Effects Of Polyacrylonitrile Modification On The Texture Of Precursor Fibers

In this paper, the preparation technology and structure at different levels of PAN-based carbon fiber precursor were investigated in detail. The preparation technology was composed of free-radical solution copolymerization, dope modification and wet-spinning.Firstly, the radical solution copolymerization of AN with comonomers IA, AIA, MIA was investigated, respectively. The reactivity rates of the three copolymerization systems were all slower than that of homo-polymerization of AN, AN/MIA5 at the high end and molecular weight distribution D at the range of 2.0-2.4. The reaction rate of aqueous suspension polymerization was the fastest of the three, with the viscosity molecular weight reaching 6.6 ×10⁵ at the high end and molecular weight distribution D at about 3.2. The viscosity molecular weight can reach 5.2 ×10⁵ at the high end and molecular weight distribution D was about 3.0.The theoretical molecular weight was calculated according to free-radical solution polymerization principle. When the monomer concentration was 20 30%(wt) respectively, the theoretical number-average molecular weight was 5.39 ×10⁴ 6.89 × 10⁴ accordingly. For the aqueous suspension polymerization products, the theoretical number-average molecular weight was 1.30×10⁵. The composition and sequence distribution of copolymers of AN with IA, MAA and AA as comonomers were simulated by Monte Carlo method.The rheological behavior of the polymer solution was studied. Effects of molecular weight, polymer concentration, comonomer concentration, temperature and additives on the viscosity of the solution were discussed. It was discovered that MIA as comonomer and surfactants as additives were both effective methods to reduce the viscosity of the solution.The polymer solution modification was carried out in this paper. When P(AN/IA)changed to P(AN/AIA) after amino-modification, the hydrophilic property of the polymer was improved dramaticly.From the view of mutual-diffusion and micro-phase separation, the protofiber coagulation was analyzed. The mutual-diffusion of the solvent DMSO and coagulate H₂O made the polymer micro-phase and the DMSO-H2O micro-phase separated from the dope, with the polymer as continuous phase, and liquid phase dispersing as micro-void.In the progress of micro-phase separation, if the hydrophilic property of the polymer was not high enough, big and loose voids were formed in the coagulation zoo. The big and loose voids were not interconnected and interfered the diffusion of DMSO and H₂O. The protofiber deformed for open new diffusion paths and the transverse section of the protofiber was no longer round. If the hydrophilic property of the polymer was high enough, small and interconnected voids were formed and the diffusion would progress smoothly, with protofiber transverse section round.A method was established for the determination of DMSO in the precursor fibers by high performance liquid chromatography (HPLC). By means of spinning processes improving, the residual of DMSO in PAN precursor fibers was reduced from 93 mg/L to 2 mg/L.The texture of precursor fibers was studied from the view of crystal structure, orientation structure and void structure. In the multi-level drawing process, the PAN chains or part of chain segments slide relatively and rearranged and oriented along the fiber axis. In the rearrangement and orientation process, chains or part of chain segments with high degree of orientation formed bundle-shaped structure, with chains or part of chain segments with low degree of orientation forming the connection structures among the bundle-shaped structures. SEM and HRTEM tests showed that the bundle structure had two levels: fibrils and micro-fibrils. The diameter of fibrils was at the range of 50-200nm, while the diameter of micro-fibrils was about 10nm. Micro-fibrils were the base structure of PAN precursor fibers. Comonomer IA or MIA could effectively decrease the crystal degree and crystal size. The increase of hydrophilicity of PAN made the polymer sustainable to high drawing-ratio and high degree of orientation.The effect of comonomers on the thermal properties of PAN fibers was studied by differential thermal analysis (DTA), differential scanning calorimetry (DSC) and thermal gravimetric analysis (TG). It was discovered that comonomers IA or MIA could make the exothermal peak shift to low temperature zone, with the exothermal peak of P(AN/MIA) and P(AN/IA) 5.7°C and 42.7°C lower than that of homo-PAN, respectively. Amino-modification didn't change the thermal properties of the precursor fibers because P(AN/MIA) changed back to P(AN/IA) in the spinning process.In the last part of the paper, fiber samples from every important thermal stabilization steps were studied by infrared spectroscopy (IR), oxygen element analysis and X-ray diffraction (XRD). The changes of -CN and other key functional groups during the reactions of thermal stabilization were tracked. Several characteristics of the reaction were summarized.