European Polymer Journal, 2017, vol 91pp. 232-241
The correlation between mechanical property and the microstructure of two-step wet spinning polyimide (PI) fibers after thermal annealing was investigated. In-situ wide-angle X-ray diffraction (WAXD) and ex-situ small angle X-ray scattering (SAXS) methods were utilized to study the liquid crystal-like structure orientation and microvoids structure (the length and the orientation angle of microvoids) for PI fibers during annealing process. The effects of annealing temperature and time on the microstructure were systematically studied. The results reveal that the liquid crystal-like structure and the orientation degree keep nearly the same after annealing. Both of the molecular chain relaxation and the evolution of aligned microvoids are attributed to the change in tensile modulus. In low pre-drawing fiber, a slight decrease in microvoid length and an obvious rise in microvoid orientation angle lead to improved tensile modulus after annealing. In highly pre-drawn fiber, the tensile modulus decreases first due to de-orientation of polymer chains and then increase owing to disordering microvoids with decreasing size. Our work demonstrates the relationship between microvoids and mechanical properties in annealed PI fibers which provides a new avenue to access high performance PI fiber by facile annealing method and a deep insight into the structure and properties of high performance PI fibers.