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In the present work, a detailed analysis of the glassy behavior and therelaxation 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 ofbroadband dielectric spectroscopy has been performed. CBO7O.Py shows threedifferent dielectric relaxation modes and two glass transition (Tg)temperatures: the higher Tg is due to the freezing of the molecular motionsresponsible of the relaxation mode with the lowest frequency (\mu 1L); thelower Tg is due to the motions responsible of the two relaxation modes withhighest frequencies (\mu 1H and \mu 2), which converge just at theircorresponding Tg. It is shown how the three modes follow a critical-likedescription via the dynamic scaling model. The two modes with lowestfrequencies (\mu 1L and \mu 1H) are cooperative in the whole range of themesophases, whereas the highest frequency mode (\mu 2) is cooperative justbelow some cross-over temperature. In terms of fragility, at the glasstransition, the ensemble (\mu 1H +\mu 2) presents a value of the steepnessindex and \mu 1L a different one, meaning that fragility is a propertyintrinsic of the molecular motion itself. Finally, the steepness index seem tohave a universal behavior with temperature for the dielectric relaxation modesof liquid crystal dimers, being almost constant at high temperatures andincreasing drastically when cooling the compound down to the glass transitionfrom a temperature about (3/4)TNI.

physical review. e

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