Radiation Effect Of Electrospun Polyacrylonitrile Nanofibers And The Subsequent Effect On Stabilization Process

Electrospun polyacrylonitrile(PAN)nanofibers have been found to be the most important precursors for producing carbon nanofibers due to its fast pyrolysis rate and high carbon yield.The thermal stabilization plays a critical role in the stucture and performance of resulting carbon nanofibers.Modification of PAN nanofibers could optimize the stabilization process and improve the performance of the carbon nanofibers.In our study,PAN nanofibers were prepared by electrospining and then were irradiated by ?-rays.Radiation effect on PAN nanofibers was studied and the subsequent influence on stabilization was also investigated.The structure and properties of free radicals induced by radiation and heat treatment were studied in detail,respectively.The thesis mainly contains two parts as follows:PAN nanofibers were irradiated by ?-rays at room temperature in air.The radiation effect on the structure and thermal properties of PAN nanofibers was studied via fourier transform infrared spectroscopy(FTIR),X-ray photoelectron spectroscopy(XPS),differential scanning calorimetry(DSC)and thermo-gravimetric analysis(TGA).It was found that ? irradiation in air led to radiation oxidation of PAN nanofibers and functional groups containing oxygen can be directly introduced onto the molecular chain of PAN through radiation oxidation,which facilitate dehydrogenation reaction in the preoxidation process.Meanwhile,cyclization of nitrile groups was induced by ?-rays irradiation at room temperature to produce-C=N-conjugate structure and the content of-C=N-group increases with absorbed dose.Isothermal stabilization method was used to study the effect of radiation on the pre-oxidation reactions.The chemical structure of pre-oxidized fibers obtained from original and irradiated PAN was characterized via FTIR.The results show that irradiated PAN nanofibers exhibited higher preoxidation extent under the same stabilization condition.Thus,radiation treatment could accelerate the stabilization process and consequently shorten the stabilization time.The free radicals induced by radiation are closely related to crosslinking and degradation reaction.The free radicals in PAN nanofibers induced by ?-ray irradiation were studied via electron spin resonance(ESR)technique.It was found that the spectra of PAN nanofibers are similar to those of PAN microns fibers after ?-ray irradiation in vacuum which contain hyperfine structure and central singlet peak assigned to alkyl radicals and polyimine radicals respectively.The free radicals in vacuum were found to be rather stable at room temperature and its half-period is more than 200 days.However,the free radicals decayed very fast when exposed to air due to the fast diffusion rate of oxygen into PAN nanofibers with small diameter and disordered structure.Besides,the free radicals induced by thermal oxidat ion above 200? were studied.The results show that a large number of polyenyl radicals were produced and its concentration increased first and then decreased with increasing temperature.This indicates that polyenyl radicals formed during stabilization further participate in preoxidation reaction with increasing temperature.This study provides a theoretical basis for the better use of radiation processing technology and it is useful for further study on the improvement of pre-oxidation by radiation processing.