Preparation And Properties Of Mesophase Pitch Doped With Polyacrylonitrile Based Carbon Fibers

Mesophase pitch based carbon fibers(MPCFs)have many advantages such as high modulus,high thermal conductivity,which can satisfy the special needs of spaceflight.The perfect graphite structure and the high axial orientation benefit from the rigid two-dimensional planar structure of the mesophase pitch molecules.During spinning,the plane pitch molecular orientates both along axial direction and along radial direction.This phenomenon leads to weak force interlayers and low tensile strength of carbon fibers.Therefore,how to reduce its radial orientation becomes the key to improve the tensile strength of pitch based carbon fiber as well as remaining the orientation degree along its axial direction.In this paper,we chose polyacrylonitrile(PAN)as doping material,forming one-dimensional rigid molecular polymer after cyclization by heating treatment,to change the mesophase molecular radial orientation effect in the spinning process and formation of graphitic carbon in carbonation.Naphthalene mesophase pitch was chosen due to its good spinning property and it was mixed with PAN by solid-liquid method.The effects of PAN content on spinning property of mixed material were tested by Fourier infrared spectrum analyzer,Soxhlet extraction,polarizing microscope and rotational rheometer.According the results,spinning temperature and doping content were initially determined.The spinning equipment was self-restraint vapor-pressure type single spinning machine.The prepared pitch fibers were treated in a drying oven to get oxidation fibers and then were carbonized in a box-tape carbonized oven with the protection of nitrogen.The change of weight,microcrystalline structure and other microstructure of oxidation fibers and carbon fibers were tested by thermo gravimetric(TG)analysis,X-ray diffraction(XRD),Raman spectrum(Raman)and field emission scanning electron microscopy(FE-SEM).The effects of doping content of PAN on the structure and performance of fiber were focused and the results were as follows:(1)The uniformity of PAN doping and its influence on spinnability was studied.It was found that the PAN could be dispersed in mesophase pitch by solid-liquid mixing,and the amount of additive did not affect its spinnability.When the doping content was below 0.3 wt.%,the mixture had good spinnability,which can acquire final pitch fiber as the un-doped pitch in the stage of spinning.When the doping content was over 0.3 wt.%,the PAN was sensitive to the elastic effect,which limited the movement of mesophase molecules and led to the unsteady flow.(2)The influence of PAN-doped on the structure of carbon fibers was studied.It was found that the PAN and mesophase pitch has a strong co-carbonization effect.Carbonization under the nitrogen showed that oxidized fibers had a sharp slope in loss weight with the increase of concentration of PAN below 450 ℃.However,the rate of loss weight became lower than un-doped sample above 450 ℃,and final yield of carbon had a slight increase with the increase of doping amount.Then,the carbon fibers were studied in detail by XRD and Raman.The results showed that La and Lc of the carbon fiber modified with 0.3 wt.% PAN(PCF-0.3)increased by 38.36 % and 16.73 % respectively compared with the pristine carbon fiber(PCF-0).Simultaneously,the Raman test results showed that the ratio of the intensity of D peak and G peak(ID/IG)decreased from 1.08 to 0.97.(3)The influence of PAN-doped on the orientation structure and mechanical properties of carbon fibers was studied.It was found that PAN could not only change the cross section structure of mesophase pitch carbon fiber,but slow down the axial splitting and improve the tensile strength of carbon fiber.The cross section of the pitch based carbon fiber doped with 0.3 wt.% has a large area of folds,and the tensile strength increased by16.56 % compared with the un-doped one.