Method for preparing polyimide film having low dielectric constant and high fracture toughness

A method for preparing a polyimide film having a low dielectric constant and high fracture toughness, comprising the steps of:

• (1) dissolving aromatic diamine in a polar organic solvent at an ambient humidity of lower than 50% under ice bath and nitrogen atmosphere, maintaining the mass concentration thereof at 6% to 10%;
• after the aromatic diamine is completely dissolved, further stirring for 25-35 min to obtain an aromatic diamine solution;
• wherein the aromatic diamine is 4,4′-diaminodiphenyl ether, p-phenylenediamine or 4,4′-diamino-2,2′-bistrifluoromethylbiphenyl;
• (2) grinding and mixing norbornene dianhydride-maleimido heptaisobutyl polysilsesquioxane alternating copolymer and aromatic dianhydride, adding to the aromatic diamine solution prepared in step (1) and stirring for 12 to 24 h to obtain a norbornene dianhydride-maleimido heptaisobutyl polysilsesquioxane alternating copolymer/polyamic acid solution;
• wherein the molar ratio of the aromatic dianhydride and the aromatic diamine is 1:1, and the mass of the norbornene dianhydride-maleimido heptaisobutyl polysilsesquioxane alternating copolymer is 1% to 6% of the total mass of the aromatic dianhydride and the aromatic diamine;
• (3) uniformly applying the norbornene dianhydride-maleimido heptaisobutyl polysilsesquioxane alternating copolymer/polyamic acid solution prepared in step (2) on a clean glass sheet, then placing it in a vacuum drying oven to remove the bubbles under vacuum for 4-6 h;
• heating and maintaining the temperature according to the preset program, which comprises maintaining at 70±2° C. for 1.8-2.2 h, maintaining at 120±2° C. for 1-1.2 h, maintaining at 150±2° C. for 1.8-2.2 h, maintaining at 200±2° C. for 1-1.2 h and maintaining at 300±2° C. for 1-1.2 h;
• after that cooling to room temperature, peeling off a film by ultrasonic in water, then drying the film under vacuum to obtain the polyimide film having a low dielectric constant and high fracture toughness.