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

Abstract

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.

Microstructural Evolution during Dry-Jet Wet Spinning Postprocessing from Coagulation Bath Fiber to High-Performance Polyacrylonitrile Precursor Fiber

“…After the coagulation bath stage (P1), lamellae perpendicular to the fiber axis formed in the whole fiber. This morphology was also described as plane-wave-type concentration fluctuations, which were generated from stress-diffusion coupling . Compared with the core region (Figure a2), the lamellar arrangement in the skin region (Figure a1) was more regular and compact.…”

“…Appreciable changes of their tensile strength (increased by 485.25%) and breaking strain (decreased by 97.1%) were obtained after the spinning process was completed. This is exactly opposite to the changes in the coagulation process of PAN nascent fibers, 24 which indicates that the spinning stage is a period of fiber strength and orientation enhancement. Based on the comprehensive analysis of microstructural characterization mentioned earlier, the increase in fiber tensile performance was mainly attributed to the continuous development and orientation of internal microcrystalline structure and microfibers.…”

“…Through these postspinning steps, the internal fiber microstructure undergoes corresponding alterations. The changes in the crystal structure, the orientation of molecular chains, the micropore, and the microfibril were particularly important for obtaining high-performance precursor fibers. , Gao et al , reported the transformation of the microfibril network during the process of concrete solidification and stretching. Furthermore, many investigations have shown that the quality and mechanical performance of PAN fibers could be improved by enhancing the microstructure of the fibers, − such as increasing the perfection of the supramolecular structure and adjusting the mesoporous structure inside the fibers .…”

Role of Small Amounts of Water in the Gelation of Polyacrylonitrile Solutions in Dimethyl Sulfoxide: Rheology, Kinetics, and Mechanism

“…The region at 2241 cm −1 is of particular interest as it reveals nitrile−nitrile interactions in solutions influenced by water presence. 37 The addition of water to the complex solvent induces a shift in the band position to higher wavenumbers (Figure 23). This shift can be distinctly observed across all concentrations studied, underscoring the impact of water on the structure of PAN solutions.…”

Evolution of polyacrylonitrile precursor fibers and the effect of stretch profile in wet spinning

“…While precursor structure has been well defined in the unoriented and oriented states, the evolution of microstructure in PAN‐based precursors, or directly connecting the path from unoriented to oriented states in fiber spinning, is rarely studied because mimicking commercial spinning processes and investigating the impact of process changes on fiber structure is capital intensive and difficult. Rather, many studies have focused efforts on nascent stages of fiber spinning where coagulation parameters can have immediate and profound impacts on the precursor structure 40–45 . In particular, much focus has been placed on the dope extrusion rate and jet stretch, or stretch taken between the spinneret and first godet out of coagulation in wet spinning 46–50 …”