Study On Preparing Polyimide Regular Fiber And Nanofiber Via Partly Imidized Process Of Poly(Amic Acid)

Polyimide fiber has been known for its high temperature resistance, radiation resistance , high intensity , high modulus and the excellent mechanical properties, which is one of the most important fibers in high-tech fields now. Polyimide fiber was invented both in the USA and Russia in 1960s, and for some reasons, such as the technologies and the costs, it had been intermitted for a long time. Later in 1990s, with the improvement both of the polymerization and spinning technologies, polyimide fiber got much more attention.The traditional two-step process of preparing polyimide fiber was used in this paper. At first, the intermediate product poly(amic acid) was synthesized from pyromellitic(acid)dianhydride (PMDA) and 4,4'-diaminodiphenylether (ODA) via copolymerization in polar solvent Dimethylacetamide (DMAc). Then the mixture of acetic anhydride and triethylamine was added into the intermediate product at 0℃. The partly imidized poly(amic acid) was spun directly after deaeration completely.A series of solvents of different methanol concentration were adopted as coagulants, and the results showed that the as-spun fiber spun from the coagulant of 40% methanol was golden with excellent mechanical properties. The interior structure of this fiber was dense and its glass transition temperature is high. But the fiber's interior structure was less dense when time went by.It was found by the method of IR, SEM and XRD that with the increasing of thermal imidization temperature the imidized extent of PAA fiber was greater, the fiber's mechanical properties were better and its decomposition temperature got higher. Also the fiber's interior structure was denser while the size of crystals became bigger.A series of partly imidized PAA solutions were synthesized for electrospinning. Both the fluidity effects of different PAA solutions and the surface characters of nanofiber were introduced. And the mechanical properties of PAA nanofiber sheets were also discussed. The results showed that the viscosity decreased when the shear rate increased, and the decreasing extent of rigid macromolecular chains was stronger than that of the less rigid ones. With the increasing viscosity of the solutions, the nanofiber's diameter also increased. The PAA nanofiber sheet's maximum strength in tension and the modulus of elasticity were about 17MPa and 255MPa, respectively. And the nanofiber's decomposition temperature rose when the solution's viscosity rose. PI nanofiber had deeper color by thermal imidization. Its temperature resistance became better while thermal imidization temperature got higher.