Light-absorbing copolymers of polyimides as efficient photothermal materials for solar water evaporation

Polyimide (PI), an important engineering polymer with a rigid chemical structure, readily has excellent chemical stability, heat resistance, and electrical insulation but lacks broadband photothermal properties. Herein, we design and synthesize PI copolymers that embrace intrinsic photothermal properties by using two diamine monomers of (Z)-2,3-bis(4-aminophenyl) acrylonitrile (CNDA) and 4,4-diphenyldiamine (NDA) with strong ultraviolet (UV), and near-infrared (NIR) absorption capabilities, respectively. Tuning the molar ratio of the two diamines can modulate UV and NIR light absorption and regulate the intrinsic photothermal properties of PIs. After condensation with pyromellitic dianhydride, the resulting PI-0.5 with a unit molar ratio of CNDA:NDA = 1 shows the best photothermal efficiency. PI-0.5 is used to construct 3D steam generators with vertically dried channels by a freeze-drying method. The 3D steam generators show a good water evaporation rate and continuously operate with excellent stability under varying salinity and pH conditions. The synthetic design herein suggests that PI can be molecularly engineered to be intrinsic photothermal materials, expanding the properties and applications of existing PIs.