Unexpected effect of tetraglyme plasticizer on lithium ion dynamics in PAMPS based ionomers

Li⁺ cation conducting ionomers based on poly(2-acrylamido-2-methyl-1-propane sulphonic acid) (PAMPS) incorporating a low molecular weight plasticizer have been characterized. Previously we have observed an apparent decoupling of ionic conductivity and lithium ion dynamics from the T_g of this ionomer along with an increase in ionic conductivity obtained by incorporating a quaternary ammonium co-cation. The incorporation of tetraglyme as a coordinating plasticizer was investigated in order to further improve the ion dissociation and dynamics. Solid-state NMR, thermal analysis, impedance spectroscopy and infrared spectroscopy were used to characterize these systems. As expected, the glass transition temperature T_g decreased upon the addition of the plasticizer. However, in contrast to the previously reported Na-conducting systems, the ionic conductivity was also decreased by several orders of magnitude, indicating that the tetraglyme recouples the conductivity back to the polymer dynamics. Temperature dependent ⁷Li NMR line width and T₁ measurements were used to probe the Li⁺ dynamics, which were found to be dependent on the Li⁺ concentration, the nature of the co-cation and the presence or absence of tetraglyme.