Fuente: Journal of applied polymer Lugar: REVIEW Kevlar is traditionally modeled as a rigid-rod polymer, yet rheological behavior deviates from this assumption. The review introduces a novel “twist-tie” entanglement theory for aramid polymer chains in the nematic liquid crystal state, providing a molecular explanation for deviations from rigid-rod models. Understanding twist-tie entanglement dynamics enables optimization of fiber processing and enhancement of yarn mechanical performance through improved chain alignment.

ABSTRACT
Aramid fibers achieve exceptional strength per unit weight through the crystalline alignment of high molecular weight polymer, like poly(p-phenylene terephthalamide) (PPTA). A comprehensive understanding of how molecular organization during fiber spinning influences material performance is essential for enhancing product functionality. This review examines how shear and extensional flows of PPTA in the nematic liquid crystal state influence molecular and rheological behavior. Traditional rigid-rod analytical models fail to predict the dual shear stress peaks at 100–110 and 200–250 strain units or their reappearance after shear cessation. This theory-experimental gap highlights the need for a more precise model, as the molecular length of PPTA chains is ca. 7–15 times longer than the persistence length. This study shows that PPTA chains can adopt helical conformations and offers the unique hypothesis that such conformations lead to twist-tie knot entanglements at concentrations and molecular weights relevant to aramid fiber production. This new perspective qualitatively explains the discrepancies between existing analytical model predictions and experimentally observed rheological behavior. Understanding PPTA molecular dynamics in shear and extensional flows is essential for enhancing alignment during processing and could enable the development of higher-performance materials.