Abstract
In the present work, the nematic glassy state of the non-symmetric LC dimer α-(4-cyanobiphenyl-4′-yloxy)-ω-(1-pyrenimine-benzylidene-4′-oxy) undecane is studied by means of calorimetric and dielectric measurements. The most striking result of the work is the presence of two different glass transition temperatures: one due to the freezing of the flip-flop motions of the bulkier unit of the dimer and the other, at a lower temperature, related to the freezing of the flip-flop and precessional motions of the cyanobiphenyl unit. This result shows the fact that glass transition is the consequence of the freezing of one or more coupled dynamic disorders and not of the disordered phase itself. In order to avoid crystallization when the bulk sample is cooled down, the LC dimer has been confined via the dispersion of γ-alumina nanoparticles, in several concentrations.
Highlights
Glass transition is one of the most interesting and intriguing phenomena in the field of materials science
In order to overcome this inconvenience, we introduce a quenched random disorder in the material by the dispersion of γ-alumina nanoparticles, which partially suppress the undesired crystallization of the liquid crystal dimer
The main milestone of the present work was the identification of two glass transition temperatures close to each other by means of several complementary experimental techniques: broadband dielectric spectroscopy, thermal stimulated depolarization current and calorimetry
Summary
Glass transition is one of the most interesting and intriguing phenomena in the field of materials science. One problem shall arise when performing these experiments: even if it is possible to vitrify the nematic (N) phase of the CBO11O.Py, the nematic glassy state can only be reached by cooling down the sample at high rates and, the supercooled N phase (Nspc) crystallizes when heating up the sample from the glassy state to some temperature above, but very close to, Tg [22,23]. This implies a great technical difficulty when trying to study the dielectric properties near the glass transition.
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