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 (τc) obtained from EPR simulation exhibited a sharp increase near 4.5 wt %, close to the entanglement concentration (ce) 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 τc changed abruptly closely matched that at which the solution became transparent. Additionally, τc 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.