Decoupling Between Structural and Conductivity Relaxation in Aprotic Ionic Liquids

Abstract

For the [CnMIm][NTf2] ionic liquids with n = 4, 6, and 8 the dynamic calorimetric glass transition temperature T g,dyn was determined in a wide frequency range from 10−2 to 105 rad s−1. The calorimetric glass transition temperature or vitrification temperature T g from standard DSC with 10 K min−1 cooling rate was determined too. The obtained value for T g in these ionic liquids is in very good agreement with the calorimetric T g,dyn at 100 s relaxation time. The obtained calorimetric data are compared to conductivity and other relaxation data available in the literature. In a relaxation map at short relaxation times (τ < 1 µs), conductivity relaxation and calorimetric relaxation show a similar behavior. However, at low frequencies a significant decoupling between conductivity and calorimetric data is observed. Similar to other ionic conductors, the conductivity relaxation has a weaker temperature dependency than structural relaxation. Interestingly, there is no break in the conductivity data when crossing T g. This is different from many other systems.