The Cyclized And Oxidized Structures Regulation Of Polyacrylonitrile Fibers In Pre-oxidation Process

The structural differences of PAN（polyacrylonitrile）fiber formed in the pre-oxidation process can affect the thermal cracking behavior of the subsequent carbonization and the properties of carbon fibers so that it is very important to control the cyclized structure and oxidized structure of PAN pre-oxidized fibers.In this thesis,the PAN fiber was modified by impregnation of KMn04 solution in different conditions and heated in different pre-oxidation temperatures to produce different pre-oxidized fibers.Using FT-IR,¹³C-NMR,XPS,Raman,TG and other characterization methods,the influence of the modification by KMnO₄ and pre-oxidation temperature on the evolution of cyclized structure,oxidized structure and skin-core structure was studied,the cyclized structure and oxidized structure of the modified pre-oxidized fiber was studied.The relationship between the thermal stability of pre-oxidized PAN fibers and the cyclized structure and oxidized structure was further investigated.The results show that:1.During pre-oxidation process,KMnO₄ plays a role in catalyzing cyclization reaction and improving the content of cyclized structure and oxidized structure with the impregnation content of KMnO₄ controlled by impregnation temperature and time in a reasonable range.The outer 3d vacant electron orbit of Mn⁷⁺ will form coordination bond with the lone pair electron of N in C=N,promoting the formation of ring structure between adjacent C=N and initiating the cyclization reaction at lower temperatures.2.The influence of temperature on cyclized structure and oxidized structure is as follows.The cyclized structure（pyridine ring,enamine ring,etc.）has been increasing and perfected continuously from 180 ℃.Oxidized structure（pyridone）is generated slowly at low pre-oxidation temperatures and speeded up its formation after 240℃.The higher the temperature is,the more oxidized structure there forms.3.The influence of modification by KMnO₄ on cyclized structure and oxidized structure is as follows.At the same pre-oxidation temperature,more cyclized structure and oxidized structure are produced under the influence of KMnO₄ and the content of cyclized structure is increased by 11.3%,the oxygen structure is increased by 2.6%maximally.However,the growth rate of the two structures are different in different impregnation conditions.When the impregnation content of Mn is less than 1.6%,the growth rate of cyclized structure is faster than that of oxygen structure,causing the different proportion of the two at last.4.The modification by KMnO₄ is also beneficial to the homogenization of fiber structure while regulating the content of cyclized structure and oxidized structure of PAN pre-oxidized fiber,making the distribution of the cyclized structure and oxidized structure more uniform in the radial direction and improving the skin-core structure.The core/shell ratio is increased by 9.7%and the standard deviation of light density is decreased by 0.0104.In combination with pre-oxidation temperature and chemical modification,the cyclized structure and oxidized structure can be regulated in quantity,type and distribution.5.The thermal stability of PAN pre-oxidized fiber is influenced by the proportion of the cyclized structure and oxidized structure,degree of cyclization and core/shell ratio.Therefore,the higher the degree of cyclization and core/shell ratio and the bigger the proportion of the cyclized structure and oxidized structure,the better the thermal stability.The thermal stability of PAN pre-oxidized fiber modified by KMnO₄ is higher and the carbon yield is increased.In this paper,the carbon yield of PAN fiber modified by 60℃KMnO₄ and pre-oxidized at 280℃ increased from 66% to 71%.In this thesis,the quantity,type and distribution of the cyclized structure and oxidized structure of PAN pre-oxidized fiber has been regulated in combination with the pre-oxidation temperature and chemical modification and the relationship between cyclized structure,oxidized structure and thermal stability is cleared,revealing the reasons for the improvement of KMnO₄ modified carbon fibers’ performance and providing theory basis for the optimization of pre-oxidation process.