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

Abstract

In the present work, a detailed analysis of the glassy behavior and the relaxation dynamics of the liquid crystal dimer α-(4-cyanobiphenyl-4'-yloxy)-ω-(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 (T_{g}) temperatures: The higher T_{g} is due to the freezing of the molecular motions responsible for the relaxation mode with the lowest frequency (μ_{1L}); the lower T_{g} is due to the motions responsible for the two relaxation modes with highest frequencies (μ_{1H} and μ_{2}), which converge just at their corresponding T_{g}. It is shown how the three modes follow a critical-like description via the dynamic scaling model. The two modes with lowest frequencies (μ_{1L} and μ_{1H}) are cooperative in the whole range of the mesophases, whereas the highest frequency mode (μ_{2}) is cooperative just below some crossover temperature. In terms of fragility, at the glass transition, the ensemble (μ_{1H}+μ_{2}) presents a value of the steepness index and μ_{1L} a different one, meaning that fragility is a property intrinsic to the molecular motion itself. Finally, the steepness index seems 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 3/4T_{NI}.

Twist‐Bend Nematic Glasses: The Synthesis and Characterisation of Pyrene‐based Nonsymmetric Dimers

“…Such an arrangement is stabilized by pyrenepyrene and cyanobiphenyl-cyanobiphenyl πÀ π interactions, and the resulting structure can be classified as an Haggregate. 40 On the other hand, along the [100] direction the molecules are arranged antiparallel, such that the pyrene and cyanobiphenyl units are interchanged leading to a number of CÀ H…π interactions (Figure 4(e)). It is interesting that there are infinite channels between the layers capable of hosting disordered solvent molecules (Figure 4(e)).…”

“…The corresponding diether linked dimer series, the 4′‐{[ω‐(4‐{(E)‐[(pyren‐1‐yl)imino]methyl}phenoxy)alkyl]‐oxy}[1,1′‐biphenyl]‐4‐carbonitriles, referred to as CBO n O.Py, is shown in Figure 2 [33] . This series is well‐known and has been the subject of a number of prior investigations [36–40] . The phase behaviour of a selection of homologues from both series are reported and compared.…”

“…[33] This series is well-known and has been the subject of a number of prior investigations. [36][37][38][39][40] The phase behaviour of a selection of homologues from both series are reported and compared.…”

DNMR measurements of an asymmetric odd liquid crystal dimer: determination of the intramolecular angle and the degree of order of the two rigid cores

“…25 This material, which presents both the nematic (N) and smectic A (SmA) mesophases below the isotropic (I) phase, is formed by a cyanobiphenyl core and a much bulkier pyrene-containing unit, linked via a flexible alkyloxy chain with seven carbons in it. 25–30 The cyanobiphenyl unit is clearly pro-mesogenic. With respect to the pyrene-containing moieties, it must be noted that strong π-interactions can be formed between them, which may help orienting these rigid cores uniaxially.…”

“…When these reorientations are not strongly coupled (as is the case for the shortest homologous of the series (CBO3O.Py), they vitrify at different, even if close to each other, temperatures. 29,30,34 Such a difference between rigid cores is key to the study we present.…”