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 degrees 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 (R) film.