Properties, synthetic methods and types of polyimide (PI)

Polyimide (PI) is a kind of polymer material with high heat resistance developed in the 1950s, which generally refers to a kind of polymer containing imide ring (-co-NH-Co -) in the main chain.

Ⅰ.Performance characteristics

Because the polyimide structure has a very stable aromatic heterocyclic ring, it has many excellent properties:

(1) Heat resistance
Polyimide has a strong heat resistance, TGA thermogritty analysis shows that polyimide decomposition temperature can reach 500℃ -600 ℃, is one of the most stable polymers at this stage.

(2) Low temperature resistance
Polyimide material has the characteristics of ultra-low temperature resistance, even in ultra-low temperature liquid nitrogen, will not be brittle, still can maintain a certain mechanical strength.

(3)Good mechanical properties
Kapton has a tensile strength of 170 MPa and a tensile modulus of 3.0 GPa, while Upilex has a tensile strength of 400 MPa and a tensile modulus of 3 ~ 4 GPa, which can be greater than 200 GPa.

(4) Good dimensional stability
Polyimide material has a very low coefficient of thermal expansion, the coefficient of thermal expansion is generally 2×105 ~ 3×10-5 /℃, the coefficient of thermal expansion of biphenyl polyimide is 1×10-6 /℃, and the coefficient of thermal expansion of individual products can be 1×10-7 /℃, which is close to the coefficient of thermal expansion of metal. It is this low coefficient of thermal expansion characteristic that PI can be widely used in the manufacture of flexible printed circuit boards.

(5) Good dielectric and insulation
The permittivity of polyimide materials is generally between 3.0 and 3.6, and when fluorine atoms are introduced or nano-scale air is dispersed in it, the permittivity can be reduced to between 2.5 and 2.7, or even lower. The dielectric loss is about 1×10-3, the dielectric strength is 100 ~ 300 Kv/mm, and the volume resistance is 1× 1017ω.cm. This low dielectric constant polyimide provides a guarantee for the application of packaging materials and insulation materials in the microelectronics industry.

(6) Good radiation resistance
Polyimide material is stable under high temperature, high vacuum and irradiation, and less volatile matter.

(7) Good chemical stability
Usually polyimides are insoluble in common organic solvents, and common soluble polyimides are only dissolved in some specific polar organic solvents, but polyimides, like other aromatic polymers, are not resistant to concentrated sulfuric acid, concentrated nitric acid and halogens. Polyimide has strong hydrolysis resistance to dilute acids and high stability to oxidants and reducing agents, especially at high temperatures. But the general varieties are not resistant to hydrolysis, especially alkaline hydrolysis.

(8) Good flame retardancy
Polyimide is a self-quenching polymer, the smoke rate is very low, the residual carbon rate after high temperature combustion is often more than 50%, is a good heat inhibitor and flame retardant.

(9) Non-toxic and biocompatible
Polyimides are non-toxic, and some polyimides have good biocompatibility. Polyimides are used in the manufacture of tableware and medical devices and can withstand thousands of sterilizations.

(10) Polyimide also has its disadvantages 

Traditional polyimide is insoluble and difficult to process; The film is hard, brittle and has poor strength. When used in microelectronics industry, its thermal expansion coefficient is not good; Used in optical communication industry, poor transparency; At the same time, its adhesion is not very good.


Ⅱ.Synthesis method

The synthesis methods of polyimide can be divided into two categories, the first is the formation of imide rings in the polymerization process or in macromolecular reaction; The second type is the synthesis of polyimide by monomers containing imide rings. The first synthesis method mainly includes: polyimide is formed by the reaction of dianhydride and diamine; Polyimide is formed by the reaction of quaternary acid and binary amine. Polyimide was obtained by reaction of diester and diamine of quaternary acid, and polyimide was obtained by reaction of dianhydride and diisocyanate.
In the second class of synthesis methods, almost all general polycondensation reactions have been used to synthesize various polymers with imide rings from monomers with imide rings, such as polyester imide, polyamide imide, polycarbonate imide, polyurethane imide, etc.



According to the chemical structure of the repeating unit, polyimides can be divided into three kinds: aliphatic, semi-aromatic and aromatic polyimides. According to the thermal properties, it can be divided into thermoplasticity (including the true sense of thermoplasticity and false thermoplasticity) and thermosetting 2. Thermoplastic polyimide can flow deformation when heated, and can maintain a certain shape after cooling. In a certain temperature range, it can be repeatedly heated to soften and cooled to harden. The ether type polyimide obtained by the reaction of diphenyl ether tetracarboxylic anhydride with aromatic diamine belongs to this class and can be molded several times at 390℃.
Thermosetting polyimide can soften the flow when it is first heated, heated to a certain temperature, resulting in a chemical reaction – cross-linking curing and hardening, this change is irreversible, and then, when it is reheated, it can no longer become soft flow.
Thermosetting polyimides are usually composed of low molecular weight polyimides or polyamides with unsaturated groups at the ends, which are then polymerized by unsaturated end groups when applied. According to the different sealing agent and synthesis method, it is mainly divided into bismaleimide resin, PMR type polyimide resin, phenylidene-terminated polyimide resin, asymmetric dianhydride based polyimide resin, imide-soluble polyimide resin and acetylidene-terminated polyimide resin.


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Post time: Jun-26-2023