CN108129836B

CN108129836B - High-refractive-index and high-transparency polyimide film and preparation method thereof

Info

Publication number: CN108129836B
Application number: CN201711344377.9A
Authority: CN
Inventors: 张玉谦; 腾国荣
Original assignee: Tianjin Tian Yuan Electrical Materials Co ltd
Current assignee: Tianjin Tian Yuan Electrical Materials Co ltd
Priority date: 2017-12-15
Filing date: 2017-12-15
Publication date: 2020-06-12
Anticipated expiration: 2037-12-15
Also published as: CN108129836A

Abstract

The invention provides a polyimide film with high refractive index and high transparency and a preparation method thereof. Specifically, the method takes 6FDA as a dianhydride monomer, adopts a two-step method to synthesize a series of PI films, and tests the thermal property, the mechanical property, the optical property and the like of the PI films. Two kinds of PI resin with excellent performance are obtained by researching the rule of the influence of the monomer molecular structure on the performance of the PI film. Based on the PI resin, the invention further uses high-refractive-index nano TiO₂And ZrO₂As an inorganic filler, a polyimide film having both a good refractive index and excellent transparency is obtained by compounding with a polyimide substrate, respectively, and is an ideal optical film material.

Description

High-refractive-index and high-transparency polyimide film and preparation method thereof

Technical Field

The invention relates to the technical field of functional polyimide films, in particular to a high-refractive-index and high-transparency polyimide film and a preparation method thereof.

Background

Optical materials are widely applied in the fields of electronics, information, building materials, coatings and the like, and are indispensable components of modern material science and technology. Refractive Index (n) is one of the most important basic properties of optical materials, and determines whether a material can be used for optical design and applications. The refractive index of a material is the ratio of the propagation velocity of light in vacuum to the propagation velocity in the material, the higher the refractive index, the stronger its ability to refract incident light.

The high refractive index material has very important practical application value, and can reduce the thickness and curvature of an optical device, thereby enabling an optical instrument to be miniaturized and lightened under the condition of not influencing the refractive power of the optical device. Therefore, high refractive Index optical materials are widely used in optoelectronic devices, optical lenses, optical waveguides, LED packaging materials, highly reflective or anti-reflective coatings, optical films, and other fields (Tomoya Higashira, Mitsuru Ueda. recording Progress in high refractive Index polymers. macromolecules,2015,48: 1915-1929). Conventional high refractive index optical materials are classified into two broad categories, inorganic materials and organic materials. The high-refractivity inorganic material has the features of relatively high refractivity, high strength and hardness, high heat resistance, etc. and has refractivity of over 2.0, such as ZrO₂(n＝2.10)、TiO₂(n ═ 2.61), PbS (n ═ 4.19), and the like. However, inorganic materials are often not highly transparent and are difficult to process and mold, and are gradually replaced by organic high refractive index optical materials. The traditional organic polymer optical material has the advantages of light weight, good transparency, impact resistance, dyeability, easy processing and forming and the like, and has wide application in the optical industry and research.

The organic optical resin is an optical material with excellent optical performance, and can be prepared into an optical film to be applied to optical lenses and displays. However, the refractive index of organic optical resins is mostly between 1.3 and 1.6, and the refractive indices of common optical materials, including polymethacrylate (PMMA, n ═ 1.49), acryl diglycol carbonate (CR-39, n ═ 1.50), and polycarbonate (PC, n ═ 1.58), cannot meet the characteristic requirements of modern optical devices, such as miniaturization and weight reduction, and the development of organic optical materials is limited. Therefore, the development of novel high refractive index optical materials having both advantages of inorganic and organic optical materials is a major research direction in the field of optical materials.

Polyimide (PI) films are widely used in the industry today. Although the refractive index of the conventional PI film can reach 1.70, the conventional PI film is dark in color and cannot be used as an optical film. Many methods of improving the color of PI films, including the introduction of fluorine-containing groups, alicyclic groups, bulky organic groups, etc., tend to lower the refractive index of the PI films. Research shows that adding high-refractive-index inorganic nanoparticles (titanium dioxide, zirconium dioxide and the like) into a colorless and transparent PI film is an effective means for preparing an optical film with good optical transparency and high refractive index. However, if the refractive index of the bulk PI film is low, then to achieve a higher refractive index, a large amount of high refractive index inorganic nanoparticles needs to be added, which greatly impairs the optical transparency of the PI film.

In order to prepare a high refractive index inorganic nanoparticle composite Polyimide (PI) film having good transparency, it is necessary to first screen a PI matrix film itself, i.e., a PI matrix film having excellent optical transparency. The main methods for improving the optical transparency of the PI film comprise: (1) introduction of highly electronegative groups, such as trifluoromethyl (-CF), into PI molecular structures₃) Sulfone group (-SO)₂-) and the like to cut off the charge transfer from the electron donor (diamine moiety) to the electron acceptor (dianhydride moiety) between PI molecular chains and inside the molecular chains, thereby preventing the generation of a Charge Transfer Complex (CTC); (2) an alicyclic structure is introduced into the PI molecular structure, particularly a diamine structural unit, so that the conjugation of an aromatic ring is reduced, and the generation of CTC can be cut off. Both the two schemes can effectively improve the optical transparency of the PI film. However, the first embodiment is more suitable as a high refractive index PI substrate in view of the heat resistance stability and refractive index of the film. In practical application, the fluorine-containing PI film, especially the PI film based on fluorine-containing dianhydride 4,4' -hexafluoroisopropyl bis (phthalic anhydride) (6FDA) has good optical transparency and excellent heat-resistant stability. Although the introduction of trifluoromethyl makes the refractive index of such films low, by using a diamine monomer having a specific structure, it is possible to compensate for the problem of the decrease in refractive index of the dianhydride moiety due to the introduction of the fluorine-containing group while maintaining the excellent optical properties of the PI film.

Refractive index is one of the most fundamental properties of optical thin film materials. According to classical electromagnetic theory, the refractive index of a material can be obtained from the following Lorentz-Lorenz equation:

wherein R is molar refraction; n is the refractive index; v is the molar volume.

As can be seen from the above relationship, the refractive index is inversely proportional to the molecular volume, proportional to the molar refraction, and proportional to the dielectric polarizability. Therefore, in order to increase the refractive index of the polymer thin film, the medium is required to have a large polarizability and a small molecular volume. The refractive index of the optical film is increased in the order of branched chain < straight chain < alicyclic ring < aromatic ring. In addition, halogen (Cl, Br), sulfur atoms, sulfone groups, condensed rings, heavy metal ions, and the like are introduced into the molecule to improve the refractive index. The molar refractive indices of common atoms or groups are given in table 1.

TABLE 1 molar refractive index R of common atoms or groups

Light transmittance is another important property indicator of optical film materials. As described above, the rigid structure of the polyimide polymer chain largely sacrifices the transmittance in the visible light region. Research shows that flexible ether bond, thioether bond, methylene and other structures are introduced into the aromatic heterocyclic polymer, so that separation of chromophoric groups can be facilitated, the intermolecular complexation effect is reduced, and the visible light transmittance is effectively improved. In addition, external factors such as the purity of the PI resin, solvent residues in the resin, the film-forming process, etc. will also affect the color of the film.

Disclosure of Invention

The invention aims to provide a polyimide film with high refractive index and high transparency and a preparation method thereof aiming at overcoming the technical defects of the prior art, and aims to solve the technical problem that the conventional PI film in the prior art is difficult to have good transparency and high refractive index at the same time.

In order to achieve the technical purpose, the invention adopts the following technical scheme:

the high-refractive-index and high-transparency polyimide film has an infrared spectrum of 3500-3100 cm^-1No N-H absorption peak of carboxyl, amide or amino groups in the range; the film has an infrared spectrum with characteristic peaks for absorption of carbonyl C ═ O vibrations on imidization, including 1787cm^-1The asymmetric stretching vibration peak of carbonyl group, 1724cm^-1A carbonyl symmetric stretching vibration absorption peak; the infrared spectrum of the film is 1375cm^-1Has an imide C-N bond stretching vibration absorption peak; the infrared spectrum of the film was 721cm^-1Having a bending vibration peak of the imide ring.

Preferably, the film has an infrared spectrum of 2959cm^-1、2930cm^-1And 2866cm^-1Has a stretching vibration absorption peak of methyl and methylene saturated C-H bonds.

Preferably, the film has an infrared spectrum of 3360cm^-1Has a stretching vibration absorption peak of an N-H bond of an imidazole ring.

Preferably, the film has an infrared spectrum of 3366cm^-1And 3420cm^-1Has an N-H stretching vibration absorption peak with a-CO-NH-bond.

Meanwhile, the invention provides a preparation method of the polyimide film, which comprises the following steps: the method comprises the following steps:

1) adding 0.1mol of first material and 300g of DMAc into a reaction container, and stirring for 0.5h at the temperature of 15-25 ℃ to obtain a transparent solution; adding 0.1mol of 6FDA, flushing a feeding vessel with 38g of DMAc, adding the eluate into a reaction container, and reacting at room temperature for 24 hours to obtain a viscous solution; adding 0.1mol of acetic anhydride and 0.33mol of pyridine into the mixture to carry out imidization reaction for 24 hours; pouring the product into 80% ethanol solution to obtain yellowish continuous filaments, soaking in ethanol solution overnight, and replacing with ethanol once; taking out the filaments, air-drying at room temperature, then drying at 120 ℃ under reduced pressure overnight, vacuumizing for a plurality of times, and cooling to obtain a light yellow filament product, namely PI resin; wherein the first material is selected from one of APBI, APBO, DABA or Cl-DABA;

2) weighing 5g of the solid of the PI resin, dissolving the solid in 20g of DMAc solvent, and preparing a PI solution with the solid content of 20 wt%;

3) filtering the PI solution by a G1 sand core funnel to remove impurities to obtain a PI film preparation solution;

4) pouring the PI film-making solution on a glass plate, and coating the PI film-making solution on an automatic film coating machine to form a uniform coating;

5) placing the glass plate in a drying box according to the proportion of 80 ℃/3h, 150 ℃/1h and 180 ℃/1 h; heating at 250 deg.C/1 h, removing DMAc solvent and solidifying PI;

6) and cooling to room temperature, soaking the glass plate in water, stripping off the PI coating and drying to obtain the polyimide film.

Another high refractive index, high transparency polyimide film having an infrared spectrum of 1778cm^-1、1717cm^-1、1375cm^-1Has characteristic absorption peaks.

Preferably, the film has an infrared spectrum of 797cm^-1Has a weak absorption peak of Ti-O bonds.

Preferably, the film has an infrared spectrum of 964cm^-1Has a weak absorption peak of Zr-O bond.

Meanwhile, the invention provides a preparation method of the polyimide film, which comprises the following steps:

1) adding 0.1mol of first material and 300g of DMAc into a reaction container, and stirring for 0.5h at the temperature of 15-25 ℃ to obtain a transparent solution; adding 0.1mol of 6FDA, flushing a feeding vessel with 38g of DMAc, adding the eluate into a reaction container, and reacting at room temperature for 24 hours to obtain a viscous solution; adding 0.1mol of acetic anhydride and 0.33mol of pyridine into the mixture to carry out imidization reaction for 24 hours; pouring the product into 80% ethanol solution to obtain yellowish continuous filaments, soaking in ethanol solution overnight, and replacing with ethanol once; taking out the filaments, air-drying at room temperature, then drying at 120 ℃ under reduced pressure overnight, vacuumizing for a plurality of times, and cooling to obtain a light yellow filament product, namely PI resin; wherein the first material is selected from one of APBO or Cl-DABA;

2) TiO is preliminarily treated₂Or ZrO₂Drying the nanoparticles at 100 ℃ for 3h, and filling into a reagent bottle;

3) weighing 40g of the PI resin, adding the PI resin into the reagent bottle, adding a DMAc solvent to 200g of the PI resin, and fully dissolving to prepare a PI resin solution with 20% of solid content;

4) weighing nanoparticles, ultrasonically dispersing the nanoparticles in 10g of DMAc solvent, and adding a drop of KH-550 coupling agent to obtain a nanoparticle dispersion liquid;

5) mixing 30g of the PI resin solution with the nanoparticle dispersion liquid, and putting the mixture into a mixing defoaming machine for fully mixing and defoaming to obtain a film-making solution with the PI content of 15%;

6) pouring the film-making solution on a glass plate, and coating the glass plate with an automatic film coating machine to form a uniform coating;

7) placing the glass plate in a drying box according to the proportion of 80 ℃/3h, 150 ℃/1h and 180 ℃/1 h; heating at 250 deg.C/1 h, and removing DMAc solvent to solidify;

8) and cooling to room temperature, soaking the glass plate in water, stripping off the coating and drying to obtain the polyimide film.

Preferably, the operation procedure of the mixing defoaming machine in the step 5) is as follows: firstly mixing for 30s at the rotating speed of 1800 r/min; then defoaming for 30s at 2000 r/min.

The invention provides a polyimide film with high refractive index and high transparency and a preparation method thereof.

Specifically, the method takes 6FDA as a dianhydride monomer, adopts a two-step method to synthesize a series of PI films, and tests the thermal property, the mechanical property, the optical property and the like of the PI films. By researching the influence rule of the monomer molecular structure on the performance of the PI film, the following conclusion is obtained:

1) the 6 FDA-based PI film has excellent heat-resistant stability, 5% weight loss temperature of the film exceeds 520 ℃, glass transition temperature of the film exceeds 345 ℃, and CTE of the film is less than 35 multiplied by 10 at 50-200 DEG C^-6/℃；

2) The 6 FDA-based PI film has excellent optical transparency, and the light transmittance at 500nm is over 84 percent. PI-2 prepared from 6FDA with 2- (4-aminophenyl) -5-aminobenzoxazole and PI-4 prepared from 6FDA with 2-chloro-4, 4' -diaminobenzanilide have the highest refractive index and good optical transparency.

Based on the above studies, it was found that a PI resin prepared from APBO or Cl-DABA as a raw material has a refractive index, and a nanocomposite film having a higher refractive index is expected to be obtained from the PI resin as a substrate.

Based on the PI resin, the invention further uses high-refractive-index nano TiO₂And ZrO₂As an inorganic filler, the composite film is compounded with a polyimide matrix respectively, and the influence rule of the content of the nano particles on the properties of the PI nano composite film such as the refractive index and the like is researched. By analyzing the results and data of the study, the following conclusions were drawn:

1) the introduction of the nano particles can not obviously influence the chemical structure and the thermal property of the PI nano composite film;

2) the introduction of the nano particles can improve the refractive index of the PI nano composite film, and the refractive index of the PI nano composite film is gradually increased along with the increase of the content of the nano particles;

the introduction of the nano particles can reduce the transparency of the PI nano composite film, but the PI nano composite film still has good light transmittance on the practical application thickness (0.1-1 mu m).

The polyimide film prepared by the invention has good refractive index and excellent transparency, and is an ideal optical film material.

Drawings

FIG. 1 is a chemical reaction formula of PI resin synthesis in example 1 of the present invention;

FIG. 2 is a FT-IR curve of a PI film in example 1 of the present invention;

FIG. 3 is a TGA curve of a PI film of example 1 of the present invention;

FIG. 4 is a DSC curve of a PI film in example 1 of the present invention;

FIG. 5 is a TMA curve of a PI film in example 1 of the present invention;

FIG. 6 is a graph showing the yellowness index of a PI film in example 1 of the present invention;

FIG. 7 is a UV-Vis spectrum curve of the PI film in example 1 of the present invention;

FIG. 8 is a graph comparing the refractive indices of PI films in example 1 of the present invention;

FIG. 9 shows PI-4/TiO in example 2 of the present invention₂FT-IR spectrum of the nano composite film;

FIG. 10 shows PI-4/ZrO in example 2 of the present invention₂FT-IR spectrum of the nano composite film;

FIG. 11 shows PI-2 and PI-2-TiO in example 2 of the present invention₂-10 TGA profile of a nanocomposite film;

FIG. 12 shows PI-4 and PI-4-TiO in example 2 of the present invention₂-TGA profile of 10 nanocomposite films.

Detailed Description

Hereinafter, specific embodiments of the present invention will be described in detail. Well-known structures or functions may not be described in detail in the following embodiments in order to avoid unnecessarily obscuring the details.

Approximating language, as used herein in the following examples, may be applied to identify quantitative representations that could permissibly vary in number without resulting in a change in the basic function. Accordingly, a numerical value modified by a language such as "about", "left or right" is not limited to the precise numerical value itself. In some embodiments, "about" indicates that the value allowed for correction varies within plus or minus ten percent (10%), for example, "about 100" indicates that any value between 90 and 110 is possible. Further, in the expression "about a first value to a second value", both the first and second values are corrected at about the same time. In some cases, the approximating language may be related to the precision of a measuring instrument.

Unless defined otherwise, technical and scientific terms used in the following examples have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs.

The test reagent consumables used in the following examples are all conventional biochemical reagents unless otherwise specified; the experimental methods are conventional methods unless otherwise specified; in the quantitative tests in the following examples, three repeated experiments are set, and the results are averaged; in the following examples,% is by mass unless otherwise specified.

Example 1

1. Reagent and apparatus

1.1 starting materials and reagents

The raw materials and reagents required for the preparation of PI resins are shown in table 2.

TABLE 2 raw materials for the synthesis of PI resins and their chemical structures

Other commercial reagents were used as received.

1.2 Experimental instruments

Three-mouth bottle, beaker, reagent bottle, analytical balance, electric stirrer, AFA-II automatic coating device, electric heating drying box.

1.3 testing and characterization

Intrinsic viscosity of PI resin [ η]Measuring with Ubbelohde viscometer at 25 deg.C, wherein the sample is polymer solution with concentration of 0.5g/dL, and NMP is solvent; thermogravimetric analysis (TGA) was measured using a thermal analyzer STA8000 PerkinElmer, USA, with a temperature rise rate of 20 ℃/min and a test environment of nitrogen or air; thermomechanical analysis (TMA) adopts a Q400 analyzer of TA company, the heating rate is 5 ℃/min, and the test environment is nitrogen; the differential thermal analysis (DSC) is measured by adopting a DSC 214 differential scanning calorimeter of German NETZSCH company, the test temperature is 20-450 ℃, and the heating rate is 20 ℃/min; the tensile property test is carried out on an Instron 3365 universal tester according to the national standard GB1447-83, the drawing rate is 5.0mm/min, and the sample size is 120 multiplied by 10 multiplied by 0.05mm³。

The Fourier infrared Spectrum (FT-IR) is measured by a Spectrum One Fourier transform infrared spectrometer, and the measuring range is 4000-400 cm^-1(ii) a Ultraviolet-visible spectrum(UV-Vis) is measured by a Hitachi U3900 spectrophotometer, the scanning range is 190-800 nm, and the thickness of a film sample is 8-12 mu m; the refractive index was measured by depositing the polymer on a 3 inch silicon wafer using a Sairon Tech Model SPA-4000 prism coupler at a wavelength of 632.8 nm. In-plane (n)_TE) And out of plane (n)_TM) The refractive index is measured using a linearly polarized laser parallel or perpendicular to the film plane, respectively. Average refractive index n_avIs defined as n_av＝(2n_TE+n_TM)/3. The birefringence Δ n is defined as Δ n ═ n_TE-n_TM. The optical permittivity (. epsilon.) is estimated from the refractive index according to Maxwell's equation, where ε is 1.1n_av ². The yellowness index and turbidity were measured using an American X-rite color tester.

The solubility test method is as follows: at room temperature, 1.0g of the polymer was dissolved in 9.0g of an organic solvent (10% by weight of solid content), and the polymer was magnetically stirred for 24 hours to observe the dissolution of the polymer. The dissolution profile is divided into three categories: total dissolution (++), partial dissolution (+) and insoluble (-). Complete dissolution (++) means that the resin is completely dissolved in the solvent to form a homogeneous and clear solution, and no phase separation, precipitation or gelation occurs.

2 preparation of polyimide film

2.1 Synthesis of polyimide resin

The first work in making PI films is to synthesize PI resins, as shown in fig. 1. PI-1-PI-4 is prepared by adopting a 6FDA dianhydride monomer and rigid heteroatom-containing aromatic diamine monomers APBI, APBO, DABA and Cl-DABA through a two-step chemical imidization method. Firstly, dissolving a diamine monomer in DMAc subjected to reduced pressure distillation, cooling a reactor to below 10 ℃ by using an ice water bath, adding an equivalent amount of dianhydride monomer in batches, reacting for 3 hours, removing the ice water bath, and naturally heating to room temperature; and (3) carrying out condensation reaction on diamine and dianhydride monomer for 24h to obtain polyamic acid, then adding acetic anhydride and pyridine, and carrying out chemical imidization to cyclize and dehydrate PAA to generate PI.

The procedure for the synthesis of PI resin is illustrated by PI-1(6 FDA/APBI):

1) 22.426g of APBI and 300g of DMAc are added into a 1000mL three-necked bottle with mechanical stirring, a thermometer and a cold water bath, and a transparent solution is obtained after stirring for 0.5h, wherein the temperature of a reaction system is about 20 ℃;

2) 44.424g of 6FDA is added, 38g of DMAc is added to wash the feeding funnel, the medicine is ensured to be completely added into the reaction system, and the reaction is carried out for 24 hours at room temperature, so as to obtain viscous solution;

3) adding 102.09g of acetic anhydride and 26.1g of pyridine into a reaction system, and carrying out chemical imidization for 24 hours, wherein a large amount of white floccules appear in the reaction system at first, and the floccules are gradually dissolved along with the stirring to obtain a transparent solution;

4) slowly pouring the obtained viscous solution into 80% ethanol solution to obtain yellowish continuous filament, soaking in ethanol solution overnight, and replacing with ethanol once;

5) drying the filaments at room temperature, then placing the filaments in a vacuum oven, drying the filaments at 120 ℃ under reduced pressure overnight, vacuumizing the vacuum oven for several times, and cooling to obtain a light yellow filament product, namely PI-1 resin;

6) PI-2, PI-3 and PI-4 resins were prepared in the same manner.

The amounts of starting materials and reagents required for the synthesis of PI-1 to PI-4 are shown in Table 3.

TABLE 3 PI resin Synthesis formulations

The intrinsic viscosity numbers of the PI-1 to PI-4 resins are measured by an Ubbelohde viscometer and respectively 0.76 dL/g, 0.85 dL/g, 0.92 dL/g and 1.06dL/g, which show that the resins have higher molecular weights, and the PI solution obtained by polymerization is precipitated in absolute ethyl alcohol to obtain the filiform PI resin.

2.2 preparation of polyimide film

The preparation process of the PI film is as follows:

1) weighing 5g of PI resin solid, dissolving the PI resin solid in 20g of DMAc solvent, and preparing a PI solution with the solid content of 20 wt%;

2) filtering the PI solution by a G1 sand core funnel to remove impurities to obtain a PI film preparation solution;

3) pouring the PI film-making solution on a clean glass plate, and coating the solution on an automatic film coating machine to form a uniform coating;

4) placing the glass plate in a clean drying box according to the proportion of 80 ℃/3h, 150 ℃/1h and 180 ℃/1 h; heating at 250 deg.C/1 h, removing DMAc solvent and solidifying PI;

5) after cooling to room temperature, the glass plate was immersed in water, and the PI coating was carefully peeled off and dried to obtain a flat, self-supporting PI film.

2.3 results and discussion

The PI film is subjected to structural characterization by using an infrared technology (IR), and the dissolution property, the thermal property, the mechanical property and the optical property of the PI film are respectively tested and analyzed.

2.3.1 structural characterization

FIG. 2 shows the IR spectra of the PI-1 to PI-4 films. It can be seen that the distance is 3500-3100 cm^-1No obvious N-H absorption peak of carboxyl, amido or amino is seen nearby, which indicates that the imidization degree of PAA converted into PI is higher. The characteristic peaks of the C ═ O vibrational absorption of the carbonyl group at imidization, including 1787cm, can be clearly observed in the figure^-1The left and right are asymmetric stretching vibration peak of carbonyl group, 1724cm^-1The left and right sides are symmetric stretching vibration absorption peaks of carbonyl. In addition, 1375cm^-1The vicinity is an imide C-N bond stretching vibration absorption peak, 721cm^-1Is the peak of flexural vibration of the imide ring.

In addition to the common characteristic absorption peaks described above, the series of PIs also exhibit individual characteristic absorption peaks. For example PI-1 at 2959cm^-1、2930cm^-1And 2866cm^-1The stretching vibration absorption peak of methyl and methylene saturated C-H bond appears; PI-2 at 3360cm^-1A stretching vibration absorption peak of an N-H bond of an imidazole ring appears; PI-4 and PI-4 are respectively at 3366cm^-1And 3420cm^-1N-H stretching vibration absorption peaks of-CO-NH-bonds appear at the positions. This indicates that we successfully produced PI resins with the expected structure.

2.3.2 solubility Properties

Since the standard PI has a strong conjugation between its molecular chains, it is difficult to dissolve the standard PI in an organic solvent. In practice, a soluble polyamic acid solution (PAA) is generally used. PAA further dehydrates during heating to form an insoluble PI coating. For example, in the passivation process of an integrated circuit chip, a PAA solution is widely coated on the chip, and then the PI film is obtained by performing imidization after the PAA solution is heated to about 300 ℃. However, PAA needs to remove small molecular water to complete imine ring during thermal curing, and the overflow of small molecular water is liable to generate defects such as tiny pinholes in the PI film, thereby adversely affecting the performance of the PI film. Imidization temperatures above 300 ℃ can also adversely affect temperature sensitive devices. In addition, due to the difference of the thermal expansion coefficients of the materials at high temperature, internal stress is easily generated between the PI film and other substrate materials in the cooling process, so that the reliability of the device is affected. Therefore, the development of soluble PI, especially PI that can be dissolved in conventional low-boiling organic solvents, and the goal of achieving low-temperature solidification thereof have been hot issues in the field of functional PI research.

The PI resin prepared in the research contains huge hexafluoroisopropyl structures, flexible ether bonds and other groups in the molecular structure, so that the molecular chain is loose in stacking and has higher free volume, and the penetration and diffusion of a solvent are facilitated, so that the solubility of PI is increased. We tested the solubility of this series of PI resins in N-methylpyrrolidone (NMP), DMAc, chloroform, cyclopentanone, tetrahydrofuran and acetone at room temperature (25 ℃ C.) with a solids content of 10 wt%. The test results are shown in Table 4 (+ +, completely dissolved at room temperature; + -. dissolved upon heating;. insoluble). It can be seen that in NMP and DMAc, the four PIs all have good solubility properties, which benefit from the spatial non-coplanar isomeric structure of 6FDA itself, resulting in loose molecular chains of the PIs that are easily penetrated by solvent molecules.

TABLE 4 solubility Properties of PI resins

2.3.3 thermal Properties

We performed thermogravimetric analysis (TGA), differential thermal analysis (DSC) and thermomechanical analysis (TMA) on the prepared PI-1 to PI-4 thin films, respectively. The TGA, TMA and DSC curves of the PI films are given in FIGS. 3-5, respectively, and the thermal performance data are summarized in Table 5.

TABLE 5 thermal Properties of PI films

It can be seen that the PI film prepared by the method has excellent heat resistance, and mainly comprises the following points:

1) the 5% weight loss temperature of several PI films in the air exceeds 520 ℃; the residual weight of the film is still over 20% by weight up to 650 ℃; the PI film is not decomposed completely until the temperature is above 700 ℃.

2) Glass transition temperature (T) of four PI films_g) Both above 345 deg.c, mainly due to the rigid structure of the diamine part of the PI molecule. T of four PIs_gIn the order PI-4<PI-2<PI-3<And (3) PI-1. PI-1 shows the highest T_gBecause the rigid benzimidazole ring and N-H bond of the imidazole ring are easy to form hydrogen bond with the carbonyl of the imide ring, the molecular structure is most stable.

3) PI films also exhibit a low Coefficient of Thermal Expansion (CTE). In general, 6 FDA-based PI films have a relatively high coefficient of thermal expansion (>45×10^-6/° c), the aromatic diamines selected for use in this study all have a relatively rigid backbone structure, and therefore the PI films prepared exhibit relatively low CTE values. The CTE sequence of the PI film is PI-4<PI-3<PI-2<And (3) PI-1. Because the molecular structures of PI-4 and PI-4 contain amido bonds, the CTE values of the PI-4 and the PI-4 within the range of 50-200 ℃ are only 27.8 multiplied by 10 respectively^-6/° C and 29.2 × 10^-6The high-temperature dimensional stability is excellent, and the high-temperature dimensional stability is significant for the application of the high-temperature dimensional stability in the optical field.

2.3.4 optical Properties

We first compared the transparency of the PI films prepared in appearance. The 6 FDA-based PI film prepared by the method has better optical transparency than the commercial PI film. In contrast, PI-2 is most optically transparent than PI-4, especially PI-2 is almost colorless and transparent. This is mainly due to the fact that alicyclic units in their molecular structure effectively prevent conjugation of the electron cloud. The PI-1 contains rigid conjugated benzimidazole rings, so that the visible light is remarkably absorbed, and the film shows light yellow. Compared with PI-4, PI-3 shows a light pink color because the molecular structures of the two films both contain amido bonds (-CONH-), and the two films can absorb visible light to a certain extent. However, the PI-4 molecular structure contains C-Cl bonds, which have high electronegativity and can cut off the conjugation of electron cloud to a certain extent, so that the PI-4 film has good transparency.

Next, we tested the yellowness and haze indices of the PI films, as shown in fig. 6. The yellowness index represents the degree of yellow color of the material, the yellowness indexes of four PI films are all very low, the change rule is that PI-4 is less than PI-2, less than PI-3 is less than PI-1, and the largest PI-1(15.93) is far smaller than that of a common commercial PI film (Kapton: 90). The change rule of the haze index (haze) is that PI-1< PI-4< PI-3< PI-2, and although the haze of PI-2 and PI-4 with lighter colors is not the lowest in the whole system, the haze index can still meet the requirement of practical application.

We further tested the UV-Vis spectrum (UV-Vis) of the PI film, which is shown in fig. 7. It can be seen that the ultraviolet cut-off wavelengths of the four PI films are less than 400nm, and the light transmittance at the wavelength of 500nm exceeds 80%. In contrast, the transmittance of PI-2 films is the worst, mainly due to the absorption of visible light by the imidazole ring. The light transmittance of the four PI films at the wavelength of 450nm is changed into PI-1, PI-4, PI-3, PI-4 and PI-2. Although the PI-1 film has optimal optical transparency, the mechanical property of the PI-1 film is poor, so that the PI-3 and PI-4 films have higher practical application value.

Finally, we tested the refractive index of the PI films, as shown in fig. 8. As can be seen, the average refractive index (n) of the four PI films_av) The change rule is PI-2>PI-4>PI-3>And (3) PI-1. PI-2 has the highest refractive index, on one hand, because benzoxazole has a higher molar refractive index and a smaller molar volume, the R/V value is higher; the-CONH-and C-Cl bonds in the PI-4 structure also have a high molar refractive index, and thus the refractive index of PI-4 is also high. PI-2 and PI-4 have been developed to have a higher refractive index than optical films commonly used at present, such as polymethacrylate (PMMA, n ═ 1.49), polycarbonate (PC, n ═ 1.58), and the like.

As a set of control, in this example, according to the preparation method in sections 2.1 and 2.2 above, the APBI component in the above is replaced by the conventionally used fluorinated diamine 2,2 '-bis (trifluoromethyl) -4,4' -diaminobiphenyl (TFDB), and the other conditions are the same, and a PI film of the following structure is prepared, the test refractive index is 1.5560, and the light transmittance at 450nm wavelength is 80.3%. The refractive index is lower than that of the PI film prepared by using 6FDA and rigid diamine.

As another set of control, in this example, according to the above preparation methods in sections 2.1 and 2.2, the APBI component in the above is replaced by the conventionally used fluoro-diamine 2, 2-bis [4- (4-aminophenoxy) phenyl ] hexafluoropropane (BDAF) in the prior art, and the same conditions are applied, so that the PI film having the following structure is prepared, the test refractive index is 1.5800, and the light transmittance at 450nm wavelength is 80.2%. The refractive index is lower than that of the PI film prepared by using 6FDA and rigid diamine.

Therefore, the PI film prepared by the method has optical properties such as light transmittance, refractive index and the like which are comprehensively superior to those of the prior art.

We summarize the optical properties of the four PI films prepared in table 6. In view of all the properties, PI-2 and PI-4 have higher refractive index and transparency, lower yellowness index and optimal comprehensive properties.

TABLE 6 optical Properties of PI films

2.4 summary

A series of PI films are synthesized by using 6FDA as a dianhydride monomer and adopting a two-step method, and the thermal property, the mechanical property, the optical property and the like of the PI films are tested. By researching the influence rule of the monomer molecular structure on the performance of the PI film, the following conclusion is obtained:

Based on the above conclusions, we intend to select PI-2 and PI-4 as the substrate in the subsequent work to prepare organic/inorganic nanocomposite films. Since PI-2 and PI-4 have high refractive indexes, a nano composite film with a higher refractive index can be expected to be prepared by using the PI-2 and the PI-4 as substrates.

Example 2

Preparation of 1 high-refractive-index polyimide nano composite film

1.1 introduction to

In general, the refractive index of a polymer nanocomposite can be calculated by the following equation:

wherein n is_comp、n_pAnd n_orgRespectively representing the refractive indexes of the nano composite film, the inorganic nano particles and the organic matrix; phi is a_pPhi and phi_orgRespectively representing the volume fractions of the inorganic nanoparticles and the organic matrix;

represents the weight percentage of the nanoparticles; rho_pAnd rho_orgRespectively, the density of the inorganic nanoparticles and the organic matrix. According to this formula, the volume fraction and weight fraction of the inorganic high refractive index nanoparticles that need to be added to achieve a particular refractive index can be calculated. It can also be seen that to achieve a particular refractive index, the higher the bulk refractive index of the organic polymer matrix, the smaller the volume or weight fraction of inorganic nanoparticles that needs to be added.

In the previous work, two resin systems of PI-2 and PI-4 with good dimensional stability, heat resistance, mechanical property, transparency and refractive index are successfully screened out. The two film substrates and the nano TiO are utilized in this chapter₂Or ZrO₂And compounding to prepare the PI nano composite film with high refractive index.

1.2 reagents and instruments

TABLE 7 raw materials for preparing PI nanocomposite films

Other commercial reagents were used as received.

1.3 preparation of polyimide nanocomposite films

The preparation process of the PI nano composite film is as follows (PI-2-TiO)₂；PI-4-TiO₂；PI-4-ZrO₂)：

1) TiO is preliminarily treated₂And ZrO₂Drying the nanoparticles at 100 deg.C for 3h, and bottling;

2) weighing 40g of PI-2 in a reagent bottle, adding DMAc solvent to 200g of the solution to be fully dissolved, preparing a PI-2 solution with 20% of solid content, and preparing a PI-4 solution with 20% of solid content in the same way;

3) a certain amount of nano particles are weighed and ultrasonically dispersed in 10g of DMAc solvent, and a drop of KH-550 coupling agent is added;

4) mixing 30g of PI solution with the nanoparticle dispersion solution, and placing the mixture into a mixing defoaming machine to be fully mixed and defoamed (firstly mixing for 30s, and rotating speed is 1800 r/min; defoaming for 30s at the rotating speed of 2000r/min) to obtain a uniform film-making solution with the PI content of 15%;

5) pouring the PI film-making solution on a clean glass plate, and coating the solution on an automatic film coating machine to form a uniform coating;

6) placing the glass plate in a clean drying box according to the proportion of 80 ℃/3h, 150 ℃/1h and 180 ℃/1 h; heating at 250 deg.C/1 h, and removing DMAc solvent to solidify;

7) after cooling to room temperature, the glass plate was immersed in water, the coating was carefully peeled off and dried to give a flat, self-supporting PI nanocomposite film.

Tables 8 and 9 for preparing PI/TiO, respectively₂And PI/ZrO₂The amount of raw materials required for the nanocomposite film.

TABLE 8 PI/TiO₂Raw materials and dosage of nano composite film

TABLE 9 PI/ZrO₂Raw materials and dosage of nano composite film

1.4 results and analysis

We also performed structural characterization, thermal performance and optical performance test analysis on the prepared PI nanocomposite film.

1.4.1 structural characterization

We performed infrared spectroscopy (FT-IR) testing and analysis of the structure of PI nanocomposite films. FIGS. 9 and 10 show PI-4/TiO, respectively₂And PI-4/ZrO₂FT-IR spectrum of the nano composite film.From the result, the PI can be accurately identified to be 1778cm^-1、1717cm^-1、1375cm^-1The characteristic absorption peak indicates that the imidization of the PI nano composite film is relatively complete. Further, it can be seen from FIG. 9 that the Ti-O bond is located at 797cm^-1A weak characteristic absorption peak at (B), while the Zr-O bond is located at 964cm in FIG. 10^-1The weak characteristic absorption peak indicates that the nano particles are successfully introduced into the PI matrix.

1.4.2 thermal Properties

Typical loading of 10% TiO in thermal performance₂PI-3-TiO of₂-10 and PI-4-TiO₂The TGA curves of-10 are shown in FIGS. 11 and 12, respectively. It can be seen that the 5% weight loss temperature of the PI film is almost unchanged before and after loading the nanoparticles, indicating that the introduction of the nanoparticles has no significant effect on the thermal properties of the PI film. For PI-2-TiO₂For-10 films, the residual weight at 750 ℃ was 8.29 wt%, closer to the theoretical loading of 10 wt%. For PI-4-TiO₂The same is true for-10 films, which have a residual weight of 9.83 wt% at 750 ℃ and are closer to the theoretical loading of 10 wt%.

1.4.3 optical Properties

With PI-2/TiO₂Taking a nano composite film as an example, we compared the transparency of PI nano composite films with different nano particle contents in appearance. For a nanocomposite film with a thickness of 25 μm, no TiO was added₂The PI-3 film of (B) is almost completely transparent, with the addition of TiO₂Then, the transparency of the PI nano composite film is in TiO₂The content of the additive is less than 15 percent, and the additive can be well maintained. Therefore, the introduction of a certain content of nanoparticles does not have a significant effect on the optical transparency of the PI composite film. In practical applications of high refractive index optical materials, such as microlens materials for image sensors, the thickness of the material is generally only 0.1 to 1 μm. High content of TiO₂The introduction of the nano particles can reduce the transparency of the PI nano composite film, but under the condition of relatively thin thickness, the light transmittance of the film in a visible light region can also reach more than 85%.

Table 10 shows the refractive index of the prepared PI nanocomposite film. It can be seen that the larger the content of the nanoparticles in the PI composite film, the higher the refractive index thereof, and the optical dielectric constant thereof is increased. The birefringence of the composite film is less than 0.1.

TABLE 10 refractive index of polyimide nanocomposite films

In summary, the refractive index of the PI nanocomposite film gradually increases with the increase of the content of the nanoparticles in the PI nanocomposite film.

1.5 summary

On the basis of the previous part of experiment, PI-2 and PI-4 with good comprehensive performance are selected as polymer matrixes for preparing polyimide nano composite films, and high-refractive-index nano TiO is adopted₂And ZrO₂As an inorganic filler, the composite film is compounded with a polyimide matrix respectively, and the influence rule of the content of the nano particles on the properties of the PI nano composite film such as the refractive index and the like is researched. By analyzing the results and data of the study, the following conclusions were drawn:

3) the introduction of the nano particles can not obviously influence the chemical structure and the thermal property of the PI nano composite film;

4) the introduction of the nano particles can improve the refractive index of the PI nano composite film, and the refractive index of the PI nano composite film is gradually increased along with the increase of the content of the nano particles;

The embodiments of the present invention have been described in detail, but the description is only for the purpose of illustrating the preferred embodiments of the present invention and is not to be construed as limiting the invention. Any modification, equivalent replacement, and improvement made within the scope of the application of the present invention should be included in the protection scope of the present invention.

Claims

1. A high-refractive-index, high-transparency polyimide film characterized in that: red color of the filmThe external spectrum is 1778cm^-1、1717 cm^-1、1375 cm^-1Has a characteristic absorption peak;

the film is prepared by a preparation method comprising the following steps:

1) adding 0.1mol of a first material and 300g N, N-dimethylacetamide (DMAc) into a reaction container, and stirring for 0.5h at the temperature of 15-25 ℃ to obtain a transparent solution; adding 0.1mol of 4,4' - (hexafluoroisopropylidene) diphthalic anhydride 6FDA, flushing a feeding vessel with 38g N, N-dimethylacetamide (DMAc), adding the eluate into a reaction container, and reacting at room temperature for 24 hours to obtain a viscous solution; adding 0.1mol of acetic anhydride and 0.33mol of pyridine into the mixture to carry out imidization reaction for 24 hours; pouring the product into 80% ethanol solution to obtain yellowish continuous filaments, soaking in ethanol solution overnight, and replacing with ethanol once; taking out the filaments, air-drying at room temperature, then drying at 120 ℃ under reduced pressure overnight, vacuumizing for a plurality of times, and cooling to obtain a light yellow filament product, namely PI resin; wherein the first material is 2-chloro-4, 4' -diaminobenzanilide Cl-DABA;

3) weighing 40g of the PI resin, adding the PI resin into a reagent bottle, adding an N, N-dimethylacetamide (DMAc) solvent to 200g of the PI resin, and fully dissolving to prepare a PI resin solution with 20% of solid content;

4) weighing nanoparticles, ultrasonically dispersing the nanoparticles in a 10g N N-dimethylacetamide DMAc solvent, and adding a drop of KH-550 coupling agent to obtain a nanoparticle dispersion liquid;

6) pouring the film-making solution on a glass plate, and coating the glass plate into a uniform coating by using an automatic film coating machine;

7) placing the glass plate in a drying box according to the proportion of 80 ℃/3h, 150 ℃/1h and 180 ℃/1 h; heating at 250 deg.C/1 h, and removing N, N-dimethylacetamide (DMAc) solvent to solidify;

2. A high refractive index, high transparent polyimide film according to claim 1, wherein: the film has an infrared spectrum of 797cm^-1Has a weak absorption peak of Ti-O bonds.

3. A high refractive index, high transparent polyimide film according to claim 1, wherein: the infrared spectrum of the film is 964cm^-1Has a weak absorption peak of Zr-O bond.

4. A high refractive index, high transparent polyimide film according to claim 1, wherein: the operation procedure of the mixing defoaming machine in the step 5) is as follows: firstly mixing for 30s at the rotating speed of 1800 r/min; then defoaming for 30s at 2000 r/min.

5. A method for producing a polyimide film according to any one of claims 1 to 4, comprising the steps of:

6. The method of claim 5, wherein: the operation procedure of the mixing defoaming machine in the step 5) is as follows: firstly mixing for 30s at the rotating speed of 1800 r/min; then defoaming for 30s at 2000 r/min.