Preparation Of Polyimide Fibers And Their Structure-property Relation

Polyimide fiber is a typical kind of high performance fiber with higher thermal stability, elastic modulus and resistance to magnetic radiation, and lower water absorption, than kevlar fibers. It is promising to be used under circumstances such as atomic energy industry, space industry, military field and so on. Now, the developed countries like USA are quickly pushing their studies on polyimide fibers. However, the related reports are rare, because the polyimide fiber is used in manufacturing of space crafts and rockets for its high performance.In this paper, a kind of poly(amic acid) (PAA) has been synthesized, with N,N-dimethylacetamide (DMAc) as solvent, from 4,4'-diamino diphenyl and pyromellitic dianhydride using solution polycondensation. The chemical structure of the polymer obtained has been characterized by FTIR. Studies are concentrated on the effects of the reaction conditions, such as reaction temperature, time, water content of the monomers etc. on intrinsic viscosity of the PAA.Experimental results show that the increasing of reaction temperature, the solid content of the solution, and the water content of monomers will all obviously decreased the intrinsic viscosity of PAA synthesized. During the reaction procedure, the intrinsic viscosity of the system increases slowly before the dissolution of the solid monomer and goes up sharply after that. The mechanical properties of the poly(amic acid) increase with the increasing of the intrinsic viscosity of PAA. Studies on the degradation of the poly(amic acid)/tertiary amine salt solution show that the addition of tertiary amine can reduce the hydrolytic degradation of PAA solution. The mechanism of the reaction has been discussed.By using water, alcohol, DMAc, and HC1 water solution as coagulants, which were mixed by some proportions or used independently, the coagulation values of the PAA and PAA/tertiary amine salt solution have been tested in varying kinds of polar organic solvent, such as DMAc ,DMSO, NMP, and so on. The feasible proportions of coagulants have been selected, which would be useful in the subsequent study on the wet spinning process.The as-spun fibers of PAA have been prepared by wet-spinning method under different coagulation conditions. The intersection of the PAA as-spun fibers have been observed by SEM.The as-spun fibers have been processed by pre-chemical treatment and thermal treatment. The structure and properties of the treated fibers have been characterized by means of FTIR,TGA, DSC, mechanical measurement and so on. FTIR and DSC results show that the as-spun fibers have been imidized after treatment. The imidization temperature is in the range of 150- 250. The imidization activation energy is about 45-66kJ/mol. The imidization reaction order n is about 0.70-0.73. The TGA results show that the thermal stability of the treated fibers has been improved. The thermal degradation energy of the treated fibers is about 10 kJ/mol higher than that of untreated one. The mechanical property of the fibers increases after treatment. The initial modulus of the treated fiber is about 3 times of the untreated one. The tensile strength of the fiber increases with the increasing of the thermal treatment temperatures.