Multiple glassy dynamics of a homologous series of triphenylene-based columnar liquid crystals -- A study by broadband dielectric spectroscopy and advanced calorimetry

TL;DR

This study investigates the complex glassy dynamics of a series of triphenylene-based liquid crystals using advanced calorimetry and dielectric spectroscopy, revealing how alkyl chain length influences phase behavior and molecular mobility.

Contribution

It provides new insights into the molecular relaxation processes and phase transitions in homologous triphenylene liquid crystals, highlighting the impact of alkyl chain length on their glassy dynamics.

Findings

Identification of multiple glassy relaxation processes.

Transition from rigid to softer systems around n=8.

Influence of alkyl chain length on phase behavior and molecular mobility.

Abstract

Hexakis(n-alkyloxy)triphenylene) (HATn) consisting of an aromatic triphenylene core and alkyl side chains are model discotic liquid crystal (DLC) systems forming a columnar mesophase. In the mesophase, the molecules of HATn self-assemble in columns, which has one-dimensional high charge carrier mobility along the columns. Here, a homologous series of HATn with different length of the alkyl chain (n=5,6,8,10,12) is investigated using differential scanning calorimetry (DSC), broadband dielectric spectroscopy (BDS) and advanced calorimetric techniques including fast scanning calorimetry (FSC) and specific heat spectroscopy (SHS). The investigation of the phase behavior was done utilizing DSC experiments and the influence of the alkyl chain length on the phase behavior was revealed. By the dielectric investigations probing the molecular mobility, a $\gamma$-relaxation due to localized fluctuations as well as two glassy dynamics the $\alpha$ core and $\alpha$ alkyl relaxation were observed in the temperature range of the plastic crystalline phase. Moreover, the observed glassy dynamics were further studied employing advanced calorimetry. All observed relaxation processes are attributed to the possible specific molecular fluctuations and discussed in detail. From the results a transition at around n=8 from a rigid constrained (n=5,6) to a softer system (n=10,12) was revealed with increasing alkyl chain length. A counterbalance of two competing effects of a polyethylene like behavior of the alkyl chains in the intercolumnar domains and self-organized confinement is discussed in the context of a hindered glass transition.