Glassy dynamics and electrical conductivity of Ionic liquid Crystals: Dependence of the cation n-alkyl side chain length*

We investigate the molecular dynamics of a homologous series of linear-shaped guanidinium based ionic liquid crystals (ILCs) that vary in alkyl chain length, R = 8, 10, 12, 14, 16,  by employing broadband dielectric spectroscopy (BDS), Fast Scanning Calorimetry (FSC), and temperature modulated FSC (TMFSC).  Besides conductivity, the dielectric dispersion reveals two relaxation modes:  a fast γ and a slow α₁ relaxation. The former is assigned to localized fluctuations while the latter is due to segmental dynamics of the alkyl chains.  Calorimetric investigation reveals one process, the α₂ process, for ILC12,14 and 16, and two processes, α₂ and α_3, for ILC8 and 10. The relaxation rates of the α₂ process have a similar temperature dependence as that of the α₁ relaxation, which indicates that both BDS and FSC probe the segmental dynamics of alkyl side chains. We interpret the α₃ process of ILC8 and 10 as the segmental dynamics of the cation core. For all ILCs, the absolute values of DC conductivity increase by 4 orders of magnitude at the transition from the plastic crystalline to hexagonal columnar phase. The α₂ process shifts to higher temperatures with increasing alkyl chain length. Conversely, the DC conductivity drops by 3 orders of magnitude from ILC8 to ILC16.