Construction of semi-fluorinated polyimides with perfluorocyclobutyl aryl ether-based side chains†
In view of cost reduction and environmental protection, achieving low dielectric constant (k value) at low fluorine content is of great significance. Incorporation of perfluorocyclobutyl (PFCB) aryl ether-containing side chains into polyimide, which is expected to impede compact stacking of polymer chains and increase the fraction of free volume (FFV) as a consequence, may be an effective strategy to reduce the k value of polyimide. A series of PFCB aryl ether-containing polyimides with various side chain lengths and side chain contents are produced. To our delight, a nearly state-of-art k value of 2.31 is achieved at a fluorine content of 23.8%, representing the highest efficiency of fluorine atom in reducing the dielectric constant of polyimide and confirming the effectiveness of side chain fluorine chemistry. Positron annihilation lifetime spectroscopy (PALS) and dual-processed weight change experiment results indicate that the dielectric constant of semi-fluorinated polyimide is determined by the opposite actions exerted by FFV and water absorption. Moreover, this kind of fluorine-containing polyimide is proved to be an eligible candidate for dielectric applications with excellent overall performance in dielectric constant as well as thermal stability and mechanical strength.