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Colorless and transparent PI film was prepared by the introduction of trifluoromethyl and flexible ether bond in molecular structurecan, but at the same time, due to the addition of flexible groups, the thermal and mechanical properties of PI films reduced, especially the introduction of trifluoromethyl groups can significantly reduce the adhesion performance of PI film on other materials, limiting its application in various aspects.
1.2 Overview of traditional polyimide
1.2.1 The development process of polyimide
PI is a kind of polymer containing imide ring, which is prepared of diamine and dianhydride by a series of reaction[3]. This kind of polymer have been reported early in 1908 , but then due to the understanding of the polymer is not deep enough, so that enough attention is paid to until in the 1960 s, DuPont applied for a series of patent and the commercialization of polyimide film (Kapton) and varnish (Pyre ml), which began the era of vigorous development of PI [4]. Research and development of PI in China was began in 1962, after enameled wire research and development in 1966 ,PI adhesive was reported, such as plastics and films. So far, Department of domestic research on PI research are Changchun research institute, Institute of Chemistry of Chinese Academy of science, Chengdu University of science and technology, Shanghai synthetic resin Research Institute, Guilin electrical equipment scientific research institute. Our country although in many areas has been close to the level of developed countries, but overall the gap with the developed countries is still far, mainly in small scale of production, the product category is few, and it is difficult to meet the requirements in some applications. In the field of PI application, the development prospects are very good, such as in the aerospace, electrical and electronic, automobile manufacturing and other fields have great value, therefore, to speed up the research and application of Chinese PI is very necessary.
1.2.2 The performance and application of polyimide
There are a lot of benzene and imide ring in the molecular structure of PI,and it has excellent comprehensive performance, mainly manifested in the following aspects:
(1) High/low temperature resistancee: for the aromatic PI, according to TG analysis, the decomposition temperature is generally at about 500 ℃, biphenyl-PI thermal decomposition temperature can reach 600 ℃, is by far one of the polymers with the highest stability varieties; in addition, it is not brittle in the thermodynamic temperature of 4K liquid states of helium.
(2) Good mechanical properties: unfilled plastic strength is over 100MPa, benzene- polyimide film is 250Mpa, biphenyl-polyimide films reaches 530Mpa. The elastic modulus is also far more than the general engineering plastics, elastic modulus of PI fiber are just inferior to carbon fiber in theory.
(3) Resistance to dilute acid, generally resistant to hydrolysis, especially alkaline hydrolysis, not concentrated nitric acid, sulfuric acid and halogen. Therefore, the use of recycled materials has become the alkaline hydrolysis of polyimide, which is different from other polymers.
(4) The solubility of different varieties of PI. Due to the differences in the structure, it can dissolve in ordinary solvents, while others are almost insoluble in all solvents.
(5) Low coefficient of thermal expansion. Thermal expansion coefficient of Biphenyl-polyimide is similar with metal, inpidual species is low 10-7/℃.
(6) Good radiation resistance. The film can still reach a high level in the absorption of a large number of electronic radiation intensity.
(7) Good dielectric properties. The relative permittivity of the ordinary aromatic polyimide is about 3.4, introducing fluorine or large side groups, the relative dielectric constant can drop to 2.5.
(8) The smoking rate is low. Polyimide is a self-extinguishing type polymer.
(9) Less amount of gas under high vacuum.