High-performance copolyimide (co-PI) fibers were prepared via the wet spinning process of co-polyamide acid precursors based on 3,3 0 ,4,4 0 -biphenyldianhydride (BPDA) and a mixture of three diamines namely p-phenylene diamine (p-PDA), 2-(4-aminophenyl)-5-aminobenzimidazole (BIA), and 4,4 0 -oxidianiline (ODA), followed by drawing and imidization at high temperatures. Effects of the ODA and BIA contents on the molecular packing, morphology, hydrogen-bonding interactions, mechanical and thermal properties of the prepared fibers were investigated. The mechanical properties of the co-PI fibers were improved with the addition of ODA and BIA, and they reached the optimum tensile strength of 2.7 GPa and modulus of 94.3 GPa. Wide-angle X-ray diffraction results (WAXD) showed that the co-PI fibers exhibited highly oriented structure along the fiber direction with low degree of lateral packing orders in the transverse direction. Two-dimensional small-angle X-ray scattering (2D-SAXS) revealed that the incorporation of ODA resulted in the reduction in radius, length, misorientation, and internal surface roughness of the microvoids in the fibers. Fourier transform infrared (FTIR) results indicated that hydrogen-bonding formed between the BIA and cyclic imide units effectively strengthened the intermolecular interactions. The co-PI fibers exhibited excellent thermal and thermal-oxidative stability, with a 5%-weight-loss temperature of 5788C under N 2 and 5728C in air. POLYM. ENG. SCI., 55:2615-2625, 2015.

Copolyimide (co-PI) fibers containing 4,4 0 -oxydiphthalic anhydride (ODPA) moiety into the 3,3 0 ,4,4 0 -biphenyltetracarboxylic dianhydride (BPDA)/p-phenylenediamine backbone were prepared via a two-step wet-spinning method. The processability and mechanical properties were improved significantly after the incorporation of ODPA, and the fibers exhibited an optimum tensile strength of 10.94 cN dtex -1 and modulus of 470.52 cN dtex -1 with elongation of 2.75 % at a BPDA/ODPA molar ratio of 7/3. Two-dimensional wide angle X-ray diffraction indicated that highly oriented structures and ordered molecular packing regions were formed in the fibers. Two-dimensional small angle X-ray scattering revealed that the incorporation of ODPA resulted in the reduction in radius, length, misorientation, and internal surface roughness of the microvoids in the fibers simultaneously, which was supposed to be mainly dominated for the drastically improved mechanical properties of PI fibers. Moreover, the co-PI fibers exhibited excellent thermal and thermal-oxidative stability, and the 5 % weight loss temperature was above 572 and 535°C under nitrogen and air, respectively.

High performance fibers based on copolyimide (co-PI) containing benzimidazole and ether moieties were prepared via a two-step wet spinning method. The effects of draw ratio on the structures and properties of the fibers are investigated. The mechanical properties of the co-PI fibers are improved greatly by increasing draw ratio at temperatures above 400 8C, reaching the optimum tensile strength of 2.8 GPa and modulus of 136 GPa, which is due to the development of the molecular orientation along the fiber direction. The hot drawing process roughens the fiber surface and stretches the molecules in the fibers. Moreover, the fiber structure evolutions during the drawing process were also discussed and a ''smectic A liquid crystal-like'' model was proposed.

A series of polyimide (PI) fibers derived from 3,3 0 ,4,4 0 -biphenyltetracarboxylic dianhydride (BPDA), 4,4 0 -oxydiphthalic anhydride (ODPA) and p-phenylenediamine (p-PDA) were successfully prepared through a partially pre-imidization process, and the effects of different amount of dehydration reagents and initial poly(amic acid) (PAA) concentrations on the structure-property relationship of the resultant PI fibers were systematically investigated. The results showed that both the increased amount of dehydration reagents and PAA concentration could result in the ordered molecular packing arrangement of the polymer chains and gradually formed homogeneous structures in the fibers, which are proposed to be essentially dominated for the effectively enhancement in the mechanical properties of PI fibers. Moreover, the PI fibers obtained through the pre-imidization process still exhibited excellent thermaloxidative stabilities, although the 5% weight loss temperature of the PI fibers was slightly decreased compared with that of the pure PI fibers as a result of the residual dehydration reagents in the fibers.Consequently, the present work provided a new approach in preparing high-performance PI fibers through a partially pre-imidization process.