Silicone grafted hyperbranched polyethylene and its use in the preparation of low-leakage paraffin-based thermally conductive phase change materials

Abstract

The application discloses a siloxane grafted hyperbranched polyethylene and application thereof in preparation of a low-leakage paraffin-based heat-conducting phase change material. The application provides a siloxane grafted hyperbranched polyethylene, which is obtained by copolymerization of ethylene and vinyltriethoxysilane according to a 'chain walking' polymerization mechanism and catalyzed by a Pd-diimine catalyst. The application provides application of the siloxane grafted hyperbranched polyethylene in preparation of a low-leakage paraffin-based heat-conducting phase change material. The application synthesizes the siloxane grafted hyperbranched polyethylene with crosslinking characteristics, uses the same to be compounded with paraffin, and forms a three-dimensional network structure through subsequent crosslinking, thereby effectively restricting macroscopic flow of paraffin above a phase change temperature, and significantly improving shape stability and anti-leakage performance of the composite material. Meanwhile, flake graphite is introduced to construct an efficient heat-conducting path, and the overall heat-conducting coefficient of the material is improved.

Description

Technical Field

[0001] This invention belongs to the field of thermally conductive phase change materials technology, specifically relating to a siloxane-grafted hyperbranched polyethylene (HBPE-co-VTES) and its application in the preparation of low-leakage paraffin-based thermally conductive phase change materials. Background Technology

[0002] With the popularization of 5G communication technology and the rapid development of the electronic information industry, various electronic components are constantly evolving towards miniaturization, high integration, and high power density. This trend results in higher heat flux generated by electronic components during service. If waste heat cannot be dissipated effectively and in a timely manner, it will lead to increased operating temperature, decreased operating efficiency, and shortened service life, and in severe cases, even safety accidents. Therefore, thermal management has become one of the key technical challenges that urgently need to be addressed in the field of high power density electronic devices.

[0003] Thermal interface materials play a crucial role in the thermal management of electronic components. Among them, thermally conductive phase change materials (PCMs) have become important functional materials in the field of thermal management due to their properties such as absorbing and releasing a large amount of latent heat within the phase change temperature range, high energy density, and near-isothermal phase change process. They are widely used in battery thermal management, energy storage systems, building energy conservation, waste heat recovery, smart textiles, and biomedicine. Paraffin wax, due to its high latent heat value, wide phase change temperature range, non-toxicity, non-corrosiveness, and low cost, has become one of the most widely researched and applied organic phase change materials. However, most paraffin-based PCMs still face several common problems in practical applications. First, high-temperature leakage. Paraffin wax is a solid-liquid phase change material. After undergoing a phase transition, liquid paraffin wax is prone to migration and leakage, which can not only contaminate surrounding electronic components and cause safety hazards such as short circuits, but also lead to a decrease in the cycle stability of composite materials. Second, low thermal conductivity. The low thermal conductivity of paraffin wax limits the rapid heat conduction and thermal management response rate. This typically requires improving the thermal conductivity of the material by introducing fillers with high thermal conductivity (such as graphene, expanded graphite, boron nitride, and metal particles). Thirdly, the paraffin encapsulation process is costly. Although paraffin itself is relatively inexpensive, the encapsulation additives and complex encapsulation processes often significantly increase the overall cost of the material, hindering large-scale application.

[0004] To address the leakage problem of paraffin wax during service, researchers have developed various encapsulation strategies, including porous adsorption, microencapsulation, and polymer-based encapsulation. Porous adsorption utilizes porous materials with high specific surface area (such as expanded graphite, SiO2 aerogel, and metal foam) as a supporting framework. Capillary forces, surface tension, and pore confinement effects confine the liquid paraffin wax within the porous network, thus maintaining the morphological stability of the composite material during phase transition. Microencapsulation uses paraffin wax as the core material and organic polymers (such as PMMA, PU, ​​and MF) or inorganic materials (such as silica) as the shell material to construct a core-shell structure at the micron to nanoscale, achieving physical isolation and encapsulation of the paraffin wax. Polymer-based encapsulation directly composites paraffin wax with polymers, utilizing the spatial confinement effect of the polymer network to restrict the migration of paraffin wax in the molten state.

[0005] The methods described above have all achieved certain results in suppressing liquid paraffin leakage, but they still have limitations in practical applications. Porous carriers have limited adsorption capacity for paraffin at high temperatures and are often accompanied by problems such as pore blockage and incomplete thermal conductivity networks. Some carriers (such as metal foam) may also face corrosion problems during long-term service. Microencapsulation is a complex process with high encapsulation costs, and the shell material may be at risk of fatigue cracking during frequent phase transition cycles; furthermore, the shell material has poor thermal conductivity, forming a thermal barrier that reduces the rate of heat absorption and release of paraffin. The polymer matrix used in traditional polymer shaping methods often cannot form a cross-linked structure or has insufficient cross-linking degree, and the encapsulation reliability under high-temperature service conditions still needs further improvement, and may significantly reduce the phase transition enthalpy of the material.

[0006] Hyperbranched polyethylene possesses a highly branched three-dimensional structure, abundant terminal functional groups, low melt viscosity, and good processing fluidity. These properties make it a potential high-performance paraffin encapsulation matrix. By introducing crosslinkable siloxane groups into the hyperbranched polyethylene molecular chain, interpenetrating or encapsulating networks can be formed after compounding with paraffin, thereby achieving microscopic encapsulation and macroscopic shaping of paraffin. However, how to prepare hyperbranched polymer-based composite phase change materials with good encapsulation effects, high thermal conductivity, and suitable processability through simple synthesis methods remains a challenge in current technology.

[0007] Therefore, developing a novel paraffin-based composite phase change material with a simple process that can simultaneously solve the problems of leakage and heat conduction has significant practical application value. Summary of the Invention

[0008] This invention provides a siloxane-grafted hyperbranched polyethylene (HBPE-co-VTES) and its application in the preparation of paraffin-based thermally conductive phase change materials with low leakage characteristics. The aim is to solve the flow and leakage problem caused by solid-liquid phase change in existing paraffin-based phase change materials under high-temperature service conditions, while simultaneously improving the thermal conductivity and cycle stability of the material. This method synthesizes siloxane-grafted hyperbranched polyethylene (HBPE-co-VTES) with crosslinking properties, and utilizes it to form a three-dimensional network structure through composite with paraffin and subsequent crosslinking. This effectively constrains the macroscopic flow of paraffin above the phase change temperature, thereby significantly improving the shape stability and leakage resistance of the composite material. Simultaneously, by introducing flake graphite to construct efficient thermally conductive pathways, the overall thermal conductivity of the material is improved.

[0009] To achieve the above objectives, the present invention adopts the following technical solution: In a first aspect, the present invention provides a siloxane-grafted hyperbranched polyethylene (HBPE-co-VTES), which is obtained by copolymerizing ethylene with vinyltriethoxysilane (VTES) according to the "chain-walking" polymerization mechanism using a Pd-diimine catalyst as shown in Formula I. .

[0010] In some embodiments, the siloxane-grafted hyperbranched polyethylene is prepared according to the following steps: In an ethylene atmosphere, Pd-diimine catalyst, anhydrous solvent, and vinyltriethoxysilane are sequentially added to a reaction vessel, such that the initial concentration of the catalyst in the polymerization reaction system is 1.5-6.0 mmol / L and the concentration of vinyltriethoxysilane is 0.5-1.5 mol / L; the reaction temperature is controlled at 10-30 °C, the ethylene pressure is maintained at 0.1-0.8 MPa, and the mixture is stirred thoroughly; under these conditions, polymerization is carried out for 12-24 h. After the reaction is completed, the reaction product is separated and purified to obtain siloxane-grafted hyperbranched polyethylene HBPE-co-VTES.

[0011] The Pd-diimine catalyst described in this invention can be synthesized with reference to the following literature: [1] Johnson, LK, Killian, CM, Brookhart, M., J. Am. Chem. Soc., 1995, 117, 6414; [2] Johnson, LK, Mecking S., Brookhart, M., J. Am. Chem. Soc., 1996, 118, 267. Preferably, the anhydrous solvent is selected from anhydrous dichloromethane, anhydrous trichloromethane, and anhydrous toluene.

[0012] Preferably, the initial concentration of the catalyst in the polymerization reaction system is 2.0-4.0 mmol / L, more preferably 3.0 mmol / L.

[0013] Preferably, the initial concentration of vinyltriethoxysilane in the polymerization reaction system is 1.0 mol / L (1M).

[0014] Preferably, the reaction temperature is 15-30 °C, more preferably 15 °C; the ethylene pressure is 0.1-0.3 MPa, more preferably 0.1 MPa; and the reaction time is 18-22 h, more preferably 20 h.

[0015] The present invention is particularly preferred in that the initial concentration of vinyltriethoxysilane in the polymerization reaction system is 1.0 mol / L, the reaction temperature is 15 °C, the ethylene pressure is 0.1 MPa, and the reaction time is 20 h.

[0016] Preferably, the crude reaction product is separated and purified according to the following steps: (1) The solvent used in the reaction is removed by rotary evaporation; (2) Dissolve the obtained crude product in chloroform; (3) Slowly add methanol until the product is completely precipitated, then gently discard the supernatant; (4) Repeat the dissolution-precipitation step three times to remove unreacted comonomers and obtain the final product.

[0017] In a second aspect, the present invention provides the application of the siloxane-grafted hyperbranched polyethylene described in the first aspect in the preparation of low-leakage paraffin-based thermally conductive phase change materials, the application comprising the following steps: (1) The paraffin is completely dissolved in the above solution of siloxane-grafted hyperbranched polyethylene, and then thermally conductive filler is added to form a uniform mixed slurry. (2) Remove the solvent from the mixed slurry obtained in step (1) to obtain a uniform polymer / thermal conductive filler / paraffin mixture; (3) Heat the polymer / thermal filler / paraffin mixture obtained in step (2) to above the melting point of paraffin for melting and mixing, then add the crosslinking catalyst and mix evenly, and let the resulting mixture cool and solidify; (4) Place the sample after cooling and shaping in step (3) in an oven and heat treat it at 60-100 ℃ for 6-24 h to allow the siloxane groups to undergo hydrolysis and condensation reaction, crosslinking to form a stable crosslinking network. After crosslinking is completed, demold to obtain the low-leakage paraffin-based thermally conductive phase change material.

[0018] Preferably, the total mass of the siloxane-grafted hyperbranched polyethylene, paraffin wax, and thermally conductive filler is 100%, the mass percentage of the siloxane-grafted hyperbranched polyethylene is 10-30%, the mass percentage of the thermally conductive filler is 5-40%, and the remainder is paraffin wax.

[0019] Preferably, the thermally conductive filler is selected from at least one of flake graphite, hexagonal boron nitride, alumina, aluminum nitride, and zinc oxide.

[0020] Preferably, the crosslinking catalyst is dibutyltin dilaurate or oleylamine, and its addition amount is 0.1-2.0% of the mass of the siloxane-grafted hyperbranched polyethylene.

[0021] Preferably, the solvent for the solution of the siloxane-grafted hyperbranched polyethylene is anhydrous chloroform or toluene, with a concentration of 5-25 wt.%, more preferably 10 wt.%. The chloroform and methanol used should be anhydrous solvents to avoid hydrolysis of the siloxane.

[0022] In step (2) of the present invention, the solvent can be removed by heating and pressurizing, preferably by rotary evaporation.

[0023] In step (3) of the present invention, the cooling and shaping of the mixture can be carried out in the following steps: pour the obtained mixture into the mold while it is hot and cool and shape it at room temperature; or cool the obtained mixture to room temperature first and then mold it.

[0024] The present invention has the following advantages over the prior art: First, compared to polymers used in conventional polymer shaping methods, hyperbranched polyethylene has structural characteristics such as low molecular chain entanglement and low melt viscosity, and exhibits good solubility in molten paraffin. This ensures that the composite after molten mixing will not undergo phase separation due to insufficient compatibility during cooling, thus achieving more complete coating of paraffin.

[0025] Second, by introducing siloxane groups onto the molecular chains of hyperbranched polyethylene, the polymer can be cross-linked through siloxane hydrolysis and condensation. After being melt-composite with paraffin, the three-dimensional network structure formed by in-situ cross-linking can effectively constrain the macroscopic flow of molten paraffin, thereby significantly suppressing the leakage of phase change materials.

[0026] Third, the cross-linked hyperbranched polyethylene network has good thermal stability and mechanical flexibility. It can still maintain the restraining effect on paraffin after multiple melt-solidification cycles, ensuring the reliability of the phase change material in long-term service.

[0027] Fourth, the present invention features a simple process, adjustable performance, and controllable cost. It employs an integrated process flow of "solution compounding-melt mixing-in-situ crosslinking," eliminating the need for complex microcapsule encapsulation or framework prefabrication steps, thus helping to reduce the overall cost of the encapsulation process. Furthermore, by adjusting the proportion of thermally conductive filler, the thermal conductivity of the material can be adjusted within a wide range, thereby meeting the differentiated thermal conductivity requirements of various thermal management scenarios. Attached Figure Description

[0028] Figure 1 Optical photographs of the HBPE-co-VTES synthesized in Example 1 and the HBPE synthesized in Comparative Example 1.

[0029] Figure 2 The HBPE-co-VTES of Example 1 and the HBPE of Comparative Example 1 1 H-NMR spectrum.

[0030] Figure 3 The images show the FTIR spectra of HBPE-co-VTES from Example 1 and HBPE from Comparative Example 1.

[0031] Figure 4 This is a comparison of the GPC effluent curves of HBPE-co-VTES synthesized at different comonomer concentrations and temperatures in Example 1.

[0032] Figure 5 Viscosity-shear rate rheological curves of HBPE-co-VTES in Example 1 and HBPE in Comparative Example 1 are shown.

[0033] Figure 6 The molecular weight and siloxane grafting rate of copolymers synthesized at different temperatures (15 °C, 20 °C and 25 °C) with three different comonomer concentrations (0.5, 1.0 and 1.5 mol / L) are shown.

[0034] Figure 7 The crosslinking gel curves of HBPE-co-VTES synthesized at 15℃ with three different comonomer concentrations (0.5, 1.0 and 1.5 mol / L) are shown.

[0035] Figure 8 This is a schematic diagram of the preparation process of the low-leakage paraffin-based thermally conductive phase change material described in this invention.

[0036] Figure 9 Comparative photographs show the leakage behavior of three phase change materials—Comparative Example 2 (pure paraffin), Comparative Example 3 (flake graphite / paraffin), and Example 2 (flake graphite / HBPE-co-VTES / paraffin)—placed on filter paper and subjected to a temperature of 80 °C over time.

[0037] Figure 10 The graph shows a comparison of the mass retention rate of the composite phase change materials obtained by different component ratios in Example 2 after (a) short-cycle heating and (b) long-cycle heating.

[0038] Figure 11 (ad) shows the cross-sectional SEM images of Comparative Example 2 (pure paraffin) and Comparative Example 4 (HBPE-co-VTES / paraffin); (e) shows the microstructure of the HBPE-co-VTES / paraffin composite phase change material drawn based on the SEM images.

[0039] Figure 12 The DSC curves of the composite phase change materials obtained in different component ratios in Example 2, along with their calculated crystallization enthalpy and melting enthalpy, are shown.

[0040] Figure 13 (a) Comparison of thermal conductivity between Comparative Example 2 (pure paraffin) and Comparative Example 3 (30% flake graphite / paraffin); (b) Thermal conductivity of composite phase change materials with different formulations in Example 2; (c) Infrared thermal images of the corresponding sample of Example 2 at different time points on an 80 °C heating stage. Detailed Implementation

[0041] The present invention will be further described in detail below with reference to specific embodiments and accompanying drawings, but the implementation of the present invention is not limited thereto.

[0042] Unless otherwise specified in the embodiments of this invention, conventional conditions or conditions recommended by the manufacturer shall apply. Reagents or instruments whose manufacturers are not specified are all conventional products that can be obtained through conventional technical means or commercially available.

[0043] Example 1, Comparative Example 1

[0044] 1. Sample preparation

[0045] (1) Example 1

[0046] Step 1, HBPE-co-VTES synthesis: (1) Dry a 100 mL Schlenk reaction flask at 120 °C for more than 2 hours, transfer it to the glove box transition chamber while hot, cool it under vacuum, and then transfer it to the glove box under nitrogen atmosphere. In the glove box, add 125 mg (0.15 mmol) of Pd-diimine catalyst to the reaction flask. (2) Transfer the reaction flask containing the catalyst out of the glove box, immediately connect it to the ethylene gas supply system, and place it in a low-temperature water bath at 5 °C. Perform three cycles of "vacuuming-filling with ethylene" to replace the nitrogen in the reaction system. (3) Under the condition of continuous ethylene supply and stirring, add 35 mL of anhydrous dichloromethane and 15 mL of comonomer VTES (concentration of about 1.5 mol / L) to the flask through a syringe. (4) Adjust the reaction temperature to 20 °C and maintain the ethylene pressure at 0.1 MPa. Under these conditions, the polymerization reaction is carried out for 20 h.

[0047] Step 2, HBPE-co-VTES separation and purification: (1) Transfer the reaction mixture to a glass beaker. While stirring, slowly add excess anhydrous methanol (pre-soaked and dried with 4Å molecular sieve) to the mixture until the polymer is completely precipitated. After standing, discard the supernatant. (2) Add an appropriate amount of anhydrous chloroform (pre-dried with 4Å molecular sieve) to the precipitated polymer and stir until it is completely dissolved. (3) Slowly add anhydrous methanol to the chloroform solution again to allow the polymer to precipitate again. (4) Repeat the "methanol precipitation-chloroform dissolution" operation three times to remove unreacted monomers. (5) Dissolve the purified polymer in anhydrous chloroform to prepare a polymer solution with a mass fraction of approximately 15%.

[0048] Step 3, obtaining pure HBPE-co-VTES: (1) Take a portion of the above polymer solution and remove most of the solvent using a rotary evaporator at 40 °C. (2) Transfer the residue to a vacuum drying oven and dry it at 40 °C to constant weight to obtain a solid HBPE-co-VTES sample for subsequent characterization.

[0049] Following similar steps to Example 1, this invention conducted nine parallel experiments with varying comonomer concentrations (0.5M, 1.0M, and 1.5M) and reaction temperatures (15℃, 20℃, and 25℃). The results are shown in Tables 1 and 2.

[0050] (2) Comparative Example 1

[0051] Step 1, HBPE homopolymer synthesis: (1) Dry a 100 mL Schlenk reaction flask at 120 °C for more than 2 h, transfer it to the glove box transition chamber while hot, cool it under vacuum, and then transfer it to the glove box under nitrogen atmosphere. In the glove box, add 125 mg (0.15 mmol) of Pd-diimine catalyst to the reaction flask. (2) Transfer the reaction flask containing the catalyst out of the glove box, immediately connect it to the ethylene gas supply system, and place it in a low temperature water bath at 5 °C. Perform three cycles of "vacuuming-filling ethylene" to replace the nitrogen in the reaction system. (3) Under the condition of continuous ethylene supply and stirring, add 50 mL of anhydrous dichloromethane to the flask through a syringe. (4) Adjust the reaction temperature to 20 °C and maintain the ethylene pressure at 0.1 MPa. Under these conditions, the polymerization reaction is carried out for 20 h.

[0052] Step 2: HBPE separation and purification: (1) Transfer the mixed solution after reaction to a glass beaker and blow it with cold air to remove most of the solvent. (2) Add tetrahydrofuran dropwise until the product is just completely dissolved, then add 1 mL of concentrated hydrochloric acid and 1 mL of hydrogen peroxide dropwise, and stir at 600 rpm for 30 min to remove the Pd catalyst in the polymerization product. (3) Add methanol dropwise to the solution until no more precipitate is formed. (4) After gently pouring off the supernatant, add tetrahydrofuran dropwise to just completely dissolve the precipitate. (5) Repeat the "methanol precipitation-tetrahydrofuran dissolution" step three times to purify the product. (6) After rotary evaporation and vacuum drying at 60 °C for 24 h, HBPE is obtained.

[0053] 2. Characterization and Testing

[0054] (1) Nuclear magnetic resonance hydrogen spectrum ( 1 H-NMR analysis

[0055] HBPE-co-VTES and HBPE homopolymer 1 H-NMR spectra were measured on a 500 MHz nuclear magnetic resonance spectrometer (Bruker, AVANCE III). Deuterated chloroform was used as the solvent, and the test temperature was 25 °C. 1 The degree of branching of the polymer and the grafting rate of the comonomer can be calculated from the H-NMR spectrum results.

[0056] (2) Gel permeation chromatography (GPC) analysis

[0057] The relative molecular weights and distributions of HBPE-co-VTES and HBPE homopolymers were determined by gel permeation chromatography (Malvern Panalytical, Omnisec Resolve multi-detector GPC). Tetrahydrofuran was used as the mobile phase at a flow rate of 1.0 mL / min, and the column temperature was 30 °C. Narrow-distribution polystyrene (PS) standards were used for calibration. The test samples were dissolved in tetrahydrofuran to prepare solutions with a concentration of 3–5 mg / mL, filtered through a 0.22 μm polytetrafluoroethylene (PTFE) membrane, and then injected for analysis.

[0058] (3) Fourier transform infrared spectroscopy (FTIR) analysis

[0059] The functional groups in the molecular structures of HBPE-co-VTES and HBPE homopolymers were characterized using FTIR (Thermo Fisher Scientific, Nicolet iS50). The test range was 400-4000 cm⁻¹. -1 The resolution is 4 cm. -1 The scan was performed 32 times, and an air background spectrum was collected before the test. The sample was prepared using the potassium bromide pellet method, in which the diluted polymer solution (concentration of about 0.1 mg / mL) was dropped onto potassium bromide, and the sample was tested after being thoroughly dried.

[0060] (4) Rotational rheological test

[0061] The rheological properties of HBPE-co-VTES and HBPE homopolymers were determined using a rotational rheology test (TA Instruments, HR-20 model). The flow test mode was employed to obtain viscosity-shear rate profiles. The test temperature was 25°C, the test fixture consisted of 25.4 mm thick stainless steel plates with a plate spacing of 0.5 mm, and the shear rate ranged from 0.1 to 100 s⁻¹. -1 .

[0062] (5) Gel time test

[0063] The gel behavior of HBPE-co-VTES under the action of a crosslinking catalyst was characterized using a rotational rheometer (TA Instruments, HR-20). The Oscillation test mode was employed, monitoring the complex viscosity of the composite over time. The gel time was defined as the time corresponding to the inflection point where the viscosity increased sharply. The test conditions were as follows: temperature 80 °C, 25.4 mm parallel plate fixture, plate spacing 0.5 mm, shear strain 0.5%, and angular frequency 0.1 rad / s. The test samples were prepared as follows: HBPE-co-TEOS, paraffin wax, and the crosslinking catalyst were weighed at a mass ratio of 30:70:0.6, rapidly mixed at 65 °C until homogeneous, and immediately poured into a mold for initial shaping before being transferred to the rheometer fixture for testing.

[0064] 3. Test Result Analysis

[0065] Table 1. Yields, grafting rates, and branching degrees of products under different reaction conditions.

[0066] Figure 2 The HBPE-co-VTES prepared in Example 1 and the HBPE homopolymer of Comparative Example 1 1 H-NMR spectrum comparison. The comparison revealed the following new characteristic signals in the spectrum of the copolymer HBPE-co-VTES: a quartet at δ 3.80-3.85 ppm attributed to the methylene proton (-O-CH2-CH3) of the ethoxy group in the comonomer VTES; a singlet at δ 1.22 ppm corresponding to a significant change in the chemical shift of the vinyl proton (CH2=CH-Si≡) in the original VTES monomer after polymerization, confirming the participation of vinyl in the copolymerization reaction; and the methyl proton signal (-O-CH2-CH3) in the ethoxy group, due to its close chemical shift to the methyl signal in the hyperbranched backbone, was contained within a broad peak at δ 0.81-0.90 ppm. These analyses indicate that VTES was successfully covalently grafted onto the hyperbranched polyethylene backbone, thus synthesizing the target product HBPE-co-VTES. Furthermore, Table 1 summarizes the results based on this... 1 The degree of branching and grafting rate were calculated from the H-NMR spectra. The results showed that the grafting rate of VTES was positively correlated with the reaction temperature and monomer concentration.

[0067] Figure 3 This is a comparison of FTIR spectra of the comonomer, HBPE-co-VTES, and HBPE homopolymer. The range is 2920-2850 cm⁻¹. -1The absorption peaks between these peaks are attributed to the saturated CH stretching vibrations in the hyperbranched polyethylene skeleton. The relative peak area decreases after grafting the comonomer. Meanwhile, in HBPE-co-VTES, the peak area is around 1200 cm⁻¹. -1 A characteristic absorption peak at 1070 cm⁻¹, attributed to the stretching vibration of the Si-C bond, appears. -1 and 910 cm -1 Characteristic absorption peaks, attributed to antisymmetric and symmetric stretching vibrations of the Si-OC bond, appeared at the points indicated. This demonstrates that VTES has been successfully copolymerized, consistent with the previous results. 1 The results correspond to the H-NMR values.

[0068] Figure 4 The GPC test results for HBPE-co-VTES synthesized at different temperatures and with different comonomer concentrations in Example 1 are shown. Based on the GPC test results, the number-average molecular weight (Mn), weight-average molecular weight (Mw), molecular weight polydispersity index (PDI), and Mark-Hovink coefficient (MHα) of the polymers can be calculated. The statistical results are shown in Table 2. The MHα values ​​of all products are in the range of 0.55 to 0.65, which confirms that the obtained polymers exhibit a highly branched and compact conformation in solution, consistent with the structural characteristics of hyperbranched polymers.

[0069] Table 2. Molecular weights of products obtained at different monomer concentrations and reaction temperatures.

[0070] Figure 5 The viscosity-shear rate curves for HBPE-co-VTES and HBPE homopolymers are shown. Under the same synthesis conditions, the viscosity of HBPE-co-VTES in the low shear rate region is significantly lower than that of the HBPE homopolymer. This is mainly because the introduction of VTES reduces the molecular weight and the degree of molecular entanglement. In addition, both polymers exhibit weak shear-thinning behavior, which further confirms the hyperbranched structural characteristics of the products.

[0071] Figure 6 The effects of copolymerization concentration and temperature on the grafting rate and molecular weight of the resulting copolymers were summarized. Analysis showed that the siloxane grafting rate of the copolymers increased with increasing comonomer concentration and reaction temperature, mainly because higher monomer concentrations and reaction temperatures are more favorable for the copolymerization reaction; among these, monomer concentration had a more significant impact on the grafting rate. The molecular weight of the polymer was less affected by the reaction temperature, but showed a decreasing trend with increasing comonomer concentration, which is related to the inhibitory effect of the monomer on catalyst activity.

[0072] Figure 7Crosslinking gel curves of HBPE-co-VTES synthesized at 15℃ with three different comonomer concentrations (0.5, 1.0, and 1.5 mol / L) are shown. The results indicate that, under the same catalyst dosage and curing temperature, the product with a monomer concentration of 1.0 mol / L gels fastest, followed by 0.5 mol / L and 1.5 mol / L. Higher molecular weight contributes to the construction of a network with higher mechanical strength, while higher grafting ratio provides denser crosslinking points. However, as mentioned above... Figure 6 The results show that increasing the monomer concentration improves the grafting rate but decreases the molecular weight. This antagonistic effect leads to a non-linear change in the gel behavior of the copolymer. Specifically, the 0.5 mol / L sample has a high molecular weight but insufficient grafting rate and sparse crosslinking points; the 1.5 mol / L sample has a high grafting rate but a low molecular weight, requiring a longer time to form an effective network. Therefore, the 1.0 mol / L sample achieves the best balance between molecular weight and grafting rate, exhibiting the fastest gel kinetics.

[0073] Based on the comprehensive evaluation of the above gel efficiency, HBPE-co-VTES synthesized at a monomer concentration of 1.0 mol / L and a temperature of 15℃ was selected as the representative sample for subsequent composite material research.

[0074] Example 2, Comparative Examples 2-4

[0075] 1. Sample preparation

[0076] (1) Example 2

[0077] Step 1, ultrasonic dispersion: Accurately weigh a certain mass of HBPE-co-VTES copolymer and flake graphite according to the proportions shown in Table 3, place them in a 100 mL sample bottle, and add 40 mL of anhydrous chloroform. Sonicate the sample (720 W) for 60 min to fully disperse the flake graphite and promote the adsorption of the copolymer on its surface.

[0078] Step 2, Solvent Displacement and Melt Blending: Accurately weighed sliced ​​paraffin wax (its mass, together with the total mass of the aforementioned HBPE-co-VTES and flake graphite, is 10 g) is added to the dispersion obtained in Step 1. The sample vial is transferred to a rotary evaporator, and the temperature is slowly increased to 80 °C under reduced pressure to allow the chloroform to evaporate completely. During this process, the molten paraffin wax, HBPE-co-VTES, and flake graphite are fully melt-blended to obtain a homogeneous mixed melt.

[0079] Step 3, catalyst mixing: The above-mentioned mixed melt is transferred to an oven at 80 °C to keep it in a molten state. The curing catalyst dibutyltin dilaurate (1% of the mass of HBPE-co-VTES) is added to the mixed melt and stirred for 3 min to ensure uniform distribution of the catalyst. Then it is cooled to obtain an uncrosslinked black solid composite.

[0080] Step 4, Molding and Cross-linking Curing: Weigh an appropriate amount of the black solid composite and place it in a tableting mold. Cold press it under a pressure of approximately 5 MPa to obtain a pre-formed disc. Place the disc into a silicone mold with a matching shape and transfer it to an oven at 100 °C for heat treatment for 12 h to allow the siloxane groups to undergo sufficient hydrolysis, condensation, and cross-linking reactions, ultimately obtaining a shaped thermally conductive phase change material disc.

[0081] This embodiment discloses a series of composite phase change materials with different mass ratios. The preparation method is the same as the process steps described in Example 2. The specific formula variables are shown in Table 3.

[0082] (2) Comparative Example 2

[0083] Comparative Example 2 is a pure paraffin wax sample, which is prepared as follows: an appropriate amount of paraffin wax is weighed and placed in a tableting mold, and then cold-pressed at room temperature under a pressure of 5 MPa. After demolding, the pure paraffin wax comparative sample is obtained.

[0084] (3) Comparative Example 3

[0085] Comparative Example 3 is a mixture of paraffin and graphite, prepared as follows: Step 1, ultrasonic dispersion: Accurately weigh a certain mass of flake graphite and place it in a 100 mL sample bottle, then add 40 mL of anhydrous chloroform. Sonicate the sample for 60 min to fully disperse the flake graphite and promote the adsorption of the copolymer on its surface.

[0086] Step 2, Solvent replacement and melt blending: Add an accurately weighed amount of sliced ​​paraffin (its mass plus the total mass of the aforementioned flake graphite is 10 g) to the above dispersion. Transfer the sample vial to a rotary evaporator and slowly heat to 80 °C under reduced pressure to remove the chloroform solvent and achieve mixing of the molten paraffin and flake graphite. Allow to cool naturally to room temperature after mixing.

[0087] Step 3, molding: Weigh an appropriate amount of the mixture from step 2, place it in a tableting mold, and cold press it at room temperature under a pressure of 5 MPa. After demolding, the flake graphite / paraffin comparison sample is obtained.

[0088] (4) Comparative Example 4

[0089] Comparative Example 4 is a mixture of paraffin and HBPE-co-VTES. Its preparation method is similar to that of Example 2, except that its formulation does not contain flake graphite. It is prepared by compounding paraffin and HBPE-co-VTES in the corresponding mass ratio and then heating and crosslinking them.

[0090] Table 3. Phase transition enthalpy, thermal conductivity, and mass retention of different phase change material samples. The test conditions were 80 °C for 120 h. 2. Characterization and Testing (1) Leakage rate test Composite materials with different proportions were prepared into discs with a diameter of 12.7 mm and a thickness of 1.5-2.0 mm. These discs were placed in glass petri dishes lined with quantitative filter paper and then placed in a forced-air drying oven at 80 °C. Every 1 h (short-cycle experiment) or 24 h (long-cycle experiment), the discs were removed, cooled to room temperature in a desiccator, and weighed. The change in mass over time was recorded. The retention rate (R) was calculated using the formula R = m t / m o Calculate by multiplying by 100%, where m0 is the initial mass, m t Let be the mass at time t.

[0091] (2) Phase change performance test

[0092] Differential scanning calorimetry (DSC 214, Netzsch) was used for the tests. Under a nitrogen atmosphere (flow rate 50 mL / min), approximately 5 mg of sample was weighed and tested according to the following procedure: the temperature was increased from 25 °C to 90 °C at a rate of 10 °C / min and held for 5 min to eliminate thermal history; then the temperature was decreased to 25 °C at a rate of 5 °C / min and held for 5 min; finally, the temperature was increased to 90 °C at a rate of 5 °C / min. The curves for the second heating process were recorded, and the phase transition temperature and enthalpy were analyzed.

[0093] (3) Thermal conductivity test

[0094] The tests were conducted using a laser flare analyzer (Netzsch, LFA 467). Samples were fabricated as circular discs with a diameter of 12.7 mm and a thickness of 1.5–2.0 mm. A graphite coating was applied to the surface before testing to ensure uniform heat absorption. Testing was conducted at 25 °C, with a flare voltage set to 180 V and a detector area of ​​3.7 mm². 2 By measuring the thermal diffusivity (α) and combining it with the sample's specific heat capacity (C... p ) and density (ρ), according to the formula λ = α × C p The thermal conductivity (λ) of the material is calculated by multiplying ρ.

[0095] (4) Infrared thermal imaging test

[0096] Thermally conductive discs with different compositions were placed on an 80 ℃ constant-temperature heating stage to heat their lower surfaces. An infrared thermal imager (Fotric, 348+ type) was used to vertically align with the upper surface of the discs and capture temperature distribution images at fixed time intervals. By analyzing the temperature uniformity and heating rate, the thermal conductivity of different samples was qualitatively compared.

[0097] (5) Microscopic morphological characterization

[0098] The cross-sectional morphology of the composite material was observed using a field emission scanning electron microscope (SEM-4000 model, QuantumCTek). The brittle fractured sample was fixed on the sample stage with the fracture surface facing upwards, and after being sputtered with gold, it was placed in the electron microscope for observation. The accelerating voltage was set to 5 kV, and images of the cross-sectional microstructure were taken at different magnifications to analyze the graphite distribution and polymer-paraffin compatibility.

[0099] 3. Comparison and Analysis of Test Results

[0100] (1) Leakage resistance

[0101] As attached Figure 9 As shown in the optical photographs, in the system without HBPE-co-VTES (Comparative Examples 2 and 3), the samples were unable to effectively confine the paraffin under heated conditions, and obvious leakage occurred within a short time at 80 °C, with the sample shape collapsing significantly. In contrast, in Example 2 with HBPE-co-VTES, the material showed a significantly reduced degree of leakage under the same conditions. Figure 10 The graphs showing the changes in mass retention of the composite phase change materials obtained with different component ratios in Example 2 after (a) short-cycle heating and (b) long-cycle heating are illustrated. Table 2 shows the mass retention rates of the corresponding samples after heating at 80 °C for 120 h. The high mass retention rate indicates that the constructed cross-linked network can effectively suppress the flow and exudation of paraffin, thereby achieving good shape stability.

[0102] (2) Microscopic morphology analysis

[0103] Figure 11 The cross-sectional micromorphology of pure paraffin (Comparative Example 2) and paraffin with added HBPE-co-VTES (Comparative Example 4) was compared. The pure paraffin fracture surface exhibited irregular wrinkles and lamellar structures, displaying a typical ductile fracture morphology. This is due to the slippage of large-sized lamellar crystals formed during the cooling process of paraffin during fracture. However, the introduction of HBPE-co-VTES significantly altered the cross-sectional morphology of the composite material. Paraffin and HBPE-co-VTES showed good compatibility, with no significant phase separation. The cross-linked polymer network segmented and encapsulated the paraffin within micro-regions, forming a capsule-like structure (e.g., ...). Figure 11 (e) indicates this). As the amount of HBPE-co-VTES added increases, the volume of the encapsulated microdomains also increases. Figure 11 (b) - (d)).

[0104] (3) Analysis of phase transition enthalpy and thermal conductivity

[0105] As attached Figure 12 As shown in the DSC curves and related data tables, the melting and crystallization temperatures of the composite material decreased slightly compared to pure paraffin after the introduction of HBPE-co-VTES. This is primarily due to the cross-linked structure formed in the system restricting the migration and rearrangement of paraffin molecular chains, making it difficult to form a complete and ordered crystal structure during crystallization, thus reducing crystallinity and leading to a decrease in phase transition temperature. Secondly, the introduction of HBPE-co-VTES with a hyperbranched structure disperses the paraffin phase to some extent, reducing the concentration of effective components that can participate in crystallization, further weakening the crystal growth process. In addition, changes in filler content and the proportion of each component also have a synergistic effect on the crystallization and melting behavior of paraffin. With the decrease in paraffin content, the phase transition enthalpy of the material shows a decreasing trend, while the shape stability and thermal conductivity of the system are correspondingly improved. Table 2 summarizes the phase transition enthalpy of each sample.

[0106] (4) Thermal conductivity analysis

[0107] Figure 13 (a) and (b) compare the thermal conductivity of pure paraffin (Comparative Example 2), 30% flake graphite / paraffin (Comparative Example 3), and Example 2 with different formulations. Figure 13 (c) Infrared thermal imaging shows the thermal response rates of the samples with different formulations in Example 2. The results show that with the increase of thermally conductive filler content, the material's response rate to changes in external temperature is significantly improved, demonstrating superior thermal conductivity. Furthermore, under the same graphite content conditions, the sample with HBPE-co-VTES exhibits higher thermal conductivity efficiency compared to the system without it. This indicates that the introduction of hyperbranched polyethylene helps improve the interfacial compatibility between components, promotes the formation of more continuous heat transfer pathways between thermally conductive fillers, and thus improves overall thermal conductivity.

Claims

1. A siloxane-grafted hyperbranched polyethylene, characterized in that: The siloxane-grafted hyperbranched polyethylene is obtained by copolymerizing ethylene with vinyltriethoxysilane shown in Formula II using the Pd-diimine catalyst shown in Formula I according to the "chain walking" polymerization mechanism. 。 2. The siloxane-grafted hyperbranched polyethylene as described in claim 1, characterized in that: The siloxane-grafted hyperbranched polyethylene is prepared according to the following steps: In an ethylene atmosphere, Pd-diimine catalyst, anhydrous solvent, and vinyltriethoxysilane are sequentially added to a reaction vessel, so that the initial concentration of the catalyst in the polymerization reaction system is 1.5-6.0 mmol / L and the concentration of vinyltriethoxysilane is 0.5-1.5 mol / L; the reaction temperature is controlled at 10-30 ℃, the ethylene pressure is maintained at 0.1-0.8 MPa, and the mixture is stirred thoroughly; under these conditions, polymerization is carried out for 12-24 h. After the reaction is completed, the reaction product is separated and purified to obtain the siloxane-grafted hyperbranched polyethylene.

3. The siloxane-grafted hyperbranched polyethylene as described in claim 2, characterized in that: The anhydrous solvent is selected from one of anhydrous dichloromethane, anhydrous trichloromethane, and anhydrous toluene.

4. The siloxane-grafted hyperbranched polyethylene as described in claim 2, characterized in that: The initial concentration of the catalyst in the polymerization reaction system is 2.0-4.0 mmol / L, preferably 3.0 mmol / L.

5. The siloxane-grafted hyperbranched polyethylene as described in claim 2, characterized in that: The initial concentration of vinyltriethoxysilane in the polymerization reaction system was set to 1.0 mol / L.

6. The siloxane-grafted hyperbranched polyethylene as described in claim 2, characterized in that: The reaction temperature is 15-30 ℃, preferably 15 ℃; the ethylene pressure is 0.1-0.3 MPa, preferably 0.1 MPa; and the reaction time is 18-22 h, preferably 20 h.

7. The siloxane-grafted hyperbranched polyethylene as described in claim 4, characterized in that: The initial concentration of vinyltriethoxysilane in the polymerization reaction system was 1.0 mol / L, the reaction temperature was 15 ℃, the ethylene pressure was 0.1 MPa, and the reaction time was 20 h.

8. The application of siloxane-grafted hyperbranched polyethylene as described in any one of claims 1-7 in the preparation of low-leakage paraffin-based thermally conductive phase change materials, the application comprising the following steps: (1) The paraffin is completely dissolved in the above-mentioned solution of siloxane-grafted hyperbranched polyethylene, and then thermally conductive filler is added to form a uniform mixed slurry. (2) Remove the solvent from the mixed slurry obtained in step (1) to obtain a uniform polymer / thermal conductive filler / paraffin mixture; (3) Heat the polymer / thermal filler / paraffin mixture obtained in step (2) to above the melting point of paraffin for melting and mixing, then add the crosslinking catalyst and mix evenly, and let the resulting mixture cool and solidify; (4) Place the sample after cooling and shaping in step (3) in an oven and heat treat it at 60-100 ℃ for 6-24 h to allow the siloxane groups to undergo hydrolysis and condensation reaction, crosslinking to form a stable crosslinking network. After crosslinking is completed, demold to obtain the low-leakage paraffin-based thermally conductive phase change material.

9. The application as described in claim 8, characterized in that: The total mass of the siloxane-grafted hyperbranched polyethylene, paraffin wax, and thermally conductive filler is 100%, the mass percentage of the siloxane-grafted hyperbranched polyethylene is 10-30%, the mass percentage of the thermally conductive filler is 5-40%, and the remainder is paraffin wax.

10. The application as described in claim 8, characterized in that: The thermally conductive filler is selected from at least one of flake graphite, hexagonal boron nitride, alumina, aluminum nitride, and zinc oxide; the crosslinking catalyst is dibutyltin dilaurate or oleylamine, and its addition amount is 0.1-2.0% of the mass of the siloxane-grafted hyperbranched polyethylene.

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