Study on Process Optimization to Enhance the Quality of PAN Fiber

Abstract

Polyacrylonitrile (PAN)-based carbon fibers are essential materials for high-performance applications in aerospace, automotive, and energy industries due to their exceptional specific strength, stiffness, and thermal stability. However, the quality of carbon fibers is fundamentally determined by the microstructure of PAN precursor fibers. Despite extensive research on PAN fiber production, the complex interplay between thermodynamic and kinetic parameters during coagulation, and their effects on the resulting fiber microstructure and properties, remains incompletely understood. Furthermore, the influence of polymer molecular characteristics on microstructural development has not been systematically investigated. This dissertation presents a systematic investigation of processing‒structure‒property relationships in PAN precursor fibers through a hierarchical three-stage optimization approach. The research systematically examines how (1) solvent/nonsolvent system selection, (2) coagulation bath conditions, and (3) polymer molecular parameters influence the phase separation behavior during fiber formation and the subsequent development of fiber microstructure and mechanical properties. The ultimate objective of this study is to establish fundamental design principles for producing high-quality PAN precursor fibers through control of processing parameters and polymer architecture. In Chapter 2, the effects of solvent/nonsolvent system selection on fiber microstructure were investigated by comparing four systems: DMF/Water, DMSO/Water, DMF/Methanol, and DMSO/Methanol. Thermodynamic compatibility was quantified using Hansen solubility parameters and Flory-Huggins interaction parameters, and kinetic behavior was determined through mutual diffusion coefficients. The results demonstrated that rapid phase separation occurred in the DMF/Water system, which was characterized by higher thermodynamic incompatibility, resulting in large, finger- like microvoids and poorly ordered crystalline structures. The DMSO/Methanol system exhibited excessively slow phase separation, which led to high residual solvent content within the fiber, thereby preventing stable fiber formation. Interestingly, the DMF/Methanol system achieved optimal phase separation behavior, effectively suppressing microvoid formation while promoting a well-ordered crystalline structure. Consequently, the resulting fibers exhibited enhanced microstructural development during post-drawing due to this optimized initial morphology. In Chapter 3, the effects of coagulation bath composition and temperature on fiber microstructure development were investigated. Three conditions were examined: pure methanol at -10°C (MeOH100_- 10), 70 vol% methanol at -10°C (MeOH70_-10), and 70 vol% methanol at 30°C (MeOH70_30). Thermodynamic and kinetic analyses revealed that the MeOH100_-10 condition exhibited faster phase separation due to a strong thermodynamic driving force and high diffusion rates. This rapid phase separation produced fibers with kidney-shaped cross-sections along with predominantly planar zigzag chain conformations and superior initial molecular orientation. In contrast, the MeOH70-based conditions resulted in slower phase separation, yielding more circular cross-sections with higher fractions of helical conformations and lower initial orientation. These findings demonstrate that the initial molecular conformation and orientation established during coagulation influence the structural development during the subsequent drawing process, thereby fundamentally determining the final fiber microstructure. The MeOH100_-10 fibers achieved the highest crystallinity, densest lateral chain packing, superior chain alignment along the fiber axis, and less pronounced microvoid morphology. This enhanced microstructural development directly led to superior mechanical properties, with MeOH100_-10 fibers exhibiting substantially higher tensile strength and modulus than MeOH70-based fibers. These results suggest that precise control of coagulation kinetics through bath composition and temperature enables optimization of molecular-level structure, which subsequently governs the microstructural evolution during the drawing process. In Chapter 4, the effects of molecular parameters (molecular weight and polymer composition) were investigated by comparing three PAN samples: low-molecular-weight homopolymer (LMW_PANHomo), high-molecular-weight homopolymer (HMW_PANHomo), and high-molecular-weight copolymer (HMW_PANCo). This study also evaluated the applicability of the hexagonal versus orthorhombic crystalline structure models for PAN through systematic peak deconvolution analysis. The orthorhombic model demonstrated superior fit quality compared to the hexagonal model. The results demonstrated that molecular weight was the dominant factor governing fiber drawability and mechanical performance. LMW_PANHomo fibers exhibited limited drawability (TDR 15) due to insufficient chain entanglements, resulting in lower orientation and mechanical properties. Both HMW- based polymers achieved significantly higher drawability (TDR 27) through enhanced entanglement networks. Notably, HMW_PANCo fibers exhibited superior chain mobility compared to HMW_PANHomo due to comonomer-induced disruption of intermolecular interactions between nitrile groups, which facilitated more extensive molecular rearrangement during drawing. This enhanced processability enabled HMW_PANCo fibers to achieve the highest degree of lateral chain packing, excellent molecular orientation, and optimal mechanical properties (tensile strength of 1.1 GPa, tensile modulus of 20.4 GPa). In conclusion, this dissertation establishes comprehensive processing‒structure‒property relationships for PAN precursor fibers through systematic optimization. The research demonstrates that high-quality precursor fibers require: (ⅰ) solvent/nonsolvent systems with balanced thermodynamic and kinetic parameters (DMF/Methanol), (ⅱ) coagulation conditions that promote rapid solidification with favorable initial molecular conformations (pure methanol at -10°C), and (ⅲ) high-molecular-weight copolymers that combine sufficient entanglement density with enhanced chain mobility (HMW_PANCo). The synergistic optimization of these parameters enables the production of PAN precursor fibers with well-developed crystalline structure, high molecular orientation, minimal microvoid defects, and superior mechanical properties suitable for manufacturing high-performance carbon fibers. These findings provide fundamental insights and practical guidelines for the design of PAN fiber production processes.