Solid-State Nuclear Magnetic Resonance Characterization of Residual ²³Na in Aramid Fibers

In aramid (para-phenylene terephthalamide) fibers, small concentrations of sodium ions are present as remnants of the neutralization step in the production. This is thought to relate to the long-term stability of the fibers. Our study shows that most of the sodium ions are accessible by water. Fitting the ²³Na NMR spectra at two magnetic fields reveals a large distribution in both quadrupolar interaction and isotropic chemical shift, indicating substantial structural variations in the direct vicinity of the ions. All these observations imply that, upon drying, the sodium ions reside in a single pool with a very disordered environment. This implies that fibers consist of crystallites that are packed in such a way that end groups are not directly exposed for coordination with sodium. The sodium resides in intercrystalline regions of the fibers, that is, cracks and voids, most likely in the form of sodium sulfate at various stages of (residual) hydration.