Preparation and characterization of low expansion transparent copolyimide films

Five transparent polyimide thin films with different proportions of 1,2,4,5-,pyromellitic dianhydride (PMDA) were made by copolymerization of 4,4'-(six fluoro isopropenyl) two phthalic anhydride (6FDA), 4,4'-two amino -2,2'-double three trifluoromethyl biphenyl (TFMB) and PMDA, and their properties were characterized. The results show that thermal stability and heat resistance are increased after the introduction of rigid monomers; the dielectric constant of the films is increased with the increase of PMDA content; homopolymer film has higher light transmission rate than copolymer films in visible light; tensile test shows that adding a small amount of PMDA, the tensile strength and elastic modulus of the film are increased, but when the content of PMDA is higher than 40%, the mechanical properties are decreased; the coefficient of thermal expansion of the films are decreased obviously while PMDA content is increased.