Evolution of Radial Aggregated Structure Within Polyacrylonitrile‐Based Fibers During Stretching

ABSTRACT The water bath stretching process offers an effective approach to tune microcrystals and orientation parameters of carbon fibers, thereby enhancing their performance. However, a comprehensive understanding of the evolution mechanism of the aggregated structures during water bath stretching governing radial heterogeneity in PAN fibers remains poorly understood. Herein, we use nascent fibers that have undergone solvent removal to stretch in a water bath at varying temperatures and quantitatively analyze the evolution of aggregated structures. The results reveal that the evolution of aggregated structures into radial structural heterogeneity stems from the radial stress gradient distribution during stretching. This is accompanied by the gradual formation of a more ordered structural stacking and molecular chain orientation in the skin region compared to the core region. The radial structural heterogeneity becomes more pronounced with the elevated bath temperature. Importantly, by adjusting the radial aggregated structures and the molecular chain alignment, the optimal precursor fibers exhibit significantly enhanced mechanical performance when stretched at 75°C. This study could provide valuable insights for the structural regulation in polyacrylonitrile‐based fibers with enhanced performance.