SCHEDULED MAINTENANCE
Evolution of the microstructure and morphology of polyimide fibers during heat-drawing process
Abstract
The development of crystalline structure and morphology for polyimide (PI) fibers in the heat-drawing process was investigated by simultaneous synchrotron wide-angle X-ray diffraction (WAXD) and small-angle X-ray scattering (SAXS). WAXD results indicated that the drawing process resulted in a high crystal orientation and ordered crystal structure. Especially, as the drawing ratio increases to 2.0, a well-defined crystalline structure forms in the fibers. We propose that the highly oriented molecular chains induce the formation of crystalline regions. Namely, an orientation-induced crystallization occur with stretching in the case of the heat-drawing polyimide fibers. The meridional scattering streaks in the SAXS patterns for the as-spun fibers suggest the presence of periodic lamellar structure in the fibers. These crystalline lamellae may evolve to more complete crystalline regions. The size of microvoids in the cross-section of the PI fibers is analyzed by SAXS. As a result, the drawing process leads to the orientation of microvoids along the fiber, and to reduced diameter of the microvoids in the fiber. Dynamic thermomechanical analysis indicates that the activation energy Ea of α relaxation increases with the increase in the crystallinity and orientation in the fibers.
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