Local environment and distribution of alkali ions in polyelectrolyte complexes studied by solid-state NMR

Abstract

Polyelectrolyte complexes (PECs) formed by the addition of substoichiometric amounts of (poly(diallyldimethyl ammonium chloride)) (PDADMAC) solutions to sodium or lithium poly(styrene sulfonate) (Na- or Li-PSS) solution contain adjustable amounts of charge balancing Li⁺ or Na⁺ cations, which possess ionic mobility of interest for solid electrolyte applications. Very little is known regarding the local environments and the spatial distributions of these cations and their interactions with the polyelectrolyte chains in these amorphous materials. To address such issues, the present work develops a comprehensive solid state NMR strategy based on complementary high-resolution magic-angle spinning (MAS) NMR and various dipolar spectroscopic techniques. ^6,7Li and ²³Na chemical shifts measured on a series of PECs with general composition described by B_(2x−1)PSS_xPDADMA_(1−x) (B = Li or Na and 0.53 ≤ x ≤ 1) reveal composition-independent local cation environments. In contrast, ⁷Li{⁶Li} spin echo double resonance (SEDOR) experiments measured on ⁶Li enriched materials and ⁷Li{¹H} rotational echo double resonance (REDOR) experiments are consistent with an approximately random ion distribution. The same conclusion is suggested by ²³Na{¹H} REDOR measurements on the analogous sodium containing system indicating a non-segregated PEC structure. In apparent contrast to this conclusion, ²³Na spin echo decay spectroscopy yields nearly constant dipolar second moments over a wide composition range. This can be explained by considering that the ²³Na spin echo decays are affected by both ²³Na–²³Na homonuclear dipolar couplings and ²³Na–¹H heteronuclear dipolar interactions in the presence of strong homonuclear ¹H–¹H spin exchange. In protonated Na-PSS both contributions are of comparable magnitude. In the PECs the contribution from ²³Na–²³Na interactions decreases, while that from ²³Na–¹H dipolar couplings with the protons from the PDADMA chains increases with decreasing Na content, resulting in superimposed opposite dependences on the ion concentration. All results for Li and Na containing PECs point at a non-phase separated polymer network with uniform ionic sites of very similar environment. The cations can be viewed as randomly distributed and located close to the polyion sulfate groups.