The Correlation Between The Composition Of PAN-based Carbon Fiber Precursor And The Structural Evolution During The Formation Of Carbon Fiber

Because of the unique properties-excellent mechanical properties,light-weight,superior temperature resistance,corrosion resistance,fatigue resistance and so on,polyacrylonitrile-based?PAN-based?carbon fiber has been used as a reinforcing material for high-performance composites which have been widely used in military field and civil field.But the tensile strength of the strongest carbon fibers commercially available in the market is only 5%of the theoretical tensile strength of carbon fiber.It is necessary to investigate some unclear issues in depth so that some guidance and suggestions can be provided for improving the overall performance of PAN-based carbon fibers.During the manufacture of PAN-based carbon fibers,the most important step is to adopt some vinyl acid monomers,such as itaconic acid?IA?,to copolymerize with acrylonitrile?AN?.This is because that the presence of IA or not decide PAN to carbonization or to pyrolysis during the high temperature treatment.Moreover,the content of IA possesses vital influences on the polymerization of PAN,and the structural evolution and the thermal behaviors of PAN during thermal treatment.So,the influences of IA on the polymerization and the thermal treatment of PAN should be taken into consideration simultaneously to give a suitable IA content for high-quality PAN-based carbon fiber precursor.Moreover,the mechanism for significant improvement of the structural evolution of PAN during thermal oxidative stabilization?TOS?by introducing a small amount of IA into PAN has not been investigated in detail.It is common that the reactivity of vinyl acid monomer is higher than that of AN,so the PAN polymer is a compound that consist of PAN polymers formed at different polymerization stages.But,there is no enough experiment data discussed whether the slight composition difference of PAN influences the structural evolution and the formed chemical structures obviously.Cyclization reaction is the most important during the manufacture of PAN-based carbon fibers because the formed cyclic structures possess excellent thermal stability and are the intermediates for the formation of graphite-like structures.However,literatures were rarely reported on the cyclization pathways and the influences of the characteristics of PAN macromolecules,such as the ends of PAN chains,and the composition of PAN on the cyclization pathways.As well-known,the properties of PAN-based carbon fibers are controlled by the defects in them.Among the defects,skin-core structure caused by the heterogeneous distribution of physical and chemical structures across the cross-section of the fibers during TOS process is the main reason for the weakened performance of PAN-based carbon fibers.To weaken even eliminate the skin-core structure during TOS process,the preferred strategy is to optimize the molecular structures of PAN by using a special monomer which on the one hand promotes the diffusion of oxygen into PAN samples,on the other hand has no negative even positive influences on the cyclization reaction.This will enlarge the adjustment room for weakening the skin-core structure during the TOS by some kinds of post treatments.To solve the above issues,the following investigations were carried out in this thesis:?1?Nuclear magnetic resonance spectroscopy?NMR?,wide angle X-ray diffraction?WAXD?,differential scanning calorimetry?DSC?,thermogravimetry analysis?TGA?,fourier transform infrared spectroscopy?FTIR?and peak fitting were adopted to investigate the effects of IA content on the polymerization and the structural evolution of poly?acrylonitrile/itaconic acid??PAI?during the TOS process.The suitable IA content was suggested under the consideration of polymerization and thermal treatment simultaneously.The results indicated that the over high IA content in the feed resulted in the formation of high-IA-content PAI and significantly decreased the polymerization rate and the average molecular weight of PAI.Moreover,the high-IA-content PAI possessed poor thermal stability,and the pyrolysis reactions occurred in the high-IA-content PAI during thermal treatment.These resulted in the lower efficiency of IA in initiating cyclization reaction.When IA content in the feed varied from 0.5 mol%to 1.0 mol%,the influence of IA on the polymerization was slight,and the cyclization and the oxygen uptake reactions in the resultant PAI copolymers were better than the other PAI copolymers.?2?By isothermal DSC?iso-DSC?,FTIR,Raman spectroscopy?Raman?and peak fitting,the mechanism of IA in the improvement of the TOS reactions and the resulted chemical structures was investigated in detail.The results showed that one IA unit could initiate multiple cyclization reactions among different chains by the ionic mechanism throuhg the jump transportation of activation species between different PAI chains.In the present work,the times of the activation species jump transportation was about two or three based on the iso-DSC data.So,a small amount IA initiated a large-scale cyclization reaction.Just because of the jump transportation of activation species,slight variation of IA content in PAI did not result in obvious difference in the chemical structures during the TOS process.But the dominant reactions were different at different TOS stages.The cyclization reaction was occurred in the early stage.With the proceeding of TOS process,the cyclization faded,and the termination of the cyclization and the dehydrogenation of the linear chains became dominant.These resulted in a decreased length of the formed cyclic structures.?3?In-situ FTIR,two-dimensional?2D?correlation analysis and DSC were adopted to investigate the thermal behaviors,the cyclization pathways and the length of cyclic structures of PAN and PAI during the thermal treatment with a temperature procedure under inert atmosphere.The results showed that the nitrile groups at the end of chains were vital for the cyclization initiated by the free radical mechanism at low temperature.Moreover,IA was converted into anhydride at first rather than initiated cyclization directly by the ionic mechanism at low temperature,and subsequently the formed anhydride initiated cyclization by the ionic mechanism at higher temperature.Both for PAN and PAI,the cyclization was initiated fully by the free radical mechanism when the thermal treatment temperature up to 240°C,and the formed free radicals were terminated at higher temperature with the formation of a large-scale?-amino nitrile(2198 cm^-1).Both for PAN and PAI,the average length of the cyclic structures formed by the free radical mechanism was about three repeated six-membered rings.However,the average length of the cyclic structures formed by the ionic mechanism was about eight repeated six-membered rings.The longer average length of the cyclic structures endowed PAI better thermal stability.?4?Poly?acrylonitrile-co-itaconic acid-co-dimethyl itaconate??PAID?copolymers were prepared by the free radical polymerization in water,and the role and the mechanism of large side group comonomer,dimethyl itaconate?DMI?,in improving TOS reactions,especially the oxidation reaction,were investigated by FTIR.The results showed that DMI slowed down the cyclization rate at low temperature but facilitated the cyclization at high temperature.Moreover,the conjugated carbonyl groups formed in the unit cyclization extent increased with the DMI increased.The cyclization pathways at different temperatures of these three PAID copolymers were analyzed by in-situ FTIR and 2D correlation in detail.The corresponding results approved that DMI slowed down the cyclization rate at low temperature but facilitated the cyclization at high temperature.