JP6940581B2 - Polyimide film - Google Patents

Description

The present invention relates to a polyimide-based film.

Conventionally, glass has been used as a material for a base material of various display members such as a solar cell and a display and a transparent member such as a front plate. However, glass has the drawback of being fragile and heavy. Further, in recent years, the display has not had a sufficient material in terms of thinning and weight reduction and flexibility. Therefore, a polyimide-based film has been studied as a transparent member of a flexible device instead of glass (see, for example, Patent Documents 1 and 2).

U.S. Pat. No. 8,207,256 Japanese Unexamined Patent Publication No. 2008-163309

However, the conventional polyimide-based film has not been sufficiently flexible.

The polyimide film according to the present invention satisfies the following condition (I).
Condition (I): Tangent line at q = 0.003 in the small-angle X-ray scattering profile I (q) represented by a log-log plot of the wave number q (unit: Å ^{-1) and the scattering intensity I of the polyimide film.} Is the tangent line F (q), and the maximum value of I (q) / F (q) exceeds 1.5 at 0.003 <q ≦ 0.018.

Here, the film preferably has a yellowness YI of 10 or less.

Further, the film preferably has a Haze of 1% or less. Further, the film preferably has a total light transmittance of 85% or more at 550 nm.

In addition, the film may contain silica particles.

The polyimide film of the present invention has good flexibility.

It is a conceptual diagram which shows an example of a small angle X-ray profile, and is a logarithmic plot on both the vertical axis and the horizontal axis.

(Polyimide film)
The polyimide film according to this embodiment satisfies the following condition (I).
Condition (I): Tangent line at q = 0.003 in the small-angle X-ray scattering profile I (q) represented by a log-log plot of the wave number q (unit: Å ^{-1) and the scattering intensity I of the polyimide film.} Is the tangent line F (q), and the maximum value of I (q) / F (q) exceeds 1.5 at 0.003 <q ≦ 0.018.

The small-angle X-ray scattering profile is obtained by small-angle X-ray scattering measurement.
The small-angle X-ray scattering profile represents the wavenumber dependence of the scattering intensity at which the scattering angle 2θ is less than 10 ° among the scattered X-rays scattered by the atoms constituting the film when the X-rays are incident on the film. The small-angle X-ray scattering profile is usually expressed not by the angle θ but by the scattering intensity I with respect to the wave number (absolute value of the scattering vector) q. The wave number q is calculated from (4π / λ) sin θ, where λ represents the wavelength of the X-ray. The wavelength of the X-ray is 0.1 to 3 Å. Further, the scattering angle 2θ is preferably 5 ° or less.
Such X-rays can be used in SPring-8, PF rings, etc., and by using synchrotron radiation X-rays, a small-angle X-ray scattering profile having sufficient intensity can be obtained in a short time.

FIG. 1 shows an example of a small-angle X-ray profile. With reference to FIG. 1, each profile a to c has a tangent F (q) at a point of ^{wavenumber q = 0.003 Å -1.} The maximum value of I (q) / F (q) in the wavelength range 0.003 <q ≦ 0.018 is the position q where the value of F (q) and the value of I (q) are farthest in the above range. It is calculated from the value of _a. The small-angle X-ray profile of the polyimide film does not have to have a peak in the range of 0.003 <q ≦ 0.018 as in profile a, and has a peak of 0.003 <q ≦ 0.018 as in profile b. It may be in the range of. When the peak is not in the above range as in profile a, the maximum value of I (q) / F (q) in the above range may be calculated. Further, in the small-angle X-ray profile of the polyimide-based film, the tangent line F (q) can have an inclination like the profile c.
The maximum value of I (q) / F (q) in 0.003 <q ≦ 0.018 is greater than 1.5, preferably greater than 1.8, more preferably greater than 2.2. The upper limit of the maximum value of I (q) / F (q) in 0.003 <q ≦ 0.018 can be 10, and may be 8. A small-angle X-ray profile of a polyimide film that satisfies the above conditions corresponds to the polyimide film according to the present embodiment.

(Polyimide-based polymer)
The polyimide-based film contains a polyimide-based polymer. In the present specification, the polyimide-based polymer means a polymer containing at least one type of repeating structural unit represented by the formula (PI), the formula (a), the formula (a') or the formula (b). .. Among them, it is preferable that the repeating structural unit represented by the formula (PI) is the main structural unit of the polyimide-based polymer from the viewpoint of film strength and transparency. The repeating structural unit represented by the formula (PI) is preferably 40 mol% or more, more preferably 50 mol% or more, still more preferably 70 mol%, based on all the repeating structural units of the polyimide polymer. The above is more preferably 90 mol% or more, and even more preferably 98 mol%.

G in the formula (PI) represents a tetravalent organic group, and A represents a divalent organic group. ^{G 2} in the formula (a) represents a trivalent organic group, and A ² represents a divalent organic group. ^{G 3} in the formula (a') represents a tetravalent organic group, and A ³ represents a divalent organic group. ^{G 4} and ^{A 4} in the formula (b) represents each a divalent organic group.

In the formula (PI), the organic group of the tetravalent organic group represented by G (hereinafter, may be referred to as the organic group of G) is derived from an acyclic aliphatic group, a cyclic aliphatic group and an aromatic group. There are groups selected from the group. From the viewpoint of transparency and flexibility of the polyimide film, the organic group of G is preferably a cyclic aliphatic group or an aromatic group. Examples of the aromatic group include a monocyclic aromatic group, a condensed polycyclic aromatic group, and a non-condensed polycyclic aromatic group in which aromatic groups are directly linked to each other or by a bonding group. From the viewpoint of transparency and suppression of coloring of the resin film, the organic group of G has a cyclic aliphatic group having a fluorine-based substituent, a monocyclic aromatic group having a fluorine-based substituent, and a fluorine-based substituent. It is preferably a non-condensable polycyclic aromatic group having a fused polycyclic aromatic group or a fluorine-based substituent. In the present specification, the fluorine-based substituent means a group containing a fluorine atom. The fluorine-based substituent is preferably a fluoro group (fluorine atom, −F) and a perfluoroalkyl group, and more preferably a fluoro group and a trifluoromethyl group.

More specifically, the organic group of G is, for example, a saturated or unsaturated cycloalkyl group, a saturated or unsaturated heterocycloalkyl group, an aryl group, a heteroaryl group, an arylalkyl group, an alkylaryl group, a heteroalkylaryl. It is selected from groups that have a group and any two of these (which may be the same) and which are linked to each other either directly or by a linking group. Examples of the bonding group include -O-, an alkylene group having 1 to 10 carbon atoms, -SO _2- , -CO- or -CO-NR- (R is a methyl group, an ethyl group, a propyl group and the like having 1 to 1 carbon atoms. (Representing an alkyl group of 3 or a hydrogen atom).

The tetravalent organic group represented by G usually has 2 to 32 carbon atoms, preferably 4 to 15 carbon atoms, more preferably 5 to 10 carbon atoms, and further preferably 6 to 8 carbon atoms. When the organic group of G is a cyclic aliphatic group or an aromatic group, at least one of the carbon atoms constituting these groups may be replaced with a heteroatom. Heteroatoms include O, N or S.

Specific examples of G include groups represented by the following formulas (20), formulas (21), formulas (22), formulas (23), formulas (24), formulas (25) or formulas (26). Be done. * In the formula indicates a bond. Z in formula (26) is a single bond, -O-, -CH _2- , -C (CH ₃ ) _2- , -Ar-O-Ar-, -Ar _{-CH 2-Ar-, -Ar-} Represents C (CH ₃ ) _2- Ar- or -Ar-SO _2- Ar-. Ar represents an aryl group having 6 to 20 carbon atoms, and examples thereof include a phenylene group. At least one of the hydrogen atoms of these groups may be substituted with a fluorine-based substituent.

In the formula (PI), the organic group of the divalent organic group represented by A (hereinafter, may be referred to as the organic group of A) is derived from an acyclic aliphatic group, a cyclic aliphatic group and an aromatic group. A group selected from the group of The divalent organic group represented by A is preferably a cyclic aliphatic group or an aromatic group. Aromatic groups include monocyclic aromatic groups, fused polycyclic aromatic groups, and non-condensed polycyclic aromatics having two or more aromatic rings and linked to each other directly or by a bonding group. The group is mentioned. From the viewpoint of transparency and suppression of coloring of the resin film, it is preferable that a fluorine-based substituent is introduced into the organic group of A.

More specifically, the organic group of A is, for example, a saturated or unsaturated cycloalkyl group, a saturated or unsaturated heterocycloalkyl group, an aryl group, a heteroaryl group, an arylalkyl group, an alkylaryl group, a heteroalkylaryl. It is selected from a group and a group having any two of these (which may be the same) and those linked to each other directly or by a binding group. Examples of the hetero atom include O, N or S, and examples of the bonding group include -O-, an alkylene group having 1 to 10 carbon atoms, -SO _2- , -CO- or -CO-NR- (R is methyl). (Including an alkyl group having 1 to 3 carbon atoms such as a group, an ethyl group, a propyl group, or a hydrogen atom).

The divalent organic group represented by A has usually 2 to 40 carbon atoms, preferably 5 to 32 carbon atoms, more preferably 12 to 28 carbon atoms, and further preferably 24 to 27 carbon atoms.

Specific examples of A include a group represented by the following formula (30), formula (31), formula (32), formula (33) or formula (34). * In the formula indicates a bond. Z ^{1 to} Z ³ are independently single-bonded, -O-, -CH _2- , -C (CH ₃ ) _2- , -SO _2- , -CO- or -CO-NR- (R is Represents an alkyl group having 1 to 3 carbon atoms such as a methyl group, an ethyl group, and a propyl group, or a hydrogen atom). In the following groups, Z ¹ and Z ² and Z ² and Z ³ are preferably in the meta or para position with respect to each ring, respectively. Further, ^{it is preferable that the single bond between Z 1} and the terminal, the single bond between Z ² and the terminal, and ^{the single bond between Z 3} and the terminal are in the meta position or the para position, respectively. One example of A is that Z ¹ and Z ³ are -O- and Z ² is -CH _2- , -C (CH ₃ ) _2- or -SO _2- . One or more of the hydrogen atoms of these groups may be substituted with fluorine-based substituents.

At least one of A and G has at least one hydrogen atom selected from the group consisting of a fluorine-based substituent, a hydroxyl group, a sulfone group, an alkyl group having 1 to 10 carbon atoms, and the like. It may be substituted with one kind of functional group. Further, when the organic group of A and the organic group of G are cyclic aliphatic groups or aromatic groups, respectively, it is preferable that at least one of A and G has a fluorine-based substituent, and both A and G have a fluorine-based substituent. It is more preferable to have a fluorine-based substituent.

^{G 2} in the formula (a) is a trivalent organic group. This organic group can be selected from the same groups as the organic group of G in the formula (PI) except that it is a trivalent group. As ^{an example of G 2} , a group in which any one of the four bonds of the groups represented by the formulas (20) to (26) given as a specific example of G is replaced with a hydrogen atom can be mentioned. can. ^{A 2} in formula (a) can be selected from the same groups as A in formula (PI).

^{G 3} in formula (a') can be selected from the same groups as G in formula (PI). ^{A 3} in formula (a') can be selected from the same groups as A in formula (PI).

^{G 4} in formula (b) is a divalent organic group. This organic group can be selected from the same groups as the organic group of G in the formula (PI) except that it is a divalent group. An example of G ⁴ is a group in which any two of the four bonds in the groups represented by the formulas (20) to (26) given as specific examples of G are replaced with hydrogen atoms. can. A ⁴ in the formula (b) may be selected from the same groups as A in the formula (PI).

The polyimide polymer contained in the polyimide film includes diamines and tetracarboxylic acid compounds (including acid chloride compounds and tetracarboxylic acid compound analogs such as tetracarboxylic acid dianhydride) or tricarboxylic acid compounds (acid chloride compounds). It may be a condensed polymer obtained by polycondensing with at least one of (including a tricarboxylic acid compound analog such as tricarboxylic acid anhydride). Further, a dicarboxylic acid compound (including an analog such as an acid chloride compound) may be polycondensed. The repeating structural unit represented by the formula (PI) or the formula (a') is usually derived from diamines and tetracarboxylic acid compounds. The repeating structural unit represented by the formula (a) is usually derived from diamines and tricarboxylic acid compounds. The repeating structural unit represented by the formula (b) is usually derived from diamines and dicarboxylic acid compounds.

Examples of the tetracarboxylic acid compound include an aromatic tetracarboxylic acid compound, an alicyclic tetracarboxylic acid compound and an acyclic aliphatic tetracarboxylic acid compound. These may be used in combination of two or more. The tetracarboxylic acid compound is preferably a tetracarboxylic dianhydride. Examples of the tetracarboxylic acid dianhydride include aromatic tetracarboxylic acid dianhydride, alicyclic tetracarboxylic acid dianhydride, and acyclic aliphatic tetracarboxylic acid dianhydride.

From the viewpoint of solubility of the polyimide-based polymer in a solvent and transparency and flexibility when a polyimide-based film is formed, the tetracarboxylic acid compound is an alicyclic tetracarboxylic compound, an aromatic tetracarboxylic acid compound, or the like. Is preferable. From the viewpoint of transparency and suppression of coloring of the resin film, the tetracarboxylic acid compound is preferably an alicyclic tetracarboxylic acid compound having a fluorine-based substituent and an aromatic tetracarboxylic acid compound having a fluorine-based substituent. , It is more preferable that it is an alicyclic tetracarboxylic acid compound having a fluorine-based substituent.

Examples of the tricarboxylic acid compound include aromatic tricarboxylic acids, alicyclic tricarboxylic acids, acyclic aliphatic tricarboxylic acids and their related acid chloride compounds, acid anhydrides and the like. The tricarboxylic acid compound is preferably an aromatic tricarboxylic acid, an alicyclic tricarboxylic acid, an acyclic aliphatic tricarboxylic acid and an acid chloride compound related thereto. Two or more kinds of tricarboxylic acid compounds may be used in combination.

From the viewpoint of solubility of the polyimide polymer in a solvent and transparency and flexibility when a polyimide film is formed, the tricarboxylic acid compound is preferably an alicyclic tricarboxylic acid compound and an aromatic tricarboxylic acid compound. .. From the viewpoint of transparency and suppression of coloring of the resin film, the tricarboxylic acid compound is preferably an alicyclic tricarboxylic acid compound having a fluorine-based substituent and an aromatic tricarboxylic acid compound having a fluorine-based substituent.

Examples of the dicarboxylic acid compound include aromatic dicarboxylic acids, alicyclic dicarboxylic acids, acyclic aliphatic dicarboxylic acids and their related acid chloride compounds, acid anhydrides and the like. The dicarboxylic acid compound is preferably an aromatic dicarboxylic acid, an alicyclic dicarboxylic acid, an acyclic aliphatic dicarboxylic acid and an acid chloride compound related thereto. Two or more kinds of dicarboxylic acid compounds may be used in combination.

The dicarboxylic acid compound is preferably an alicyclic dicarboxylic acid compound or an aromatic dicarboxylic acid compound from the viewpoint of solubility of the polyimide-based polymer in a solvent and transparency and flexibility when a polyimide-based film is formed. .. From the viewpoint of transparency and suppression of coloring of the resin film, the dicarboxylic acid compound is preferably an alicyclic dicarboxylic acid compound having a fluorine-based substituent and an aromatic dicarboxylic acid compound having a fluorine-based substituent.

Examples of diamines include aromatic diamines, alicyclic diamines and aliphatic diamines. Two or more kinds of diamines may be used in combination. From the viewpoint of the solubility of the polyimide polymer in the solvent and the transparency and flexibility when the polyimide film is formed, the diamines may be an alicyclic diamine or an aromatic diamine having a fluorine-based substituent. preferable.

The polyimide-based polymer may be a copolymer containing a plurality of the above-mentioned repeating structural units of different types. The weight average molecular weight of the polyimide polymer is usually 10,000 to 500,000. The weight average molecular weight of the polyimide polymer is preferably 50,000 to 500,000, more preferably 70,000 to 400,000. The weight average molecular weight is a standard polystyrene-equivalent molecular weight measured by GPC. If the weight average molecular weight of the polyimide polymer is large, high flexibility tends to be obtained, but if the weight average molecular weight of the polyimide polymer is too large, the viscosity of the varnish tends to increase and the processability tends to decrease. be.

The polyimide-based polymer may contain a halogen atom such as a fluorine atom that can be introduced by the above-mentioned fluorine-based substituent or the like. When the polyimide polymer contains a halogen atom, the elastic modulus of the resin film can be improved and the yellowness can be reduced. As a result, it is possible to suppress the occurrence of scratches and wrinkles on the resin film and improve the transparency of the resin film. The halogen atom is preferably a fluorine atom. The content of the halogen atom in the polyimide polymer is preferably 1% by mass to 40% by mass, more preferably 1 to 30% by mass, based on the mass of the polyimide polymer.

The polyimide film may contain one kind or two or more kinds of ultraviolet absorbers. The ultraviolet absorber can be appropriately selected from those usually used as an ultraviolet absorber in the field of resin materials. The ultraviolet absorber may contain a compound that absorbs light having a wavelength of 400 nm or less. Examples of the ultraviolet absorber that can be appropriately combined with the polyimide polymer include at least one compound selected from the group consisting of benzophenone compounds, salicylate compounds, benzotriazole compounds, and triazine compounds. ..
As used herein, the term "system compound" refers to a derivative of a compound to which the "system compound" is attached. For example, the "benzophenone-based compound" refers to a compound having benzophenone as a maternal skeleton and a substituent attached to benzophenone.

The amount of the ultraviolet absorber is usually 0.5% by mass or more, preferably 1% by mass or more, more preferably 2% by mass or more, still more preferably 3% by mass, based on the total mass of the resin film. % Or more. The amount of the ultraviolet absorber is usually 10% by mass or less, preferably 8% by mass or less, and more preferably 6% by mass or less. By including the ultraviolet absorber in these amounts, the weather resistance of the polyimide film can be particularly effectively enhanced.

(Inorganic particles)
The polyimide film may further contain inorganic particles from the viewpoint of increasing the strength. Examples of the inorganic particles include particles containing a silicon atom, and examples of the particles containing the silicon atom include silica particles. Examples of other inorganic particles include titania particles, alumina particles, zirconia particles and the like.

The average primary particle size of the inorganic particles is usually 100 nm or less. When the average primary particle size of the inorganic particles is 100 nm or less, the transparency of the film tends to be improved. The primary particle size of the inorganic particles can be measured with a directional diameter using a transmission electron microscope (TEM). The average primary particle size can be obtained by measuring 10 points of the primary particle size by TEM observation and as an average value thereof.

In the polyimide film, the compounding ratio of the polyimide and the inorganic particles is preferably 1: 9 to 10: 0, more preferably 3: 7 to 10: 0, in terms of mass ratio, from 3: 7 to 3: 7. It is even more preferably 8: 2, and even more preferably 3: 7 to 7: 3. When the compounding ratio of the polyimide and the inorganic particles is within the above range, the transparency and the mechanical strength tend to be improved.

Inorganic particles may be bonded to each other by a molecule having a siloxane bond (−SiOSi−).

The polyimide-based film may further contain other components as long as the transparency and flexibility are not impaired. Examples of other components include colorants such as antioxidants, mold release agents, stabilizers and bluing agents, flame retardants, lubricants, thickeners, leveling agents and the like.

The polyimide-based film can also contain a quaternary alkoxysilane such as tetraethyl orthosilicate (TEOS) and an organosilicon compound such as a silsesquioxane derivative.

The components other than the polyimide and the inorganic material are preferably more than 0% and 20% by mass or less with respect to the mass of the polyimide film. More preferably, it is more than 0% and 10% by mass or less.

The thickness of the polyimide-based film is appropriately adjusted according to the intended use, but is usually 10 to 500 μm, preferably 15 to 200 μm, and more preferably 20 to 100 μm.

This polyimide-based film preferably has a total light transmittance of 85% or more, more preferably 90% or more, in accordance with JIS K7105: 1981.
Further, this polyimide-based film preferably has a Haze according to JIS K 7105: 1981 of 1% or less, and more preferably 0.9% or less.
Further, this polyimide-based film preferably has a yellowness YI according to JIS K 7373: 2006 of 10 or less, more preferably 5 or less, and further preferably 3 or less.

Such a polyimide-based film has excellent flexibility. The reason why such a polyimide film is excellent in flexibility is not clear, but it has some structure that affects X-ray scattering such as agglomerated structure and fine structure on a nanoscale, and the structure is a size that does not scatter visible light. Furthermore, it is considered that the fact that the structure exists with a uniform size distribution to some extent contributes to the improvement of the flexibility.

In the small-angle X-ray scattering profile, wavenumber q = 0.003 and wavenumber q = 0.018 correspond to lengths of 209 nm and 35 nm, respectively. Having a structure such as a peak in the X-ray scattering profile in this range means having a periodic structure in that range. Therefore, F (q) is defined as a tangent line at q = 0.003 corresponding to 209 nm, and having a portion where I (q) is largely separated from the tangent line is 0.003 <q ≦ 0.018. , Indicates that the film has a certain amount of structure of the above size.

Further, even when the structure has the above-mentioned size, it tends to be difficult to obtain sufficient flexibility when the size of the structure is non-uniform. When the size of the structure is non-uniform, the peak of the X-ray scattering profile becomes broad, and as a result, the maximum value of I (q) / F (q) becomes small. Therefore, having a maximum value of I (q) / F (q) greater than 1.5 means that it contains a certain amount of nano-sized structure that does not impair the transparency of the material, and that it It means that the size of the structure has a certain degree of uniformity.

(Production method)
Next, an example of the method for producing the polyimide film of the present embodiment will be described. The polyimide-based film is a solidified solid content contained in the polyimide-based polymer varnish.
The polyimide-based polymer varnish is prepared by dissolving a solvent-soluble polyimide polymerized using a known polyimide synthesis method in a solvent. The solvent may be any solvent that dissolves polyimide, for example, N, N-dimethylacetamide (DMAc), N, N-dimethylformamide (DMF), dimethyl sulfoxide (DMSO), γ-butyrolactone (GBL), N. -Methylpyrrolidone (NMP), ethyl acetate, methyl ethyl ketone (MEK), tetrahydrofuran, 1,4-dioxane, acetone, cyclopentanone, dimethyl sulfoxide, xylene and combinations thereof can be used.
The polyimide may be any solvent-soluble polyimide and may have the above-mentioned structure.

The polyimide-based polymer varnish may further contain water. The water content is usually 0.1 to 10% by mass with respect to the mass of the polyimide-based polymer varnish.
Since the varnish contains water, it is easy to obtain a film having a structure showing the above-mentioned small-angle X-ray profile. The reason why the structure of the polyimide film is changed by the addition of water is not clear, but it is considered that the presence of water when the solvent is dried affects the structure formation of the polyimide.

The polyimide-based polymer varnish can further contain the above-mentioned inorganic particles. When the polyimide-based polymer varnish contains inorganic particles, the addition of the fact that the polyimide-based polymer varnish contains water suppresses the gelation of the inorganic particles, which affects the structure formation of the polyimide-based polymer. It is possible to give.

When the polyimide-based polymer varnish contains inorganic particles, the polyimide-based polymer varnish may contain a metal alkoxide such as alkoxysilane in order to improve solution stability. An alkoxysilane having an amino group is preferable. Alkoxysilane contributes to the formation of bonds between inorganic particles.

The amount of the metal alkoxide added is usually 0.1 to 10 parts by mass and preferably 0.5 to 5 parts by mass with respect to 100 parts by mass of the inorganic particles.

Further, additional additives may be added to the polyimide-based polymer varnish, and the additives include, for example, antioxidants, mold release agents, stabilizers, colorants such as brewing agents, flame retardants, and lubricants. , Thickener, leveling agent and the like may be added.

The prepared polyimide-based varnish is then overcoated on a PET substrate, a SUS belt, or a glass substrate by a known roll-to-roll or batch method to form a coating film. This coating film is dried and peeled from the substrate to form a film. The film may be further dried after the peeling.

The coating film is dried at a temperature of 50 to 350 ° C. by appropriately evaporating the solvent under conditions of an inert atmosphere or reduced pressure. The solvent may be evaporated in the atmosphere. When it is carried out in the atmosphere, it is preferable that the temperature is 230 ° C. or lower from the viewpoint of coloring.

(Use)
Since such a polyimide film is transparent and has excellent flexibility, it can be used as a component of a flexible display. For example, it can be used as a front plate (window film) for protecting the surface of a flexible display.
Further, it is also possible to obtain a laminate in which various functional layers such as an ultraviolet absorbing layer, a hard coat layer, an adhesive layer, a hue adjusting layer and a refractive index adjusting layer are added to this polyimide film.

Hereinafter, the present invention will be described in more detail with reference to Examples and Comparative Examples, but the present invention is not limited to the following Examples.

(Example 1)
The compound represented by the formula (1), the compound represented by the formula (2), the compound represented by the formula (3), the solvent (γ-butyrolactone and dimethylacetamide) and the catalyst were charged into the nitrogen-substituted polymerization tank. .. The amount charged was 75.0 g of the compound represented by the formula (1), 36.5 g of the compound represented by the formula (2), 76.4 g of the compound represented by the formula (3), 438.4 g of γ-butyrolactone, and dimethyl. The amount of acetamide was 313.1 g and the amount of the catalyst was 1.5 g. The molar ratio of the compound represented by the formula (2) to the compound represented by the formula (3) is 3: 7, the total of the compound represented by the formula (2) and the compound represented by the formula (3). The molar ratio with the compound represented by the formula (1) was 1.00: 1.02.

After stirring the mixture in the polymerization tank to dissolve the raw material in the solvent, the temperature of the mixture was raised to 100 ° C., then raised to 200 ° C. and kept warm for 4 hours to polymerize the polyimide. During this heating, water in the liquid was removed. Then, it was purified and dried to obtain a polyimide.

Next, a γ-butyrolactone solution of polyimide adjusted to a concentration of 20% by mass, a solution in which silica particles having a solid content concentration of 30% by mass are dispersed in γ-butyrolactone, a dimethylacetamide solution of alkoxysilane having an amino group, and water are mixed. , Stirred for 30 minutes.

Here, the mass ratio of silica to polyimide is 60:40, the amount of alkoxysilane having an amino group is 1.67 parts based on 100 parts by mass of silica and polyimide, and water is 100 parts by mass of silica and polyimide. On the other hand, it was set to 10 parts by mass.

The mixed solution was applied to a glass substrate and heated at 50 ° C. for 30 minutes and 140 ° C. for 10 minutes to dry the solvent. Then, the film was peeled off from the glass substrate, a gold frame was attached, and the film was heated at 210 ° C. for 1 hour to obtain a transparent polyimide film having a thickness of 50 μm.

(Example 2)
As the polyimide, "Neoprim" manufactured by Mitsubishi Gas Chemical Company was used, and the same as in Example 1 except that the mass ratio of silica to polyimide was 55:45.

(Example 3)
The same procedure as in Example 2 was carried out except that the mixed solution prepared in Example 2 was stored at 40 ° C. for 24 hours before being applied to the glass substrate.

(Comparative Example 1)
The mixed solution was prepared in the same manner as in Example 1 except that water was not added, and the mixed solution was stored at 40 ° C. for 24 hours before being applied to the glass substrate.

(Measurement of small-angle X-ray scattering profile)
The small-angle X-ray scattering profile of each of the obtained films was acquired by the USAXS apparatus of SPring-8 under the following conditions. The wavelength λ of the X-ray was 2 Å, and the range of the wave number q was 0.002 to 0.02 Å ^-1 .

From the obtained profile, the maximum value of I (q) / F (q) in 0.003 <q ≦ 0.018 was obtained. As a result, Example 1 was 2.5 and Comparative Example 1 was 1.1. rice field.

(Evaluation of flexibility)
A sample (10 mm × 100 mm) cut out from each film was bent 180 ° at the central portion in the longitudinal direction, and the bent portion was further pressed with a finger to observe the bent portion. The results are shown in Table 1. The case where the bent portion is broken is referred to as C, and the case where the bent portion is not broken is referred to as A.

(YI evaluation)
The yellowness (Yellow Index: YI) of the films of Examples and Comparative Examples was measured by an ultraviolet-visible near-infrared spectrophotometer V-670 manufactured by JASCO Corporation in accordance with JIS K 7373: 2006. After performing the background measurement in the absence of the sample, the film was set in the sample holder, the transmittance for light of 300 to 800 nm was measured, and the tristimulus values (X, Y, Z) were determined. YI was calculated based on the following formula.
YI = 100 × (1.2769X-1.0592Z) / Y

(Evaluation of Haze)
Haze (%) of the film was measured by a fully automatic direct reading haze computer HGM-2DP manufactured by Suga Test Instruments Co., Ltd. in accordance with JIS K 7105: 1981.

(Evaluation of total light transmittance Tr)
The total light transmittance of the film was measured by a fully automatic direct reading haze computer HGM-2DP manufactured by Suga Test Instruments Co., Ltd. in accordance with JIS K7105-1: 1981. These results are shown in Table 1.

Claims (5)

A front plate of a flexible display characterized by satisfying the following condition (I).
The front plate is made of a polyimide film.
The polyimide-based film contains a polyimide-based polymer and contains
The polyimide polymer is represented by the formula (PI), Ri polymer der containing formula (a) or formula (a ') a repeating structural unit represented by at least one or more,
The thickness of the polyimide film is Ru 10~500μm der,
Flexible display front plate

[G in the formula (PI) represents a tetravalent organic group, and A represents a divalent organic group. ^{G 2} in the formula (a) represents a trivalent organic group, and A ² represents a divalent organic group. ^{G 3} in the formula (a') represents a tetravalent organic group, and A ³ represents a divalent organic group . ] .
Condition (I): In the small-angle X-ray scattering profile I (q) represented by a log-log plot of the wave number q (unit: Å ^{-1) and the scattering intensity I on the front plate of the flexible display, q = 0.003.} When the tangent line in is tangent line F (q),
When 0.003 <q ≦ 0.018, the maximum value of I (q) / F (q) exceeds 1.5. The front plate of the flexible display according to claim 1, wherein the yellowness YI is 10 or less. The front plate of a flexible display according to claim 1 or 2, wherein the haze is 1% or less. The front plate of a flexible display according to any one of claims 1 to 3, wherein the total light transmittance is 85% or more. The front plate of a flexible display according to any one of claims 1 to 4, which contains silica particles.

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