The Study Of Formation And Development Mechanism Of Polyacrylonitrile Precursor For Carbon Fiber

Properties of carbon fiber based on polyacrylonitrile（PAN） or copolymer are dominated by protofilament quality.During wet-spinning process,when PAN-dope entering in coagulation bath,the protofilament is formed via mass and thermal transferring with stretching.Some related questions have been studied in this dissertation,such as the struction and properties of polyacrylonitrille chain,condensed state of PAN membrane or fiber and wet spinning technique.An attempt to construct the relation between macromolecule and material properties was done.Firstly,the conformation of isotactic and syndiotacticn PAN has been studied.The characteristic ratio of isotactic PAN is smaller than syndiotactic PAN.As to oxidation of PAN,syndiotactic PAN is easy to form cross-linking among macromolecules.Flory-Huggins theory is typically used to describe the thermodynamic behavior of polymer/solvent/nonsolvent system.Gibbs free energy of PAN/DMSO/water system can be formulated on the basis of Tompa’s theory. Mass transferring equation for cylinder fiber can be written according to Fick’s second law.Thermal transferring equation is written according to Fourier’s heat conducting law.Macromolecular structures and monomer contents of two kinds of copolymers were examined by analysis of IR and ¹³C NMR.The IR spectrum shows that interaction of PAN compounded with DMSO is stronger.The interaction parameters of PAN/DMSO and PAN/Water were calculated according to Hansene’s equation.The interaction parameters of PAN/Water is higher than that of PAN/Water.Necessary experiments on rheological property using RSI rheometer were performed,the results showed that molecular weight and content of the second monomer methyl acrylate in chain could affect the rheological and viscoelastic properties of solutions.Morphologies of membranes formed by precipitation were observed using scanning electron microscopy（SEM）.Based on analysis of thermomechanical and kinetic property,phase diagrams of ternary system for PAN/DMSO/Water showed that molecular weight and the ratio of methyl acrylate affected the morphology of membrane during phase separation,which resulted in relative bigger holes in membrane for sample PAN2.Microfibre orientation can be preserved by phase separation in bath with higher content water.The effect of residue stress on morphology was observed using SEM.In order to reveal the influence of phase inversion on morphology of fiber, stretching hasn’t been taken into consideration and consequently the research on fiber is equivalent to studying the mechanism for membrane.it’s difficult to obtain morphology by spinodal decomposition from core to surface in fiber, while easily to form skin-core structure.In this study,under the assumed condition that the streamlet of polymer solution,without stretching,quickly travels through several consecutive coagulation baths,the kinetic of different layer of fiber at consecutive coagulation baths was discussed based on modelling and step-mass-thermal transferring was proposed to control the morphology of fiber for PAN/DMSO/Water system.The principal points of step-mass-thermal transferring are summed up.The coarsening of droplets can be controlled by lowering temperature.During phase separation in another coagulation bath,the time should be intercepted during PAN volume fraction increasing,which is equivalent to the increase in PAN concentration of polymer solution.The composition of fiber in the third bath should be at unstable region of phase diagram in order to change situ-growth of droplets.Viscoelastic phase separation has been introduced into phase separation. The phase separation related to viscosity behavior has been simulated.In view of various voids,the mechnism of using chemical fractural and viscoelastic phase separation can give a good understanding for control morphology of membrane.With stretching,macrovoid in fiber can be eliminated during initial state of phase separation due to interface stress,friction and viscosity.The wet spinning has been studied referred to Newton model,Maxwell vicoelastic model,molecule model.The diffusion and thermal transferring combined with phase separation has been considered.Some variables in equation set,such as viscosity,friction coefficient and relaxation time,are related to the polymer volume fraction of polymer rich phase,which results in sudden change while phase separation begins.The diffusion boundary has been divided into two parts with different diffusion coefficient,and then combined together during wet-spinning.Metastable state indicates that the fiber can be stretched even though polymer rich phase is approaching vitrification.Under stretching,the outmost layer sustains the main stress for rapid increasing in water volume fraction.The macromolecule orientation in outer layer of fiber is higher than that of inner layer.Osmotic pressure can make macromolecule contract.In conclusion,this paper revealed or verify some important phenomena about wet-spinning,such as molecule properties,step-spinning,phase separation,growth of hole and wet-spinning technique.