Structure Evolution Of GO/PAN Composites During Heat Treatment

Carbon fiber(CF)is one of the major high-performance fibers due to its excellent mechanical property,low density and chemical stability.It's an important reinforcement material used in composites modification.With the growing application of carbon fiber and its composites in commercial and civil fields,the demands for the quality and output of low-cost carbon fiber is also increasing.Based on the above problem,low-cost modification of PAN based CF foucses on two aspects.One is using textile grade PAN fiber as CF precursor,another is heat treatment process optimization.PAN fiber usually needs a series of heat treatments including oxidative stabilization and carbonization to transform into PAN-based carbon fiber.Oxidative stabilization is a key process of the fabrication of CF.PAN undergoes many complex chemical changes in this process including cyclization,dehydrogenation,and oxidation reactions.After these changes,the linear PAN molecular chains convert into crosslinked ladder structures,which can prevent PAN from melting or fusing during the carbonization process.During oxidative stabilization,oxygen diffusion is one key factor for the formation of crosslinked ladder structures in stabilized PAN fiber.Graphene oxide(GO)has excellent dispersibility in polar solvents due to the rich oxygen-containing functional groups on its surface.Moreover,the oxygen-containing functional groups are easily decomposed under thermal treatment.So,GO is considered to be a perfect modification filler for PAN based carbon fiber.However,current studies on GO modified PAN based carbon materials are very limited.The structure evolution and the related mechanism of GO/PAN materials during heat treatment have not been systematically studied.In this paper,textile grade PAN is used as the matrix material and GO is used as the modification filler to fabricate GO/PAN blending films.During the whole PAN based CF fabrication process,the structure evolution and the related mechanisms have been systematically studied,which provides some new research ideas for the preparation and structure modification of low cost CF.The related experiments and conclusions are shown as follows:(1)The GO/PAN blending solutions were prepared by solution blending method in DMSO solvent.It has been found that GO sheets dispersed in the DMSO solvent and PAN solution uniformly by polarization microscope.The rheological behaviors of GO/PAN blending solutions were investigated.The study found that the blending solution had good stability and fluidity.The apparent viscosity value of the blending solution was decreased,but the non-newton exponent(n)was increased after adding GO.For example,the zero shear viscosity of GP-0.8 solution was 34Pa�s which was decreased 50%compared with PAN solution,the n value of GP-0.8 solution was0.985 which was larger than PAN solution(0.969).The flow activation energy(E_?)of GO/PAN blending solution is also increased after adding GO,the E_?value of GP-0.8 solution was 30.4kJ/mol which was increased 32.7%than PAN solution,this result indicated the rheological behavior of blending solutions were more sensitive to the temperature with a high GO content.It was also found that GO sheets and PAN macromolecules mainly showed electrostatic interaction in the solution.(2)The GO/PAN films were prepared by evaporating DMSO solvent from GO/PAN blending solutions.During the oxidative stabilization,GO/PAN films were transformed into GO/PAN stabilized(SGP)films.The crosslinking structure and cyclization structure were separated by a solubility measurement,the result showed that GO could change the proportion of the two kinds of structures in the stabilized film,the proportion of crosslinking structure increased with GO content increasing.There had 88.5%crosslinking structure in the SGP-5 film which was increased 23%than that of SGP-0 film.The DSC and XPS tests showed that GO could initiate the cyclization reaction of PAN at a lower temperature due to its functional groups,and the cyclizing structure of PAN could be combined with the graphite structure of GO to form a new kind of crosslinking structure.In comparison with PAN,exothermic peak temperature(Tp),exothermic enthalpy(?H)and the exothermic rate(?H/?T)decreased with the increase of GO content.The Tp,?H and?H/?T of GP-10 film decreased respectively by 11�C,152 J/g and 5.2 J/(g??)than that of PAN film.Moreover,GO could effectively improve the structural uniformity of stabilized PAN film and reduce the formation of skin-core structure.(3)The GO/PAN based carbon(FSC)films were prepared by carbonization of the SGP films.GO sheets as template could induce an ordered graphite structure formed in the carbon film during carbonization.The graphite structure of FSC films were studied by Raman spectrum and XRD test.It was found that graphite crystallite domain sizes(L_a)of FSC films were increased with increasing of GO contents.The R value of FSC-G10 film was 1.17 which was decreased by 0.56than that of FSC-G0 film.The La value of FSC-G10 film was 3.76 nm,which was increased 1.22nm than that of FSC-G0.The graphite crystallite dimensions of FSC-G10 film were much bigger than that of FSC-G0 film,the stacking height,lateral size and the average interlayer spacing of FSC-G10 film were 5.25,1.66 and 0.353 nm,respectively.It was also found that GO could effectively improve the flexibility of PAN based carbon film through tensile test.The elongation of FSC-G10 film was 1.31%which was increased by 92.6%than that of FSC-G0 film.(4)GO was assembled on the surface of PAN fiber to form the PAN-PG fiber.Through DSC test,it was found that the Tp,?H and?H/?T of PAN-PG fiber increased respectively by2�C,45.1 J/g and 0.5 J/(g??)than that of PAN fiber,which indicated that the uniform dispersion of GO sheets played a key role in the structure evolution of PAN during heat treatment.PAN-PGH fiber showed good electrical conductivity after thermal reduction of PAN-PG fiber.When the PAN-PG fiber reduced under 200�C,the resistivity of PAN-PGH fiber reached 5.50�10~2?�cm.