Low relative dielectric permittivities of polyimides and copolyimides derived from non-coplanar diamines and 4,4′-(hexafluoroisopropyl)diphthalic anhydride

Two series of polyimides and copolyimdes were solution cast into transparent, flexible and tough films. The glass transition temperatures T_g obtained from dynamic mechanical analysis were moderate compared with commercial polyimides such as Dupont Vespel®. The T_gs for the majority of polymers derived from monomer 1 (2′,5′-bis(p-aminophenoxy)-[1,1′;4′,1″]terphenyl) (1A-F) were higher than those of polymers derived from monomer 2 (2,5-bis(p-amino-phenoxy)biphenyl) (2A-F). The thermal expansion coefficients of the two series of polymers were moderate and comparable. The maximum elongations and moduli of polymers 2A-F were larger than those of polymers 1A-F possibly because polymers 2A-F were more flexible than polymers 1A-F. The relative dielectric permittivities of both series of polymers were measured and exhibited a low value from 2.49 to 3.01 at 100 kHz. In contrast, the relative permittivities calculated using the Clausius-Mosotti relationship were higher. Incorporation of non-coplanar diamines into the polymer chain causes an increase in free volume, which may account for the decrease in relative permittivities.