preparation of Boron Nitride Nanosheet-Liquid Crystalline Elastomer Film A. pre-crosslinking an olefinic liquid crystal monomer (M1), a crosslinking agent (J1), a carboxyl liquid crystal monomer (M2), and polymethyl hydrogen siloxane (PMHS) to obtain a single-crosslinked liquid crystal elastomer film; B. dispersing the BNNS-NH2 prepared in step 1-1) into the liquid crystal elastomer film prepared in step A by an impregnation method to obtain a boron nitride nanosheet-liquid crystal elastomer film; 1-3) Preparation of Boron Nitride Nanosheets / Carbon Nanotubes-Liquid Crystalline Elastomer Films Coating carbon nanotubes on the upper and lower surfaces of the boron nitride nanosheet-liquid crystal elastomer film prepared in step 1-2) to obtain a boron nitride nanosheet / carbon nanotube-liquid crystal elastomer composite film: using the boron nitride nanosheet-liquid crystal elastomer film as a substrate, coating the upper and lower surfaces of the boron nitride nanosheet-liquid crystal elastomer film with a 20-30 mg / ml carbon nanotube chloroform dispersion using an automatic coating machine, and drying to obtain a boron nitride nanosheet / carbon nanotube-liquid crystal elastomer composite film; In step (2), the preparation of the three-layer structure composite material with the boron nitride nanosheet / carbon nanotube-liquid crystal elastomer film as the intermediate layer is carried out as follows: the boron nitride nanosheet-liquid crystal elastomer film is prepared by steps 1-2), and then placed on the upper and lower surfaces of the boron nitride nanosheet / carbon nanotube-liquid crystal elastomer film prepared in steps 1-3), respectively, and then hot-pressed to obtain a sandwich structure liquid crystal high thermal conductivity composite film material, the hot pressing process being: temperature of 70-105°C, pressure of 2-7MPa, and hot pressing time of 20-30min.

2. The preparation method according to claim 1, wherein In step A of step 1-2), the olefinic liquid crystal monomer (M1) is at least one selected from propyldicyclohexylbutene, vinylcyclohexylbenzonitrile and methyldiphenylbutene; In step A of step 1-2), the cross-linking agent (J1) is 1,11-dodecadiene; In step A of step 1-2), the molar ratio of the polymethyl hydrogen siloxane to the crosslinking agent, the vinyl liquid crystal monomer, and the carboxyl liquid crystal monomer is 0.471:0.3:(2.682-2.664):(0.018-0.036).

3. The preparation method according to claim 1, wherein In step B of step 1-2), the amount of BNNS-NH2 used is 8-20% of the mass of the once-crosslinked liquid crystal elastomer film.

4. The preparation method according to claim 1, wherein in step 1-3), the amount of carbon nanotubes used is 1% to 5% of the mass of the boron nitride nanosheet-liquid crystal elastomer film.

5. The preparation method according to claim 1, wherein Step A of steps 1-2) can be performed as follows: A vinyl liquid crystal monomer, a cross-linking agent, a carboxyl liquid crystal monomer, and polymethyl hydrogen siloxane were dissolved in toluene, and the mixed solution was ultrasonically treated for 5 to 10 minutes to remove bubbles in the solvent. Then, 0.5 mL of a 5 mg / L tetrahydrofuran solution of hexachloroplatinic acid was added. The mixed solution was then transferred to a square mold made of polytetrafluoroethylene and placed in an oven to react at 55 to 65°C for 8 to 12 hours. After the mold was removed from the oven, a large amount of n-hexane was immediately dripped into the mold for demolding. The removed film was naturally dried to obtain a once-crosslinked liquid crystal elastomer film.

6. The preparation method according to claim 1, wherein Step B of steps 1-2) can be performed as follows: The BNNS-NH2 prepared in step 1-1) is added to a toluene solution and ultrasonicated for 30 to 60 minutes to obtain a uniformly dispersed BNNS-NH2 toluene dispersion. The film prepared in step A is then placed in the toluene solution of BNNS-NH2 and soaked for 1 to 2 hours. The film is then taken out and dried. After the film is uniaxially stretched to 150% of its original length using an external force and kept fixed, it is then placed in an oven and reacted at 55 to 65°C for 12 to 24 hours to obtain a boron nitride nanosheet-liquid crystal elastomer film.

7. A sandwich structure liquid crystal high thermal conductivity composite film material, wherein: The composite film material is prepared by the method according to any one of claims 1 to 6.

8. Use of the sandwich structure liquid crystal high thermal conductivity composite film material prepared by the method according to any one of claims 1 to 6 in a flexible thermal interface material.

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