Four different types of cross-linked polyimides based on 4,4-diphenylmethane diisocyanate (MDI) were prepared by the reaction of different types of conventional poly(amic acid) intermediates with MDI as a cross-linking agent. Subsequently, they were thermally imidized in order to obtain corresponding cross-linked polyimide structure. The results of FTIR-ATR showed that MDI can effectively react with carboxylic acid groups of PAA to form cross-linked polyimide films. TGA, FTIR-ATR and SEM analyses were carried out for characterization of cross-linked polyimide (CPI) films. Moreover, the electrical properties such as dielectric breakdown strength, dielectric constant, I–V characteristics and loss factor of MDI based cross-linked polyimides have been checked. In addition, some physical properties such as water uptake, adhesion, hardness and solubility properties of the films were investigated.
The results showed that all CPI films have good insulating properties such as high dielectric breakdown voltage, low loss factor (tan δ), leakage density and excellent physical properties.