Comparative investigations on the effects of pendent trifluoromethyl group to the properties of the polyimides containing diphenyl-substituted cyclopentyl Cardo-structure

Two series of Cardo-type polyimides (PIs), fluorinated or nonfluorinated, were prepared from condensation of the fluorinated Cardo-type diamine 1,1-bis[4-(4-amino-2-trifluoromethylphenoxy)phenyl]cyclopentane (BATFPCP) or the analogous nonfluorinated Cardo-type diamine 1,1-bis[4-(4-aminophenoxy)phenyl]cyclopentane (BAPCP) with four aromatic dianhydrides PMDA, ODPA, BTDA and BPDA, respectively. Both fluorinated and nonfluorinated PIs with Cardo-structure in polymer backbone generally retained good thermal properties comparable to aromatic PIs, with glass transition temperature (Tg) of 227–270 °C and 234–340 °C, 5% weight loss temperature in nitrogen (air) of 471–483 °C (448–474 °C) and 472–482 °C (464–492 °C), respectively. The fluorinated and nonfluorinated PIs with Cardo-structure in the polymer backbone showed tensile strengths of 90.1–101.7 MPa and 78.6–99.6 MPa, tensile modulus of 1.44–1.83 GPa and 1.25–1.50 GPa, respectively, also comparable to those of the traditional aromatic PIs. In comparison with nonfluorinated series, the fluorinated PIs possessed better solubility, higher optical transparency and lower water absorption. The fluorinated PIs were readily soluble in a variety of organic solvents, exhibited high optical transparency and light color with the cut off wavelength at 373–424 nm and transmittance at 500 nm generally higher than 80%, and the water absorptions were measured as low as 0.18–0.93%. Additionally, the fluorinated series showed higher hydrophobicity by water contact angles and water absorption measurements and lower dielectric constant by dynamic dielectric measurements.