Dynamics in discotic liquid crystals by dielectric spectroscopy and advanced calorimetry

Dynamics in discotic liquid crystals by dielectric spectroscopy and advanced calorimetry

Source: BAM, Division Physical and chemical analysis of polymers

Disk-like molecules with aromatic cores and alkyl substituents like triphenylenes can form columnar structures by self-assembling into columns in a liquid crystalline columnar mesophase. These innovative martials are called discotic liquid crystals (DLCs). The self-assembly into columns is mainly driven by the π π interactions of the aromatic cores of discotics, whereas the intercolumnar space is filled by the alkyl substituents of the molecules where the rigid columns superimpose a nanometer confinement to the alkyl chains. Because the columns carry some disorder each column can be considered as a one-dimensional liquid.
Due to their specialized structure DLCs have properties such as one-dimensional charge transport and/or light harvesting ability, making them promising materials for electronic applications such as in field effect transistors, photovoltaic cells or as nanowires in molecular electronics. For all of these applications the molecular mobility of these innovative materials is a key property.

Here a selected series of dipole functionalized triphenylene-based DLCs was synthesized and studied in a systematic way to reveal the phase behavior and molecular dynamics. The phase behavior was investigated by X-ray scattering, polarizing optical microscopy, and conventional differential scanning calorimetry. The results obtained from these investigations were compared to that of the corresponding unfunctionalized DLC.
Special attention is paid to investigate the molecular mobility of these materials in a broad temperature and frequency range. For that a combination of broadband dielectric spectroscopy (10-1 Hz. – 109 Hz) and advanced calorimetry is employed for the first time. For advanced calorimetry a combination of differential scanning calorimetry (DSC), temperature modulated DSC, Hyper-DSC, fast scanning calorimetry, and specific heat spectroscopy is used.

As a result, surprisingly besides a process related the localized fluctuations two different relaxation processes due to cooperative glassy dynamics were observed. The first process observed at higher temperatures was assigned to a glass transition in the intercolumnar space and therefore to a glass transition under a nanoconfinement. The second process found at lower temperatures was related to small scale fluctuations of aromatic cores taking place in the disordered parts of the columns and can therefore be considered as a glass transition in a one-dimensional liquid.

Multiple glassy dynamics in dipole functionalized triphenylene-based discotic liquid crystals revealed by broadband dielectric spectroscopy and advanced calorimetry – assessment of the molecular origin
Arda Yildirim, A. Bühlmeyer, S. Hayash, J. C. Haenle, K. Sentker, Christina Krause, Patrick Huber, Sabine Laschat, Andreas Schönhals
published in Physical Chemistry Chemical Physics, Vol. 21, Issue 33, pp 18265-18277, 2019.
BAM, Division Physical and Chemical Analysis of Polymers