Study On Preparing Polyimide Derived From Pre-imidization Poly(Amic Acid) Ammonium Salt

Polyimide(PI) has been received more and more attention for their excellent thermalstability, chemical stability, good modulus of elasticity and high mechanical properties,along with good radiation performance and low water absorption. Owing to theseoutstanding properties, PI are being used for a variety of applications such as atomicenergy industry, aerospace programs, defense construction, new construction, high-speedtransport, ocean development, sports equipment, new energy, environment and industrialtools, etc. However, traditional preparation approach of PI by poly(amic acid)(PAA)precursors contains some disadvantages, such as, low hydrolytic stability of PAAprecursors, which cannot be storage too long, affected by the synthesis and process, themechanical properties of PI far from the value of the theoretical strength, it was difficult toindustrial production of PI fibers and the price of PI fibers was more expensive. Toovercome these drawbacks, considerable attention has been drawn to the other precursorsof PI, so we synthesized a novel polyimide precursor pre-imidization poly(amic acid)ammonium salt (PI-PAAS).The main research content and conclusions are as follows:1. A novel polyimide precursor—PI-PAAS, was synthesized, and its chemicalstructure was confirmed by1H NMR and FT-IR spectra.2. The intrinsic viscosity of PAA solution and PI-PAAS solution along with themechanical properties of PAA film and PI-PAAS film were studied. It was foundthat the intrinsic viscosity of PAA solution and the mechanical properties of PAAfilm obviously decreased with storage time during30days, especially at the first5days, however, no obviously decreased of PI-PAAS with storage time during30days. The results suggested that the PI-PAAS has good hydrolytic stability.3. Different PI films were prepared by the precursor of PAA and PI-PAAS, it wasfound that the PI films come from PI-PAAS whose internal structure was moredense and have better crystalline performance by XRD and SEM. It also found thatthe mechanical properties of PI films come from PI-PAAS were better than thatcome from PAA precursor. 4. Spinning behaviour of PAA and PI-PAAS solutions was studied, and it was foundthat PAA spinning solution has poor spinnablity in the coagulation bath, and it wasdifficult to spin continuously, however, PI-PAAS spinning solution has betterspinning performance, and it was easy to spin fibers. Wet-spinning processes werealso studied, and using aqueous methanol as coagulation bath, the PI-PAAS fiberswere prepared, then treated by heating and stretching, final PI fibers were prepared.On the basis of PI fibers, PI/PAN composite fiber was also prepared.5. The effect of temperature on the PI and PI/PAN fibers was studied, and it wasfound that the color of the fibers are apparently changed along with temperature,and the internal molecular structure are changed at different temperatures, butwhen the fibers were treated at the temperature above350oC, PI-PAAS andPI/PAN fibers can be fully imidized. It was also found that the mechanical andthermal properties of PI/PAN fiber were better.