Multiscale Molecular Orientation in Polyacrylonitrile Precursors for Carbon Fibers: Processing Regulation, Characterization Methodologies, and Structure-Property Correlations.

Polyacrylonitrile (PAN) precursor fibers represent the dominant feedstock for manufacturing high-performance carbon fibers, and the hierarchical molecular orientation established during precursor processing fundamentally determines the tensile strength and elastic modulus of the final products. However, the precise regulation of multiscale orientational structures across spinning, hot-drawing, and thermal treatment stages remains a long-standing bottleneck for breaking the performance ceiling of carbon fibers. This review systematically elucidates the formation and evolution mechanisms of oriented structures at four hierarchical levels: molecular chains, crystalline domains, microfibrils, and fiber assemblies. Flow-induced orientation behaviors in three mainstream spinning routes (conventional wet spinning, dry-jet wet spinning, and electrospinning) are comparatively analyzed, followed by a discussion on crystallization-driven orientation during hot stretching and orientation inheritance throughout pre-oxidation and carbonization processes. Quantitative evidence from reported PAN precursor datasets confirms that increasing the orientation factor from 0.7 to 0.9 is associated with a tensile-strength gain from approximately 2.4 to 4.7 GPa and a modulus gain from 230 to 550 GPa in the corresponding carbon fibers, illustrating the magnitude of the orientation-performance relationship within specific material systems. We critically appraise widely used characterization techniques including X-ray scattering, polarized spectroscopy, optical birefringence, and electron microscopy, with particular focus on their spatial resolution, applicable scenarios, and inherent limitations, and propose combined characterization strategies for multiscale orientation analysis. Critical bottlenecks, including cross-scale modeling gaps, dynamic orientation kinetics under multi-field coupling, and industrial scalability limitations, are identified, and future research directions such as in situ synchrotron characterization and machine learning-assisted process optimization are outlined. This review builds a systematic framework linking molecular-level orientation control to macroscopic carbon fiber properties, providing theoretical guidance for the rational design of high-performance PAN precursors.