Structure–property relationship of polyimide fibers containing ether groups

The copolyimide (co-PI) fibers with outstanding mechanical properties were prepared by a two-step wet-spinning method, derived from the design of combining 4,4′-oxydianiline (ODA) with the rigid 3,3′,4,4′-biphenyltetracarboxylic dianhydride (BPDA)/p-phenylenediamine (p-PDA) backbone. The mechanical properties of PI fibers were drastically improved with the optimum tensile strength of 2.53 GPa at a p-PDA/ODA molar ratio of 5/5, which was approximately 3.7 times the tensile strength of BPDA/p-PDA PI fibers. Two-dimensional wide-angle X-ray diffraction indicated that the highly oriented structures were formed in the fibers. Two-dimensional small-angle X-ray scattering revealed the existence of the needle-shaped microvoids aligned parallel to the fiber axis, and the introduction of ODA led to the reduction in the size of the microvoids. As a result, the significantly improved mechanical properties of PI fibers were mainly attributed to the gradually formed homogeneous structures. The co-PI fibers also exhibited excellent thermal stabilities of up to 563°C in nitrogen and 536°C in air for a 5% weight loss and glass transition temperatures above 279°C. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015, 132, 42474.