ABSTRACT
Polyimide materials have been extensively used in the aerospace and electronics fields because they are thermally stable, mechanically strong, and electrically insulating [1–3]. These fields also need transparent polyimide materials to cover solar cells and polyimides that have low permittivity to decrease the delay time of electrical circuits. Optoelectronics needs polyimide materials with transparency at wavelengths longer than those of the visible light region and a controllable refractive index. One of the most effective ways to satisfy these needs is to introduce fluorine into polyimide materials [4,5]. Fluorinated polymers, such as poly(tetrafluoroethylene), have attractive features such as low water uptake, water and/or oil repellence, low permittivity, low refractive index, resistance to wear and abrasion, and both thermal and chemical stability. The fluorine atom inducing these properties is the second smallest atom, and its 2s and 2d electrons are close to the nucleus. Its electric polarity is therefore small, and it is the most electronegative of all the elements. This high electronegativity results in strong bonds between carbon and fluorine atoms, giving fluorocarbon materials high thermal and chemical stability. The low polarity of fluorine gives fluorocarbons a low refractive index and low dielectric constant, and the low cohesive energy and surface free energy due to the low polarity result in a low uptake of water, water and oil repellence, and resistance to wear and abrasion. The introduction of fluorine atoms into polyimides is therefore expected to produce polyimides with many attractive features. However, there is also a danger that the resultant polyimides may have some undesirable properties such as low adhesion strength, low mechanical strength, or a high coefficient of thermal expansion(CTE). The goal of fluorinated polyimide research is to obtain the advantages of introducing fluorine atoms without sacrificing the many advantages of polyimides.
