Structures and properties of polyimide fibers prepared via gel spinning induced by chemical imidization

A series of 3,3′,4,4′-biphenyldianhydride (BPDA)/p-phenylene diamine (PDA) polyimide (PI) fibers were prepared through a novel gel spinning method induced by chemical imidization. The effects of draft ratios, solvent content and thermal treatment temperature on the aggregation structure and properties of PI fibers in the gel spinning were systematically investigated by experiment and molecular dynamics (MD) simulation. The mechanical properties of the fibers are significantly improved with increasing draft ratio due to the development of the molecular orientation along the fiber direction during large-ratio drafting process. The MD simulation results indicate that the nascent gel fiber with more solvent content has larger interchain space and weaker interchain interaction and molecular chains tend to be arranged along the drafting direction. The highly oriented PI fibers are completely imidized after thermal treatment at 320 °C, reaching the optimum tensile strength of 1.43 GPa and modulus of 121 GPa. The surface and internal defects of the highly oriented PI fiber gradually increased with increasing the thermal treatment temperature, resulting in a slight decrease in mechanical properties. In a word, the fiber orientation is carried out before the thermal imidization in the gel spinning process, which can greatly increase fiber draft ratio and reduce the subsequent thermal treatment temperature and energy consumption.