What are the properties of polyimide?

Introduction to the properties of polyimide

1. According to thermogravimetric analysis, the decomposition temperature of fully aromatic polyimide is generally around 500°C. The polyimide synthesized from pyromellitic dianhydride and p-phenylenediamine has a thermal decomposition temperature of 600°C and is one of the polymers with the highest thermal stability to date.

2. Polyimide can withstand extremely low temperatures, such as not brittle in liquid helium at -269°C.

3. Polyimide has excellent mechanical properties. The tensile strength of unfilled plastics is above 100MPa, the film of pyromellitic polyimide (Kapton) is above 170MPa, and the impact strength of thermoplastic polyimide (TPI) is as high as 261kJ/m2. The biphenyl polyimide (Upilex S) reaches 400MPa. As an engineering plastic, the elastic modulus is usually 3-4GPa, and the fiber can reach 200GPa. According to theoretical calculations, the fiber synthesized from pyromellitic anhydride and p-phenylenediamine can reach 500GPa, second only to carbon fiber.

4. Some polyimide varieties are insoluble in organic solvents, stable to dilute acid, and generally not very resistant to hydrolysis. This seemingly disadvantageous property makes polyimide a great feature different from other high-performance polymers, that is, the raw materials dianhydride and diamine can be recovered by alkaline hydrolysis. For example, for Kapton film, the recovery rate can reach 80%-90%. Changing the structure can also obtain varieties that are quite resistant to hydrolysis, such as being able to withstand 120℃, 500 hours of boiling.

5. Polyimide has a wide solubility spectrum. Depending on the structure, some varieties are almost insoluble in all organic solvents, while others can be dissolved in common solvents, such as tetrahydrofuran, acetone, chloroform, and even toluene and methanol.

6. The thermal expansion coefficient of polyimide is 2×10-5-3×10-5/℃, thermoplastic polyimide is 3×10-5/℃, biphenyl type can reach 10-6/℃, and some varieties can reach 10-7/℃.

7. Polyimide has high radiation resistance, and its film has a strength retention rate of 90% after 5×109rad fast electron irradiation.

8. Polyimide has good dielectric properties, with a dielectric constant of about 3.4. By introducing fluorine or dispersing air nanometers in polyimide, the dielectric constant can be reduced to about 2.5. The dielectric loss is 10-3, the dielectric strength is 100-300kV/mm, and the volume resistivity is 1017Ω·cm. These properties can still be maintained at a high level over a wide temperature and frequency range.

9. Polyimide is a self-extinguishing polymer with low smoke generation rate.

10. Polyimide has very little outgassing under extremely high vacuum.

11. Polyimide is non-toxic and can be used to make tableware and medical instruments, and can withstand thousands of sterilizations. Some polyimides also have good biocompatibility. For example, they are non-hemolytic in blood compatibility experiments and non-toxic in in vitro cytotoxicity experiments.