Study On Preparing Polyimide Fibers Via Partly Imidized Process Of Poly(Amic Acid)

Polyimide fibers have been known for their high temperatureresistance, radiation resistance, high intensity and high modulus andexcellent mechanical properties, which are one of the major varieties inhigh-tech fibers. In 1960s, Polyimide fibers have come out, but for somereasons, such as technologies and costs, it has been intermitted for along time. Later in 1990s, as the improvement of polymerization andspinning technologies, also for its outstanding performance, polyimidefibers were received attention again. At present, the research andexploitation about polyimide fibers are been in the ascendant. Also, thereare many reports about polyimide fibers on the magazines or internets.The traditional two-step process of preparing polyimide fibers wasused in this paper. At first, the intermediate product poly(amic acid)was synthesized from pyromellitic dianhydride (PMDA) and4,4'-diaminodiphenyl oxide (ODA) via copolymerization reaction in polarsolvent DMAc. Meanwhile, a novel method was adopted. The mixture of aceticanhydride and triethylamine was added into the intermediate product at0℃to get partly imidized poly(amic acid), which can be spun directlyafter degassing completely. It was found that the intrinsic viscosity ofpolymer was influenced by molar ratio of PMDA and ODA, solvent system,reaction temperature and time and so on. In addition, whatever intrinsicviscosity of PAA or partly imidized PAA, the viscosity always decreasedalong with the time at room temperature. But the decreasing tendency ofthe viscosity of partly imidized PAA was more moderated than PAA because of added triethylamine.Different solvent was adopted as coagulants, such as water, methanol,ethanol, DMAc. It was found that water had good ability to remove solvent,but it was too fast. Once the skin of the fiber surface formed, the innersolvent was hard to diffuse. So it was disadvantageous for the elongation.If ethanol or DMAc was adopted as coagulants, the double-diffusing speedwas relatively low. The result by the experiment was that certainconcentration of methanol aqueous solution could increase diffusingvelocity, so the methanol aqueous solution as coagulant was better.The structure and performance of as-spun fibers and attenuationmechanism were characterized by SEM, IR spectrum, DTA and mechanicalmeasurement. It was found that the intensity and initial modulus ofas-spun fibers decreased clearly along with the time at room temperature.While it maintained the original properties after freezing at about-15℃. It was obvious that low temperature could restrain attenuation ofas-spun fibers.Different methods were introduced to implement imidization for partlyimidized PAA. The result by IR spectrum and DTA analysis was that theas-spun fibers had been imidized at 300℃. The SEM result showed thatthrough heat treatment the fiber structure became denser than that withoutthe process. Also the result by mechanical measurement was that afterthermal imidization, the elongation decreased from 32.9% to 12.8%, theintensity increased from 2.23 cN.dtex^-1 to 3.12 cN.dtex^-1, the initialmodulus increased from 10.18 cN.dte^-1 to 44.36 cN.dtex^-1.