Multiple glass transitions in the plastic crystal phase of triphenylene derivatives

Abstract

The dynamics and phase behavior of the discotic liquid crystalline compound hexahexyloxytriphenylene (HAT6) and a derivative were studied by broad-band dielectric spectroscopy, differential scanning calorimetry, X-ray diffraction and optical microscopy. While the pristine compound HAT6 forms both a columnar mesophase (Col h) and a plastic crystal phase, no liquid crystallinity was observed for the highly asymmetric compound HAT6-C 10Br. This paper focuses on the dielectric relaxations in the plastic crystal phase. For HAT6-C 10Br, a ‘high temperature’ glass transition, manifested by a Vogel–Fulcher–Tammann (VFT) type α 2-process, was found at −31 °C that was assigned to the columnar glass transition in accordance with previous literature. The main result of our study is the observation of a second, low-temperature VFT process ( α 1) for both compounds, which indicates co-operative liquid dynamics within the framework of the plastic crystal order at temperatures as low as −100 °C. Comparison of these fast dynamics with relaxation data from polyethylene and polymer series with long alkyl groups identifies this process as a ‘hindered’ polyethylene-like dynamic glass transition that originates from the nanophase-separated, spatially confined fraction of aliphatic tails.