Effects Of Chemical Imidization And Drawing On The Structural Properties Of Polyimide-based Graphite Fibers

Polyimide(PI)not only has high mechanical properties and excellent high temperature resistance,but also is a very excellent new carbon material precursor,such as high thermal conductivity graphite film and carbon foam widely used in the field of electronic heat dissipation.Therefore,it is possible to prepare high performance carbon fibers using PI fibers as precursors.In this paper,aiming at the problems of low orientation and poor performance of PI-based graphite fibers in the previous study of our research group,the microstructure and properties of PI-based graphite fibers were effectively improved by using chemical imidization method and drawing in heat treatment.This paper mainly studies from the following three aspects.Firstly,PI-based graphite fibers were prepared by chemical imidization.Polyamide acid(PAA)spinning solution was synthesized from pyromellitic anhydride(PMDA)and 4,4’-oxydianiline(ODA).Different content of dehydrating agent acetic anhydride reacted with pyridine catalyst to obtain partially chemically iminated PAA-PI spinning solution,and the primary fiber was prepared by dry-wet spinning.The PI-based graphite fiber was obtained by thermal imidization,carbonization and graphitization.The structure and properties of PI-based graphite fiber were tested by infrared spectroscopy,X-ray diffraction,scanning electron microscope and monofilament strength tester.We found that the chemical imidization method plays a positive role in improving the properties of PI-based graphite fibers.The imidization degree of the fiber can be controlled by controlling the appropriate content of acetic anhydride,and the appropriate ratio of acetic anhydride to catalyst pyridine can increase the reaction rate and avoid the damage to the structure caused by the excessive addition of small molecules.When the content of acetic anhydride is 50 mol%,the ratio of pyridine to acetic anhydride is 2: 1,the properties of graphite fiber are the best,and the thermal conductivity can reach 126 W·m-1·k-1.This is because the chemical imidization removes the small molecules and the imidization reaction in which PAA is closed into cyclic PI is carried out in the solution ahead of time,which avoids the damage to the microstructure of the fiber caused by the removal of simple thermal imidizing small molecules from the solid phase.At the same time,some rigid PI segments can limit the free entanglement of PAA molecules and drive other molecular chains to be oriented under the huge shear force in the spinneret process,and finally obtain a regular graphite microcrystal structure,which can improve the thermal conductivity.After determining the suitable chemical imidization reagent formula,we studied the effects of different drawing tension(from 0 to fracture limit)in the process of thermal imidization and carbonization on the structure and properties of PI-based graphite fiber.The graphite fiber which received the limit drawing tension in the process of thermal imidization and carbonization,the diameter of the sample was reduced to 9.48 um,the thermal conductivity reached 373 W·m-1·k-1,and the tensile strength reached 1331 MPa,increased by 73% and 131% respectively.This is due to the fact that under the maximum axial force,the disorientation trend in the PI heat treatment process and the disturbance caused by the escape of small molecules in the carbonization process are greatly alleviated.At the same time,under the axial stress,the hexagonal carbon network structure of the fiber can be arranged along the fiber axis,and finally a regular graphite crystal is formed.In the above study,we confirmed that chemical imidization and heat treatment limit drawing play a significant role in improving the structure and properties of PI-based graphite fibers.There is still much room for improvement in the properties and preparation effect of the obtained fibers.Therefore,we use chemical imidization and drawing method to prepare high thermal conductivity PI-based graphite fibers in the process of preparing graphene/PI composite graphite fibers.It is found that the addition of graphene has an important effect on the improvement of the properties of PI-based graphite fibers.The thermal conductivity of the fiber with 2% graphene content reaches 435 W·m-1·K-1,which is 16.62% higher than that without graphene.This is mainly due to the induced orientation of graphene,which is composed of SP2 hybrid carbon atoms and is connected to the six-membered ring of PI by strong π-π conjugation.Under the action of shearing and drawing in the spinning process,the planar molecules of graphene are easily oriented along the fiber axis,and at the same time drive the PI molecules to arrange orderly along the fiber axis.At the same time,graphene can also induce graphite microcrystal crystallization.In short,this paper studies the effects of chemical imidization and heat treatment limit drawing on PI-based graphite fibers,which provides a new method for the preparation of PI-based graphite fibers with high axial orientation,and will promote the further research and preparation of high thermal conductivity PI-based graphite fibers.