Investigation Of The Solidification Forming And Phase Separation Of Polyacrylonitrile Precursor

Due to their excellent performances,carbon fibers are widely used in all kinds of fields.Polyacrylonitrile(PAN)-based carbon fibers with many advantages have become one of most important varieties in carbon fiber industries.There are large gaps in the quality of PAN-based carbon fibers between the homemade and foreign products.As is well known,PAN precursors have been the key factor for producing high-performance carbon fibers,and the property of PAN precursors depends largely on their inner microstructure.In fiber-forming process,the microstructure of PAN precursors is constantly evolving and transforming,but their structural defects will be likely genetic to carbon fibers.The appearance of structural defects should be avoided in spinning process,in that case,the excellent precursors with ideal microstructure can be fabricated.Therefore,it can't be achieved to foresee the fibers' performance and provide greater researching space until phase separation mechanism in wet or dry-jet-wet spinning process has been clearly clarified.This paper is divided into four major parts,in the first part of the study,various experimental methods have been adopted to investigate the thermodynamic and hydrodynamic properties of PAN spinning solution,deeply expounded the PAN-DMSO-H₂O system of thermodynamic stability and rheological properties and gelation mechanism.In the second part,based on binary interaction parameters and the extended Flory-Huggins theory,the theoretical ternary phase diagrams at different temperatures for PAN-DMSO-H₂O system have been constructed.In the third part, the ternary cloud point phase diagrams for this system have been determined by cloud point titration and Boom's linear cloud point curve correlation.In the fourth part, according to the constructed theoretical and cloud point phase diagrams,the fiber-forming mechanism in wet or dry-jet-wet spinning process has been discussed. Furthermore,by analyzing influence of spinning technical parameters on fiber's cross-section morphology,microstructure and performance,the cross-section microstructure formation mechanism of PAN precursors was described.In Chapter Two,by using viscosity method,variable temperature FTIR analysis, dynamic light scattering and dynamic/static rheological measurement,the thermodynamic and hydrodynamic properties of PAN spinning solution were investigated.Judging from the viscosity results,PAN-DMSO-H₂O system with the upper critical solution temperature(UCST) behavior,which was proved to be thermodynamic stable one,is usually used as PAN spinning solution.The variable temperature FTIR analysis proved that the hydrogen bonding interaction between the PAN and DMSO in the vicinity of 35℃is at maximum,which is in good agreement with the numerical calculation results.Dynamic light scattering studies showed that the sol-gel conversion is a long,dynamic and reversible process.Rheological measurement showed that the PAN-DMSO-H₂O solution has better ability to maintain orientation,and this system in the low-frequency region is in line with the linear viscoelastic theory.In Chapter Three,the experimental and calculated methods were introduced for nonsolvent-solvent,nonsolvent-polymer and solvents-polymer interaction parameters. Based on the extended Flory-Huggins theory and binary interaction parameters,the ternary theoretical phase diagrams at different temperatures for PAN-DMSO-H₂O system were achieved by successive iterations calculation.Based on the constructed ternary theoretical phase diagrams,the temperature dependence of this system was to be further discussed.With increasing the temperature,the theoretical binodal curve gradually approaches PAN-H₂O axis.Meanwhile,the results show that both higher values of g12 and lower values ofχ13 can shift the binodal curve toward the PAN-H₂O axis and also produce larger homogeneous regions in the phase diagram of this system.In Chapter Four,by using cloud point titration and cooling cloud point methods, the cloud point data for PAN-DMSO-H₂O system with low concentration were determined.According to Boom's linear cloud point curve correlation(LCP),the whole experimental binodal curve was completed by using extrapolation method, which is in accordance with the theoretical binodal curve.Therefore,the results validated the rationality of the binary interaction parameters determined by various experimental methods.Finally,based on the theoretical spinodal curve,a series of ternary phase diagrams,with critical point,binodal curve,spinodal curve and phase separation trajectory,were determined.In Chapter Five,the morphology of PAN fibers was investigated by using scanning electron microscopy(SEM).With increasing the concentration of PAN spinning solution as well as the quenching depth,the morphology of PAN fibers turns from large open channels to small bead-like structures,accompanying with a reduction of the porosity of PAN fibers.As well as the cross-section shape of PAN fibers changes from nephroid shape to elliptical shape or circular shape due to a variation in the DMSO-H₂O exchange rate at the phase separation stage.The different pore morphologies of PAN fibers can be assigned to different demixing types.More dilute PAN solution has a higher probability to demix by SD,giving rise to channel-like pores,while NG predominated in more concentrated PAN solution, resulting in bead-like pores.This paper deeply investigated phase separation mechanism in PAN fiber forming process and influence of spinning technical parameters on PAN fibers' structure and performance.The phase separation mechanisms were used to further demonstrate the nature of micro structure formation.Comprehensive and in-depth understanding of wet or dry-jet-wet spinning technical conditions,formation mechanism,structure,performance and the inevitable connection between them,the theoretical foundation was provided for preparing excellent PAN precursors.