| The structural characteristics of polyacrylonitrile(PAN)precursor are the basis structure and performance of PAN-based carbon fiber,and round,homogeneous,dense,and low-defect PAN nascent fiber is the prerequisite for the preparation of PAN precursor with excellent structure.However,it's easy to produce defect structure in PAN nascent fiber formed by the spinning solution flow through coagulation due to the strong coagulation ability of the precipitant(water)in the traditional coagulation bath.For this reason,the organic coagulant is preferred in this study to explore the mechanism of PAN nascent fiber coagulation and molding,and the structure of PAN nascent fiber under the new coagulation system,which is of great significance for the preparation of high-performance PAN precursor.This study focus on the external conditions of the coagulation bath of fiber forming,changing the coagulation bath system,temperature and time,the composition of the PAN nascent fiber was analyzed by ultraviolet(UV),the morphology of PAN fiber was observed through optical microscope(OM)and scanning electron microscopy(SEM),the crystalline structure and pore structure of the PAN nascent fiber was characterized by wide-angle X-ray diffraction(XRD)and fully automatic specific surface and porosity analyzer(BET).Through the experiment of cloud point about PAN-DMSO solution with different non-solvents,the relevant particle size parameters of the cloud point system formed by different non-solvents were studied,and glycerol was determined as a suitable coagulation bath medium.By characterizing the composition,crystallization parameters,cross-sectional morphology and pore structure of nascent fiber,the effect of coagulation bath temperature,coagulation time and coagulation bath medium on PAN nascent fiber structure and forming process were studied.(1)Water,ethanol,and glycerol are all PAN precipitant.In the cloud point system formed by the polymer solution and different coagulants,although the particle size of the precipitated polymer in the glycerol system is slightly larger than that of the ethanol system,the particle size dispersion coefficient of the precipitated polymer is the lowest,which has weaker coagulation ability than the traditional coagulant(in water).(2)In the glycerol coagulation,the residual solvent content in PAN nascent fiber increases with the temperature of the coagulation bath and decreases with the extension of the coagulation time;while the residual solvent content in PAN nascent fiber in the DMSO-H2O system decreases with the increase of coagulation bath temperature,the critical temperature is 40?.If the temperature of the coagulation bath is further increased,the residual solvent content of the fiber will increase,prolonging the coagulation time,which is conducive to the further diffusion of the solvent in the fiber.The PAN nascent fiber prepared in the glycerol coagulation bath has the characteristics of low residual solvent content.(3)In the glycerol coagulation,the PAN nascent fiber has crystalline diffraction peaks at 2?=16°and 2?=29°,respectively,and has small dispersion peaks around 2?=25° representing the amorphous structure.Under this system,crystalline structure of the PAN nascent fiber changes with the coagulation bath temperature and coagulation time.Lowering the temperature and prolonging the time are beneficial to the preparation of nascent fiber with high crystallinity and large grain size.The crystallinity of PAN nascent fiber varies between 83.95?89.25%,and the grain size varies between 8.27?11.29 nm.The PAN nascent fiber formed by the DMSO-H2O system only show a crystalline diffraction peak at 2?=16°,and the dispersion peak at 2?=25° is larger.The crystallinity of the PAN nascent fiber first increases and then decreases with temperature.The prolongation of coagulation time is beneficial to the perfection of fiber crystal structure.However,in this system,the PAN nascent fiber has a crystallinity of 44.98%and a grain size of 8.54 nm.(4)In the glycerol coagulation,as the temperature of the coagulation bath increases from 20? to 60?,the cross-section of the PAN nascent fiber gradually tends to be round,with skin-core structure generated above 50?,and its diameter increases from 48 ?m to 62 ?m.With the extension of the coagulation time,the cross-section of the PAN nascent fiber changes to a circle,and the fiber diameter tends to decrease.But the temperature of the coagulation bath has a more significant influence on the cross-section of the PAN nascent fiber.The cross-section of the PAN nascent fiber in the DMSO-H2O system changes from a loose kidney shape to a dense circular shape with increasing temperature.Insufficient coagulation of the fiber core above the critical temperature results in a loose porous structure.Prolonging the coagulation time is beneficial to obtain PAN nascent fiber with a continuous dense structure.Increasing the coagulation temperature and prolonging the coagulation time reduce the diameter of PAN nascent fiber.(5)In the glycerol coagulation,PAN nascent fiber formed in a low-temperature coagulation bath(20?40?)has a small specific surface area and a cumulative pore volume in the range of 0.0013?0.0159 cm3/g,mainly between 2?4 nm.When the temperature is increased to 60?,the cumulative pore volume is 0.0875 cm3/g,which mainly contributes to mesopores and even large pores of different sizes.Prolonging the coagulation time helps to eliminate the pores in the fiber and reduce the pore size.In the DMSO-H2O system,the cumulative pore volume and pore size of PAN nascent fiber decrease as the temperature of the coagulation bath increases.The maxium cumulative pore volume of PAN nascent fiber forming in glycerol is 0.0875 cm3/g,and the maxium specific surface area is 17.29 m2/g.The cumulative pore volume of PAN nascent fiber in DMSO-H2O system is from 0.0915 to 0.3339 m2/g and the specific surface area is more than 35 m2/g.The PAN nascent fiber prepared by using glycerol as the coagulation bath has a denser fiber structure. |
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