Cylindrical sub-micrometer confinement results for the odd-symmetric dimer alpha,omega-Bis[(4-cyanobiphenyl)-4 '-yloxy]undecane (BCB.O11)

Abstract

Broadband dielectric spectroscopy (102 Hz to 1.9 × 109 Hz) and specific heat measurements have been performed on the odd-symmetric dimer R,ω-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.