Phase separation behavior of polyacrylonitrile solution and microstructural changes of fibers according to the solvent/nonsolvent system

Abstract

This study examines the influence of different solvent/nonsolvent systems on the microstructure, microvoid morphology, and structural evolution of polyacrylonitrile (PAN) precursor fibers during stabilization. PAN fibers were prepared using three solvent/nonsolvent pairs: DMF/Water, DMSO/Water, and DMF/MeOH. In the DMF/Water system, rapid phase separation due to the weaker interactions between PAN and the solvent/nonsolvent resulted in as-spun fibers with relatively high crystallinity and molecular orientation. However, the rapid phase separation also led to the formation of finger-like microvoid during initial coagulation, which caused greater microvoid misalignment after post-drawing. In contrast, the DMF/MeOH system produced as-spun fibers with the lowest initial orientation but demonstrated superior drawability during post-drawing. This is attributed to the formation of a gel-like network structure, which facilitated greater polymer chain mobility and alignment under tensile stress. The structural evolution of PAN precursor fiber during stabilization is strongly influenced by their microstructural characteristics. In the DMF/Water precursor fiber, the presence of large, poorly aligned microvoids hindered effective oxygen diffusion, resulting in reduced oxidation and dehydrogenation reactions. Consequently, the stabilized fiber exhibited smaller crystal sizes and lower orientation of the (002) planes, despite its initially high molecular orientation—unlike the DMSO/Water fiber, which showed more effective structural development during stabilization. Conversely, the DMF/MeOH precursor fiber, characterized by smaller microvoids and higher chain alignment, retained the highest orientation after stabilization. These findings highlight that the final microstructure of stabilized PAN fibers is governed by the interplay between microvoid morphology formed during phase separation, and molecular orientation induced through post-drawing.