Strain-induced crystallization of polyimide fibers containing 2-(4-aminophenyl)-5-aminobenzimidazole moiety

Structural evolution induced by cyclization reaction and heat-drawing of polyimide (PI) fibers containing the 2-(4-aminophenyl)-5-aminobenzimidazole moiety was studied by means of synchrotron radiation wide-angle X-ray diffraction (WAXD) and small-angle X-ray scattering (SAXS). WAXD results indicate that the drawing process induces an increase in the crystallinity and crystal orientation of PI fibers, leading to improvement of the mechanical properties. SAXS results reveal that lamellar structures formed upon heat-drawing. A mechanism for strain-induced phase transitions has been proposed to correlate the structural information with WAXD/SAXS patterns. Dynamic mechanical analysis (DMA) of the obtained fibers shows an obvious a relaxation process above room temperature, which is found to be crystallinity dependent, the intensity of tan d decreases significantly after heat-drawing. The stretched fiber exhibits a negative value of the linear coefficient of thermal expansion (CTE), while the unstretched PI fiber exhibits a positive value. Moreover, heat-drawing increases the thermal stability of the prepared fibers. These improvements may be related to the aggregation structure of the high performance fibers. (C) 2015 Elsevier Ltd. All rights reserved.