JP2018028076A - Polyimide precursor and polyimide resulting therefrom - Google Patents

Abstract

An object of the present invention is to provide a polyimide and a precursor thereof which are excellent in low linear expansion property and film forming property, can be easily peeled from a supporting substrate, and are excellent in heat resistance and high transparency. A polyimide precursor having a structural unit derived from a diamine and a structural unit derived from an acid dianhydride, wherein i) a structural unit derived from an aromatic diamine represented by the following formula (1): ii) a polyimide precursor having a structural unit derived from 2,2'-bis (trifluoromethyl) benzidine or a structural unit derived from 1,2,3,4-cyclobutanetetracarboxylic dianhydride, and an Polyimide obtained by conversion. (In the formula (1), Z is NH or O.)

Description

  The present invention relates to a polyimide useful as a support base material for forming a display device having high transparency, low thermal expansion coefficient, high heat resistance, and laser lift-off characteristics, and a precursor thereof.

  Organic EL devices used for various displays, including large displays such as televisions and small displays such as mobile phones, personal computers and smartphones, generally form thin film transistors (TFTs) on a glass substrate, which is a support substrate. Further, an electrode, a light emitting layer, and an electrode are sequentially formed thereon, and these are hermetically sealed with a glass substrate or a multilayer thin film. The structure of the organic EL device includes a bottom emission structure in which light is extracted from the support substrate side, and a top emission structure in which light is extracted from the side opposite to the support substrate. A transparent material is required for the support substrate having the bottom emission structure, but a non-transparent material may be used for the support substrate having the top emission structure.

  By replacing the supporting base material of such an organic EL device with a resin from a conventional glass substrate, the organic EL device can be made thin, light and flexible, and the application of the organic EL device can be further expanded. However, since resin is generally inferior in dimensional stability, transparency, heat resistance, moisture resistance, film strength and the like as compared with glass, various studies have been made.

  A polyimide film is known as a material useful as a plastic substrate for flexible displays. For example, Patent Document 1 is obtained by casting a polyimide precursor solution having a specific structure onto an inorganic substrate, drying and imidizing, and a laminate composed of a polyimide film and an inorganic substrate, and peeling from the laminate. A polyimide film is disclosed. This polyimide film is said to have high light transmittance and low outgas, but since the coefficient of thermal expansion (CTE) exceeds 40 ppm / K, the difference in CTE from an inorganic substrate such as a glass substrate is large. It is difficult to obtain a device such as an organic EL device having excellent shape stability, such as warpage is likely to occur and peeling or cracking occurs after device formation.

  Patent Documents 2 and 5 disclose a polyimide precursor resin composition for forming a flexible device substrate such as a display device or a light receiving device, which is manufactured by peeling a polyimide film from a carrier substrate, and the polyimide film has a temperature of 300 ° C. or higher. The glass transition temperature and the thermal expansion coefficient of 20 ppm / K or less are described.

  The organic EL device has low resistance to moisture, and the characteristics of the EL element that is the light emitting layer are degraded by moisture. Therefore, when a resin is used as the supporting base material, a resin having a low moisture absorption rate is preferred in order to prevent moisture and oxygen from entering the organic EL device. In general, inorganic materials typified by silicon oxide and silicon nitride are used as the organic EL substrate, and these CTEs are usually 0 to 10 ppm / K. On the other hand, since a general polyimide has a CTE greater than 10 ppm / K, if the polyimide is simply applied to the support substrate of the organic EL device, warping or cracking may occur due to thermal stress, or it may be peeled off. May occur. Patent Document 6 uses 2,3,6,7-naphthalenetetracarboxylic dianhydride as a polyimide material, but its thermal expansion coefficient is larger than 20 ppm / K, which may cause warping. .

In response to the touch panel installed on the display device, the study of thinning, lightening, and flexibility has been actively conducted.
The main methods of touch panels are roughly classified into a method for detecting a change in light and a method for detecting a change in electrical characteristics. As a method for detecting a change in electrical characteristics, a resistance film method and a capacitive coupling method are known. In addition, there are two types of capacitive coupling methods: surface type and projection type, but projection type from the viewpoint of being suitable for multi-touch recognition (multi-point recognition), which has become an indispensable function for smartphones and the like. Is attracting attention.
The projected capacitive coupling type touch panel is provided with two electrode rows (first and second electrodes) in the vertical and horizontal directions, and by measuring the change in capacitance of the electrode when the finger touches the screen, the touch position can be determined. It can be detected precisely. The specific structure is a structure in which a first substrate on which a first electrode is formed and a second substrate on which a second electrode is formed are joined via an insulating layer (dielectric layer). Thinning, lightening, and flexibility can be realized by replacing a substrate on which an electrode is formed with a flexible resin base material from a conventional glass substrate. In addition, the first electrode and the second electrode are formed on one substrate to further reduce the thickness and weight.
When part or all of the touch panel substrate is replaced with a resin base material, polyimide is attracting attention. Also in this case, it is desired that the polyimide has a CTE of 35 ppm / K or less from the viewpoint of preventing warpage.

  Patented polyimide film obtained from aromatic tetracarboxylic dianhydride including 3,3 ', 4,4'-biphenyltetracarboxylic dianhydride and 2- (4'-aminophenyl) -5-aminobenzimidazole Although disclosed in Document 6, it is limited to circuit board applications. Patent Document 7 discloses a polyimide obtained from isopropylidenebis (4-phenyleneoxy-4-phthalic acid) dianhydride and 6-amino-2- (p-aminophenyl) benzimidazole. This is also limited to the metal laminated substrate application.

  Patent Documents 8 and 9 disclose a polyimide precursor obtained by the reaction of an aromatic diamine having a benzoxazole structure and an acid dianhydride and a polyimide resulting therefrom, but with a specific acid dianhydride or diamine. I don't teach the combination.

  A method is known in which a solution of a polyimide precursor is cast on a support substrate and heat imidized to obtain a laminate (Patent Document 3). In addition, it has been reported that a polyimide film having a low linear expansion coefficient is directly laminated on a support substrate (Patent Document 4). However, depending on the combination of the support substrate and the polyimide precursor, peeling becomes difficult.

  In consideration of the above points, at the time of replacing the support substrate for the display device from the glass substrate to the resin substrate, at least the low CTE, high heat resistance, high transparency, and the property of being easily peeled off from the support substrate at the same time. Although it must be satisfactory, it has been difficult to produce materials that satisfy all of these requirements.

JP 2012-40836 JP 2010-202729 A JP-A-64-774 JP 2012-35583 A JP-A-2015-93915 Japanese Patent No.5716493 Japanese Patent No. 4433655 Japanese Patent No. 5699607 JP-A-2015-214122

  An object of the present invention is to provide a polyimide and a precursor thereof that are low in thermal expansion, excellent in film forming properties, easily peelable from a supporting substrate, and excellent in heat resistance and high transparency. Another object of the present invention is to provide a polyimide having good laser lift-off characteristics and a precursor thereof for facilitating peeling from a supporting substrate.

  As a result of intensive studies, the present inventors have found that a polyimide having a specific structure or a precursor thereof can satisfy the above characteristics, and have completed the present invention.

（式（１）中、ＺはＮＨ又はＯである。） That is, the present invention is a polyimide precursor having a structural unit derived from a diamine and a structural unit derived from an acid dianhydride, i) a structural unit derived from an aromatic diamine represented by the following formula (1) And ii) a polyimide having a structural unit derived from 2,2′-bis (trifluoromethyl) benzidine or a structural unit derived from 1,2,3,4-cyclobutanetetracarboxylic dianhydride It is a precursor.

(In the formula (1), Z is NH or O.)

It is desirable that the polyimide precursor satisfies one or more of the following.
1) The structural unit derived from 1,2,3,4-cyclobutanetetracarboxylic dianhydride should contain 50 mol% or more of the structural unit derived from total acid dianhydride.
2) Containing 50 mol% or more of structural units derived from 2,2′-bis (trifluoromethyl) benzidine derived from all diamines.
3) Containing 5 mol% or more of the structural unit derived from the aromatic diamine represented by the above formula (1) based on the total structural unit derived from the diamine.
4) The polyimide obtained by imidizing the polyimide precursor in a nitrogen atmosphere has a yellowness of 6 or less and a linear expansion coefficient of 35 ppm / K or less when changed from 250 ° C. to 100 ° C. about.
5) The polyimide obtained by imidizing the polyimide precursor has a light transmittance of 308 nm of 5% or less and a light transmittance of 430 nm of 70% or more.

  The present invention is a polyimide having a structural unit derived from a diamine and a structural unit derived from an acid dianhydride, i) a structural unit derived from an aromatic diamine represented by the above formula (1); ii) A polyimide having a structural unit derived from 2,2′-bis (trifluoromethyl) benzidine or a structural unit derived from 1,2,3,4-cyclobutanetetracarboxylic dianhydride .

The polyimide desirably satisfies one or more of the following.
1) Yellowness is 6 or less, and the coefficient of thermal expansion when changing from 250 ° C. to 100 ° C. is 35 ppm / K or less.
2) The light transmittance at 308 nm is 5% or less, and the light transmittance at 430 nm is 70% or more.

Other aspects of the present invention or inventions related to the present invention are shown below.
1) A polyimide film formed from the above polyimide.
This polyimide film is excellent for flexible devices.
2) A polyimide film with a functional layer in which a functional layer is provided on the polyimide film.
3) Use of the polyimide film as a flexible substrate for flexible display, touch panel use, color filter use, or display element, light emitting element, circuit, conductive film, metal mesh, hard coat film or gas barrier film of the polyimide film Use as a flexible substrate to form functional layers.

  Furthermore, the other aspect is a resin composition characterized by containing said polyimide precursor and a solvent.

  Furthermore, another aspect is a polyimide film formed by applying the resin composition on the surface of a support substrate and then imidizing the polyimide precursor by heating the resin composition.

  Furthermore, another aspect includes a step of applying the above resin composition on the surface of a support substrate, a step of heating this to imidize a polyimide precursor to form a polyimide film, and this polyimide film as a support substrate. And a step of obtaining a polyimide film by peeling from the substrate.

  Furthermore, another aspect is obtained by providing a support substrate and a polyimide film, spreading the resin composition on the surface of the support substrate, and heating it to imidize the polyimide precursor to form a polyimide film. It is the laminated body characterized by this. Also, a step of developing the resin composition on the surface of the support substrate, and heating this to imidize the polyimide precursor to form a polyimide film to obtain a laminate composed of the support substrate and the polyimide film It is a manufacturing method of a layered product characterized by comprising a process.

  Furthermore, another aspect is a substrate, an image display device, an optical material, or an electronic device containing or having the above polyimide or polyimide film.

  Furthermore, in another embodiment, after the polyimide film is obtained by applying, drying, and heat-treating the above resin composition on the support substrate, the functional layer is formed on the polyimide film and then peeled off from the support substrate together with the functional layer. Thus, the method for producing a polyimide film with a functional layer is obtained.

  In the method for producing a polyimide film with a functional layer, the support substrate is preferably composed of an inorganic substrate, and the functional layer is a display element, a light emitting element, a circuit, a conductive film such as ITO, a metal mesh, a hard coat film, moisture, oxygen, or the like. It is preferable that the gas barrier film prevent permeation of water.

  The polyimide precursor of the present invention or the polyimide obtained therefrom has high heat resistance, high transparency, low thermal expansion, and is easy to peel off from the support substrate. Since it is suitable and can provide a polyimide film excellent in productivity, such as a display element, a light emitting element, a circuit, a conductive film such as ITO, a metal mesh, a hard coat film, or a gas barrier film that prevents permeation of moisture, oxygen, etc. It can be suitably used as a flexible substrate for forming a functional layer.

The polyimide precursor of the present invention comprises a structural unit (U1) derived from an aromatic diamine represented by the above formula (1) and 2,2′-bis (trifluoromethyl) benzidine (also known as 2,2′- It has a structural unit (U2) derived from bis (trifluoromethyl) -4,4′-diaminobiphenyl) or a unit (U3) derived from 1,2,3,4-cyclobutanetetracarboxylic dianhydride. The polyimide precursor of the present invention has a structural unit (U1) and a structural unit (U2), a structural unit (U1) and a structural unit (U3), a structural unit (U1) and a structural unit (U2). ) And structural units (U3).
In formula (1), Z is NH or O.

Examples of structural units contained in the polyimide precursor of the present invention are shown in formulas (2) and (3).

As can be seen from the above formulas (2) and (3), the polyimide precursor can be represented by the following formula (4).
[-OCX (COOH) ₂ CO-HN-Y-NH-] (4)
On the other hand, polyimide can be represented by the following formula (5).
[-N (OC) ₂ X (CO) ₂ NY-] (5)
In the formulas (2) to (5), X is a tetravalent residue formed by removing two acid dianhydride groups from the acid dianhydride. In formulas (4) to (5), Y is a divalent residue formed by taking two amino groups from diamine.

When the acid dianhydride is 1,2,3,4-cyclobutanetetracarboxylic dianhydride, it becomes a 1,2,3,4-cyclobutane-tetrayl group represented by the following formula (6).

  In the description of the structural unit, terms such as a structural unit derived from a diamine and a structural unit derived from an acid dianhydride are used. However, this is for convenience, and the formulas (2) to (6) As long as the structural units as shown are given, the structural units are not limited to these. Specifically, in the above formulas (4) to (5), it is understood that the structural unit derived from acid dianhydride is X, and the structural unit derived from diamine is Y, meaning a raw material or a production method. Then it should not be understood. And since the structural unit and its ratio of a polyimide precursor are decided by the kind and usage ratio of diamine and acid dianhydride, description of a structural unit is demonstrated by diamine and acid dianhydride. The proportion of diamine and acid dianhydride used is the proportion of structural units derived from each.

  The polyimide precursors represented by the above formulas (2) and (3) are examples in which the aromatic diamine represented by the formula (1) is used as the diamine, but other diamines include 2,2′-bis. It can be (trifluoromethyl) benzidine. In formulas (2) and (3), the position of the bond in the rightmost benzene ring is not limited, but the 4-position is preferred.

The polyimide precursor and polyimide of the present invention include an aromatic diamine represented by the formula (1) as a diamine, or a diamine containing the aromatic diamine and 2,2′-bis (trifluoromethyl) benzidine, , 3,4-cyclobutanetetracarboxylic dianhydride can be obtained by reacting with or without using acid dianhydride. When using a diamine mixture containing 2,2'-bis (trifluoromethyl) benzidine as the diamine, the acid dianhydride may not contain 1,2,3,4-cyclobutanetetracarboxylic dianhydride, When a diamine containing an aromatic diamine represented by the formula (1) but not 2,2′-bis (trifluoromethyl) benzidine is used, the acid dianhydride is 1,2,3,4- Includes cyclobutanetetracarboxylic dianhydride.
That is, a polyimide precursor (also called polyamic acid or polyamic acid) is obtained by reaction of tetracarboxylic dianhydride and diamine, which is known as a general production method, and this is converted into polyimide by a ring-closing reaction by heat treatment. The method can be used.

  As the diamine, an aromatic diamine represented by the formula (1) or a mixture containing this aromatic diamine and 2,2′-bis (trifluoromethyl) benzidine is used. The aromatic diamine represented by the formula (1) includes a benzimidazole-type diamine in which Z is NH and a benzoxazole-type diamine in which Z is O, either of which may be used together. .

The amount of the aromatic diamine represented by the formula (1) is 5 mol% or more, preferably 20 mol% or more, more preferably 50 mol% or more of the total diamine, from the viewpoints of heat resistance and low thermal expansion. Good.
In another preferred embodiment, the total amount of the aromatic diamine represented by the formula (1) and 2,2′-bis (trifluoromethyl) benzidine is used in view of the balance between heat resistance, low thermal expansion and transparency. Therefore, the content is preferably 50 mol% or more of the total diamine, more preferably 80 mol% or more.
In another preferred embodiment, the amount of 2,2′-bis (trifluoromethyl) benzidine used is 5 mol% or more, preferably 20 mol% or more of the total diamine, from the viewpoint of transparency at 430 nm and low yellowness. More preferably, it is 50 mol% or more, and still more preferably 80 mol% or more.

As the tetracarboxylic dianhydride, 1,2,3,4-cyclobutanetetracarboxylic dianhydride is preferably used from the viewpoint of low thermal expansion or transparency.
The amount of 1,2,3,4-cyclobutanetetracarboxylic dianhydride used is preferably 50 mol% or more, more preferably 80 mol% or more of the total acid dianhydride.
When 2,2'-bis (trifluoromethyl) benzidine is used as the diamine, it gives heat resistance, low thermal expansion or transparency, so 1,2,3,4-cyclobutanetetracarboxylic dianhydride is Although it is not necessary to use, it is good to use 10 mol% or more of total acid dianhydride, Preferably it is 50 mol% or more.

  The amount of 1,2,3,4-cyclobutanetetracarboxylic dianhydride used is 50 mol% or more, preferably 80 mol% or more of the total acid dianhydride, and the aromatic represented by the formula (1) The amount of diamine used is 5 mol% or more, preferably 50 mol% or more, more preferably 80 mol% or more of the total diamine, or the aromatic diamine represented by the formula (1) and 2,2′-bis The total amount of (trifluoromethyl) benzidine used is 50 mol% or more, preferably 80 mol% or more of the total diamine. When the aromatic diamine represented by the formula (1) and 2,2′-bis (trifluoromethyl) benzidine are used in combination, the ratio of the amount used is in the range of 0.1 to 20 mol with respect to 1 mol of the former. Is good.

The proportion (mol%) of 1,2,3,4-cyclobutanetetracarboxylic dianhydride used in total acid dianhydride is 1,2,3,4-cyclobutane in all structural units derived from acid dianhydride. It is the content (mol%) of the structural unit derived from tetracarboxylic dianhydride. The same applies to the content (mol%) of the structural unit derived from the aromatic diamine represented by the above formula (1) in all structural units derived from diamine.
And the aromatic diamine represented by the above formula (1) is used in an amount of 5 mol% or more, preferably 50 mol% or more of the total diamine, and the structural unit (U1) is 5 mol of all the structural units derived from the diamine. % Or more is preferable. Further, it may contain a structural unit (U2) derived from 2,2′-bis (trifluoromethyl) benzidine, and the structural unit (U2) and 1,2,3,4-cyclobutanetetracarboxylic dianhydride More preferably, the derived structural units (U3) each contain 50 mol% or more of all structural units derived from diamine or all structural units derived from acid dianhydride.

As tetracarboxylic dianhydride, when 2,2'-bis (trifluoromethyl) benzidine is used, there is no particular limitation, but 1,2,3,4-cyclobutanetetracarboxylic dianhydride is used. It is preferable to do.
Whether or not 1,2,3,4-cyclobutanetetracarboxylic dianhydride is used, other tetracarboxylic dianhydrides can be used. When using another tetracarboxylic dianhydride, it is good to use in the range of 10-70 mol% of all the acid dianhydrides. Preferably it is 50 mol% or less, More preferably, it is 20 mol% or less. When 2,2'-bis (trifluoromethyl) benzidine is used and other tetracarboxylic dianhydrides are used, use within the range of 10 to 100 mol% of the total acid dianhydrides. However, it is preferably at most 50 mol%, more preferably at most 20 mol%.

Examples of the other tetracarboxylic dianhydrides include 4,4 ′-(2,2′-hexafluoroisopropylidene) diphthalic dianhydride, naphthalene-2,3,6,7-tetracarboxylic dianhydride. Anhydride, naphthalene-1,2,5,6-tetracarboxylic dianhydride, naphthalene-1,2,6,7-tetracarboxylic dianhydride, pyromellitic dianhydride, 3,3 ', 4 , 4'-biphenyltetracarboxylic dianhydride 3,3 ', 4,4'-benzophenone tetracarboxylic dianhydride, 2,2', 3,3'-benzophenone tetracarboxylic dianhydride, 2,3 , 3 ', 4'-Benzophenonetetracarboxylic dianhydride, naphthalene-1,2,4,5-tetracarboxylic dianhydride, naphthalene-1,4,5,8-tetracarboxylic dianhydride, naphthalene -1,2,6,7-tetracarboxylic dianhydride, 4,8-dimethyl-1,2,3,5,6,7-hexahydronaphthalene-1,2,5,6-tetracarboxylic acid bis Anhydride, 4,8-dimethyl-1,2,3,5,6,7-hexa Dronaphthalene-2,3,6,7-tetracarboxylic dianhydride, 2,6-dichloronaphthalene-1,4,5,8-tetracarboxylic dianhydride, 2,7-dichloronaphthalene-1,4 , 5,8-tetracarboxylic dianhydride, 2,3,6,7-tetrachloronaphthalene-1,4,5,8-tetracarboxylic dianhydride, 1,4,5,8-tetrachloronaphthalene -2,3,6,7-tetracarboxylic dianhydride, 2,2 ', 3,3'-biphenyltetracarboxylic dianhydride, 2,3,3', 4'-biphenyltetracarboxylic dianhydride 3,3``, 4,4 ''-p-terphenyltetracarboxylic dianhydride, 2,2 '', 3,3 ''-p-terphenyltetracarboxylic dianhydride, 2, 3,3``, 4 ''-p-terphenyltetracarboxylic dianhydride, 2,2-bis (2,3-dicarboxyphenyl) -propane dianhydride, 2,2-bis (3,4 -Dicarboxyphenyl) -propane dianhydride, bis (2,3-dicarboxyphenyl) ether dianhydride, bis (2,3-dicarboxyphene) Nyl) methane dianhydride, bis (3.4-dicarboxyphenyl) methane dianhydride, bis (2,3-dicarboxyphenyl) sulfone dianhydride, bis (3,4-dicarboxyphenyl) sulfone dianhydride, 1,1-bis (2,3-dicarboxyphenyl) ethane dianhydride, 1,1-bis (3,4-dicarboxyphenyl) ethane dianhydride, perylene-2,3,8,9-tetracarboxylic Acid dianhydride, perylene-3,4,9,10-tetracarboxylic dianhydride, perylene-4,5,10,11-tetracarboxylic dianhydride, perylene-5,6,11,12-tetra Carboxylic dianhydride, phenanthrene-1,2,7,8-tetracarboxylic dianhydride, phenanthrene-1,2,6,7-tetracarboxylic dianhydride, phenanthrene-1,2,9,10- Tetracarboxylic dianhydride, cyclopentane-1,2,3,4-tetracarboxylic dianhydride, pyrazine-2,3,5,6-tetracarboxylic dianhydride, pyrrolidine-2,3,4, 5-tetracarbo Acid dianhydride, thiophene-2,3,4,5-tetracarboxylic dianhydride, 4,4'-oxydiphthalic dianhydride, (trifluoromethyl) pyromellitic dianhydride, di (trifluoromethyl) ) Pyromellitic dianhydride, di (heptafluoropropyl) pyromellitic dianhydride, pentafluoroethyl pyromellitic dianhydride, bis {3,5-di (trifluoromethyl) phenoxy} pyromellitic dianhydride 2,2-bis (3,4-dicarboxyphenyl) hexafluoropropane dianhydride, 5,5'-bis (trifluoromethyl) -3,3 ', 4,4'-tetracarboxybiphenyl dianhydride 2,2 ', 5,5'-tetrakis (trifluoromethyl) -3,3', 4,4'-tetracarboxybiphenyl dianhydride, 5,5'-bis (trifluoromethyl) -3, 3 ', 4,4'-tetracarboxydiphenyl ether dianhydride, 5,5'-bis (trifluoro Til) -3,3 ', 4,4'-tetracarboxybenzophenone dianhydride, bis {(trifluoromethyl) dicarboxyphenoxy} benzene dianhydride, bis {(trifluoromethyl) dicarboxyphenoxy} trifluoromethyl Benzene dianhydride, bis (dicarboxyphenoxy) trifluoromethylbenzene dianhydride, bis (dicarboxyphenoxy) bis (trifluoromethyl) benzene dianhydride, bis (dicarboxyphenoxy) tetrakis (trifluoromethyl) benzene Anhydride, 2,2-bis {(4- (3,4-dicarboxyphenoxy) phenyl} hexafluoropropane dianhydride, bis {(trifluoromethyl) dicarboxyphenoxy} biphenyl dianhydride, bis {(tri Fluoromethyl) dicarboxyphenoxy} bis (trifluoromethyl) bi Examples thereof include phenyl dianhydride, bis {(trifluoromethyl) dicarboxyphenoxy} diphenyl ether dianhydride, bis (dicarboxyphenoxy) bis (trifluoromethyl) biphenyl dianhydride, and the like. Moreover, these may be used independently or can also be used together 2 or more types.
Naphthalene-2,3,6,7-tetracarboxylic dianhydride, pyromellitic dianhydride, or 3,3 ', 4,4'-biphenyltetracarboxylic acid from the viewpoint of high heat resistance and low thermal expansion A dianhydride is preferred. From the viewpoint of transparency, 4,4 ′-(2,2′-hexafluoroisopropylidene) diphthalic dianhydride or 4,4′-oxydiphthalic dianhydride is preferable.

Other preferred tetracarboxylic dianhydrides include pyromellitic dianhydride and 3,3 ', 4,4'-biphenyltetracarboxylic dianhydride for the purpose of imparting strength and flexibility to the polyimide film. 4,4′-oxydiphthalic dianhydride and the like. Of these, pyromellitic dianhydride or 3,3 ′, 4,4′-biphenyltetracarboxylic dianhydride is preferable since a low thermal expansion polyimide film can be obtained.
Incidentally, 1,2,3,4-cyclobutanetetracarboxylic dianhydride is excellent in that it provides high heat resistance, low thermal expansion and transparency.

  As the diamine, in addition to the aromatic diamine represented by the formula (1) or the aromatic diamine and 2,2′-bis (trifluoromethyl) benzidine, other diamines can be used. When using other diamine, it is good to use in 10-70 mol% of total diamine, Preferably it is less than 50 mol%.

  As the other diamine, a diamine having one or more aromatic rings is suitable. Examples of such diamines include 2,2′-dimethyl-4,4′-diaminobiphenyl (also known as 2,2′-dimethyl-benzidine), 3,3′-dimethyl-4,4′-diaminobiphenyl, 4,4'-diaminodiphenyl ether, 3,4'-diaminodiphenyl ether, 4,6-dimethyl-m-phenylenediamine, 2,5-dimethyl-p-phenylenediamine, 2,4-diaminomesitylene, 4,4'- Methylenedi-o-toluidine, 4,4'-methylenedi-2,6-xylidine, 4,4'-methylene-2,6-diethylaniline, 2,4-toluenediamine, m-phenylenediamine, p-phenylenediamine, 4,4'-diaminodiphenylpropane, 3,3'-diaminodiphenylpropane, 4,4'-diaminodiphenylethane, 3,3'-diaminodiphenylethane, 4,4'-diaminodiphenylmethane, 3,3'-diamino Diphenylmethane, 2,2-bis [4- (4-aminophenoxy) phenyl] pro 4,4'-diaminodiphenyl sulfide, 3,3'-diaminodiphenyl sulfide, 4,4'-diaminodiphenyl sulfone, 3,3'-diaminodiphenyl sulfone, 4,4'-diaminodiphenyl ether, 3,3'- Diaminodiphenyl ether, 1,3-bis (3-aminophenoxy) benzene, 1,3-bis (4-aminophenoxy) benzene, 1,4-bis (4-aminophenoxy) benzene, benzidine, 3,3'-diamino Biphenyl, 3,3'-dimethyl-4,4'-diaminobiphenyl, 3,3'-dimethoxybenzidine, 4,4'-diamino-p-terphenyl, 3,3'-diamino-p-terphenyl, bis (p-β-amino-t-butylphenyl) ether, bis (p-β-methyl-δ-aminopentyl) benzene, p-bis (2-methyl-4-aminopentyl) benzene, p-bis (1, 1-dimethyl-5-aminopentyl) benzene, 1,5-diaminonaphthalene, 2,6-diaminonaphtha 2,4-bis (β-amino-t-butyl) toluene, 2,4-diaminotoluene, m-xylene-2,5-diamine, p-xylene-2,5-diamine, m-xylylenediamine , P-xylylenediamine, 2,6-diaminopyridine, 2,5-diaminopyridine, 2,5-diamino-1,3,4-oxadiazole, piperazine and the like.

  Of these, preferably 2,2'-dimethyl-4,4'-diaminobiphenyl, 4,4'-diaminodiphenyl ether, 4,6-dimethyl-m-phenylenediamine, 2,5-dimethyl-p-phenylenediamine, 2,4-diaminomesitylene, 2,4-toluenediamine, m-phenylenediamine, or p-phenylenediamine, but 2,2'-dimethyl-4,4 from the viewpoint of fast reaction and high transparency '-Diaminobiphenyl or 4,4'-diaminodiphenyl ether is suitable.

  The polyimide precursor of the present invention can be produced by a known method in which diamine and acid dianhydride are used in a molar ratio of 0.9 to 1.1 and polymerized in an organic polar solvent. Specifically, after dissolving diamine in an aprotic amide solvent such as N, N-dimethylacetamide and N-methyl-2-pyrrolidone under a nitrogen stream, acid dianhydride is added, and at room temperature, 3 to It can be obtained by reacting for about 20 hours. At this time, the molecular ends may be sealed with an aromatic monoamine or aromatic monocarboxylic dianhydride. Other examples of the solvent include dimethylformamide, 2-butanone, diglyme, xylene, γ-butyrolactone, and the like, which can be used alone or in combination of two or more.

  The polyimide of the present invention is obtained by imidizing the polyimide precursor of the present invention. The imidization can be performed by a thermal imidization method or a chemical imidization method. Thermal imidation is performed by applying a polyimide precursor on an arbitrary support substrate such as glass, metal, resin, etc. using an applicator and pre-drying at a temperature of 150 ° C. or lower for 2 to 60 minutes, and then removing the solvent. For imidation, it is usually carried out by heat treatment at room temperature to about 360 ° C. for about 10 minutes to 20 hours. In chemical imidization, a dehydrating agent and a catalyst are added to a polyimide precursor (also referred to as “polyamic acid”) solution, and dehydration is performed chemically at 30 to 60 ° C. A typical dehydrating agent is acetic anhydride, and a catalyst is pyridine. Thermal imidation can be completed in a relatively short time by selecting a combination of acid dianhydride, diamine, and solvent, and heat treatment including preheating can be performed within 60 minutes. is there. In addition, when apply | coating a polyimide precursor, you may apply | coat as a polyimide precursor solution which dissolved the polyimide precursor in the well-known solvent.

  The preferred degree of polymerization of the polyimide precursor or polyimide of the present invention is 1,000 to 40,000 cP, and preferably 3,000 to 5,000 cP, as measured by an E-type viscometer of the polyimide precursor solution. The molecular weight of the polyimide precursor can be determined by the GPC method. The preferred molecular weight range (polystyrene equivalent) of the polyimide precursor is preferably in the range of 15,000 to 250,000 in terms of number average molecular weight and in the range of 30,000 to 800,000 in terms of weight average molecular weight. Not all are unusable. The molecular weight of polyimide is also in the same range as the molecular weight of its precursor.

In the polyimide precursor of the present invention, the polyimide obtained by imidizing it in a nitrogen atmosphere has a yellowness of 6 or less, preferably 4 or less. If it is this range, it can be used conveniently for the board | substrates which are required to be transparent and colorless, such as a TFT substrate for organic EL devices, a touch panel substrate, and a color filter substrate.
And it is good that a thermal expansion coefficient (CTE) is 35 ppm / K or less. Preferably it is 30 ppm / K or less. Here, CTE is a linear expansion coefficient (also referred to as “thermal expansion coefficient”) when changed from 250 ° C. to 100 ° C. unless otherwise specified. If it is this range, the curvature of a board | substrate can be suppressed at the time of manufacture of flexible devices, such as a functional layer lamination | stacking in the TFT substrate for organic EL apparatuses, a touch panel substrate, a color filter, etc., and it is excellent in the manufacture yield of a flexible device.
Furthermore, the polyimide obtained by imidizing this has a light transmittance of 308 nm of 5% or less, and a light transmittance of 430 nm of 70% or more, preferably 80% or more. The light transmittance at 308 nm is more preferably 3% or less, still more preferably less than 1%, and further preferably less than 0.1%. Here, when the polyimide of the present invention is used as a polyimide film, preferably when used for a flexible device, it is more preferably used as a polyimide film with a functional layer provided with a functional layer on the polyimide film. In this case, the light transmittance is a value measured for the film unless otherwise specified. In a preferred embodiment, it is a value measured in the state of a film having a thickness of 10 to 15 μm, and the transmittance may be given in any of the above thickness ranges. In a more preferred embodiment, it is a value measured in the state of a film having a thickness of 13 μm. In this case, the value measured in the state of a film having a thickness of about 13 μm can be a value converted to a 13 μm film.

  In this way, light in the near ultraviolet region is absorbed, and the transmittance for light in the visible region is increased. Within this range, 308 nm laser light can be absorbed while maintaining transparency in the visible light region. As a result, without damaging the display device on the transparent polyimide (PI) layer by irradiating a flexible substrate such as a substrate for organic EL devices, a touch panel substrate, and a color filter substrate having a top emission structure with a laser, PI can be peeled from glass and can be suitably manufactured by a laser lift-off method.

  The polyimide of the present invention is a polyimide having a structural unit derived from a diamine and a structural unit derived from an acid dianhydride, i) a structural unit derived from an aromatic diamine represented by the above formula (1), ii) having a structural unit derived from 2,2′-bis (trifluoromethyl) benzidine or a structural unit derived from 1,2,3,4-cyclobutanetetracarboxylic dianhydride; Here, the description regarding the structural unit derived from the diamine and the structural unit derived from the acid dianhydride is the same as the description of the polyimide precursor. The polyimide of the present invention is also an imidized product of the polyimide precursor of the present invention. Although imidization is performed by dehydrating and ring-closing the polyimide precursor, the arrangement of the structural units is similarly maintained.

The polyimide of the present invention has a yellowness of 6 or less, preferably 4 or less. Moreover, CTE is 35 ppm / K or less, Preferably it is 30 ppm / K or less, More preferably, it is good that it is 20 ppm / K or less.
The polyimide of the present invention preferably has a light transmittance of 308 nm of 5% or less and a light transmittance of 430 nm of 70% or more. From the viewpoint of transparency, the light transmittance of all light rays in the visible region is 70% or more, preferably 80% or more. And from a heat resistant viewpoint, it is good that a glass transition temperature is 360 degreeC or more, Preferably it is 400 degreeC or more. The Td1 thermal decomposition temperature (1% weight loss temperature) is preferably 300 ° C. or higher. The light transmittance at 308 nm is more preferably 3% or less, still more preferably less than 1%, and further preferably less than 0.1%.

  The polyimide precursor and polyimide satisfying the above characteristics are obtained by setting the content of structural units (U1) to (U3) included as essential or preferable structural units in the polyimide precursor and polyimide of the present invention to a certain level or more. It is done.

Although there is no restriction | limiting in the method which uses a polyimide precursor as a polyimide, When using a polyimide as a support base material, obtaining as a laminated body containing a film form or a polyimide layer is advantageous.
Preferably, a resin solution (resin composition) containing a polyimide precursor is applied on a substrate and then dried and heat-treated, or a resin solution that has been completed up to imidization in a liquid phase is applied and dried on a substrate. Alternatively, a polyimide laminate can be obtained by pasting a separately prepared polyimide film on another substrate. From the viewpoint of production efficiency, it is desirable that imidization is performed on the substrate as described above to obtain a laminated body as it is, and this is peeled off if necessary to form a film.

  Moreover, the polyimide of this invention is suitable as a polyimide film with a functional layer. The polyimide film in this case may be made of a plurality of layers of polyimide. In the case of a single layer, it is preferable to have a thickness of 3 μm to 50 μm. On the other hand, in the case of a plurality of layers, the main polyimide layer may be a polyimide film having the above thickness. Here, the main polyimide layer refers to a polyimide layer occupying the largest proportion of the thickness of the polyimides in a plurality of layers, and is a layer made of the polyimide of the present invention, and preferably has a thickness of 3 μm to 50 μm. More preferably, it is 4-30 micrometers.

  The polyimide of the present invention can be a laminate having this polyimide layer, and element layers and the like (functional layers) having various functions can be formed on the surface of the polyimide layer. Examples of the functional layer include display devices such as liquid crystal display devices, organic EL display devices, touch panels, and electronic paper, and include display devices such as color filters or these components. In addition, the display device including an organic EL lighting device, a touch panel device, a conductive film laminated with ITO, a touch panel film, a gas barrier film that prevents permeation of moisture, oxygen, and the like, and components of a flexible circuit board Various functional devices used accompanying the above are also included. That is, the functional layer mentioned here is not only a component such as a liquid crystal display device, an organic EL display device, and a color filter, but also an organic EL lighting device, a touch panel device, an electrode layer or a light emitting layer of the organic EL display device, a gas barrier. A combination of one or more of a film, an adhesive film, a thin film transistor (TFT), a wiring layer of a liquid crystal display device or a transparent conductive layer is also included.

  In addition, the formation method of the functional layer is appropriately set according to the target device, but in general, after forming a metal film, an inorganic film, an organic film, etc. on the polyimide film, It can be obtained by a known method such as patterning into a predetermined shape or heat treatment as required. That is, the means for forming these display elements is not particularly limited, and is appropriately selected, for example, sputtering, vapor deposition, CVD, printing, exposure, immersion, etc. If necessary, in a vacuum chamber, etc. These process processes may be performed. The substrate and the polyimide film may be separated immediately after the functional layer is formed through various process treatments. For example, immediately before being used as a display device, the substrate is integrated with the base material for a certain period of time. It may be separated and removed.

  As a method for facilitating peeling of the polyimide film from the substrate and preventing stretching, there is a laser lift-off method. For example, in Japanese Translation of PCT International Publication No. 2007-512568, a polyimide yellow film is formed on a glass plate, and then a thin film electronic device is formed on the yellow film, and then the bottom surface of the yellow film is irradiated with UV laser light through the glass. The glass and the yellow film can be peeled off. According to this method, since the polyimide film is separated from the glass by UV laser light, no stress is generated at the time of peeling, which is a preferable method. However, it is also disclosed that, unlike a yellow film, a transparent plastic does not absorb UV laser light, and therefore it is necessary to previously provide an absorption / release layer such as amorphous silicon under the film.

  In Japanese translation of PCT publication 2012-511173, it is disclosed that a laser having a range of 300 to 410 nm is used in order to peel off a glass plate and a polyimide layer by irradiation with UV laser light. By the way, as a peeling method, after irradiating a laser from the glass side and separating the resin base material provided with the display part from the glass, a release layer is applied to a glass substrate and formed, and then polyimide is formed on the release layer. After the resin is applied and the manufacturing process of the organic EL display device is completed, the polyimide layer is peeled from the release layer, the surface of the inorganic layer is treated with a coupling agent, and then the coupling agent is patterned by UV irradiation or the like. Examples of the method include performing a treatment to form a laminate having a good adhesion portion and an easy peel portion having different peel strengths, and then peeling the laminate.

  Since the wavelength of light emitted from the light emitting layer of the organic EL device is mainly 440 nm to 780 nm, the average transmittance in this wavelength region is at least 80% or more as a support substrate used in the organic EL device. Is required. On the other hand, when the glass and polyimide layers are peeled off by irradiation with the UV laser light described above, if the transmittance at the wavelength of the UV laser light is high, it is necessary to provide a separate absorption / peeling layer. Productivity decreases. Currently, a 308 nm laser apparatus is generally used for this peeling. In order to perform peeling without providing this, it is necessary that the polyimide itself absorbs 308 nm laser light sufficiently, and it is desirable that light is not transmitted as much as possible.

  The outline of the manufacturing method will be described below using a bottom emission organic EL display device as a functional layer as a representative example.

  A gas barrier layer is provided on the resin base material forming the functional layer so that moisture and oxygen can be prevented from permeating. Next, a circuit configuration layer including a thin film transistor (TFT) is formed on the upper surface of the gas barrier layer. In this case, in the organic EL display device, an LTPS-TFT having a high operation speed is mainly selected as the thin film transistor. In this circuit configuration layer, an anode electrode made of, for example, an ITO (Indium Tin Oxide) transparent conductive film is formed on each of the plurality of pixel regions arranged in a matrix on the upper surface thereof. Further, an organic EL light emitting layer is formed on the upper surface of the anode electrode, and a cathode electrode is formed on the upper surface of the light emitting layer. This cathode electrode is formed in common in each pixel region. Then, a gas barrier layer is formed again so as to cover the surface of the cathode electrode, and a sealing substrate is installed on the outermost surface for surface protection. It is desirable from the viewpoint of reliability that a gas barrier layer for preventing moisture and oxygen from permeating is laminated on the surface of the sealing substrate on the cathode electrode side. The organic EL light-emitting layer is formed of a multilayer film (anode electrode-light-emitting layer-cathode electrode) such as a hole injection layer-hole transport layer-light-emitting layer-electron transport layer. Since it deteriorates due to moisture and oxygen, it is generally formed by vacuum deposition, and it is generally formed continuously in vacuum including electrode formation.

  Next, an outline of a touch panel used as input means of a display device such as a liquid crystal display device or an organic EL display device will be described. As described above, in addition to the reduction in thickness and weight of the display device, the progress of flexibility has been remarkable. However, the touch panel installed on the display device has been actively studied for reduction in thickness, weight, and flexibility.

  There is a similar request for a touch panel, and the above-described studies have been made.

  Hereinafter, based on an Example and a comparative example, this invention is demonstrated concretely. The present invention is not limited to these contents.

Abbreviations and evaluation methods of materials used in Examples and Comparative Examples are shown.
(Acid dianhydride)
・ BPDA: 3,3 ', 4,4'-biphenyltetracarboxylic dianhydride ・ CBDA: 1,2,3,4-cyclobutanetetracarboxylic dianhydride ・ 6FDA: 4,4'-(2,2 '-Hexafluoroisopropylidene) diphthalic dianhydride (diamine)
・ AAPBZI: 5-amino-2- (4-aminophenyl) benzimidazole ・ AAPBZO: 5-amino-2- (4-aminophenyl) benzoxazole ・ TFMB: 2,2'-bis (trifluoromethyl) benzidine ( solvent)
・ NMP: N-methyl-2-pyrrolidone

(Light transmittance and YI)
The light transmittance (T308, T355, T400, T430) at 308 nm, 355 nm, 400 nm, and 430 nm of the polyimide film (50 mm × 50 mm) was obtained using a SHIMADZU UV-3600 spectrophotometer.
Further, YI (yellowness) was calculated based on the following calculation formula.
YI = 100 × (1.2879X−1.0592Z) / Y
X, Y, and Z are the tristimulus values of the test piece, specified in JIS Z 8722.
Further, the film was heated from room temperature to 300 ° C. at a constant rate (3 ° C./min) in an N ₂ atmosphere, held for 30 minutes, then returned to room temperature, the film was taken out, and YI (YI300 after heat treatment) ° C).

(CTE)
A polyimide film having a size of 3 mm × 15 mm was heated from 30 ° C. to 280 ° C. at a constant heating rate (10 ° C./min) while applying a 5.0 g load with a thermomechanical analysis (TMA) apparatus, and then The temperature was lowered from 250 ° C. to 100 ° C., and the thermal expansion coefficient was measured from the amount of elongation of the polyimide film at the time of temperature reduction.

(Pyrolysis temperature)
Under a nitrogen atmosphere, a change in weight was measured when a 10-20 mg polyimide film was heated from 30 ° C. to 550 ° C. at a constant rate with a thermogravimetric analysis (TG) device TG / DTA6200 made by SEIKO, and 200 The temperature at 0 ° C. was defined as zero, and the temperature at a weight reduction rate of 1% was defined as the thermal decomposition temperature (Td 1%).

(Glass transition temperature; Tg)
The dynamic viscoelasticity of a polyimide film (5 mm × 70 mm) was measured with a dynamic thermomechanical analyzer from 23 ° C. to 500 ° C. at a rate of 5 ° C./minute, and the glass transition temperature (tan δ maximum value: ° C. )

(Laser lift off; LLO)
A laminate of a polyimide layer and a glass substrate is irradiated with a laser having a beam size of 14 mm × 1.2 mm and a moving speed of 6 mm / s from the support substrate (glass substrate) side using an excimer laser processing machine (wavelength 308 nm). The state where the support base material and the polyimide layer are completely separated (determine the peeling range with a cutter, and the polyimide film spontaneously peels from the glass after one cut is made), the entire surface of the coating base material and the polyimide layer or The state where some separation was impossible or the polyimide layer was discolored was rated as x.
(LED): Laser irradiation energy density (mJ / cm ² ) until the polyimide layer and the glass substrate can be peeled is abbreviated as LED. The higher the energy density, the more difficult it is to peel off. Considering the lifetime of the laser irradiation apparatus, those having a low irradiation energy density are preferable.

  According to the following synthesis examples 1-9, the resin solution (polyimide precursor solution) for forming the polyimide layer which concerns on the polyimide laminated body of an Example and a comparative example was prepared. In addition, the weight composition of the monomer in each polyimide precursor solution is put together in Table 1 and Table 2, and is shown.

Synthesis example 1
Under a nitrogen stream, 8.5575 g of TFMB was dissolved in 85 g of NMP in a 100 ml separable flask. To this solution was then added 0.6678 g of AAPBZI. After stirring for 10 minutes, 5.7757 g of CBDA was added. The molar ratio (a / b) between the acid dianhydride (a) and the diamine (b) was 0.990. This solution is heated at 40 ° C. for 10 minutes to dissolve the contents, and then the solution is continuously stirred at room temperature for 24 hours to carry out a polymerization reaction to obtain a high-polymerization degree polyimide precursor A (viscous colorless solution). Got.

Synthesis Examples 2-8
Except having changed the acid dianhydride and diamine into the mass composition shown in Table 1, the polyimide precursor solution was prepared like the synthesis example 1, and polyimide precursor BI was obtained.

Example 1
A solvent NMP is added to the polyimide precursor solution A obtained in Synthesis Example 1 and diluted to have a viscosity of 4000 cP, and then a glass substrate (Corning E-XG, size = 150 mm × 150 mm, thickness = 0) The thickness of the polyimide after curing was about 13 μm using a spin coater. Subsequently, heating was performed at 100 ° C. for 15 minutes. In a nitrogen atmosphere, the temperature is raised from room temperature to 300 ° C. at a constant rate of temperature increase (3 ° C./min), held at 130 ° C. for 10 minutes, and a 150 mm × 150 mm first polyimide layer (150 mm × 150 mm) Polyimide A) was formed to obtain a polyimide laminate A.

Examples 2-6, Comparative Examples 1-3
Except having replaced the polyimide precursor with any of polyimide precursor BI, it operated similarly to Example 1 and obtained polyimide laminated body BI. The code | symbol of a polyimide precursor and a polyimide laminated body respond | corresponds, it means that the polyimide laminated body B was obtained from the polyimide precursor B, and it is the same also about the code | symbol C or less.

  The obtained polyimide laminates A to I were measured for laser lift-off (LLO) and LED. The results are shown in Tables 3 and 4.

For the measurements other than the above, the polyimide film was peeled off from the laminate. In this case, the laminate was prepared in accordance with the above except that a 75 μm polyimide film was used as the substrate instead of the glass substrate. . Detailed preparation conditions are shown below.

A solvent NMP is added to the polyamic acid solutions A to I obtained in Synthesis Examples 1 to 9 to dilute the viscosity to 3000 cP, and then coated on a 75 μm polyimide film (Upilex-S) substrate. Worked. Subsequently, heating was performed at 100 ° C. for 15 minutes. Then, in a nitrogen atmosphere, the temperature is raised from room temperature to 300 ° C. (Comparative Examples 2 and 3 is 360 ° C.) at a constant rate of temperature increase (3 ° C./min), and held at 130 ° C. for 10 min. Obtained. Then, the polyimide substrate was peeled off to form polyimide films A to I. The above-described peeling was performed by peeling only the formed polyimide layer once with a cutter to determine the range to be peeled off, and then peeling from the substrate with tweezers. The thickness of these films is shown in the section of thickness.

The obtained polyimide films A to I were subjected to various evaluations such as CTE, light transmittance, YI, Td1, and Tg. The results are shown in Tables 3 and 4.

Claims (9)

A polyimide precursor having a structural unit derived from a diamine and a structural unit derived from an acid dianhydride, i) a structural unit derived from an aromatic diamine represented by the following formula (1); A polyimide precursor comprising a structural unit derived from 2'-bis (trifluoromethyl) benzidine or a structural unit derived from 1,2,3,4-cyclobutanetetracarboxylic dianhydride.

(In the formula (1), Z is NH or O.)   The polyimide precursor of Claim 1 which contains 5 mol% or more of the structural units derived from aromatic diamine with respect to all the structural units derived from diamine.   The polyimide precursor according to claim 1 or 2, wherein the structural unit derived from 1,2,3,4-cyclobutanetetracarboxylic dianhydride contains 50 mol% or more of all structural units derived from the acid dianhydride.   The polyimide precursor according to any one of claims 1 to 3, comprising a structural unit derived from 2,2'-bis (trifluoromethyl) benzidine in an amount of 50 mol% or more of all structural units derived from diamine.   The polyimide obtained by imidizing the polyimide precursor in a nitrogen atmosphere has a yellowness of 6 or less and a thermal expansion coefficient of 35 ppm / K or less when changed from 250 ° C to 100 ° C. The polyimide precursor in any one of -4.   The polyimide precursor according to any one of claims 1 to 5, wherein the polyimide obtained by imidizing the polyimide precursor has a light transmittance of 308 nm of 5% or less and a light transmittance of 430 nm of 70% or more. A polyimide having a structural unit derived from a diamine and a structural unit derived from an acid dianhydride, i) a structural unit derived from an aromatic diamine represented by the following formula (1); and ii) 2,2 ′ -A polyimide having a structural unit derived from bis (trifluoromethyl) benzidine or a structural unit derived from 1,2,3,4-cyclobutanetetracarboxylic dianhydride.

(In the formula (1), Z is NH or O.)   The polyimide according to claim 7, which has a yellowness of 6 or less and a thermal expansion coefficient of 35 ppm / K or less when the temperature is changed from 250 ° C to 100 ° C. The polyimide according to claim 7 or 8, wherein the light transmittance at 308 nm is 5% or less, and the light transmittance at 430 nm is 70% or more.