|Neuber, C.; Schmidt, H.-W.; Giesa, R.: Polyimide Fibers Obtained by Spinning Lyotropic Solutions of Rigid-Rod Aromatic Poly(amic ethyl ester)s, Macromolecular Materials and Engineering, 291(11), 1315-1326 (2006) -- DOI: 10.1002/mame.200600256|
Summary: Fibers were spun from a lyotropic solution of a high-molecular-weight ( = 5.89 g · dL-1), rigid-rod, fully aromatic polyimide precursor polymer in a dry-jet, wet-spinning process in NMP. Acetone was identified as the coagulant of choice since fibers could be drawn extensively in this solvent, resulting in improved mechanical properties (tensile modulus: E = 17 GPa, strength at break: break = 400 MPa, elongation at break: break = 5.3%) and orientation, which was shown by WAXS patterns. SEM images showed a layered, skin-core morphology without any visible fibrillation. Additional processing of these fibers by step-wise hot-drawing up to 400 °C under tension rendered oriented polyimide fibers with excellent mechanical properties. (E = 68 GPa, break = 700 MPa, break = 1%). An analysis of the WAXS diffraction patterns showed an improved orientation of the fibers in the axial and lateral directions; however, probably due to the CF3 side groups, the lateral distance was still too large for crystallization. SEM images of these imidized fibers showed, for the first time, a fibrillar morphology in addition to the typical, skin-core, sheet-like morphology.