The Orientation Of Primary Fibers In Gel Spinning Process Studied By Infrared Dichroic Measurement

As a high-performance material,carbon fiber has been widely used in military and civil fields.Although the performance of domestic PAN based carbon fiber become stable,it still has a significant gap with foreign technique.The current status of technical blockade leads to an urgent demand of high-strength and high-modulus carbon fiber which its mechanism is the key question.Previous studies indicated that the basic structure of carbon fiber is a graphite-like structure which its graphite sheets are distributed along the axial direction.The key to produce high-performance carbon fibers is to obtain a network structure with high degree of orientation.The PAN precursor produced by the traditional wet-spinning process has an obvious skin-core structure.In the coagulation process,the tight skin layer would hinder the double diffusion process of the core,and resulted in the generation of defects in the layer.structure.At the same time,the skin is more easily graphitized during the carbonization process,and the degree of graphitization has a significant effect on the performance of the final carbon fiber.Due to its unique phase transition process,there is no difference in the structure of the skin layer and core layer of the carbon fiber produced by gel spinning process,and was pointed out to keep high crystallinity and crystal region integrity.Considering that carbon fiber properties are determined by the orientation of primary fibers,it is of great significance to study the orientation of primary fibers obtained by the process of gel spinning for the subsequent preparation of high-performance carbon fibers.In this thesis,the changing of the orientation of primary fibers produced under different external field factors by gel spinning was studied.Infrared dichroism technique was employed as the main research method to study the orientation change characteristics of gel-spun primary fibers.The orientation of primary fibers was characterized from four factors:drafting,shearing,temperature of coagulation bath and spinning temperature.On the basis of this,the wide-angle X-ray diffraction(WAXD)was used to study the orientation change in the crystalline region formed in gel-spining zone of gel-spun primary fibers,and the basic research on the radial distribution of fiber orientation was carried out by polarized Raman.A comprehensive study of the orientation of gel-spun primary fibers was carried out and the results showed that:(1)A series of characterization methods of gel spinning systems were established.The gel transition point was determined by measuring the viscoelastic intersection of the spinning solution,that is the critical temperature while the spinning solution transform from fluid to gel state,which provided a basis for the establishment of gel spinning temperature conditions.SEM observation showed that there was no obvious defect of the cross-section of the gel-spun primary fibers,which had a great advantage in draw ratio and breaking strength compared with the common skin-core structure of primary fiber produced by wet spinning.Polarized Raman spectroscopy was used to characterize the radial distribution of the primary fiber orientation,which further shows that the fibers produced by gel spinning are homogeneous in structure.(2)The influence of the force field on the orientation of the primary fibers was studied.The molecular chains are arranged orderly along the direction of the force,and the orientation increased with the increase of drafting ratio and shearing rate.The shearing rate is negatively correlated with the drafting ratio.When the speed of shearing rate is low,the drafting ratio at a high stage.(3)The effect of temperature field on the orientation of gel-spun primary fiber was studied.The fiber was formed at low temperature during gelspinning process,and the degree of orientation increased with the increasing of the temperature of spinning,and the decreasing of the temperature of coagulation bath.In the spinning solution,the main role of temperature is to help macromolecular chains to obtain kinetic energy and move along the direction of the force.In the coagulation bath,the low temperature is favorable for the formation of the gel state and maintains the inherent orientation of the fiber.The orientation of the crystalline region was characterized by WAXD,and it was found that the gel-spinning primary fibers had both good molecular chain orientation and crystalline orientation.