Structures and properties of polyimide fibers containing fluorine groups

A series of copolyimide (co-PI) fibers containing fluorine groups were successfully obtained based on 3,3′,4,4′-biphenyltetracarboxylic dianhydride (BPDA), 4,4′-(hexafluoroisopropylidene)diphthalic anhydride (6FDA), p-phenylenediamine (p-PDA), 2-(4-aminophenyl)-5-aminobenzimidazole (BIA) and 4,4′-oxydianiline (ODA) via a typical two-step wet-spinning method. The increased 6FDA moieties in the system resulted in unexpected great changes on the structures and properties of the resultant PI fibers. Regarding mechanical performances of the PI fibers, the tensile strength and initial modulus of the fibers decreased from 2.56 to 0.13 GPa and 91.55 to 2.99 GPa, respectively. Two-dimensional wide angle X-ray diffraction (2D WAXD) confirmed the existence of highly oriented structures along the fiber axial direction, while this feature gradually disappeared after the introduction of bulky trifluoromethyl pendant groups. SEM results suggested the presence of defects such as macrovoids structures with the increased 6FDA moieties. Besides, the dielectric permittivity was found to decrease from 3.46 to 2.78 in the frequency of 10 MHz as a result of the incorporation of 6FDA. Moreover, the co-PI fibers possessed excellent thermal-oxidative stabilities with the 5% weight loss temperature ranging from 495 to 552 °C under nitrogen atmosphere.