Study On Controlling Thermal Relaxation Behavior Of PAN Oriented Molecules

Polyacrylonitrile(PAN)fiber is an important precursor for carbon fiber.And its orientation structure affects the orientation structure of PAN-based carbon fiber.The orientation structure of the PAN fiber is formed by stretching during the spinning process.However,PAN fibers,which exist forced high-elastic deformation during the spinning process,will appear the thermal relaxation behaviors in the heat conditions,such as thermal shrinkage with disorientation and a series of structural rearrangement in the fibers.Therefore,by controlling the thermal relaxation behavior of the PAN oriented molecules,the aggregation structure of the PAN fibers can be rearranged,and it is helpful to obtain the better structure of PAN precursors and carbon fibers.In this paper,the thermal relaxation behavior of PAN fiber was studied by dynamic mechanical analyzer(DMA)and thermomechanical analyzer(TMA).The high temperature viscoelasticity of PAN fiber was studied by high temperature creep experiment of PAN monofilament.Based on this,this paper innovatively employed the high temperature reaction of PAN fibers to generate the cross-linked structures to control the thermal relaxation behavior of the oriented molecules along with the physical means of tension and temperature.Finally,the fiber aggregation structure and mechanical properties after controlling the thermal relaxation behavior were characterized.And the following conclusions were obtained:1.In this study,it indicated that the thermal relaxation behavior of PAN orientation molecules is the recovery process of high-elastic deformation which generated during the spinning process.And the greater the high-elastic deformations,the greater the internal stress and dimensional shrinkage of the fiber.The recovery process of high-elastic deformation is mainly related to temperature and tension imposed on fibers.The high-elastic deformation that can be recovered in the fiber is generally around 10%.The chemical reaction of PAN fiber at high temperature has an inhibitory effect on the thermal relaxation behavior of the fiber.2.At high temperatures,the elastic modulus,high-elastic modulus and bulk viscosity of PAN fibers decrease in temperature as the temperature rises.But the effect of chemical reactions causes the relaxation time of PAN fibers to decrease with temperature increase and then increase after transition point.Treatment of the fiber by applying tension and increasing temperature can control the thermal relaxation behavior of the PAN alignment molecule.3.Increasing the heat treatment temperature during the spinning process,the heat condition promotes the chemical reaction of the surface molecules of PAN fibers.And the treatment reduce the internal stress and dimensional shrinkage of the fiber by more than 50%,control the thermal relaxation behavior of the orientation molecules,and the total fiber orientation degree increased from 49%to 69%.4.The use of tension and temperature to control the thermal relaxation behavior of oriented molecules of PAN fibers will have an effect on fiber properties.The microcrystals of carbon fiber which prepared by the PAN fibers treated after the high temperature heat treatment with tension imposed were more regular along the molecular chain arrangement.The orientation of the fiber crystal regions increased with the treated temperature increasing.And the tensile strength of carbon fibers increased from 4.23GPa to 4.7GPa.