Preparation of crystalline polyimide nanofibers via solution crystallization

Abstract

Two crystalline polyimide nanofibers (PINFs) with different morphologies were prepared. The crystalline unit cells of the aromatic PI crystals and the crystal morphologies of the fabricated PINFs were examined. PINF-I (lengths = 305 ± 152 nm and diameters = 12 ± 2 nm) was crystallized from crystalline PI dissolved in a concentrated sulfuric acid solution. The resulting PINF-I was isolated from this solution, and it did not aggregate in water. PINF-II with diameters of 105 ± 99 nm was prepared by dispersing PINF-I in a mixed water and t-butanol (TBA) solution (water:TBA = 4:1), followed by freeze-drying. Then, the PINF-II was heated to enhance its crystallinity. X-ray diffraction and transmission electron microscopy studies of the heat-treated PINF-II revealed a PI crystalline unit cell [orthorhombic, a = 1.21 nm, b = 0.88 nm, and c = 2.23 nm (molecular chain axis direction)]. The crystal structure of the heat-treated PINF-II suggested that highly crystalline PINFs were fabricated in which the PI molecular chains were oriented along the direction of the fiber lengths.