Effect of trifluoromethyl groups on properties of aromatic and semi-aromatic polyimides for space applications

Aromatic polyimides have become popular due to their exceptional properties, including superior thermal resistance. However, these materials have a strong absorption in the UV–vis region, which limits their applications, including those for a space environment. The effect of trifluoromethyl groups in aromatic and semi-aromatic polyimide structures on their final characteristics, specifically optical, thermal, and mechanical properties, solar absorptance, emissivity, and outgassing, was investigated to develop high-performance materials suitable for space applications. As the starting dianhydride monomers, 1,2,4,5-cyclohexanetetracarboxylic dianhydride (CHDA) and 4,4′-(hexafluoroisopropylidene)diphthalic anhydride (6FDA) were used to react with different diamine monomers, specifically 4,4′-oxydianiline (ODA), 4,4′-oxybis[3-(trifluoromethyl)aniline] (6FODA), 1,4-bis(4-aminophenoxy)benzene (APB), and 1,4-bis(4-amino-2-trifluoromethylphenoxy)benzene (6FAPB). It was found that trifluoromethyl groups positively affected the thermal and optical properties of polyimide films. The thermal stability of the 6FDA-based polyimides exceeded 500 °C. The fully fluorinated polyimide films of poly(6FDA-6FODA) and poly(6FDA-6FAPB) exhibited transmittance at 400 nm (T 400) of 53% and 44%, respectively, whereas their partly fluorinated analogues of poly(6FDA-ODA) and poly (6FDA-APB) showed T 400 of 3% and 10%, respectively. Moreover, the presence of trifluoromethyl groups improved outgassing results, where the fully aromatic poly(6FDA-6FAPB) film achieved the best performance. In contrast to the CHDA-based polyimides, all 6FDA-based polyimides achieved the total mass loss values (TML) < 1, which meets one of the essential requirements for space applications.