From Microfibrillar Network to Lamellae during the Coagulation Process of Polyacrylonitrile Fiber: Visualization of Intermediate Structure Evolution

Coagulation process is one of the most important stages of the microstructure formation and evolution in polyacrylonitrile (PAN) fibers during the spinning process. The network and lamellae were fundamental superstructures of PAN nascent fibers. Nevertheless, due to complex phase transitions, the mechanism of the microstructure formation and evolution in PAN nascent fiber was not comprehensively elucidated yet. In this work, a full account of microstructure formation and transformation of PAN nascent fiber by electron microscopic analysis was presented and the underlying mechanism of their formation and transformation during the coagulation process was proposed. During the coagulation process, phase separation, gelation, crystallization, and concentration fluctuations occurred. The densified and elongated domains of the polymer-rich phase were solidified into the microfibrils, and they were cross-linked to form the microfibrillar network. The transverse domains induced by the concentration fluctuation acted as the precursors of the lamellae, and subsequently the lamellae were stacked closely and regularly. Meanwhile, crystallization occurred along the whole process, and the crystal layers formed and aggregated within the microfibrils. Our work therefore revealed the existing morphological types in PAN nascent fibers and their underlying formation mechanism.