Effect of Molecular Flexibility on the Nematic-to-Isotropic Phase Transition for Highly Biaxial Molecular Non-Symmetric Liquid Crystal Dimers.

Nerea Sebastián,Sergio Diez-Berart,David Orencio López,María Blanca Ros,Josep Salud,Miguel Angel Pérez-Jubindo,María Rosario De La Fuente

doi:10.3390/ma4101632

Nerea Sebastián, Sergio Diez-Berart + Show 5 more

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https://doi.org/10.3390/ma4101632

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Abstract

In this work, a study of the nematic (N)–isotropic (I) phase transition has been made in a series of odd non-symmetric liquid crystal dimers, the α-(4-cyanobiphenyl-4’-yloxy)-ω-(1-pyrenimine-benzylidene-4’-oxy) alkanes, by means of accurate calorimetric and dielectric measurements. These materials are potential candidates to present the elusive biaxial nematic (NB) phase, as they exhibit both molecular biaxiality and flexibility. According to the theory, the uniaxial nematic (NU)–isotropic (I) phase transition is first-order in nature, whereas the NB–I phase transition is second-order. Thus, a fine analysis of the critical behavior of the N–I phase transition would allow us to determine the presence or not of the biaxial nematic phase and understand how the molecular biaxiality and flexibility of these compounds influences the critical behavior of the N–I phase transition.

Highlights

IntroductionLiquid crystals are one kind of molecular materials that, depending on temperature and on concentration in some solvent (lyotropic liquid crystals) or on temperature alone (thermotropic liquid crystals), are able to form intermediate states of matter (mesophases) between the ordered solid crystalline state and the isotropic liquid state
Liquid crystals are one kind of molecular materials that, depending on temperature and on concentration in some solvent or on temperature alone, are able to form intermediate states of matter between the ordered solid crystalline state and the isotropic liquid state
The critical behavior was analyzed both by means of calorimetric and dielectric data

Summary

Introduction

Liquid crystals are one kind of molecular materials that, depending on temperature and on concentration in some solvent (lyotropic liquid crystals) or on temperature alone (thermotropic liquid crystals), are able to form intermediate states of matter (mesophases) between the ordered solid crystalline state and the isotropic liquid state. Liquid crystal dimers are formed when two rigid cores are linked via a flexible spacer These materials were first conceived as parts of polymeric structures in order to facilitate their study [2] and have attracted considerable attention during the last years [3,4,5,6,7,8,9,10,11,12,13,14]. Accurate experimental studies on the nature of the N–I phase transition in liquid crystal dimers may elucidate if this is weakly first or second-order For such purposes, in this work three odd members of the non-symmetric liquid crystal dimer series α-(4-cyanobiphenyl-4’-yloxy)-ω-(1-pyrenimine-benzylidene-4’-oxy) alkanes, are considered.

Specific Heat Measurements

Dielectric Measurements

Results and Discussion

Conclusions