Preparation and properties of pale-colored and optically transparent fluoro-containing polyimide films with low solar absorptivity and enhanced high-temperature dimensional stability for potential applications in space environments

Two organo-soluble polyimide (PI) resins were prepared from a tricyclic fluoro-containing dianhydride 9,9-bis(trifluoromethyl)xanthene-2,3,6,7-tetracarboxylic dianhydride (6FCDA) and aromatic diamine of 2,2-bis[4-(4-aminophenoxy)phenyl]hexafluoropropane (BDAF) for PI-IIa and 1,3-bis(3-aminophenoxy)benzene (133APB) for PI-IIb, respectively. For comparison, two analogous PIs were prepared from 4,4 '-(hexafluoroisopropylidene)diphthalic anhydride (6FDA) and the same diamines to afford PI-Ia (6FDA-BDAF) and PI-Ib (6FDA-133APB), respectively. Incorporation of the rigid and planar xanthene units in the 6FCDA-PIs obviously decreased the solubility of the resins in organic solvents. Flexible and tough PI films were fabricated from the PI solutions in N,N-dimethylacetamide (DMAc). The PI films derived from the rigid and planar 6FCDA dianhydride exhibited obviously enhance thermal stability, and high-temperature dimensional stability. For example, PI-IIa (6FCDA-BDAF) showed the glass transition temperature (Tg) and coefficient of linear thermal expansion (CTE) values of 311.5 degrees C and 50.5 x 10-6/K, respectively, which were all superior to those of the PI-Ia counterpart (Tg = 264.6 degrees C; CTE = 55.4 x 10-6/K). In addition, the 6FCDA-PI films maintained the intrinsically excellent optical transparency of the fluoro-containing PI films and showed the comparable optical properties to those of the 6FDA-PI films. As for the properties highly related to the space applications, all the PI films exhibited low solar absorptivity (alpha) values of 0.18-0.20 and the good thermal emissivity (epsilon) values in the range of 0.68-0.70. PI-Ia (6FDA-BDAF) and PI-IIa (6FCDA-BDAF) films showed the similar atomic oxygen erosion behaviors with the standard Kapton (R) type of PI film derived from pyromellitic dianhydride (PMDA) and 4,4 '-oxydianiline (ODA). Fluoro-containing polyimide film with improved thermal stability and low solar absorptivity image