Cooperative behavior of molecular motions giving rise to two glass transitions in the same supercooled mesophase of a smectogenic odd liquid crystal dimer

Abstract: In the present work, a detailed analysis of the glassy behavior and the relaxation dynamics of the liquid crystal dimer {\alpha}-(4-cyanobiphenyl-4'-yloxy)-{\omega}-(1-pyrenimine-benzylidene-4'-oxy) heptane (CBO7O.Py) throughout both nematic and smectic A mesophases by means of broadband dielectric spectroscopy has been performed. CBO7O.Py shows three different dielectric relaxation modes and two glass transition (Tg) temperatures: the higher Tg is due to the freezing of the molecular motions responsible of the relaxation mode with the lowest frequency (\mu 1L); the lower Tg is due to the motions responsible of the two relaxation modes with highest frequencies (\mu 1H and \mu 2), which converge just at their corresponding Tg. It is shown how the three modes follow a critical-like description via the dynamic scaling model. The two modes with lowest frequencies (\mu 1L and \mu 1H) are cooperative in the whole range of the mesophases, whereas the highest frequency mode (\mu 2) is cooperative just below some cross-over temperature. In terms of fragility, at the glass transition, the ensemble (\mu 1H +\mu 2) presents a value of the steepness index and \mu 1L a different one, meaning that fragility is a property intrinsic of the molecular motion itself. Finally, the steepness index seem to have a universal behavior with temperature for the dielectric relaxation modes of liquid crystal dimers, being almost constant at high temperatures and increasing drastically when cooling the compound down to the glass transition from a temperature about (3/4)TNI.