Studies On The Rheological Behavior Of Polyacrylonitrile And Cellulose Solutions And Composite Fiber Spinning

Carbon fiber with high strength,high modulus,low density and corrosion,and many other advantages to be applied in aerospace,military industry and high-end sports goods,and many other areas,the carbon fiber production process mainly made by solution spinning preparation of raw fibers,raw fibers after oxidation and carbonization process further preparation of the carbon fiber;Spinning fluid rheological behavior will determine the process of raw fibers processing conditions will influence the properties of carbon fiber,spinning solution as the initial part of the preparation of carbon fiber,therefore,research on its rheological behavior is of great significance for the preparation of high performance carbon fiber.This work is conducted based on the study of the rheological properties of polyacrylonitrile(PAN)and cellulose solutions and its gelation mechanism.PAN/cellulose fibers were prepared by gel-spun method and its morphology,structure and mechanical properties were investigated throughout the study.First,dimethyl sulfoxide(DMSO),N,N-dimethylformamide(DMF)and N,Ndimethylacetamide(DMAc)were used as solvent to prepare 20 wt% PAN solutions.By analyzing the rheological properties of PAN solutions,we found that the PAN solution,which had a smaller Hansen solubility parameter distance between the solvent and PAN,showed better solution homogeneity.Thus,PAN/DMSO solution has the best solution homogeneity among these solvents.Additionally,we observed that both heating and cooling were able to cause the gelation of PAN solution.The gelation temperature of the PAN solution exhibits strong dependence on its solvent polarity.The gelation caused by heating was ascribed to the nitrile–nitrile coupling of PAN chains,whereas the gelation caused by cooling was due to the solvent bridge effect.Second,cellulose was dissolved in LiCl/DMAc solvent system at various concentrations.During rheology measurements,we observed that the cellulose solution becomes gelled upon heating and jelly-like during cooling.The cooling induced elastic transition is reversible and the transition temperature was increased when solution concentration increases.The heating induced sol-gel transition is irreversible and the transition temperature significantly decreases when solution concentration was increased.We propose different mechanisms between heating and cooling induced rheological transitions.For heating induced sol-gel transition,the association between Li cations and DMAc is disrupted and the hydrogen bonding between cellulose chains irreversibly forms which leads to phase separation and the formation of a cellulose physical network.Whereas,for cooling induced elastic transition,the association between Cl anions and the hydroxyl groups in cellulose and the solvation of Li cations in DMAc become stronger,their interactions bridge cellulose chains into an elastic network.This bridging effect depends on temperature and is totally reversible.Finally,we prepared a mixed solution of PAN and cellulose and studied its rheology properties.We found that the homogeneity of the blending solution was decreased while the concentration of the cellulose was increased.The mixed solution containing 95 wt% of PAN and 5 wt% of cellulose exhibits the best homogeneity.The blending solutions were not able to form gel during heating,whereas,all the solutions formed gel when the temperature was decreased to below-40 ℃.Based on these rheology experiments,PAN/cellulose fibers were gel-spun through a methanol gelation bath maintained at-50 ℃.The maximum proportion of cellulose in the composite fibers is 10.1 wt%.and its morphology,structure and mechanical properties were investigated throughout the study.