Dynamics and ionic conductivity of ionic liquid crystals forming a hexagonal columnar mesophase

Area of Science:

• Materials Science
• Physical Chemistry
• Condensed Matter Physics

Background:

• Ionic liquid crystals (ILCs) exhibit unique self-assembly properties, forming ordered mesophases like hexagonal columnar structures.
• Understanding molecular dynamics is crucial for ILC applications, particularly concerning phase transitions and conductivity.

Purpose of the Study:

• To investigate the molecular mobility of linear tetramethylated guanidinium triflate ILCs with varying alkyl chain lengths.
• To elucidate the distinct relaxation processes and their relationship with different mesophases (plastic crystalline, hexagonal columnar).
• To characterize the electrical conductivity changes across phase transitions and understand the underlying charge transport mechanisms.

Main Methods:

• Broadband dielectric spectroscopy (BDS) to probe dielectric-active molecular relaxations.
• Specific heat spectroscopy (SHS) to complement dielectric studies and identify additional relaxation processes.
• Analysis of temperature-dependent relaxation times and DC conductivity measurements.

Main Results:

• Three dielectric-active processes (γ, α1, α2) were identified, with distinct temperature dependencies for α1 (BDS) and α2 (SHS).
• The γ-process was attributed to localized methyl group fluctuations in the plastic crystalline state.
• A four-orders-of-magnitude increase in DC conductivity was observed at the transition to the hexagonal columnar mesophase.

Conclusions:

• The study reveals complex molecular dynamics in ILCs, with different relaxation processes linked to specific mesophases.
• The significant conductivity jump is attributed to a shift in charge transport from electron hopping to ionic conduction within 1D channels.
• These findings provide insights into structure-property relationships in ILCs, relevant for their technological applications.