Formation And Evolution Of Sp~2 Hybrid Conjugated Structure During Pre-oxidation Of Pan Precursor

Polyacrylonitrile(PAN)filaments undergo a chemical reaction due to the effect of time and temperature during thermal stabilization,which converts the macromolecular chain structure into a heat-resistant looped trapezoidal structure,thereby reducing the defect structure contained in the carbon fiber produced.During the carbonization process of PAN stabilized fibers,the original double bond on the ring structure is opened,and a cross-linking reaction occurs between the molecules to generate a graphite-like structure.The main factor affecting the existence of carbon fiber defect structure is the cyclization rate of the PAN fiber structure,and the sp2 hybrid conjugate structure generated by the chemical reaction of the carbon-containing structure is the effective basic structure of the final carbon fiber six-member ring plane Therefore,the cyclization rate is positively correlated with the content of sp2 hybrid conjugated structure generated during thermal stabilization of PAN filaments.To sum up,it can be seen that investigating the effect of time and temperature on the formation and evolution of sp2 hybrid conjugated structure of PAN fibers during the thermal stabilization process is good for the production of carbon fibers with good performance.In this paper,micro confocal Raman spectroscopy(Raman),ultraviolet-visible absorption spectroscopy(UV-vis)and 13C-solid nuclear magnetic carbon spectroscopy(13C-NMR)are used to characterize the formation and evolution of sp2 hybrid conjugated structure of PAN fibers.Among them:Raman spectrum characterizes the process of formation and evolution of sp2 hybrid structure;UV-vis absorption spectrum characterizes the size of conjugated ring structure;13C-NMR characterizes the structure content of sp2 hybrid C atoms.The study results indicated that:(1)When the thermal stabilization time is constant,the conjugation degree of the sp2 hybrid structure in PAN fibers increase linearly with the increase of the thermal stabilization temperature due to the temperature effect.The structure content of sp2 hybridized C atoms showed an "S-shaped"increasing trend with the increase of thermal stabilization temperature.Among them,when the heat treatment temperature is 180-230'C,with the increase of the thermal stabilization temperature,the sp2 hybrid conjugated ring structure formed by the PAN fiber is converted from a single ring plus two double bonds cross-linked structure to three Ring conjugated structure,the structure of the double ring plus two double bonds is converted into a three ring conjugated structure.When the heat treatment temperature is 240-280?,the two-ring conjugated structure generated in the low temperature range is cleaved and transformed to form a structure with a single ring plus a double bond and a structure with two rings plus a double bond;The yoke structure is cleaved and transformed to form a structure with two rings plus a double bond,and a structure with three rings plus a double bond.(2)When the thermal stabilization temperature is constant,the PAN fiber is affected by the time effect,so that the degree of conjugation of the sp2 hybrid structure increases rapidly with the thermal stabilization time and then tends to stabilize.(3)With the extension of thermal stabilization time,the amount of conjugated ring structures generated by PAN stabilized fibers has increased,but the size has not changed,so the effect of time on the size of the conjugated ring structure is not obvious.The size of the conjugated ring structure mainly depends on the temperature effect,in which the effect is obvious in the low temperature region(180-230?).