A thermally adhesive polyimide / thermotropic liquid crystal polymer composite film containing a coordination unit and a preparation method thereof

Abstract

This invention relates to the field of polymer composite materials technology, and provides a thermoplastic polyimide / thermotropic liquid crystal polymer composite film containing coordination units and its preparation method. The composite film is obtained by melt extrusion casting of raw materials containing the following components: (a) a thermoplastic polyimide resin containing coordination units; (b) a thermotropic liquid crystal polymer resin; wherein the thermoplastic polyimide resin containing coordination units comprises structural units derived from nitrogen-containing heterocyclic diamine monomers, and the nitrogen-containing heterocyclic diamine monomers contain at least one heteroatom coordination group selected from pyridine rings, thiazole rings, imidazole rings, or benzothiazole rings. This invention overcomes the shortcomings of the prior art, has a reasonable design, and a compact structure. By introducing nitrogen-containing heterocyclic (pyridine, thiazole, etc.) coordination units into the polyimide molecular chain, this invention enables the resin to form stable coordination bonds with the copper foil surface during hot pressing, thereby significantly enhancing the interfacial adhesion.

Description

Technical Field

[0001] This invention belongs to the field of polymer composite material technology, specifically relating to a composite film with high thermal viscosity, low dielectric constant, and low dielectric loss, and its preparation method. It is mainly used in adhesive-free double-sided flexible copper clad laminate (2L-FCCL) and high-frequency communication substrates. Background Technology

[0002] With the rapid development of fifth-generation mobile communication technology (5G) and future 6G technology, electronic devices are evolving towards higher frequency, higher speed, and thinner and lighter designs, which puts forward higher performance requirements for flexible circuit substrates used for signal transmission. These requirements mainly include low dielectric constant (Dk), low dielectric loss (Df), high heat resistance, high dimensional stability, and good mechanical properties.

[0003] Polyimide (PI) films are widely used in flexible copper clad laminates (FCCLs) due to their excellent thermal stability, chemical stability, mechanical properties, and low dielectric constant. However, traditional three-layer FCCLs use epoxy resin or acrylate adhesives to bond copper foil to the PI film. The presence of adhesives not only limits heat resistance but also increases dielectric loss and thickness, making it difficult to meet the requirements of high-frequency communication. In recent years, two-layer FCCLs (2L-FCCLs) have been developed, eliminating the intermediate adhesive layer and directly laminating copper foil and PI film, becoming the mainstream direction for high-frequency flexible circuit substrates.

[0004] However, PI films used in 2L-FCCL face a trade-off between thermal adhesion and dimensional stability: to enhance adhesion to copper foil, flexible segments are typically introduced, but this leads to an increase in the coefficient of thermal expansion and a decrease in thermal stability. Although there are reports in the prior art of enhancing adhesion to copper foil by introducing heteroatom coordination units containing N, O, etc., their thermal and dielectric properties still need further improvement.

[0005] Thermotropic liquid crystal polymers (TLCPs) have become another hot material for high-frequency communication substrates due to their excellent dielectric properties (Dk approximately 3.5, Df approximately 0.0015), low moisture absorption, and dimensional stability. However, TLCPs have relatively rigid molecular chains, low melt viscosity, and insufficient melt strength, which makes film formation difficult and results in poor adhesion to copper foil.

[0006] Therefore, how to organically combine the high adhesion of PI with the excellent dielectric properties of TLCP, while solving the compatibility problem between the two, and prepare composite films with high thermal adhesion, low dielectric loss and high dimensional stability has become a technical problem that urgently needs to be solved in this field. Summary of the Invention

[0007] The purpose of this invention is to overcome the shortcomings of the prior art and provide a thermo-adhesive polyimide / thermotropic liquid crystal polymer composite film containing coordination units and its preparation method. This composite film possesses high thermo-adhesiveness, low dielectric constant, low dielectric loss, high heat resistance, and high tensile strength, and can be applied to the hot-pressing method for fabricating adhesive-free double-sided flexible copper clad laminates (2L-FCCL).

[0008] To achieve the above objectives, the present invention provides the following technical solution: a thermosetting polyimide / thermotropic liquid crystal polymer composite film containing coordination units, wherein the composite film is obtained by melt extrusion casting after blending raw materials containing the following components:

[0009] (a) A thermoplastic polyimide resin containing coordinating units;

[0010] (b) Thermotropic liquid crystal polymer resin;

[0011] The thermoplastic polyimide resin containing a coordinating unit comprises a structural unit derived from a nitrogen-containing heterocyclic diamine monomer, wherein the nitrogen-containing heterocyclic diamine monomer contains at least one heteroatom coordinating group selected from a pyridine ring, a thiazole ring, an imidazole ring, or a benzothiazole ring.

[0012] The mass ratio of the thermo-adhesive polyimide resin containing coordination units to the thermotropic liquid crystal polymer resin is (10-40):(60-90).

[0013] Preferably, the thermoplastic polyimide resin containing coordinating units is prepared by a method comprising the following steps:

[0014] S1: In an aprotic polar solvent, a nitrogen-containing heterocyclic diamine monomer and a dianhydride monomer are reacted at 0–5 °C to generate a polyamic acid solution;

[0015] S2: Add functional materials to the polyamic acid solution and disperse them evenly;

[0016] S3: After step S2, a chemical imidizing agent is added to carry out a chemical imidization reaction. After precipitation, drying and pulverization, a thermoplastic polyimide resin powder containing coordinating units is obtained.

[0017] Preferably, the functional material comprises silica aerogel, and the functional material is pretreated with an aminosilane coupling agent before being added to the polyamic acid solution.

[0018] Preferably, the chemical imidizing agent is a mixture of acetic anhydride and pyridine, wherein the amount of acetic anhydride used is 2 to 4 times the initial molar amount of the dianhydride monomer.

[0019] Preferably, the nitrogen-containing heterocyclic diamine monomer is selected from at least one of 2,6-diaminopyridine, 2-aminobenzothiazole, and 2-aminothiazole.

[0020] Preferably, the thermotropic liquid crystal polymer resin is a fully aromatic copolyester copolymerized from p-hydroxybenzoic acid, 6-hydroxy-2-naphthoic acid and diacid monomers containing an imide structure.

[0021] Preferably, the composite film has a dielectric constant (Dk) ≤ 3.2 and a dielectric loss (Df) ≤ 0.0022 at a frequency of 10 GHz;

[0022] The peel strength of the composite film and copper foil after hot pressing at 350℃ and 0.8MPa for 90 seconds is ≥1.3 N / mm.

[0023] A method for preparing a composite thin film, characterized by comprising the following steps:

[0024] Step 1: Mix the thermoplastic polyimide resin powder containing coordination units with the thermotropic liquid crystal polymer resin granules in a certain proportion to obtain a mixture;

[0025] Step 2: The mixture is fed into a twin-screw extruder and melt-blended and extruded at a temperature of 330℃~360℃ to obtain composite granules;

[0026] Step 3: After drying the composite granules, feed them into a single-screw casting machine, and obtain the composite film through melt casting, traction, and winding.

[0027] A non-adhesive double-sided flexible copper-clad laminate includes copper foil and an insulating dielectric layer located between two copper foil layers, wherein the insulating dielectric layer is a thermo-adhesive polyimide / thermotropic liquid crystal polymer composite film containing coordination units as described in any one of claims 1 to 7.

[0028] A method for preparing adhesive-free double-sided flexible copper-clad laminate involves stacking the composite film between two copper foils and performing vacuum hot pressing at a temperature of 330℃~360℃ and a pressure of 0.5~1.0 MPa for 30~120 seconds.

[0029] Preferably, during the hot pressing process, the heteroatom coordinating groups in the thermoplastic polyimide containing coordinating units undergo a chemical coordination reaction with the copper element on the surface of the copper foil to form N→Cu coordination bonds.

[0030] This invention provides a thermosetting polyimide / thermotropic liquid crystal polymer composite film containing coordinating units and its preparation method. It possesses the following beneficial effects:

[0031] 1. This invention introduces nitrogen-containing heterocyclic (pyridine, thiazole, etc.) coordination units into the polyimide molecular chain, enabling the resin to form stable coordination bonds with the copper foil surface during hot pressing, thereby significantly enhancing interfacial adhesion. As shown in Table 2, the peel strength of Examples 1-8 all reached above 1.2 N / mm, with a maximum of 1.7 N / mm, representing a 6-8 times improvement compared to Comparative Example 1 (pure TLCP, only 0.2 N / mm). Furthermore, the adhesion performance of most examples was superior to or equal to that of pure hot-adhesive polyimide film (Comparative Example 2, 1.4 N / mm). This indicates that the introduction of coordination bonds effectively solves the technical problem of poor interfacial adhesion between TLCP and copper foil, allowing the composite film to be directly used to manufacture adhesive-free double-sided flexible copper-clad laminates (2L-FCCL) without additional adhesives.

[0032] 2. Benefiting from the synergistic effect of the low-dielectric-loss TLCP matrix and the porous functional material, the composite film of this invention exhibits outstanding dielectric performance at 10 GHz. As shown in Table 2, the dielectric constant (Dk) of Examples 1-8 is ≤3.2, and the dielectric loss (Df) is ≤0.0022. In particular, the Df values ​​of Examples 1-6 are not higher than those of pure TLCP (0.0019) and are significantly lower than those of pure MPI (0.0025), achieving the goal of low signal transmission loss at high frequencies. The introduction of nanoporous silica aerogel (comparison between Examples 1 and 7) further reduces the film density and polarizability, decreasing Dk and Df by approximately 15.6% and 27.3%, respectively, meeting the stringent dielectric performance requirements of 5G communication and high-speed, high-frequency flexible circuit boards.

[0033] 3. This invention introduces rigid rod-shaped TLCP segments into the composite system, effectively suppressing the dimensional changes of the MPI flexible segments when heated. Table 2 shows that the glass transition temperature (Tg) of all embodiments is above 300℃ (up to 325℃), and the coefficient of thermal expansion (CTE) remains below 35 ppm / K (minimum 25 ppm / K), indicating that the composite film maintains high heat resistance while exhibiting significantly better dimensional stability than pure MPI film (CTE 21 ppm / K, but Tg 340℃ requires a trade-off in adhesion). Example 3, with an MPI:TLCP ratio of 40:60, still maintains a CTE of 26 ppm / K, close to the level of pure TLCP (20 ppm / K), making it suitable for high-temperature lead-free soldering processes.

[0034] 4. The tensile strength of the composite film falls between that of pure MPI (150 MPa) and pure TLCP (240 MPa). In Example 8 (10:90 ratio), the tensile strength reaches 205 MPa, with the rigid TLCP segments playing a significant reinforcing role. Simultaneously, the molten liquid crystal properties of TLCP endow the composite material with excellent melt flowability, enabling continuous film preparation via conventional twin-screw extrusion and casting processes. This overcomes the processing limitations of pure MPI, which is difficult to thin due to its high melt viscosity. By adjusting the mass ratio of MPI to TLCP (10:90~40:60), adhesion, dielectric properties, dimensional stability, and mechanical strength can be flexibly balanced according to actual application requirements, resulting in a wide process window and broad adaptability.

[0035] 5. The composite film of this invention combines the thermal bonding function of MPI with the low dielectric and high heat resistance characteristics of TLCP. It can be directly hot-pressed with copper foil, eliminating the epoxy or acrylic adhesive coating process required for traditional three-layer flexible copper clad laminates. This simplifies the production process and avoids the problems of dielectric performance degradation and heat resistance reduction caused by adhesive layers. At the same time, it conforms to the development trend of halogen-free and environmentally friendly electronic packaging materials. Detailed Implementation

[0036] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions of the embodiments of the present invention will be clearly and completely described below in conjunction with the embodiments of the present invention. Obviously, the described embodiments are only some embodiments of the present invention, not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0037] I. Preparation of Thermo-adhesive Polyimide (MPI) Resin Powder Containing Coordinating Units

[0038] Preparation Example 1: Preparation of MPI-1 Resin Powder

[0039] Under nitrogen protection, 100 parts by weight of the polar aprotic solvent N-methylpyrrolidone (NMP) was added to a reactor equipped with a mechanical stirrer and a thermometer, followed by 10.9 parts by weight (0.1 mol) of the nitrogen-containing heterocyclic diamine monomer 2,6-diaminopyridine, and stirred until completely dissolved. Under ice-water bath cooling conditions, 29.4 parts by weight (0.1 mol) of the dianhydride monomer 3,3',4,4'-biphenyltetracarboxylic dianhydride (BPDA) was added in batches, and the system temperature was maintained at 0-5°C for 2 hours. Then, the reaction was brought back to room temperature and stirred for another 8 hours to obtain a viscous polyamic acid (PAA) solution.

[0040] Five parts by weight (based on a theoretical PAA yield of 100 parts by weight) of functional material silica aerogel (pre-treated with KH550 aminosilane coupling agent, amount 2 parts by weight) were added to the above PAA solution and dispersed uniformly by high-speed stirring. After the functional material was uniformly dispersed, a chemical imidizing agent was added. The chemical imidizing agent was a mixture of acetic anhydride and pyridine at a volume ratio of 2:1. The amount of acetic anhydride added in the chemical imidizing agent was 2 to 4 times, preferably 3 times, the initial molar amount of the dianhydride monomer. The reaction was carried out by stirring at 80°C for 4 hours to perform imidization. This order of addition ensures that the functional material is fully dispersed in the PAA matrix and encapsulated by PAA. Subsequently, the functional material is fixed in situ in the polyimide matrix by chemical imidization, which is beneficial to improving the interfacial bonding force between the filler and the resin. The reaction solution was poured into a large amount of methanol to precipitate the solid, which was then filtered. The resulting solid was washed several times with methanol and then dried in a vacuum drying oven at 150°C for 12 hours. The solid was then pulverized and sieved to obtain a thermoplastic polyimide resin powder containing coordinating units, denoted as MPI-1.

[0041] Preparation Example 2: Preparation of MPI-2 Resin Powder

[0042] The preparation method is the same as in Preparation Example 1, except that the diamine monomer is replaced with an equimolar amount of 2-aminobenzothiazole. The resulting resin powder is designated as MPI-2.

[0043] Preparation Example 3: Preparation of MPI-3 Resin Powder

[0044] The preparation method is the same as in Preparation Example 1, except that the diamine monomer is replaced with an equimolar amount of 2-aminothiazole. The resulting resin powder is designated as MPI-3.

[0045] Preparation Example 4: Preparation of MPI-4 Resin Powder (excluding functional materials)

[0046] The preparation method is the same as in Preparation Example 1, except that silica aerogel and coupling agent KH550 are not added. The resulting resin powder is designated as MPI-4.

[0047] II. Preparation of Thermotropic Liquid Crystal Polymer (TLCP) Resin

[0048] Preparation Example 5: Preparation of TLCP Resin

[0049] In a reactor equipped with a nitrogen inlet, mechanical stirring, and distillation condenser, the following monomers are added:

[0050] p-Hydroxybenzoic acid (HBA): 69 parts by weight (0.5 mol);

[0051] 6-Hydroxy-2-naphthoic acid (HNA): 47 parts by weight (0.25 mol);

[0052] Diacid monomer containing imide structure (N,N'-(4,4'-diphenyl ether) bis(phthalimide) diacid): 13 parts by weight (0.025 mol);

[0053] Acetic anhydride: 90 parts by weight (as an acetylation reagent).

[0054] Under a nitrogen atmosphere, the mixture was slowly heated to 150°C and refluxed for 1 hour to carry out an acetylation reaction. Subsequently, the temperature was increased to 320°C at a rate of 1°C / min, and at this temperature, a vacuum of less than 100 Pa was maintained for a melt polycondensation reaction for 2 hours. The reaction was stopped after a significant increase in stirring torque. The product was extruded into strips from the bottom of the reactor under nitrogen pressure, cooled, and granulated to obtain thermotropic liquid crystal polymer resin granules, denoted as TLCP.

[0055] III. Preparation of Composite Thin Films

[0056] Example 1

[0057] The MPI-1 resin powder and TLCP resin granules prepared above were mixed evenly in a high-speed mixer at a mass ratio of 30:70. The mixture was then added to a twin-screw extruder (set temperature 340℃) for melt blending and extrusion granulation. The resulting composite granules were dried at 120℃ for 4 hours and then fed into a single-screw casting machine (die temperature 345℃). After melt casting, traction, and winding, a composite film with a thickness of 25μm was obtained.

[0058] Examples 1A~1C

[0059] The preparation process was the same as in Example 1, except for the set temperature of the twin-screw extruder. Example 1A had a set temperature of 330°C, Example 1B had a set temperature of 340°C (same as Example 1), and Example 1C had a set temperature of 360°C. The performance test results of the obtained composite films are as follows:

[0060] Example Extrusion temperature (°C) Peel strength (N / mm) Dk@10GHz Df@10GHz 1A 330 1.5 2.7 0.0016 1B 340 1.6 2.7 0.0016 1C 360 1.6 2.7 0.0017

[0061] The results show that high-performance composite films can be obtained within an extrusion temperature range of 330–360 °C. Too low a temperature may lead to insufficient melt plasticization, while too high a temperature may cause slight thermal degradation of the resin. Therefore, a melt blending temperature of 340–360 °C is preferred.

[0062] Examples 2-8

[0063] The preparation process is the same as in Example 1, and the specific raw material composition and ratio are detailed in Table 1 below. Among them:

[0064] Example 2: The MPI:TLCP mass ratio was adjusted to 20:80;

[0065] Example 3: Adjust the MPI:TLCP mass ratio to 40:60;

[0066] Example 4 uses MPI-2 resin;

[0067] Example 5 uses MPI-3 resin;

[0068] Example 6 reduces the amount of functional material silica aerogel and coupling agent used;

[0069] Example 7 uses MPI-4 resin without functional materials;

[0070] Example 8: The MPI:TLCP mass ratio was adjusted to 10:90.

[0071] Table 1. Raw material composition and proportions for Examples 1-8

[0072] Example Diamine monomer type in MPI MPI: TLCP mass ratio Functional materials (per set) Coupling agent (parts) 1 2,6-Diaminopyridine 30:70 5 2 2 2,6-Diaminopyridine 20:80 5 2 3 2,6-Diaminopyridine 40:60 5 2 4 2-Aminobenzothiazole 30:70 5 2 5 2-Aminothiazole 30:70 5 2 6 2,6-Diaminopyridine 30:70 2.5 1 7 2,6-Diaminopyridine 30:70 0 0 8 2,6-Diaminopyridine 10:90 5 2

[0073] *Note: All parts are relative to 100 parts by weight of polymer resin (MPI+TLCP total).

[0074] Comparative Example 1 (Pure TLCP film)

[0075] Using only the TLCP resin granules obtained in Preparation Example 5, a pure TLCP film with a thickness of 25 μm was prepared according to the casting process of Example 1 (casting temperature adjusted to 330 °C).

[0076] Comparative Example 2 (Pure MPI Thin Film)

[0077] Using only the MPI-1 resin powder obtained in Preparation Example 1, a pure MPI film with a thickness of 25 μm was prepared by single-screw extrusion casting (casting temperature 360 ​​°C).

[0078] IV. Performance Testing and Application Evaluation

[0079] The composite films prepared in the above examples and comparative examples were cut and stacked with 18μm thick electrolytic copper foil, then placed in a vacuum hot press. The hot pressing conditions were: temperature 350℃, pressure 0.8MPa, and time 90 seconds. After hot pressing, the films were cooled to room temperature to obtain adhesive-free double-sided flexible copper clad laminate (2L-FCCL) test samples. Performance tests were conducted according to the following standards or methods:

[0080] Peel strength: The 90° peel strength between the copper foil and the film was tested according to IPC-TM-650 2.4.9 standard.

[0081] Dielectric properties: The dielectric constant (Dk) and dielectric loss (Df) at a frequency of 10 GHz were tested using the Split Dielectric Resonant Cavity (SPDR) method under environmental conditions of 23℃±2℃ and 50%±5% relative humidity.

[0082] Tensile strength: The tensile strength of the film was tested according to ASTM D882 standard.

[0083] Glass transition temperature (Tg): Measured using a dynamic thermomechanical analyzer (DMA) at a heating rate of 5℃ / min and a frequency of 1Hz. The peak temperature of the loss tangent (tanδ) was taken.

[0084] Coefficient of thermal expansion (CTE): The average linear expansion coefficient in the range of 50~200℃ was measured using a thermomechanical analyzer (TMA).

[0085] The test results are summarized in Table 2 below.

[0086] Table 2 Performance test results of Examples 1-8 and comparative examples

[0087] Example Peel strength (N / mm) Dk@10GHz Df@10GHz Tensile strength (MPa) Tg (°C) CTE (ppm / K) 1 1.6 2.7 0.0016 185 318 28 2 1.4 2.9 0.0018 192 315 30 3 1.7 2.6 0.0015 178 322 26 4 1.6 2.8 0.0017 190 325 25 5 1.5 2.8 0.0017 182 310 32 6 1.3 2.9 0.0019 186 316 29 7 1.2 3.2 0.0022 195 312 31 8 1.2 3.1 0.0020 205 308 33 Comparative Example 1 0.2 3.5 0.0019 240 280 20 Comparative Example 2 1.4 3.2 0.0025 150 340 21

[0088] V. Results Analysis

[0089] As can be seen from the data in Table 2:

[0090] Adhesion performance: The peel strength of Examples 1-8 of this invention is significantly higher than that of Comparative Example 1 (pure TLCP), and most examples are superior to or equal to Comparative Example 2 (pure MPI). Specifically, the peel strength of Examples 1-5 is 1.4-1.7 N / mm, which is more than 7 times higher than that of Comparative Example 1 (0.2 N / mm). This is attributed to the formation of stable coordination bonds between the heteroatom coordination units such as pyridine and thiazole in MPI and the copper foil surface during hot pressing, which effectively enhances the interfacial adhesion. XPS analysis results (see Section VI) further confirm the formation of N→Cu coordination bonds.

[0091] Dielectric properties: Thanks to the low dielectric loss of TLCP as the substrate, the composite film of this invention has a dielectric constant Dk ≤ 3.2 and a dielectric loss Df ≤ 0.0022 at 10 GHz. The Df of Examples 1 to 5 is lower than that of pure TLCP (0.0019), the Df of Example 6 is the same as that of pure TLCP, and the Df of all examples is much lower than that of pure MPI (0.0025), showing the advantage of the two working together to reduce high-frequency loss.

[0092] Thermal and mechanical properties: The glass transition temperature (Tg) of all embodiments was above 300℃, and the coefficient of thermal expansion (CTE) remained below 35 ppm / K, indicating that the introduction of TLCP effectively suppressed the dimensional changes caused by the flexible MPI segments while maintaining a high heat resistance. The tensile strength was between that of pure MPI and pure TLCP, indicating that the rigid TLCP segments played a reinforcing role.

[0093] Effects of component ratio and additives: Comparing Examples 1, 2, 3, and 8, it is evident that increasing the MPI content is beneficial to peel strength, but the CTE slightly increases. Comparing Examples 1, 6, and 7, it is evident that introducing silica aerogel functional materials with porous structures and coupling agents helps to further reduce the dielectric constant and dielectric loss, and improve peel strength. This may be because the nanoporous structure reduces the film density, while the coupling agent enhances the filler-resin interface compatibility.

[0094] VI. Spectroscopic Verification of Coordination Bond Formation

[0095] To further verify the coordination bonds formed between the coordinating units and the copper foil surface, the peeling interface of the copper-clad laminate sample after hot pressing in Example 1 was analyzed. After the copper foil was peeled off from the composite film, X-ray photoelectron spectroscopy (XPS) was used to analyze the elemental chemical states of the peeled surface on the film side. The results showed that the N 1s binding energy peak of pyridine nitrogen in the pure MPI film was located at 398.7 eV, and the N 1s peak of the peeled surface after hot pressing shifted to 399.5 eV, with a chemical shift of 0.8 eV, indicating that an N→Cu coordination bond was formed between the nitrogen atom in the pyridine group and the copper element on the copper foil surface. At the same time, a distinct Cu²⁺ co-peak feature appeared at approximately 935 eV in the Cu 2p spectrum, further confirming the formation of the coordination bond.

[0096] In contrast, no chemical shift characteristics were detected at the peeling interface between the film and copper foil in Comparative Example 1 (pure TLCP), proving that the adhesion between TLCP and copper foil is merely physical adsorption and relatively weak. The above spectroscopic evidence corroborates the significant improvement in peel strength presented in this application, strongly supporting the technical principle of coordination bond-enhanced interfacial adhesion.

[0097] VII. Verification of Hot Pressing Process Parameter Range

[0098] To verify the applicability of the hot-pressing process parameters, copper-clad laminate samples were prepared using the composite film from Example 1 under different hot-pressing conditions, and their peel strength was tested. The results are as follows:

[0099] Hot pressing temperature (°C) Hot pressing pressure (MPa) Hot pressing time (s) Peel strength (N / mm) 330 0.8 90 1.4 340 0.8 90 1.5 350 0.8 90 1.6 360 0.8 90 1.5 350 0.5 90 1.3 350 1.0 90 1.6 350 0.8 30 1.3 350 0.8 60 1.5 350 0.8 120 1.6

[0100] The results show that a peel strength of over 1.3 N / mm can be obtained within a temperature range of 330–360℃, a pressure range of 0.5–1.0 MPa, and a time range of 30–120 seconds, meeting the requirements for use of adhesive-free copper-clad laminates. The preferred hot-pressing conditions are a temperature of 340–360℃, a pressure of 0.8–1.0 MPa, and a time of 60–120 seconds.

[0101] In summary, the thermo-adhesive polyimide / thermotropic liquid crystal polymer composite film containing coordination units provided by the present invention has high thermal adhesion, low dielectric constant, low dielectric loss and excellent heat resistance and dimensional stability, and is particularly suitable for the manufacture of adhesive-free double-sided flexible copper clad laminates.

[0102] It should be noted that, in this document, relational terms such as "first" and "second" are used only to distinguish one entity or operation from another, and do not necessarily require or imply any such actual relationship or order between these entities or operations. Furthermore, the terms "comprising," "including," or any other variations thereof are intended to cover non-exclusive inclusion, such that a process, method, article, or apparatus that comprises a list of elements includes not only those elements but also other elements not expressly listed, or elements inherent to such a process, method, article, or apparatus. Without further limitations, an element defined by the phrase "comprising one..." does not exclude the presence of other identical elements in the process, method, article, or apparatus that includes said element.

[0103] The above embodiments are only used to illustrate the technical solutions of the present invention, and are not intended to limit it. Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some of the technical features. Such modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the spirit and scope of the technical solutions of the embodiments of the present invention.

Claims

1. A thermosetting polyimide / thermotropic liquid crystal polymer composite film containing coordinating units, characterized in that, The composite film is obtained by melt extrusion casting of raw materials containing the following components: (a) A thermoplastic polyimide resin containing coordinating units; (b) Thermotropic liquid crystal polymer resin; The thermoplastic polyimide resin containing a coordinating unit comprises a structural unit derived from a nitrogen-containing heterocyclic diamine monomer, wherein the nitrogen-containing heterocyclic diamine monomer contains at least one heteroatom coordinating group selected from a pyridine ring, a thiazole ring, an imidazole ring, or a benzothiazole ring. The mass ratio of the thermoplastic polyimide resin containing coordinating units to the thermotropic liquid crystal polymer resin is (10-40):(60-90).

2. The composite film according to claim 1, characterized in that, The thermoplastic polyimide resin containing coordinating units is prepared by a method comprising the following steps: S1: In an aprotic polar solvent, a nitrogen-containing heterocyclic diamine monomer and a dianhydride monomer are reacted at 0–5°C to generate a polyamic acid solution; S2: Add functional materials to the polyamic acid solution and disperse them evenly; S3: After step S2, a chemical imidizing agent is added to carry out a chemical imidization reaction. After precipitation, drying and pulverization, a thermoplastic polyimide resin powder containing coordinating units is obtained.

3. The composite film according to claim 2, characterized in that, The functional material includes silica aerogel, and the functional material is pretreated with an aminosilane coupling agent before being added to the polyamic acid solution.

4. The composite film according to claim 2, characterized in that, The chemical imidizing agent is a mixture of acetic anhydride and pyridine, wherein the amount of acetic anhydride used is 2 to 4 times the initial molar amount of the dianhydride monomer.

5. The composite film according to claim 1, characterized in that, The nitrogen-containing heterocyclic diamine monomer is selected from at least one of 2,6-diaminopyridine, 2-aminobenzothiazole, and 2-aminothiazole.

6. The composite film according to claim 1, characterized in that, The thermotropic liquid crystal polymer resin is a fully aromatic copolyester copolymerized from p-hydroxybenzoic acid, 6-hydroxy-2-naphthoic acid and diacid monomers containing imide structures.

7. The composite film according to any one of claims 1 to 6, characterized in that, The composite film has a dielectric constant (Dk) ≤ 3.2 and a dielectric loss (Df) ≤ 0.0022 at a frequency of 10 GHz. The peel strength of the composite film and copper foil after hot pressing at 350℃ and 0.8MPa for 90 seconds is ≥1.3 N / mm.

8. A method for preparing the composite thin film according to any one of claims 1 to 7, characterized in that, Includes the following steps: Step 1: Mix the thermoplastic polyimide resin powder containing coordination units with the thermotropic liquid crystal polymer resin granules in a certain proportion to obtain a mixture; Step 2: The mixture is fed into a twin-screw extruder and melt-blended and extruded at a temperature of 330℃~360℃ to obtain composite granules; Step 3: After drying the composite granules, feed them into a single-screw casting machine, and obtain the composite film through melt casting, traction, and winding.

9. A glue-free double-sided flexible copper-clad laminate, characterized in that, It comprises copper foil and an insulating dielectric layer located between two copper foil layers, wherein the insulating dielectric layer is a thermosetting polyimide / thermotropic liquid crystal polymer composite film containing coordination units as described in any one of claims 1 to 7.

10. A method for preparing the adhesive-free double-sided flexible copper-clad laminate of claim 9, characterized in that, The composite film is stacked between two layers of copper foil and vacuum hot-pressed at a temperature of 330℃~360℃ and a pressure of 0.5~1.0 MPa for 30~120 seconds. During the hot pressing process, the heteroatom coordinating groups in the thermoplastic polyimide containing coordinating units undergo a chemical coordination reaction with the copper element on the surface of the copper foil to form N→Cu coordination bonds.