Multiple glass transitions in the plastic crystal phase of triphenylene derivatives
Yildirim, Z × Wübbenhorst, Michael Mendes, E Picken, SJ Paraschiv, I Marcelis, ATM Zuilhof, H Sudholter, EJR #
Elsevier science bv
Journal of non-crystalline solids vol:351 issue:33-36 pages:2622-2628
BDS 2004 location:Delft date:23-26 August 2004
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-C10Br. This paper focuses on the dielectric relaxations in the plastic crystal phase. For HAT6-C10Br, a 'high temperature' glass transition, manifested by a Vogel-Fulcher-Tammann (VFT) type alpha(2-)process, was found at -31 degrees 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 (alpha(1)) for both compounds, which indicates co-operative liquid dynamics within the framework of the plastic crystal order at temperatures as low as -100 degrees 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. (c) 2005 Elsevier B.V. All rights reserved.