Preparation and properties of thermally stable polyimides derived from asymmetric trifluoromethylated aromatic diamines and various dianhydrides

A novel method for the preparation of an asymmetric fluorinated aromatic diamine, 3,4′-bis(4-amino-2-trifluoromethylphenoxy)-benzophenone was investigated. This new diamine containing trifluoromethyl side group was synthesized from the nucleophilic substitution reaction of 2-chloro-5-nitrobenzotrifluoride and 3,4′-dihydroxybenzo phenone in the presence of potassium carbonate, followed by catalytic reduction with SnCl2·6H2O and concentrated hydrochloric acid. This novel diamine was used to react with different commercially available aromatic tetracarboxylic dianhydrides to prepare polyimides via thermal or chemical imidization. The polyimide properties such as inherent viscosity, solubility, thermal and surface properties were investigated to illustrate the contribution of the trifluoromethyl group and the asymmetry structure of the polyimide. The polyimides obtained had good thermal stability and the glass transition temperature values ranged from 225 to 267 °C. All of these novel polyimides held 10% weight loss at the temperature above 543 °C in air and left more than 47% residue even at 800 °C in nitrogen. The inherent viscosities of the obtained polyimides were above 0.73 dL/g and were easily dissolved in both polar, aprotic solvents and some low-boiling-point solvents. Moreover, these PI films had dielectric constants of 2.94-3.53 (1 kHz), with moisture absorption in the range of 0.07-0.34 wt%. In comparison of the PIs (5) series with the analogous symmetric PIs (6) series based on 4,4′-bis(4-amino-2-trifluoromethylphenoxy)-benzophenone, the (5) series revealed better solubility, low dielectric constant and moisture absorption.