Structure–property relationship of polyimide fibers containing ether groups

ABSTRACT 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.