Polyimide has a wide variety and various forms, and has a variety of ways in synthesis. Therefore, it can be selected according to various application purposes. This synthetic variability is also difficult for other polymers. The synthesis is described as follows:
Polyimide is mainly synthesized from binary anhydride and binary amine. Compared with many other heterocyclic polymers, such as polybenzimidazole, polybenzimidazole, polythiazole, Polyquinoline and other monomers, these two monomers have a wide source of raw materials and are easy to synthesize. There are many kinds of dianhydride and diamine. Polyimides with different properties can be obtained by different combinations.
Polyimide can be polymerized at low temperature by dianhydride and diamine in polar solvents, such as DMF, DMAC, NMP or the / methanol mixed solvent to obtain soluble polyamide acid. After film-forming or spinning, it is heated to about 300 ℃ for dehydration to form a ring and transformed into polyimide; Acetic anhydride and tertiary amine catalysts can also be added to polyamide acid for chemical dehydration and cyclization to obtain polyimide solution and powder. Diamine and dianhydride can also be heated and polycondensated in high boiling solvents, such as phenolic solvents, to obtain polyimide in one step. In addition, polyimide can be obtained by the reaction of binary ester of Quaternary acid with binary amine; It can also be converted from polyamide acid to polyimide, and then to polyimide. These methods bring convenience to processing. The former is called PMR method, which can obtain low viscosity and high solid content solution. There is a window with low melt viscosity during processing, which is especially suitable for the manufacture of composite materials; The latter increases the solubility and does not release low molecular compounds in the process of transformation.