Preparation Of Polyimide Fibrids And Their Formation Mechanism

Aromatic polyimide(PI)fibers,as a type of heterocyclic polymer fibers,have exhibited some attractiveness of their properties,such as the outstanding thermal stability and chemical resistance,and the excellent mechanical properties as well as good irradiation resistance.The short PI fiber may be widely utilized in the fields of preparation of insulated paper,high-performance printed circuit boards,non-asbestos friction material and composites because of its special feature of the large specific surface area.The spinning process and cyclization reaction of the PI fiber are so complex that the preparation of PI short fibers is difficult and thus the corresponding research and application of the short fiber are limited.A new method to prepare the short PI fiber by the partially cyclized PAA solutions was introduced in this paper and the short fibers made by this method are called fibrids.The partially cyclized PAA solution was injected into coagulation bath and was stirred with a high-speed In the coagulation bath,partial cyclized PI drops were elongated by the shear force to deform,coagulate and fibrillate.This method is convenient for preparing short PI fibers on a large scale for leaving out the complicated spinning process.The related research includes the following three parts.At first,the equimolar amounts of pyromellitic dianhydride(PMDA)and 4,4'-diaminodiphenylether(ODA)were dissolved in polar solvent dimethylacetamide(DMAc)to produce poly(amic acid)(PAA).The PAA solution was diluted by DMAc into 5 wt%.Then the mixture of acetic anhydride and triethylamine with various amounts was added into diluted PAA solution to prepare the partially imidized PAA with various imidization degree.The effect of imidization degree on polymer intrinsic viscosity was characterized by the ubbelohde viscometer.The result showed that the intrinsic viscosity of polymers increased with the increase of the cyclization degree.The structure and properties of PAA-PI polymer were characterized by FTIR and TGA.TGA results showed that,compared with PAA,the initial decomposition temperatures of partially imidized PAA-PI showed a lower value.This was because that a little of dehydrating reagents were left in polymers.Mechanical properties of partially imidized PAA-PI film were characterized by a tensile testing machine and the result showed that the mechanical properties of the PAA-PI films were enhanced by the imidization process.Short PAA-PI fibers were prepared by the method of solution precipitation.In this method,PAA-PI solutions with various pre-imidization degrees were injected into the high-speed stirred coagulation bath with the mixture of H20 and DMAc.Then short PAA-PI fibers precipited from the system.Three main factors,including the shear rate,the concentration of PAA-PI in the solution and DMAc concentration in the coagulation bath were investigated,which affect the preparation of the fibrids by characterizing the average lengths and diameters of fibrids and calculating the aspect ratio of fibrids.The result showed that the concentration of PAA-PI was about 3%and the concentration of DMAc was between 20%and 30%,the aspect ratio of PAA-PI fibrids was the largest and the properties of the fibrids were the best.The morphology of fibrids was characterized by the PLM.The result showed the formation of partially imidized PAA-PI fiber became better with the increase of the pre-imidization degree.In view of the result of pre-imidization contributing to the formation of the short fibers,the formation mechanism of short PAA-PI fibers was investigated by the cloud point method and the solidification kinetics experiment.The cloud point curve and ternary phase diagrams of(PAA-PI)-DMAc-H20 were obtained by use of the cloud point method to show the changes in the content of each component of the system.The influence of the pre-imidization degree on coagulation values in the coagulation bath was discussed by analyzing the cloud point curve and ternary phase diagram.The result showed that binodal curve was more far away from(PAA-PI)-H20 axis and homogeneous area decreased,leading to the compatibility(PAA-PI)-DMAc-H20 weakened.This demonstrated that,compared with PAA,the compatibility between DMAc and PAA weaken.Then it contributed to the diffusion of DMAc of PAA-PI solution to the coagulation bath and made the fibrids formation better.The solidification rate of PAA-PI was characterized by the solidification kinetics experiments.As a result,the square of the displacement of the solidification layer is proportional to the diffusion time.