Glasslike character of molecular ordering in discotic lyomesophases.

Abstract

We studied dynamic light scattering from isotropic solutions of triphenylene derivative in dodecane at concentrations close to the isotropic-cholesteric phase transition that takes place at c=25 wt. % at room temperature. The correlation decay g((2))(t) reveals the presence of three dynamic modes, from which two appear in polarized (VV) and one in depolarized (VH) scattering. The VH mode, which is independent of the scattering wave vector q, exhibits an Arrhenius type temperature dependence and shows a strong deviation from the exponential relaxation associated with the stretching-exponent parameter beta(VH) approximately 0.5 for all the investigated concentrations. The VV modes both exhibit a quadratic dispersion of the inverse relaxation time 1/tau on the scattering wave vector q. For concentrations 5 wt. %<c<25 wt. % the temperature dependence of the fast VV mode obeys the Vogel-Fulcher-Tamman relation that is correlated with the nonexponential character of the relaxation. The corresponding stretching-exponent parameter beta(fast) decreases by increasing concentration and decreasing temperature. The slow VV mode, on the contrary, is always close to exponential behavior and shows an Arrhenius type temperature dependence, but it exhibits strong variations in relation with sample annealing and preparation procedures. The relaxation rates of all the modes strongly decrease with increasing triphenylene concentration. The observed dynamical properties suggests on the glasslike structure of large columnar aggregates formed in the pretransitional phase.