CN116162243B - Cardo ring structure copolyimide and preparation method thereof - Google Patents

Abstract

The invention provides a Cardo ring structure copolyimide and a preparation method thereof, and relates to the technical field of polyimide materials. According to the invention, 4-diaminodiphenyl ether is used as diamine monomer, pyromellitic anhydride is used as dianhydride monomer, 9-bis (4-aminophenyl) fluorene is added as third monomer for copolymerization and introduction of a Cardo ring structure, and random copolyimide with the Cardo ring structure is prepared; the steric effect of the Cardo ring can reduce the influence of the charge transfer complex on the color of the polyimide; and the thermal stability of polyimide is improved under the interaction of the bit resistance effect, the inter-chain conjugation and the Cardo ring rigid structure. In addition, the introduction of the Cardo ring can also reduce the surface contact angle of polyimide, and change the surface energy of polyimide so that the polyimide has better adhesiveness. The Cardo-ring structure copolyimide prepared by the invention has excellent comprehensive performance while weakening CTC effect.

Description

Cardo ring structure copolymer polyimide and preparation method thereof

Technical Field

The invention relates to the technical field of polyimide materials, and in particular to a Cardo ring structure copolymer polyimide and a preparation method thereof.

Background technique

Polyimide (PI) is a polymer containing imide five-membered heterocyclic ring (-CO-N-CO-) on the main chain of the molecule. The maximum thermal decomposition temperature exceeds 600℃. Its heat resistance is one of the best among polymer materials. It also has excellent low-temperature resistance without brittle fracture in liquid nitrogen at -269℃. In addition, its mechanical properties, solvent resistance, radiation stability, dielectric properties, etc. are also excellent. These properties are mainly attributed to the high density of aromatic heterocyclic rings and polar groups on the polyimide molecular chain, which enhances the rigidity of the chain and the interaction between molecules. Aliphatic PI lacks aromatic rings on the molecular chain, and its performance is poor, inferior to aromatic PI, and its use value is very low. In addition, the aliphatic reaction monomer is very active, and it is easy to form nylon salts in the early stage of the reaction, causing agglomeration and affecting the normal reaction. There is less research, so the PI commonly mentioned is generally aromatic PI.

The excellent performance of aromatic PI depends on its aromatic heterocyclic structure, but the rich aromatic heterocyclic structure also brings other inevitable problems, such as the darkening of the product color. The reason is that a large number of benzene rings in the PI molecular chain are structurally conjugated with the five-membered heterocyclic ring of imide to form a charge transfer complex (CTC). CTC helps to improve the thermal performance of PI, but it also increases the absorption of visible light, making PI appear dark brown, which will have a serious impact on PI used in the optoelectronic field, and even make it unusable. How to reduce the influence of CTC effect on PI color, researchers in academia and industry have carried out a lot of work for this purpose, such as introducing strong electron-withdrawing groups such as halogen, sulfone group, and fluorine-containing group into the PI molecular structure. The electron-withdrawing nature of these groups hinders the charge transfer within and between molecules, and can inhibit the generation of CTC, but these groups will lead to a decrease in the thermal stability of PI and weaken the overall performance of PI.

Summary of the invention

In view of this, the present invention aims to provide a Cardo ring structure copolymer polyimide and a preparation method thereof. The Cardo ring structure copolymer polyimide prepared by the present invention has excellent thermal stability while weakening the CTC effect and has excellent comprehensive performance.

In order to achieve the above-mentioned invention object, the present invention provides the following technical solutions:

The present invention provides a method for preparing a Cardo ring structure copolymer polyimide, comprising the following steps:

4,4-diaminodiphenyl ether, 9,9-bis(4-aminophenyl)fluorene, pyromellitic anhydride and an organic solvent are mixed for polycondensation to obtain polyamic acid;

The polyamic acid is thermally imidized to obtain Cardo ring structure copolymer polyimide.

Preferably, the ratio of the sum of the molar amounts of 4,4-diaminodiphenyl ether and 9,9-bis(4-aminophenyl)fluorene to the molar amount of pyromellitic anhydride is 1:1.00-1.05.

Preferably, the molar amount of the 9,9-bis(4-aminophenyl)fluorene is 10 to 50% of the sum of the molar amounts of 4,4-diaminodiphenyl ether and 9,9-bis(4-aminophenyl)fluorene.

Preferably, the molar amount of the 9,9-bis(4-aminophenyl)fluorene is 30% of the sum of the molar amounts of 4,4-diaminodiphenyl ether and 9,9-bis(4-aminophenyl)fluorene.

Preferably, the organic solvent includes one or more of N,N-dimethylacetamide, N,N-dimethylformamide and N-methylpyrrolidone.

Preferably, the polycondensation reaction is carried out at a temperature of 0 to 10° C. and for a time of 7 to 8 hours.

Preferably, the thermal imidization comprises: heating from room temperature to 90°C at a heating rate of 1 to 2°C/min, and keeping warm for 1 hour; heating from 90°C to 120°C at a heating rate of 1 to 2°C/min, and keeping warm for 1 hour; heating from 120°C to 150°C at a heating rate of 1 to 2°C/min, and keeping warm for 1 hour; heating from 150°C to 180°C at a heating rate of 1 to 2°C/min, and keeping warm for 1 hour; heating from 180°C to 210°C at a heating rate of 1 to 2°C/min, and keeping warm for 1 hour; heating from 210°C to 240°C at a heating rate of 1 to 2°C/min, and keeping warm for 1 hour; heating from 240°C to 270°C at a heating rate of 1 to 2°C/min, and keeping warm for 1 hour; heating from 270°C to 300°C at a heating rate of 1 to 2°C/min, and keeping warm for 1 hour.

Preferably, the polyamic acid is laid on a substrate and subjected to a vacuum degassing treatment before the thermal imidization is performed.

The present invention provides a Cardo ring structure copolymer polyimide prepared by the preparation method described in the above technical scheme.

Preferably, the Cardo ring structure copolymerized polyimide comprises a Cardo ring structure copolymerized polyimide film, and the film thickness is 240-270 μm.

The present invention provides a preparation method of a Cardo ring structure copolymer polyimide, comprising the following steps: 4,4-diaminodiphenyl ether, 9,9-bis(4-aminophenyl)fluorene, pyromellitic anhydride and an organic solvent are mixed for polycondensation to obtain polyamic acid; the polyamic acid is thermally imidized to obtain a Cardo ring structure copolymer polyimide. The present invention uses 4,4-diaminodiphenyl ether as a diamine monomer (ODA), pyromellitic anhydride (PMDA) as a dianhydride monomer, and adds 9,9-bis(4-aminophenyl)fluorene (BAFL) as a third monomer for copolymerization to introduce a Cardo ring structure, thereby preparing a random copolymer polyimide having a Cardo ring structure; the steric effect of the Cardo ring can weaken the influence of the charge transfer complex on the color of the polyimide, so that the polyimide changes from dark brown to golden yellow; and the thermal stability of the polyimide is also improved under the interaction of the steric effect, interchain conjugation and the rigid structure of the Cardo ring. In addition, the random distribution and steric effect of the Cardo rings destroy the ordered arrangement of the molecular chains, and the amorphous structure of the copolymerized polyimide is significant, which can form a dense and flexible film; the introduction of the Cardo ring can also reduce the surface contact angle of the polyimide, change the surface energy of the polyimide, and make the polyimide film material have better adhesion.

The present invention provides a Cardo ring structure copolymer polyimide prepared by the preparation method described in the above technical solution. The Cardo ring structure copolymer polyimide provided by the present invention has excellent thermal stability while weakening the CTC effect, and has excellent comprehensive performance. The results of the embodiments show that the T _d5% (temperature for decomposing 5% of the mass) of the Cardo ring structure copolymer polyimide provided by the present invention is greater than 540°C, and the temperature corresponding to the maximum thermal decomposition rate is higher than that of the polyimide without adding BAFL; the color of the Cardo ring structure copolymer polyimide is lighter than that of the polyimide without adding BAFL; the water contact angle of the Cardo ring structure copolymer polyimide is smaller than that of the polyimide without adding BAFL, the surface free energy is larger, and its adhesion is better.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG1 is a structural formula and a ball-and-stick model of 9,9-bis(4-aminophenyl)fluorene, wherein (a) is the structural formula of 9,9-bis(4-aminophenyl)fluorene, and (b) to (d) are ball-and-stick models of 9,9-bis(4-aminophenyl)fluorene;

FIG2 is an infrared spectrum of the polyimide film obtained after thermal imidization in each embodiment;

FIG3 is an X-ray diffraction diagram of the polyimide film obtained in each embodiment;

FIG4 is a TGA curve of the polyimide film obtained in each embodiment;

FIG5 is a DTG curve of the polyimide film obtained in each embodiment;

FIG6 is a color comparison diagram of the polyimide films obtained in various embodiments;

FIG. 7 is a comparison diagram of contact angles of polyimide films obtained in various examples.

Detailed ways

The present invention provides a method for preparing a Cardo ring structure copolymer polyimide, comprising the following steps:

4,4-diaminodiphenyl ether, 9,9-bis(4-aminophenyl)fluorene, pyromellitic anhydride and an organic solvent are mixed for polycondensation to obtain polyamic acid;

The polyamic acid is thermally imidized to obtain Cardo ring structure copolymer polyimide.

The present invention mixes 4,4-diaminodiphenyl ether (ODA), 9,9-bis(4-aminophenyl)fluorene (BAFL), pyromellitic anhydride (PMDA, also known as pyromellitic dianhydride) and an organic solvent for polycondensation to obtain polyamic acid. In the present invention, the ratio of the sum of the molar amounts of the 4,4-diaminodiphenyl ether and the 9,9-bis(4-aminophenyl)fluorene to the molar amount of pyromellitic anhydride is preferably 1:1.00 to 1.05, more preferably 1:1.02; the molar amount of the 9,9-bis(4-aminophenyl)fluorene is preferably 10 to 50% of the sum of the molar amounts of the 4,4-diaminodiphenyl ether and the 9,9-bis(4-aminophenyl)fluorene, specifically 10%, 20%, 30%, 40%, 50%, more preferably 30%. In the present invention, the organic solvent preferably includes one or more of N,N-dimethylacetamide (DMAc), N,N-dimethylformamide and N-methylpyrrolidone, and more preferably N,N-dimethylacetamide; the solid content of the mixed solution obtained by mixing the 4,4-diaminodiphenyl ether, 9,9-bis(4-aminophenyl)fluorene, pyromellitic anhydride and the organic solvent is preferably 15wt%. In the present invention, the specific operation of the mixing is preferably: placing the 4,4-diaminodiphenyl ether and 9,9-bis(4-aminophenyl)fluorene in a flask, adding an organic solvent thereto, and obtaining a mixed solution of 4,4-diaminodiphenyl ether and 9,9-bis(4-aminophenyl)fluorene; and adding pyromellitic anhydride to the mixed solution of 4,4-diaminodiphenyl ether and 9,9-bis(4-aminophenyl)fluorene. In the present invention, the organic solvent is preferably added under stirring. The present invention has no special requirements for the stirring speed and time, as long as 4,4-diaminodiphenyl ether and 9,9-bis(4-aminophenyl)fluorene are fully dissolved and evenly mixed in the organic solvent; the pyromellitic anhydride is preferably added in batches, and the number of batches is preferably 4 times, and the time interval between each batch is preferably 20 to 25 minutes, ensuring that the pyromellitic anhydride added in the previous batch is completely dissolved when each batch of pyromellitic anhydride is added. In the present invention, the temperature of the polycondensation reaction is preferably 0 to 10°C, and the time is preferably 7 to 8 hours; in an embodiment of the present invention, the polycondensation reaction is specifically carried out under ice-water bath conditions. After the polycondensation reaction is completed, the present invention preferably does not perform any post-treatment and directly enters the next step of reaction, at which time the obtained polyamic acid (PAA) is a polyamic acid in the form of a viscous solution.

After obtaining the polyamic acid, the present invention performs thermal imidization on the polyamic acid to obtain Cardo ring structure copolymer polyimide. The present invention preferably lays a film of the polyamic acid on a substrate, and after vacuum degassing treatment, performs the thermal imidization. The present invention has no special requirements for the substrate. In the embodiment of the present invention, a glass plate is used; the film laying preferably adopts a casting method; the present invention preferably places the polyamic acid material after film laying in a vacuum oven for vacuum degassing treatment, and the time of the vacuum degassing treatment is based on the complete removal of bubbles. In the present invention, the thermal imidization is preferably carried out in a box-type resistance furnace; the thermal imidization preferably includes: heating from room temperature to 90°C at a heating rate of 1 to 2°C/min, and keeping warm for 1h; heating from 90°C to 120°C at a heating rate of 1 to 2°C/min, and keeping warm for 1h; heating from 120°C to 150°C at a heating rate of 1 to 2°C/min, and keeping warm for 1h; heating from 1 to 2°C/min at a heating rate of 1 to 2°C/min Heat from 50℃ to 180℃, keep warm for 1h; heat from 180℃ to 210℃ at a heating rate of 1-2℃/min, keep warm for 1h; heat from 210℃ to 240℃ at a heating rate of 1-2℃/min, keep warm for 1h; heat from 240℃ to 270℃ at a heating rate of 1-2℃/min, keep warm for 1h; heat from 270℃ to 300℃ at a heating rate of 1-2℃/min, keep warm for 1h. Thermal imidization is the process in which the carboxyl groups on the polyamic acid molecular chain are stripped of hydroxyl groups, and the carbon atoms are connected to the nitrogen atoms to gradually form an imide five-membered ring. The present invention adopts a step-by-step heating method for thermal imidization, which is conducive to the stable ring-forming reaction to obtain high-quality polyimide.

After the thermal imidization is completed, the present invention preferably cools naturally to room temperature, and then immerses the substrate composited with the polyimide film in water, and after the edge of the film is lifted up, the film is peeled off with a scraper, and then the obtained film material is dried to dry the water to obtain a polyimide film, that is, a Cardo ring structure copolymer polyimide in the form of a film material.

In the present invention, the reaction formula involved in preparing the Cardo ring structure copolymer polyimide is as follows:

In the process of preparing polyimide, the present invention introduces a large volume, asymmetric, non-coplanar structure, namely 9,9-bis(4-aminophenyl)fluorene (BAFL), into the polyimide molecular chain. The Cardo ring in the structure has a large volume, strong steric hindrance, and a high benzene ring density, which reduces the flexibility of the molecular chain, expands the chain spacing, and reduces the packing density. In addition, the non-coplanar structure destroys the regularity of the molecular chain, which is very helpful for weakening the CTC effect to synthesize light-colored PI, and can ensure the comprehensive performance of the polyimide. Figure 1 is the structural formula and ball-and-stick model of 9,9-bis(4-aminophenyl)fluorene, and Figure 1 (a) is the structural formula of 9,9-bis(4-aminophenyl)fluorene, and (b) to (d) are ball-and-stick models of 9,9-bis(4-aminophenyl)fluorene.

The present invention provides a Cardo ring structure copolymer polyimide prepared by the preparation method described in the above technical solution. In the present invention, the Cardo ring structure copolymer polyimide preferably includes a Cardo ring structure copolymer polyimide film, and the film thickness is preferably 240 to 270 μm. The Cardo ring structure copolymer polyimide provided by the present invention has excellent thermal stability (T _d5% >540°C) while weakening the CTC effect, and has excellent comprehensive performance, and can be widely used in the optoelectronic field and other fields, such as as a display screen.

The Cardo ring structure copolymer polyimide and the preparation method thereof provided by the present invention are described in detail below in conjunction with the examples, but they should not be construed as limiting the scope of protection of the present invention.

In each embodiment, the experimental drugs and experimental instruments used are as follows:

Experimental drugs:

4,4-Diaminodiphenyl ether (ODA, analytical grade, Shanghai Aladdin Reagent Co., Ltd.); pyromellitic dianhydride (PMDA, analytical grade, Shanghai Aladdin Reagent Co., Ltd.); 9,9-bis(4-aminophenyl)fluorene (BAFL, analytical grade, Tianjin Zhongtai Material Technology Co., Ltd.); N,N-dimethylacetamide (DMAc, analytical grade, Shanghai Aladdin Reagent Co., Ltd.). All drugs were dried.

laboratory apparatus:

Thermal collector constant temperature magnetic stirrer (DF-101D, Beijing Kewei Yongxing Instrument Co., Ltd.); box-type resistance furnace (SX2-10, Shenyang Energy-saving Electric Furnace Factory); Hicon ice maker (HZB-30F, Ningbo Wellcome International Industrial Co., Ltd.); contact angle meter (SL250, Kono Scientific Instrument Co., Ltd.); Fourier transform infrared spectrometer (FTIR-7600, Lambda); X-ray diffractometer (D8ADVANCE, Bruker); comprehensive thermal analyzer (STA449F5, Netzchi Scientific Instrument Co., Ltd.).

Example 1

Weigh 0.91 g of 4,4-diaminodiphenyl ether (ODA) and 0.17 g of 9,9-bis(4-aminophenyl)fluorene (BAFL) and place them in a flask, add a certain amount of DMAc and stir to fully dissolve and mix them evenly, weigh pyromellitic dianhydride (PMDA) according to the ratio and add it to the flask 4 times with an interval of 20 to 25 minutes each time, and ensure that the PMDA added last time is completely dissolved before adding PMDA each time. After PMDA is added, the molar ratio of dianhydride (PMDA) and diamine (ODA and BAFL) in the system is 1.02:1, and the solid content of the system is 15wt%. Place the flask in an ice water bath and stir to react for 8 hours to obtain a light yellow viscous polyamic acid (PAA) solution;

The PAA solution was slowly poured onto a clean glass plate and filmed by a casting method, then placed in a vacuum oven to remove air bubbles, and then placed in a box-type resistance furnace for stepwise heating for thermal imidization, with a heating rate of 1°C/min, and kept at 90°C, 120°C, 150°C, 180°C, 210°C, 240°C, 270°C, and 300°C for 1 hour each, and the thermal imidization was completed. After the temperature dropped to room temperature, the glass plate was immersed in water, and the film edge was lifted up and peeled off with a scraper, and the water was dried to obtain a Cardo ring structure copolymer polyimide in the form of a film material, which was recorded as PI ₁ . The thickness of the obtained polyimide film (PI ₁ ) was measured to be 240-270 μm.

Embodiments 2 to 5

The addition amounts of 4,4-diaminodiphenyl ether (ODA) and 9,9-bis(4-aminophenyl)fluorene (BAFL) were adjusted according to the data in Table 1, and the rest was the same as in Example 1, to obtain Cardo ring structure copolymer polyimide films, which were respectively recorded as PI ₂ , PI ₃ , PI ₄ , and PI ₅ . In Table 1, n _BAFL /n _diamine represents the percentage of the molar amount of 9,9-bis(4-aminophenyl)fluorene to the sum of the molar amounts of 4,4-diaminodiphenyl ether and 9,9-bis(4-aminophenyl)fluorene.

Comparative Example 1

The addition amounts of 4,4-diaminodiphenyl ether (ODA) and 9,9-bis(4-aminophenyl)fluorene (BAFL) were adjusted according to the data in Table 1 (the addition amount of BAFL was 0), and the rest was the same as in Example 1 to obtain a polyimide film, which was recorded as PI ₀ . The reaction formula involved is as follows:

Table 1 Amount of diamine monomer used and ratio of _nBAFL / _ndiamine in Examples 1 to 5 and Comparative Example 1

In the present embodiment, 4,4-diaminodiphenyl ether (ODA) is used as a diamine monomer, pyromellitic anhydride (PMDA) is used as a dianhydride monomer, N,N-dimethylacetamide (DMAc) is used as a solvent, and different proportions of 9,9-bis(4-aminophenyl)fluorene (BAFL) are added as a third monomer for copolymerization to prepare random copolymer PI with different BAFL contents, and then the effect of the Cardo ring on its structure and performance is characterized. The details are as follows:

(1) Fourier transform infrared spectroscopy (FTIR) characterization

The PI film was scanned in the range of 4000 to 500 cm ^-1 using a FTIR-7600 Fourier transform infrared spectrometer produced by Lambda Company of Australia with a resolution of 1.0 cm ^-1 .

FIG2 shows the infrared spectra of the polyimide films obtained in various embodiments after thermal imidization, wherein the wave numbers 1720 cm ^-1 and 1780 cm ^-1 are respectively the symmetric and asymmetric stretching vibration peaks of C=O on the imide ring, 720 cm ^-1 is the characteristic peak of the out-of-plane bending vibration of C=O, and 1370 cm ^-1 is the stretching vibration peak of the CN bond on the imide ring. The above characteristic peaks prove the formation of the imide five-membered ring in the molecular chain; while no stretching vibration absorption peak of the C=O bond in the carboxyl group is found at the wave number 1645 cm ^-1 , and no OH bond absorption peak is found in the range of 3100-3500 cm ^-1 , indicating that PAA is completely converted into PI after thermal imidization by step-wise heating to 300°C.

(2) X-ray diffraction (XRD) characterization

The PI film was scanned in the range of 5 to 80° using a D8ADVANCE X-ray diffractometer from Bruker, Germany, at a scanning speed of 10°/min to characterize the effect of BAFL on the crystallinity of PI.

FIG3 shows the X-ray diffraction patterns of the polyimide films obtained in each embodiment. As can be seen from FIG3, PI ₁ to PI ₅ films have only large broad peaks in the range of 10 to 40° without other sharp diffraction peaks, which is a characteristic of the typical amorphous structure of the polymer, indicating that the PI synthesized in Examples 1 to 5 are all amorphous; and compared with the polyimide film with BAFL added, the PI without BAFL added has a large diffraction peak in the range of 10 to 40°. The diffraction peak of ₀ is sharper. This is because BAFL, as the third monomer, destroys the original regularity of the molecular chain in the random copolymerization due to its cardo ring structure, making the molecular chain unable to arrange in order. Therefore, the XRD curve of the ternary copolymer PI is relatively flat. Among them, the XRD curve of the PI film is the flattest when the BAFL addition is 30%, indicating that its amorphous structure is most significant. The steric effect of the Cardo ring structure should affect the molecular chain spacing, but according to the Bragg equation 2dsinθ＝nλ, combined with the 2θ angle corresponding to the highest peak in the curve, it can be concluded that the molecular chain spacing of all PIs is not much different. It is speculated that although the steric effect of the Cardo ring structure will destroy the close arrangement of the molecular chains, it is easy to produce interchain conjugation with the adjacent molecular chains, which offsets part of the steric effect. Therefore, the average chain spacing does not change much. With the increase of BAFL content, the interchain conjugation and the rigid structure of BAFL cooperate with each other to enhance the orderliness of the molecular chain.

(3) Thermogravimetric analysis (TGA)

The thermal stability of the PI film was tested in the range of 25 to 800°C using a NETZSCH STA449F5 comprehensive thermal analyzer in a _N2 atmosphere with a heating rate of 20°C/min.

FIG. 4 shows the TGA curves of the polyimide films obtained in various examples, and FIG. 5 shows the DTG curves of the polyimide films obtained in various examples.

As can be seen from Figure 4, the initial decomposition temperature of all PI films is above 520°C, and the final decomposition temperature is about 650°C. The carbon residue rate increases with the increase of BAFL addition, up to 65.6% (PI ₄ ), indicating that the imidization degree is relatively high; after adding BAFL, the heat resistance of copolymerized PI is limited compared with pure PI, mainly because -NH ₂ in BAFL belongs to the para structure, and the PI with the maximum symmetric structure has higher order and correspondingly improved heat resistance. In addition, the abundant benzene rings in the Cardo ring structure are easy to form interchain conjugation with adjacent molecular chains, which helps to improve the thermal stability of copolymerized PI. It can also be seen from the DTG curve in Figure 5 that the initial decomposition temperature of all PIs is basically the same, but the temperature corresponding to the maximum thermal decomposition rate of PI copolymerized with BAFL is higher than that of PI without BAFL, which also confirms the view that the cardo ring structure helps to improve the stability of copolymerized PI.

(4) Effect of BAFL content on PI film color

Select areas where the PI film is uniform and of similar thickness and cut them into uniform shapes. Take photos to compare their appearance colors and analyze the influence of the cardo ring steric effect on the color of the PI film.

FIG6 is a color comparison diagram of the polyimide films obtained in various embodiments. From the comparison of each group of PI, it can be seen that the PI film (PI ₀ ) without adding BAFL presents a dark brown-yellow color. Because ODA and PMDA have no side groups and the structure has high symmetry, the PI molecular chain synthesized by the two also shows high symmetry, the molecular chain has good regularity, and can form an orderly arrangement. In addition, the molecular chain contains more benzene rings, and there is a strong CTC effect, which increases the absorption of visible light, so the color is darker. The color of the copolymerized PI with BAFL added first becomes lighter and then deepens with the increase of BAFL content. When the BAFL content is low, the steric effect of its Cardo ring structure destroys the regularity of the molecular chain, making it unable to arrange in order, weakening the generation of CTC, so the color of the PI film becomes lighter; when the BAFL addition amount is 30%, the effects of the two reach a balance, and the PI film is golden yellow; when the BAFL content increases again, the CTC effect brought by the interchain conjugation is enhanced, and the color of the PI film becomes darker, but it is lighter than the PI film without adding BAFL.

(5) Contact angle (CA) measurement

The contact angle of PI film to water was measured using a Kono Scientific Instruments SL250 contact angle meter. Three different points were measured in each group, and the average value was taken to analyze the effect of the Cardo ring structure on the hydrophilicity/hydrophobicity of the PI film.

Figure 7 is a contact angle comparison diagram of the polyimide films obtained in various embodiments. As can be seen from Figure 7, with the increase of BAFL content, the water contact angle of the copolymerized PI film first decreases and then increases, but it is lower than the PI film without adding BAFL. The reason is that when a small amount of BAFL is added, the random distribution and steric effect of the Cardo ring structure in the molecular chain destroy the regularity of the molecular chain, making it impossible to arrange in order, the molecular chain is loosely stacked, the free volume increases, the contact surface between the water molecules and the PI molecular chain is larger, and the carbonyl group on the imide ring is easy to form hydrogen bonds with the water molecules, thereby showing a lower contact angle; at the same time, the irregular arrangement of the molecular chain increases the roughness of the PI surface. According to Wenzel's theory, when the contact angle is <90°, the increase in surface roughness can increase the hydrophilicity of the PI film, thereby reducing the surface contact angle. When the BAFL content is 30%, its surface contact angle reaches 32.42°. The small surface contact angle indicates that the surface free energy of the copolymerized PI is larger than that of the PI film without adding BAFL, and its adhesion is better, which is very helpful for improving the adhesion of the prepared polyimide flexible copper-clad material and avoiding the coating from falling off after multiple etchings. With the increase of BAFL content, the structural rigidity and interchain conjugation effect are gradually enhanced, which improves the orderly arrangement of the molecular chains and increases the surface contact angle, which also confirms the XRD characterization results.

It can be seen from the above examples that the Cardo ring steric hindrance effect is strong, and due to the random copolymerization, the regularity of the molecular chain is destroyed and cannot be arranged in order. When the BAFL content is 30%, its amorphous state is most significant; the thermal stability is improved under the interaction of the steric effect, the interchain conjugation and the rigid structure of the Cardo ring; when the BAFL content is 30%, the surface contact angle of the copolymerized PI is the smallest, indicating that it has high surface energy and good wettability, which is helpful for preparing flexible copper-clad materials with good adhesion; the Cardo ring steric hindrance effect can inhibit CTC, but the interchain conjugation brought by the high benzene ring density enhances CTC, and the two effects are basically balanced when the BAFL content is 30%.

The above is only a preferred embodiment of the present invention. It should be pointed out that for ordinary technicians in this technical field, several improvements and modifications can be made without departing from the principle of the present invention. These improvements and modifications should also be regarded as the scope of protection of the present invention.

Claims (5)

1. A preparation method of a Cardo ring structure copolyimide comprises the following steps:

4, 4-diaminodiphenyl ether, 9-bis (4-aminophenyl) fluorene, pyromellitic anhydride and an organic solvent are mixed for polycondensation reaction to obtain polyamide acid; the ratio of the sum of the molar amounts of the 4, 4-diaminodiphenyl ether and 9, 9-bis (4-aminophenyl) fluorene to the molar amount of pyromellitic anhydride is 1:1.00-1.05; the molar amount of the 9, 9-bis (4-aminophenyl) fluorene is 10-30% of the sum of the molar amounts of the 4, 4-diaminodiphenyl ether and the 9, 9-bis (4-aminophenyl) fluorene; the temperature of the polycondensation reaction is 0-10 ℃ and the time is 7-8 h;

Carrying out thermal imidization on the polyamide acid to obtain a copolyimide with a Cardo ring structure; the thermal imidization includes: heating from room temperature to 90 ℃ at a heating rate of 1-2 ℃/min, and preserving heat for 1h; heating from 90 ℃ to 120 ℃ at a heating rate of 1-2 ℃/min, and preserving heat for 1h; heating from 120 ℃ to 150 ℃ at a heating rate of 1-2 ℃/min, and preserving heat for 1h; heating from 150 ℃ to 180 ℃ at a heating rate of 1-2 ℃/min, and preserving heat for 1h; heating from 180 ℃ to 210 ℃ at a heating rate of 1-2 ℃/min, and preserving heat for 1h; heating from 210 ℃ to 240 ℃ at a heating rate of 1-2 ℃/min, and preserving heat for 1h; heating from 240 ℃ to 270 ℃ at a heating rate of 1-2 ℃/min, and preserving heat for 1h; heating from 270 ℃ to 300 ℃ at a heating rate of 1-2 ℃/min, and preserving heat for 1h.

2. The method according to claim 1, wherein the organic solvent comprises one or more of N, N-dimethylacetamide, N-dimethylformamide and N-methylpyrrolidone.

3. The method according to claim 1, wherein the polyamic acid is deposited on a substrate, and the thermal imidization is performed after a vacuum bubble removal treatment.

4. The Cardo ring structure copolyimide prepared by the method of any one of claims 1 to 3.

5. The Cardo-structure copolyimide according to claim 4, wherein the Cardo-structure copolyimide comprises a Cardo-structure copolyimide film having a film thickness of 240 to 270 μm.