Polyimide has many varieties and forms, and there are many ways to synthesize. Therefore, it can be selected according to various application purposes. This synthetic flexibility is also difficult for other polymers to have. The synthesis is introduced as follows:
1. Polyimides are mainly synthesized from dianhydrides and diamines. These two monomers are combined with many other heterocyclic polymers, such as polybenzimidazole, polybenzimidazole, polybenzothiazole, and polyquinone Compared with monomers such as quinoline and polyquinoline, the raw materials are widely sourced and the synthesis is relatively easy. There are many varieties of dianhydride and diamine, and different combinations can obtain polyimides with different properties.
2. Polyimide can be polycondensed from dianhydride and diamine in polar solvents such as DMF, DMAC, NMP or THE / methanol mixed solvent at low temperature to obtain soluble polyamic acid, after film formation or spinning Heating to about 300 ° C dehydration to form a ring into polyimide; you can also add acetic anhydride and tertiary amine catalyst to the polyamic acid, chemical dehydration cyclization to obtain polyimide solution and powder. Diamine and dianhydride can also be polycondensed by heating in a high boiling point solvent, such as phenolic solvent, to obtain polyimide in one step. In addition, polyimide can also be obtained by reacting the dibasic ester of tetrabasic acid with diamine; it can also be converted from polyamic acid to polyisoimide first, and then to polyimide. These methods are convenient for processing. The former is called PMR method, which can obtain low viscosity and high solid solution. There is a window with low melt viscosity during processing, which is especially suitable for the manufacture of composite materials; the latter is increased In order to improve the solubility, low molecular compounds are not released during the conversion process.
3. As long as the purity of dianhydride (or tetraacid) and diamine is qualified, no matter what polycondensation method is adopted, it is easy to obtain a sufficiently high molecular weight. The addition of monoanhydride or monoamine can also easily control the molecular weight.
4. Polycondensation of dianhydride (or tetraacid) and diamine, as long as it reaches an equimolar ratio, heat treatment in vacuum can greatly increase the molecular weight of the solid low molecular weight prepolymer, so as to process and powder Come convenient.
5. It is easy to introduce reactive groups at the chain ends or chains to form active oligomers, thereby obtaining thermosetting polyimides.
6. Use the carboxyl group in the polyimide to esterify or form a salt, introduce a photosensitive group or a long-chain alkyl group to obtain an amphiphilic polymer, and obtain a photoresist or be used for the preparation of an LB film.
7. The general process of synthesizing polyimide does not produce inorganic salts, which is particularly advantageous for the preparation of insulating materials.
8. The dianhydride and diamine as monomers are easily sublimated under high vacuum, so it is easy to form a polyimide film on the workpiece, especially the device with uneven surface by vapor deposition method.