Molecular Alignment and Ion Transport in Rigid Rod Polyelectrolyte Solutions

Combining molecular alignment with selective ion transport can increase the freedom to design ion-conducting polymeric materials and thus enhance applications such as battery electrolytes, fuel cells, and water purification. Here we employ pulsed-field-gradient (PFG) NMR diffusometry, ²H NMR spectroscopy, polarized optical microscopy, and small-angle X-ray scattering to determine relations between counterion transport, dynamic coupling of water, and molecular alignment in aqueous solutions of a rigid rod sulfonated-aramid polyelectrolyte: poly­(2,2′-disulfonyl-4,4′-benzidine terephthalamide) (PBDT). ²³Na PFG NMR on PBDT solutions and simple sodium salt solutions shows significantly slower Na⁺ counterion diffusion in PBDT, providing agreement between counterion condensation theory and quantitative transport information. Strikingly, from ²H NMR spectroscopy we observe that the orientational order parameter of partially aligned solvent D₂O molecules increases linearly with polymer weight percentage over a large concentration range (1.4 to 20 wt %), while the polymer chains possess essentially a large and fixed order parameter S_matrix = 0.76 as observed using both SAXS and ²H NMR on labeled polymers. Finally, we apply a two-state model of water dynamics and a physical lattice model to quantitatively relate D₂O spectral splittings and nematic rod–rod distance. These studies promise to open new pathways to understand a range of anisotropic polymer systems including aligned polymer electrolyte membranes, wood composites, aligned hydrogels, liquid crystals, and stretched elastomers.