Solid state nuclear magnetic resonance investigation of polymer backbone dynamics in poly(ethylene oxide) based lithium and sodium polyether-ester-sulfonate ionomers

Abstract

Polymer backbone dynamics of single ion conducting poly(ethylene oxide) (PEO)-based ionomer samples with low glass transition temperatures (T(g)) have been investigated using solid-state nuclear magnetic resonance. Experiments detecting (13)C with (1)H decoupling under magic angle spinning (MAS) conditions identified the different components of the polymer backbone (PEO spacer and isophthalate groups) and their relative mobilities for a suite of lithium- and sodium-containing ionomer samples with varying cation contents. Variable temperature (203-373 K) (1)H-(13)C cross-polarization MAS (CP-MAS) experiments also provided qualitative assessment of the differences in the motions of the polymer backbone components as a function of cation content and identity. Each of the main backbone components exhibit distinct motions, following the trends expected for motional characteristics based on earlier Quasi Elastic Neutron Scattering and (1)H spin-lattice relaxation rate measurements. Previous (1)H and (7)Li spin-lattice relaxation measurements focused on both the polymer backbone and cation motion on the nanosecond timescale. The studies presented here assess the slower timescale motion of the polymer backbone allowing for a more comprehensive understanding of the polymer dynamics. The temperature dependences of (13)C linewidths were used to both qualitatively and quantitatively examine the effects of cation content and identity on PEO spacer mobility. Variable contact time (1)H-(13)C CP-MAS experiments were used to further assess the motions of the polymer backbone on the microsecond timescale. The motion of the PEO spacer, reported via the rate of magnetization transfer from (1)H to (13)C nuclei, becomes similar for T≳1.1 T(g) in all ionic samples, indicating that at similar elevated reduced temperatures the motions of the polymer backbones on the microsecond timescale become insensitive to ion interactions. These results present an improved picture, beyond those of previous findings, for the dependence of backbone dynamics on cation density (and here, cation identity as well) in these amorphous PEO-based ionomer systems.