Study On The Construction And Formation Mechanism Of Polar Surface Structure Of High Modulus Carbon Fibers

Polyacrylonitrile(PAN)based high modulus carbon fibers typically have a tensile modulus exceeding 350 GPa.Due to their advantages such as high modulus,good thermal stability,and good dimensional stability,they are commonly used in high-end fields such as aerospace.However,while increasing the modulus of carbon fibers,their surface polarity will inevitably decrease,which will affect the interface performance of their composite materials,and the interface performance will directly affect or even control the macroscopic performance of carbon fiber composite materials.The carbonization temperature,heat treatment time,and tension during the preparation of high modulus carbon fibers directly affect the internal structure and surface polarity of carbon fibers.In this paper,the effects of carbonization temperature,heat treatment time,and tension on the internal structure and surface polarity of carbon fibers were studied.Finally,high modulus carbon fibers with different surface polarities were prepared under reasonable processing conditions,and the formation mechanism was explored,providing a theoretical basis for achieving adjustable and controllable properties of carbon fibers.The changes in the internal aggregation structure and surface polarity of carbon fibers under different process conditions were studied through X-ray diffraction,Raman spectroscopy,droplet debonding,dynamic contact angle,and other tests.The changes in the modulus of carbon fibers were studied through mechanical property testing,and the following conclusions were obtained:1.The content of active carbon atom on surface(Sac value)can effectively characterize the surface polarity of high modulus carbon fibers,which using Raman spectroscopy testing.The heat treatment temperature,time,tension and other processes have a significant impact on the modulus and Sac value,but the relationship between Sac value-surface energy and Sac value-interface shear strength does not change significantly with the change of heat treatment process.Therefore,the Sac value can be used as a quantitative indicator for surface polarity evaluation and is also the most important factor affecting the surface and interface properties of high modulus carbon fibers.2.The main factors affecting the modulus of carbon fibers are crystallinity and microcrystalline orientation.Under a single process variable,with the increase of heat treatment temperature or time,amorphous carbon continuously transforms into graphite carbon,and the number or size of microcrystals increases.The crystallinity of carbon fibers increases,and the effect of time on crystallinity is more significant.Simultaneously,due to the internal stress generated by graphitization transformation,it promotes the degree of microcrystalline orientation.The combined effect of crystallization and orientation enhances the fiber modulus;The tension mainly improves the modulus by increasing the orientation of microcrystals,and has a certain effect on microcrystalline growth,but the effect is minimal.3.The main factors affecting the Sac value are crystallinity and microcrystalline size.Under single process variables,as the heat treatment temperature or time increases,the crystallinity of carbon fibers increases,thus the active amorphous carbon content decrease,simultaneously,the active carbon atoms at the microcrystalline boundary on the fiber surface decrease,due to microcrystalline growth.Ultimately leading to a decrease in the surface Sac value.To some extent,tension promotes the growth of microcrystals and reduces the amount of edge activated carbon.However,under tension,axial tension causes radial contraction,which in turn causes microcrystals to have a certain degree of orientation along the radial direction,resulting in more microcrystalline boundary activated carbon atoms being exposed to the surface.Overall,tension has little effect on Sac value.4.Based on the differences in the effects of temperature and time on modulus and Sac,synergistic regulation of modulus and Sac is carried out through different time-temperature combinations.Under long-term low temperature conditions,the crystallinity of carbon fibers remains unchanged,achieving fiber microcrystalline refinement and effectively increasing microcrystalline boundaries.The microcrystalline boundaries have more active carbon atoms,which can improve the surface polarity of fibers under the same modulus.The Sac value increases from 0.55 of 72s-2100 K to 0.59 of 360 s-1900 K,the surface energy increases from 24.1 m N/m to 25.7 m N/m,and the interface shear strength increases from 39.2 MPa to 45.2 MPa,which modulus of carbon fiber is about 360 GPa.5.Based on the differences in the effects of temperature and time on modulus and Sac,different temperature-tension combinations are used to synergistically regulate modulus and Sac.Under high tension low temperature conditions,tension promotes an increase in the orientation of carbon microcrystals,thereby achieving high modulus.Low temperature also promotes a decrease in the overall crystallinity of the fiber,a decrease in the size of carbon microcrystals,and an increase in surface active carbon,thereby achieving an increase in fiber surface polarity under the same modulus.The Sac value increases from 0.57 of 22.7 N-2000 K to 0.62 of 71.0 N-1900 K,the surface energy increases from 24.9 m N/m to 26.4 m N/m,and the interface shear strength increases from 42.9 MPa to 47.2 MPa.which modulus of carbon fiber is about 350 GPa.It is close to the interfacial shear strength of high strength medium mold carbon fiber(50.7 MPa).