Study On The Characterization Methods And Influencing Factors Of The Structure And Properties Of Polyacrylonitrile Coagulated Fiber

As a new material with excellent mechanical properties,carbon fiber has attracted much attention in the fields of aerospace,medical instruments and national defense security.The preparation of carbon fiber from polyacrylonitrile(PAN)is a long and complicated process,and each process will affect the structure and properties of the final carbon fiber.The structure of the coagulated fiber is one of the key factors affecting the properties of the nascent fiber.Therefore,a detection method for PAN coagulated fiber structure and properties was established and optimizing the technology of dry jet wet spinning extruding,air section drawing and solidification process.Because of the large amount of solvent in PAN coagulated fiber,the conventional detection method is not completely applicable to the PAN coagulated fiber.On the other hand,the test results may vary greatly due to the non-standard test methods.Therefore,a set of testing methods suitable for the structure and properties of PAN coagulated fiber was established in this paper by optimizing the sample treatment methods and experimental conditions.The residual solvent in the coagulated fiber was extracted by Soxhlet extractor and measured by ultraviolet spectrophotometer.Combined with the weighing method,the three-component content in the coagulated fiber can be obtained.The solvent residual rate of the coagulated fiber was affected by wiping times.When the coagulated fiber was wiped for more than 4 times,the residual rate of the solvent is between 22 ± 1.2%.The extraction time is not less than three hours.In order to reduce the error of the diameter test,a little glycerol can be dropped on the surface of the coagulated fiber.The difference in light transmittance between the skin and the core layer can clearly see the skin-core structure through a microscope.Scanning electron microscope can observe the microscopic network structure of the cross section of the coagulated fiber.And the sample shall be brittle broken in liquid nitrogen.At the same time,it shall be placed in a desiccant for 12 h to remove the remaining moisture in the coagulated fiber to avoid damage to the sample and the instrument.The crystallinity of coagulated fiber can be measured and calculated directly by XRD.The fiber birefringent interferometer is used to test the orientation degree of the coagulated fiber which is washed by deionized water.And the deionized water of different water temperatures does not have much effect on the experimental results.The natural stretching ratio obtained by S-S curve was positively correlated with the stretching ratio in the water bath.And the stretchability of the coagulated fiber can be understood as having both sufficient ability to withstand tensile stress and a certain tensile multiple in the subsequent stretching process.Therefore,the yield strength and natural tensile multiple on S-S curve can be used as the measurement index of the stretchability of PAN coagulated fiber.The tensile properties of PAN coagulated fiber under different conditions and the structural factors affecting the tensile properties were discussed.The results show that under the conditions of spinning dope of 21wt%,air section length of 30 mm,jet velocity of 10.7m/min,and the drawing ratio of 2.0,PAN coagulated fiber has a lower birefringence.And the PAN coagulated fiber has a larger natural drawing ratio and a larger stretch ratio in water bath.At the same time,the PAN coagulated fiber has the highest crystallinity,dense structure,high yield strength and large stress bearing.Under these conditions,PAN coagulated fiber has the best stretchability.The effect of increasing the concentration of the spinning dope on the stretchability of the coagulated fiber and the structural factors affecting the stretchability were studied.Firstly,in order to study the spinnability of spinning dope with different concentrations,the rheological test was performed.It was found that PAN/DMSO solutions with a concentration of 21wt%?26wt% in the temperature range of 0?80? are relatively stable for dry jet wet spinning.The dry jet wet spinning experiments were carried out with 21wt% and 26wt% of PAN/DMSO solutions.It was found that with the increase of the concentration of spinning solution,the natural tensile ratio and the stretch ratio in water bath of PAN coagulated fiber were increased,and the yield strength was higher.Therefore,the stretchability of PAN coagulated fiber was enhanced.