Polyimide film

1. A polyimide film, which is manufactured from a copolymer of aromatic dianhydrides and an aromatic diamine, and has an average coefficient of linear thermal expansion of 35.0 ppm/° C. or less, when measured at 50-250° C. for a film thickness of 50-100 μm according to a TMA method, with a yellowing index of 4.78 or less,

wherein the polyimide film has color coordinates in which L is 90 or more, a is 5 or less and b is 5 or less, when measured using a UV spectrophotometer for the film thickness of 50-100 μm;

wherein the aromatic dianhydrides comprise (i) one or more selected from the group consisting of 2,2-bis(3,4-dicarboxyphenyl)hexafluoropropane dianhydride, 4-(2,5-dioxotetrahydrofuran-3-yl)-1,2,3,4-tetrahydronaphthalene-1,2-dicarboxylic anhydride, and 4,4′-(4,4′-isopropylidenediphenoxyl)bis(phthalic anhydride), and (ii) one or more selected from the group consisting of pyromellitic dianhydride, biphenyltetracarboxylic dianhydride, and oxydiphthalic dianhydride;

wherein the aromatic diamine comprises (i) 2,2-bis[4-(4-aminophenoxy)-phenyl]propane 2,2′-bis(trifluoromethyl)-4,4′-diaminobiphenyl or 3,3′-bis(trifluoromethyl)-4,4′-diaminobiphenyl and (ii) one or more selected from the group consisting of 4,4′-bis(3-aminophenoxy)diphenylsulfone, bis(3-aminophenyl)sulfone, bis(4-aminophenyl)sulfone, 1,3-bis(3-aminophenoxy)benzene, 1,4-bis(4-aminophenoxy)benzene, 2,2′-bis[3(3-aminophenoxy)phenyl]hexafluoropropane, 2,2′-bis[4(4-aminophenoxy)phenyl]hexafluoropropane, 2,2′-bis(3-aminophenyl)hexafluoropropane, 2,2′-bis(4-aminophenyl)hexafluoropropane and oxydianiline;

wherein a molar ratio of the (i) aromatic dianhydride and the (ii) aromatic dianhydride is 4:6-8:2; and

wherein the polyimide film is obtained by polymerizing the aromatic diamine and the aromatic dianhydrides in presence of a first solvent to obtain a polyamic acid solution, imidizing the polyamic acid solution, adding the imidized solution to a second solvent, performing filtration and drying to obtain a solid polyimide resin, dissolving the solid polyimide resin in the first solvent to give a polyimide solution, which is then subjected to a film forming process.

2. The polyimide film according to claim 1 , which has an average transmittance of 85% or more at 380-780 nm, when measured using a UV spectrophotometer for the film thickness of 50-100 μm.

3. The polyimide film according to claim 1 , which has a transmittance of 88% or more at 550 nm and a transmittance of 70% or more at 440 nm, when measured using a UV spectrophotometer for the film thickness of 50-100 μm.

4. The polyimide film according to claim 1 , which has a 50% cut-off wavelength of 400 nm or less, when measured using a UV spectrophotometer.

5. The polyimide film according to claim 1 , wherein the second solvent has a polarity lower than that of the first solvent.

6. The polyimide film according to claim 1 , wherein the first solvent comprises one or more selected from the group consisting of m-cresol, N-methyl-2-pyrrolidone, dimethylformamide, dimethylacetamide, dimethylsulfoxide, acetone, diethylacetate; a low boiling point solvent; and a low absorbing solvent, and the second solvent comprises one or more selected from the group consisting of water, alcohols, ethers, and ketones.

7. A substrate for a display, comprising the polyimide film of claim 1 .

8. An alignment layer, comprising the polyimide film of claim 1 .

9. A protective film, comprising the polyimide film of claim 4 .

10. The substrate according to claim 7 , wherein the polyimide film has an average transmittance of 85% or more at 380-780 nm, when measured using a UV spectrophotometer for the film thickness of 50-100 μm.

11. The substrate according to claim 7 , wherein the polyimide film has a transmittance of 88% or more at 550 nm and a transmittance of 70% or more at 440 nm, when measured using a UV spectrophotometer for the film thickness of 50-100 μm.

12. The substrate according to claim 7 , wherein the polyimide film has a 50% cut-off wavelength of 400 nm or less, when measured using a UV spectrophotometer.