title: Vibrational density of states of triphenylene based discotic liquid crystals: dependence on the length of the alkyl chain.
abstract: The vibrational density of states of a series of homologous triphenylene-based discotic liquid crystals HATn (n = 5, 6, 8, 10, 12) depending on the length of the aliphatic side chain is investigated by means of inelastic neutron scattering. All studied materials have a plastic crystalline phase at low temperatures, followed by a hexagonally ordered liquid crystalline phase at higher temperatures and a quasi isotropic phase at the highest temperatures. The X-ray scattering pattern for the plastic crystalline phase of all materials shows a sharp Bragg reflection corresponding to the intercolumnar distance in the lower q-range and a peak at circa 17 nm(-1) related to intracolumnar distances between the cores perpendicular to the columns as well as a broad amorphous halo related to the disordered structure of the methylene groups in the side chains in the higher q-range. The intercolumnar distance increases linearly with increasing chain length for the hexagonal columnar ordered liquid crystalline phase. A similar behaviour is assumed for the plastic crystalline phase. Besides n = 8 all materials under study exhibit a Boson peak. With increasing chain length, the frequency of the Boson peak decreases and its intensity increases. This can be explained by a self-organized confinement model. The peaks for n = 10, 12 are much narrower than for n = 5, 6 which might imply the transformation from a rigid system to a softer one with increasing chain length. Moreover the results can also be discussed in the framework of a transition from an uncorrelated to a correlated disorder with increasing n where n = 8 might be speculatively considered as a transitional state.
authors.affiliation: BAM Federal Institute for Materials Research and Testing, D-12205, Berlin, Germany. Andreas.Schoenhals@bam.de.
authors.forename: Christina
authors.initials: C
authors.lastname: Krause
authors.forename: Reiner
authors.initials: R
authors.lastname: Zorn
authors.forename: Franziska
authors.initials: F
authors.lastname: Emmerling
authors.forename: Jana
authors.initials: J
authors.lastname: Falkenhagen
authors.forename: Bernhard
authors.initials: B
authors.lastname: Frick
authors.forename: Patrick
authors.initials: P
authors.lastname: Huber
authors.forename: Andreas
authors.initials: A
authors.lastname: Schönhals