Viscoelastic Response Of Polyacrylonitrile Fiber During Strentching Process

Orientation structure of polyacrylonitrile(PAN)precursor,which is formed during the stretching process,evolves continuously in the postprocessing,thermal stabilization and graphitization.It can be inherited into the structure of final carbon fiber and plays a decisive role on the mechanical properties of carbon fiber.From the perspective of macromolecule motion,the essence of stretching process is the combination of viscous and elastic motion along the direction of stress generated by the molecular chains of PAN nascent fiber under tension.This will determine the formation of orientation structure.Therefore,to study viscoelasticity is of great significance to know the orientation structure of PAN fibers during the stretching process.In this thesis,a specific four-components viscoelastic model was established to simulate the response mechanism of the motion of PAN molecular chains during the stretching process,based on which the viscoelastic constitutive equation of the stretching process was obtained.Dynamic mechanical analysis(DMA)method was applied to describe the dynamic mechanical parameters including storage modulus,loss modulus and loss angle tangent(tanδ)through the temperature scanning.The influence of stretching on the viscoelastic properties of PAN fiber was studied and the essential reason was analyzed.Meanwhile,the effects of coagulation environment,including concentration and temperature of the bath,on the viscoelasticity of PAN fiber during stretching was investigated.Polarized infrared spectroscopy was also applied to characterize the orientation factor of PAN fiber.The relationship between the formation of orientation structure and the change of viscoelasticity of PAN chains was established.The results indicated that:1.The viscoelastic model of "spring-spring/dashpot parallel connectiondashpot" were used to describe the response mechanism of the PAN molecular chain motion during the stretching process.During this process,the instantaneous stress in the fiber was the sum of the stress relaxation of the PAN linear molecular chain and the residual instantaneous stress produced by the physical cross-link points.The instantaneous modulus has a corresponding mathematical relationship with initial modulus,equilibrium modulus and relaxation time.2.The storage modulus of PAN fiber decreased with the increase of temperature because the free volume of molecular vibration increases to a certain extent and the larger segments begin to move.The loss modulus first increased and then decreases with the increase of temperature.The reason is that the viscous response of the fiber cannot keep up with the stress at the same time of phase transition,resulting in the generation of loss modulus peak while the peak of loss angle tangent corresponds to the sub relaxation transition and glass transition.3.The larger the stretching,the higher the storage modulus of PAN fiber at room temperature while the faster the decreasing rate of the storage modulus with temperature,which is due to the larger entropy elastic recovery of molecular chains caused by stretching.With the increase of drawing ratio,the relaxation transition temperature at which the storage modulus of PAN fiber decreased rapidly showed a downward tendency because of the decreasing of the physical crosslinking points amount.The larger the drawing ratio,the larger the loss modulus of PAN fiber at room temperature while the opposite tendency appeared at high temperature.The glass transition temperature also decreases,which is due to the characteristics of viscous movement.The tanδ of PAN fiber increased with the increase of drawing ratio at low temperature while the opposite tendency appeared at high temperature.4.With the increase of coagulation concentration or temperature,the storage modulus and loss modulus of PAN fiber decreased when the drawing ratio is same.The influence of coagulation bath concentration or temperature increased with the increase of drawing ratio,which is due to the different coagulation extent of fiber caused by the difference of the aggregation structure and extension state of PAN molecular chains in different environments.5.The decrease of concentration and temperature of coagulation bath was beneficial to the improvement of orientation factor.The variety rate of loss modulus,which represented the viscosity of molecular chains,had a large contribution to the formation of orientation structure that is related to plastic deformation.There was a pseudo-linear relationship between the orientation factor and the variety rate of loss modulus G".The lower the coagulation concentration or temperature,the greater the influence on the correlation between the formation of orientation structure and the variety rate of loss modulus.