Dynamics at the Nematic-Isotropic Phase Transition in Aerosil Dispersed Liquid Crystal

Abstract

Photopyroelectric high resolution ac calorimetry has been used to study the nematic-isotropic phase transition in octylcyanobiphenyl liquid crystal incorporating an aerosil soft gel. We have found that the sample is not in thermodynamic equilibrium in the two phase coexistence region, as shown by the frequency dependence of the double peak in the specific heat and by its thermal hysteresis. The results suggest that the dynamics of the system becomes slower as it is cooled over the transition temperature. An explanation for the formation of a nematic glassy phase in liquid crystals with quenched disorder based on the similarity of these results with the ones obtained in other disordered materials is attempted.

Invited Review Article: Photopyroelectric calorimeter for the simultaneous thermal, optical, and structural characterization of samples over phase transitions

“…99 Systematic studies were carried out by this upgraded calorimetric BPPE setup in these kind of samples as a function of particle concentration in the sample and of modulation frequency. 97,100 The characteristics of the material over the LTP were shown to be markedly different from those over the HTP and strain induced glassy behaviour was invoked during nucleation over the former peak. The disordering associated effects, caused by the introduction of the nanoparticles, were also investigated over the rotator phase transitions in alkane samples.…”

“…Its characteristics were, in fact, similar to those of bulk material. 97 On the contrary, the material nucleating over the LTP is the one growing close to the particle surface, with respect to which the molecular long axis is forced to locally align perpendicularly, with a consequent substantial amount of strain building up in that material. This causes its transition temperature to be lower with respect to the bulk like material and eventually leads to the fragmentation of the grown domains.…”

Molecular dynamics of a short-range ordered smectic phase nanoconfined in porous silicon

“…4 Although it seems a quite general behavior, the connection between the glass-like 4 dynamics of confined LC's and the nature of the porous topology, surface interactions and confinement-induced quenched disorder effects remains challenging. 21,22,23,24 The individual molecular dynamics of confined LC's is also strongly modified. Dielectric relaxation spectroscopy has been extensively used since it allows one to probe the rotationallibrational dynamics of the dipolar molecules around their short axis as well as tumbling relaxation modes around the local nematic director.…”

Dynamics of Nematic Liquid Crystal with Quenched Disorder in the Random Dilution and Random Field Regimes

“…It has been proposed, in fact, that the molecular coupling with disorder then changes from the RD regime to the RF one across the two phase coexistence region of the transition. This mechanism has been suggested as the origin of the striking behavior induced by disorder on the specific heat over the two phase coexistence region of the transition where a double peak structure has been reported, with no analogy with the LC bulk material, indicating that the transition takes place in two steps [8,13,14]. In Ref.…”

“…In Ref. [8], though the RD to RF crossover regime is invoked in the same qualitative way to explain the double peak structure, it was shown that it induces a reduction of the nematic material grain size with the consequent increase of its distortion eventually leading to the observed glassy behavior.…”