Synthesis and properties of the novel polyimides containing cyano and biphenyl moieties

A series of cyano-functionalized polyimides (CN-PIs) were prepared and derived from a novel diamine monomer having cyano and biphenyl groups. Meanwhile, another series of PIs without cyano groups (H-PIs) were prepared as counterparts for comparison purposes. Both series of PIs exhibited good solvent resistance, good thermal stability, remarkable film-forming ability, and excellent tensile properties. The T_g values of CN-PIs were 260–330°C, which were 10–17°C higher than the corresponding –CN free H-PIs. The CN-PIs exhibited no obvious decomposition below 470°C, and their 5% weight loss temperatures were above 568°C under nitrogen atmosphere. All the CN-PI films had high tensile strength and Young’s moduli, which were related to their rigid backbones and strong interactions between molecular chains. Due to their higher molar polarization, CN-PIs expressed the higher dielectric constants (3.29–3.69 at 1 MHz) in comparison with the corresponding H-PIs. CN-PIs exhibited the improved transparency, and both CN-PIs and H-PIs had better transparency than commercial Kapton^® film. Introducing –CN groups could have an effect on the coefficient of thermal expansion and peel strength of the PIs. The peel strength of the PI, which was derived from cyano-functionalized diamine monomer and 4,4′-oxydiphthalic anhydride, could even reach 1.10 N mm^-1, suggesting its strong interaction to copper matrix and potential use in flexible circuit board.