Segmental Dynamics of Poly(m-Phenylene Isophthalamide) Unveiled by EPR Spectroscopy

Poly( m -phenylene isophthalamide) (PMIA) is a high-performance meta-aramid polymer whose processing is governed by chain segmental dynamics. Herein, PMIA segmental dynamics were evaluated by combining spin-labeled PMIA and continuous-wave electron paramagnetic resonance (CW-EPR) spectroscopy. The rotational correlation time (τ c ) obtained from EPR simulation exhibited a sharp increase near 4.5 wt %, close to the entanglement concentration ( c e ) determined from rheological measurements, demonstrating the reliability of spin-label EPR in determining segmental motion and entanglement. The dissolution behavior of PMIA was monitored by EPR spectroscopy, and the solubilizing salt concentration at which τ c changed abruptly closely matched that at which the solution became transparent. Additionally, τ c increased rapidly at low molecular weights before leveling off at higher molecular weights, reflecting the development and saturation of entanglement. These features establish spin-label CW-EPR as a complementary tool for elucidating polymer solution behavior at the microscopic scale in systems containing additives and for guiding polymer processing optimization.