Showing papers by "Josep Salud published in 2010"
TL;DR: In this article, the authors present an experimental study on the critical behaviour of the nematic-toisotropic (N-I) and smectic A-to-nematic (SmA-N) phase transitions of the liquid crystal 4-nonyloxy-4′-cyanobiphenyl (9OCB) liquid crystal.
Abstract: As a humble contribution to this special commemorative issue to Professor Alfred Saupe, we present an experimental study on the critical behaviour of the nematic-to-isotropic (N–I) and smectic A-to-nematic (SmA–N) phase transitions of the liquid crystal 4-nonyloxy-4′-cyanobiphenyl (9OCB) Measurements of the specific heat and the dielectric permittivity were performed for bulk 9OCB as well as for 9OCB confined to Anopore membranes, and the influence of the slight-restrictive confinement was analysed The possibility of driving the weakly first-order SmA–N phase transition to second-order in nature by means of the confinement was confirmed
14 citations
TL;DR: Dynamic dielectric measurements have probed the different molecular motions in both confinements and how these motions are developed in a way similar to the bulk-dimer and proved that the surface-pinned molecular layer adjacent to the pore-wall is temperature-dependent as already found previously for liquid crystal monomers.
Abstract: Broadband dielectric spectroscopy (10(2) Hz to 1.9 x 10(9) Hz) and specific heat measurements have been performed on the odd-symmetric dimer alpha,omega-bis[(4-cyanobiphenyl)-4'-yloxy]undecane (BCB.O11) in the isotropic (I) and nematic (N) phases confined to 200 nm diameter parallel cylindrical pores of Anopore membranes. Unlike previous studies on liquid crystal monomers, untreated and hexadecyltrimethylammonium bromide-treated membranes give rise to radial and axial confinements, respectively. An attempt is made to explain these unexpected results by means of a qualitative model for the dimer arrangement on alumina substrates. The model suggests that the population of conformers, which follow the bulk-like dynamics, is modified by confinement. Such a fact seems to be consistent with other distinctive features attributed to confinement, as for example, the increasing of the entropy change at the N-to-I phase transition for both axial and radial confinements. Specific-heat measurements have shown how confinement affects the N-to-I phase transition by a downward shift in transition temperature as well as by broadened and rounded specific-heat peaks. However, these modifications are revealed to be substantially different from what has been found previously in similar studies on liquid crystal monomers. Dynamic dielectric measurements have probed the different molecular motions in both confinements and how these motions are developed in a way similar to the bulk-dimer. Dielectric results have also proved that the surface-pinned molecular layer (where molecular motions are very restricted) adjacent to the pore-wall is temperature-dependent as already found previously for liquid crystal monomers.
7 citations