Synthesis and properties of cyano group–containing polyimides with high peel strength

In order to investigate the effect of cyano groups (–CN) on the properties of the polymers, two series of polyimides (PIs) based on a –CN-containing diamine monomer and a diamine monomer without –CN group were synthesized and characterized, respectively. After studying the structure–property relationships of these polymers, it was found that some properties could be greatly improved with the incorporation of –CN groups. Firstly, the tensile test showed that the –CN-containing polymeric films had higher tensile strength and Young’s moduli. The tensile strength and the Young’s modulus of 4,4′-(hexafluoroisopropylidene) diphthalic with –CN groups anhydride reached 154.1 MPa and 1.66 GPa, respectively. Secondly, thermogravimetric analysis results revealed that the –CN-containing polymers (CN-PI polymers) had good thermal stability and exhibited no decomposition below 470°C under nitrogen atmosphere. Thirdly, the dielectric constants of the CN-PI polymers were in the range of 3.67–3.84 at 10 MHz, which had an increase of 0.5 in comparison with the corresponding PIs without –CN groups. Fourthly, the CN-PI polymers had higher peel strength to copper foils. The peel strength of 4,4′-biphenyltetracarboxylic dianhydride-containing –CN groups could reach 1.28 N mm −1 , suggesting its strong interaction to copper matrix and the potential use in flexibility circuit board. Finally, all the films showed better optical transparency compared with DuPont’s Kapton® film.