The Scaling of the Molecular Dynamics of Liquid Crystals as Revealed by Broadband Dielectric, Specific Heat, and Neutron Spectroscopy

Abstract

A combination of different complementary methods is employed to investigate scaling of the molecular dynamics of two different liquid crystalsLiquid crystals . Each method is sensitive to different kind of fluctuations and provides therefore a different window to look at the molecular dynamics. In detail, broadband dielectric spectroscopyBroadband dielectric spectroscopy is combined with specific heat spectroscopySpecific heat spectroscopy and neutron scatteringNeutron scattering . As systems, the nematic liquid crystal E7 and a discotic liquid crystalline pyrene are considered. First of all, it was proven that both systems show all peculiarities which are characteristic for glassy dynamicsGlassy dynamics and the glassy stateGlassy state . Especially for the nematic liquid crystalNematic liquid crystal E7, it could be unambiguously shown by a combination of dielectric and specific heat spectroscopy that the tumbling mode is the underlying motional process responsible for glassy dynamics. Dielectric investigations on the discotic liquid crystalline pyrene reveal that at the phase transition from the plastic crystalline to the hexagonal columnar liquid crystalline phaseHexagonal columnar liquid crystalline phase , the molecular dynamics changes from a more strong to fragile temperature dependence of the relaxation rates. Moreover, a combination of results obtained by specific heat spectroscopy with data from structural methods allows an estimation of the length scale relevant for the glass transition.