From Molecular Structure to Fiber Morphology: Kinetic Vitrification in Dry-Spun Polyimide Fibers

Abstract Despite the widespread use of dry spinning in the high-performance fiber industry, the fundamental principles governing the evolution of fiber cross-sectional morphology remain largely elusive. This morphological evolution is intrinsically governed by the transient filament dynamics during desolvation, a process that remains difficult to probe with conventional in situ experimental methods. In this study, we combine all-atom molecular dynamics simulations with dry-spinning experiments to establish a microscopic criterion for skin vitrification. Among the kinetic signatures examined, the kinetic transition temperature (Tc), identified from the change in slope in the Arrhenius plot of the solvent diffusion coefficient, is consistent with the crossover revealed by the van Hove correlation function and with the change in slope of the apparent segmental relaxation time, while requiring only nanosecond-scale trajectories. Using this criterion, we determined Tc for polyimides with different copolymer compositions and experimentally demonstrated that fiber cross-sectional irregularity correlates with Tc – Tchannel, where Tchannel denotes the spinning-channel temperature. Beyond elucidating the formation mechanism in polyimide systems, this work establishes a general molecular-level criterion that can guide the rational design of profiled-cross-section fibers from a wide range of dry-spun polymer–solvent systems.