Preparation and properties of heat-sealable polyimide films with comparable coefficient of thermal expansion and good adhesion to copper matrix

A series of thermoplastic polyimide (PI) films (PI-1~PI-5) with intrinsic heat sealability have been synthesized by the high-temperature polycondensation reaction of asymmetrical 2,3,3ꞌ,4ꞌ-oxydiphthalic anhydride (aODPA) and various aromatic diamines in order to meet the demands of advanced flexible copper clad laminates (FCCL). The derived PI resins had good solubility in polar aprotic solvents, such as N-methyl-2-pyrrolidone (NMP) and N,N-dimethylacetamide (DMAc) at a solid content up to 20 wt%. Flexible and tough films could be cast from the PI/NMP solution and the films exhibited tensile strength higher than 83 MPa. Among the developed PIs, those derived from aODPA and benzimidazole-containing diamines, including PI-4 from aODPA and 2-(4-aminophenyl)-5-aminobenzimidazole (4APBI) and PI-5 from aODPA and 2-(3-aminophenyl)-5-aminobenzimidazole (3APBI) exhibited the highest thermal stability (glass transition temperatures, Tg > 340°C), lowest linear coefficients of thermal expansion (CTE < 35·10–6 1/K), and superior adhesion to copper foil (peeling strength >1.0 N/mm). Flexible copper clad laminate (FCCL) with no curling was successfully prepared from PI-4 and copper foil.