Diego, J.; Sellarès, J.; Diez, S.; Salud, J.; Cañadas, J.C.; Mudarra, M.; Lopez, D.; de la Fuente, M.; Ros, M. Liquid crystals Vol. 44, num. 6, p. 1007-1022 DOI: 10.1080/02678292.2017.1309099 Data de publicació: 2017-04-11 Article en revista
Dielectric measurements (thermally stimulated depolarisation currents and broadband dielectric spectroscopy) have been performed near the glass transition to study the glass transition on the odd non-symmetric liquid crystal (LC) dimers of the series -(4-cyanobiphenyl-4'-oxy)--(1-pyreniminebenzylidene-4'-oxy) alkanes (CBOnO.Py) with n ranging from 3 to 9. A previous study [S. Diez-Berart et al., Materials 8 (2015) 3334] carried out in CBO11O.Py showed the presence of two glass transition temperatures, attributed to different molecular motions of the terminal groups. The study performed allows us to analyse the molecular dynamics in the rest of the series and determine the role played by the flexible spacer. Parallel and perpendicular orientations of the molecular director with regards to the probe electric field have been investigated. The low and intermediate observed relaxations are explained in the framework of Stocchero's theoretical model [M. Stocchero, J. Chem. Phys. 121 (2004) 8079] for the dielectric behaviour of non-symmetric LCs dimers, as independent end-over-end rotations of each terminal semi-rigid unit. As the length of the spacer chain in the series of compounds decreases, the different relaxations become progressively more coupled at the glass transition. Numerical simulations of the calorimetric response from the obtained kinetic parameters show good agreement with experimental behaviour.
Sebastian, N.; B. Robles-Hernández; Diez, S.; Salud, J.; Luckhurst , G.; Dunmur; Lopez, D.; de la Fuente, M. Liquid crystals Vol. 44, num. 1, p. 177-190 DOI: 10.1080/02678292.2016.1218963 Data de publicació: 2017-01-01 Article en revista
We provide an overview of the effect of the molecular structure on the dielectric properties of dimers exhibiting nematic and twist-bend nematic phases with special focus on how the conformational distribution changes are reflected by the dielectric behaviour. Nematic dimers show distinctive dielectric properties which differ from those of archetypical nematic liquid crystals, as for example, unusual temperature dependence of the static permittivity or dielectric spectra characterised by two low-frequency relaxation processes with correlated strengths. The interpretation of such characteristic behaviour requires that account is taken of the effect of molecular flexibility on the energetically favoured molecular shapes. The anisotropic nematic interactions greatly influence the conformational distribution. Dielectric behaviour can be used to track those conformational changes due to dependence of the averaged molecular dipole moment on the averaged molecular shape. Results for a number of dimers are compared and analysed on the basis of the influence of details of the molecular structure, using a recently developed theory for the dielectric properties of dimers.
Salud, J.; Lopez, D.; Sebastian, N.; de la Fuente, M.; Diez, S.; Ros, M. Liquid crystals Vol. 43, num. 1, p. 102-111 DOI: 10.1080/02678292.2015.1066890 Data de publicació: 2016-01-02 Article en revista
This paper describes a detailed study of the nematic (N)-isotropic (I) phase transition in the homologous series of liquid crystal dimers, the a-(4-cyanobiphenyl-4'-oxy)-¿-(1-pyreniminebenzylidene-4'-oxy)alkanes (CBOnO.Py) by means of calorimetric and dielectric measurements as a function of temperature. It is concluded that for this transition, the latent heat or the entropy change decreases as the chain length of the odd dimers decreases, and this decrease is consistent with the observed tricritical behaviour.
The present work arises from the significant difference between the experimental Landau tricritical point (LTCP) in the Sm A-to-N phase transition in binary mixtures of butyloxybenzylidene octylaniline (4O.8) and hexyloxybenzylidene octylaniline (6O.8) predicted by Stine and Garland, and that arising from the thermodynamic assessment using the Oonk's Equal Gibbs Curve method. By use of specific heat measurements the 4O.8 + 6O.8, phase diagram has been determined anew. The results of the subsequent application of Oonk's thermodynamic analysis are discussed and their compatibility with the behaviour universally exhibited by other liquid crystal binary mixtures for which a LTCP in the SmA-to-N phase transition has been experimentally determined are analysed.