Achieving an Ultralow‑κ yet Strong and\nTransparent Film by Antisolvent-Confined Nanowelding of Electrospun\nPolyimide Nanofibers

Developing\nultralow-κ (dielectric constant) polyimides (PIs)\nthat are mechanically robust while also being optically transparent\nis challenging. For the first time, we report a nanoporous PI film\nwith an ultralow κ of 1.8 in combination with a tensile strength\nof up to 180 MPa, a Young’s modulus of up to 6 GPa, and a transmittance\nof ∼88%. This is achieved by direct nanowelding of a porous\nelectrospun PI nanofiber membrane using a simple mixture of ethanol-dominating\nDMAc. Benefiting from the effective evaporation of the antisolvent\nethanol upon heating, the proposed nanowelding approach allows for\nthe localized surface dissolution of the PI nanofibers, which enables\nthe dissolved PI to “glue” the nanofibers and occupy\nvacant space in the membrane, resulting in the formation of a dense\nbut nanoporous self-reinforced nanocomposite film. Our findings provide\na renewed understanding of the potential of electrospun nanofibrous\nmaterials, and the underlying principle can hopefully be applied to\nother commodity polymers.