Hyperbranched polysiloxane (HBPSi)-based colorless copolyimide films with atomic oxygen (AO) erosion resistance

Colorless polyimides (CPIs) with excellent optical transparency and atomic oxygen (AO) erosion resistance have potential applications in optoelectronic devices in low earth orbit (LEO) such as flexible thin film solar cells. Herein, 2-trifluoromethyl-4,4′-diaminodiphenyl ether (3F-ODA) and amine-terminated HBPSi were synthesized as amino monomers. After co-polymerizing with 1,2,4,5-cyclohexanetetracarboxylic dianhydride (CHDA) and 4,4′-(hexafluoroisopropylidene)diphthalic anhydride (6FDA) in a typical sequential reaction manner, a series of CPI films with various HBPSi contents were fabricated. The resulting CPIs show high optical transmittance (84.8%–86.8% at 400 nm), outstanding atomic oxygen (AO) erosion resistance (AO erosion rate 4% that of pure CPI), satisfactory mechanical properties (tensile strength ca. 80 MPa, tensile modulus ca. 2.8 GPa), good thermal stability (T_d ≥ 467 °C, T_g ≥ 304 °C) and desirable solubility. More importantly, the resulting CPIs show increasing AO resistance with the increase of HBPSi amount, and a fairly high optical transmittance can still be retained after AO exposure, far better than that of pure CPI. These impressive performances enable HBPSi CPIs capable of being used as substrate for lightweight solar cell arrays onboard spacecraft in LEO environment.