Ingenieurwissenschaften und zugeordnete Tätigkeiten
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We investigate the molecular dynamics and electrical conductivity for a homologous series of linear shaped guanidinium based cyclic ILCs that vary in alkyl chain length, CLCRs (R = 8, 10, 12, 14, 16) by employing broadband dielectric spectroscopy (BDS), and calorimetry comprised of Fast Scanning Calorimetry (FSC) and temperature modulated FSC (TMFSC). Besides conductivity at high temperatures, the dielectric dispersion reveals two relaxation modes: the fast γ and the slow α1 relaxation. The former is assigned to the localized fluctuations while the latter is due to segmental dynamics of the alkyl chains. The γ mode slows down for long chain length CLCs (12,14,16) compared to their shorter analogues. Calorimetric investigation reveals one process, the α2 process, for all ILCs. The α2 process for CLC10, 12, and 14 has a similar temperature dependence as the dielectric α1 relaxation, while for the CLC8 and 10, it's shifted to a higher temperature. For all CLCRs, the absolute values of DC conductivity increase by 4 orders of magnitude at the transition from the plastic crystalline to hexagonal columnar phase. This increase is due to the change in the underlying conduction mechanism from delocalized electron hopping in the crystalline phase to one-dimensional ion mobility in the columnar phase. The glassy dynamics shifts to higher temperatures with increasing alkyl chain length. Conversely, the DC conductivity drops by 3 orders of magnitude from CLC8 to CLC16.
We study the molecular mobility and electrical conductivity of a homologous series of linear shaped columnar ionic liquid crystals ILCn, (n = 8, 10, 12, 14, 16) using Broadband Dielectric Spectroscopy (BDS), Specific Heat Spectroscopy (SHS), and X-ray scattering. We aim to understand how the alkyl chain length influences the dynamics and electric conductivity in this system. Two dielectrically active relaxation modes are observed, the and the αcore process, that correspond to the localized relaxation of the alkyl chains, and cooperative motions of the aromatic core in the columns respectively. Both the relaxation and the αcore process slow down with increasing alkyl chain length. SHS reveals one relaxation process, the αalkyl process that has similar temperature dependence as that of the αcore process for ILC12, 14, 16 but shifts to higher temperature for ILC8 and 10. For ILC12, 14, 16 the absolute values of DC conductivity increase by 4 orders of magnitude at the transition from the plastic crystalline to hexagonal columnar phase. For ILC8, 10 the DC conductivity behavior is similar to ionic liquids, where the conductivity is coupled with structural relaxation. Small-Angle X-ray (SAXS) investigations reveal that both the intercolumnar distance and disorder coherence length increase with alkyl chain length, conversely the DC conductivity decreases monotonically.
Ionic Liquid Crystals (ILCs) are materials that combine the properties of liquid crystals together with ionic conduction. It is known that liquid crystal mesophases in confinement exhibit anomalous dynamics and phase behavior. However, similar studies about factors that control the macroscopic properties of ILCs in confinement are limited. Here, Broadband Dielectric Spectroscopy (BDS), X-ray scattering, and calorimetry were applied to study the molecular dynamics, and phase behavior of a guanidinium based columnar ionic liquid crystal confined in self-ordered alumina oxide nanopores of pore sizes ranging from 180 nm down to 25 nm. It is aimed to understand how pore size and pore surface wettability (hydrophilic or hydrophobic) influence the molecular dynamics, and phase behavior for this system which are crucial for applications. The DSC measurements show: (i) the crystalline-liquid crystalline transition temperature has non-monotonic dependence on inverse pore diameter and (ii) the liquid crystalline-isotropic transition is completely suppressed for all the confined samples. This thermally suppressed transition was detected by BDS and X-ray scattering and is considered as a continuous phase transition instead of a discontinuous first order transition. BDS investigations reveal several relaxation processes for the bulk and confined scenarios. The relaxation modes are suppressed and become slower for the confined scenarios compared to the bulk. Possible molecular origins for these relaxation processes are discussed, and it is shown that the self-assembly of these ILCs is dynamic in nature.