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 Smatrix = 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.