Broadband Dielectric Spectroscopy Study of the Ion Dynamics in Blends of Polymerized and Molecular Ionic Liquids

Abstract

The primary focus in describing the decoupling of ion dynamics from the structural relaxation and dynamic glass transition (Tg) in polymerized ionic liquids (PILs) has been the effect of polymer molecular weight and chemistry of the ionic repeat unit. A particular aspect that has not been studied in attempt to understand the physicochemical properties of these fascinating materials is the concentration dependence of PILs with their low molecular weight or monomeric derivatives. Here we present studies of mixtures containing the low molecular weight monomer and the corresponding polymer of [(methacryloyloxy)ethyl]dimethylammonium (DMAEMA+), the quaternized ammonium cation of dimethylaminoethyl methacrylate, with the bis[(trifluoromethyl)sulfonyl]imide (NTf2) counter anion. The temperature dependent ion dynamics were measured using broadband dielectric spectroscopy. Our results show that there is an exponential relationship between the dc conductivity and concentration which decreases with temperature. At ~80mol% PIL this this inverse relationship between log σ0 and concentration “saturates” then reverses approaching 100% PIL. Only a single calorimetric glass transition temperature was observed for each of the concentrations measured. The Tg is concentration independent from 0-75% (~-60°C) then rapidly increases to that of the polymer (~55°C). The S-N-S bending modes, as observed in the Raman spectrum of the blends, blueshifts with increasing concentration, but exhibits a slight redshift from 85-100% polymer. This suggests the anion becomes more coordinated to the cation with increasing polymer concentration, but then reverses approaching pure polymer. In concert with results from a recent simulation study, we suggest that the ion hopping mechanism changes drastically from dilute to concentrated PIL/IL blends at a characteristic concentration (~85% for DMAEMA+ NTf2), but then this mechanism reverses as the ion dynamics and structural relaxation become decoupled in the pure polymer. A continuation of these concentration dependent studies of PIL and low molecular weight ionic liquid blends is a promising direction toward fully understanding the dynamics in these systems for rational design of PILs. Figure 1