TECHNOLOGY FOCUS - POLYMERS - Preferred Components: The aromatic tetracarboxylic acid dianhydride is 4,4'-(hexafluoroisopropylidene)diphthalic anhydride (6FDA), pyromellitic dianhydride (PMDA), 3,3',4,4'-biphenyl dianhydride (BPDA), 2,3,3',4'-biphenyltetracarboxylic dianhydride ( alpha -BPDA), 2,2',3,3'-biphenyltetracarboxylic dianhydride, 3,3',4,4'-biphenyltetracarboxylic dianhydride, 2,3,3',4'-diphenyl ether tetracarboxylic acid dianhydride, 2,2',3,3'-diphenyl ether tetracarboxylic dianhydride, 3,3',4,4'-benzophenonetetracarboxylic dianhydride, 2,3,3',4'-benzophenonetetracarboxylic dianhydride, 2,2',3,3'-benzophenonetetracarboxylic dianhydride, 3,3',4,4'-diphenylsulfonetetracarboxylic dianhydride, 4,4'-sulfonyl diphthalic anhydride or 1,4,5,8- naphthalenetetracarboxylic dianhydride. The aromatic diamine is m-phenylenediamine, p-phenylenediamine, 4,4'-diaminodiphenyl ether, 3,4'-diamino diphenyl ether, diamino 4,4'-diaminodiphenyl sulfide, 4,4'-diaminodiphenylsulfone, 4,4'-diamino benzophenone, 1,3-bis(3-aminophenoxy)benzene, 1,3-bis (4-aminophenoxy benzene), 1,4-bis (3-aminophenoxy benzene), 1,4-bis (4-aminophenoxy benzene), bis(4-(4-aminophenoxy)phenyl)sulfone, 4,4'-bis(3-aminophenoxy)diphenyl sulfone, 4,4-bis(4-aminophenoxy)biphenyl, 2,2'-bis(trifluoromethyl)-(1,1'-biphenyl)-4,4'-diamine, 3,3-bis(trifluoromethyl)-(1,1'-biphenyl)-4,4'-diamine, 2,2-bis(3-(3-aminophenoxy)phenyl)hexafluoropropane, 2,2-bis(4-(4-aminophenoxy)phenyl)hexafluoropropane)2,2-bis(3-aminophenyl)hexafluoropropane or 2,2-bis(4-aminophenyl)hexafluoropropane. Preferred Condition: The inorganic material or inorganic nanomaterial are added to modify the polymer. The thickness of the polyimide film is 15-100 mu m, glass transition temperature is 180-350 degrees C. The content of polyimide film below glass transition temperature is 3-30ppm. The thermal expansion coefficient of the film is greater than 85% and is measured by ultraviolet spectrophotometer with cutoff wavelength of less than 550 nm, average transmittance in the range of 380-780nm. Preferred Components: The inorganic material is glass fiber powder, mineral fiber powder, glass bead, aramid fiber, ceramic fiber, boron fiber, talc, carbonate calcium, alumina powder, graphene, inorganic clay or mica powder. The inorganic nanomaterial is nano-silica, nano-alumina, nano-titanium dioxide, nano-zinc oxide, nano-zirconium oxide, nano-wollastonite, nano limestone, carbon nanotubes or carbon nanofiber. The inorganic material or inorganic nanomaterial is added to modified coupling agent. The reaction solution is stirred and vigorously mixed. The molar ratio of aromatic diamine and aromatic tetracarboxylic acid dianhydride is 0.97-1.03. The amount of the inorganic material or inorganic nano material is 0-10 wt.% or 0.5-5 wt.%. The inorganic material or inorganic nano-material is dispersed in mixed solvent of ethanol and water. The solution is stirred with an electric mixer to obtain homogeneous coupling agent. The homogeneous coupling agent is added to the dispersion medium. The mixture is filtered and dried to constant weight to obtain a modified inorganic material or inorganic nanomaterials. The coupling agent is silane coupling agent, titanate coupling agent, chromium complex coupling agent or zirconium coupling agent. The solvent is m-cresol, N-methylpyrrolidone, dimethyl formamide, dimethylacetamide, dimethylsulfoxide, acetone or diethyl acetate.