The Influence Of Densified Thermal Environment On Radial Distribution Of The Structure Of Pan Monofilament

Carbon filament is a kind of material with a radial structure.It's known as an obvious "skin-core" structure in the radial direction,which makes the mechanical properties of the carbon filament distributes unevenly in the radial direction.Polyacrylonitrile(PAN)is used as the precursor of carbon filament,theremore,its structure has a decisive influence on the structure of carbon filament.In previous studies,it has been shown that PAN monofilament has "skin-core" structure at the initial stage,which will continue to evolve as wet-spinning process going on.Densification is an important step in wet-spinning,which has an important influence on the crystallization and dehydration of PAN filament,as well as the elimination of pores.In this study,the transmission electron microscope(TEM)was used for the observation on the radial microstructure of the densified PAN monofilament.Nano IR,Micro-Raman and Polarized Raman were used for characterizing the radial distribution on molecular chains structure,crystallization structure and orientation structure.Besides,Nanoindentation was used for studing the distribution of elastic modulus along the radial direction of PAN filament.The relationship between the distribution of the radial aggregation structure in the densified PAN monofilament with temperature and time has been revealed.The results indicated that:(1)The skin layer of densified PAN filament shows the closer moleculars stack,preferable crystallinity,higher elastic modulus,preferable orientation,as well as faster water removal and pores closure;however,the core layer shows the looser moleculars stack,unpreferable crystallinity,lower elastic modulus,unpreferable orientation,and lower densification.(2)With the temperature increasing,the difference in molecular chains structure,crystalline structure and modulus along the radial direction of the filament increase.This is because the rate of water removal between the skin layer with the core layer as well as the mobility of the molecular chains increase as temperature rises.The difference in orientation structure along the radial direction increases at first and then decreases.Due to the process from 90? to 140?,the difference in the mobility of the molecular chains between the skin layer with the core layer is the dominant factor,it caused the difference in the radial distribution of orientationan increasing.During the process from 140? to 150?,too fast speed of the heat transferring causes the internal organization of the filament to orient too quickly.The distribution gradient of radial orientation decreases.(3)As densification time increases,the difference in molecular chains structure,crystalline structure and modulus along the radial direction of the filament increase.The reason is the difference in the sufficiency of water removal and the adequacy of molecular chains motion increase as the densified time increases.The difference in orientation structure along the radial direction decreases at first and then increases.During the densified time from 16s to 24s,the difference in the adequacy of the molecular chains motion along the radial direction increases.The extension of time leads to a decrease in the diffrence of orientation along the radial direction.During 24s to 40s,molecular chains motion difference along the radial direction accounts for the main reason,therefore,the difference in the orientation of the densified PAN filament along the radial direction increases.