Thermodynamic and Dielectric Studies Concerning the Influence of Cylindrical Submicrometer Confinement on Heptyloxycyanobiphenyl

Abstract

Measurements of the specific heat and the static dielectric permittivity of heptyloxycyanobiphenyl (7OCB) confined to the 0.2 microm diameter parallel cylindrical pores of Anopore membranes in the isotropic phase and nematic mesophase, are presented. A comparison between the bulk and the confined 7OCB in treated and untreated pore wall surfaces using a chemical surfactant (HTBA) is performed. Both the treated and untreated membrane confinements seem to affect the nematic-to-isotropic phase transition by a downshift in transition temperature and some rounding at the specific-heat maximum, in a way similar to that which was earlier published for other liquid crystals confined in the same geometry. The static dielectric measurements clearly point out that untreated membrane confinement is axial, with the nematic director aligned parallel to the pore axis being homeotropic bulklike, i.e., with the nematic director aligned perpendicular to the electrode cell surfaces. After chemical surfactant treatment, the nematic director is constrained in a radial alignment being perpendicular to the pore walls. The dielectric measurements are revealed to be specially sensible to analyze the surface-induced nematic order due to the pore wall. The tricritical nature of the nematic-to-isotropic phase transition in bulk 7OCB as well as in treated and untreated Anopore confined geometries is discussed through both the specific heat and the static dielectric data.

Dielectric relaxation in bulk and cylindrically confined octylcyanobiphenyl (8CB)

“…The filling ratio was about 85% in all cases, similar to that obtained by other authors [4,17]. As we shall see below and according to the literature [3,4,9,17,18], with the untreated membranes the liquid crystal molecules align mainly parallel to the pore axis, axial configuration, allowing us to measure e d . For the treated membranes, the HTBA aligns the molecules mainly perpendicularly to the pore axis, allowing us to measure e ) .…”

“…The value obtained, around 18u, is quite high for an nCB compound and it should be taken with care because De 2 is very sensitive to the fitting procedure (depending much on a, b and e ' ). Nevertheless, other authors have estimated values ranging from 13u to 21u for similar compounds [2,9,19]. Figure 2 (b) shows, for the parallel alignment, both components of the dielectric permittivity in the N phase, where the results are also fitted to two near-Debye modes; a51, b51 (both fixed) for the high frequency mode and a5.98 b51 for the low frequency mode.…”

“…However, some basic problems concerning the interpretation of the experimental results are still the source of controversy [2]. Recently, much attention has been paid to the effect of confinement of liquid crystals in porous matrices, with calorimetric and dielectric techniques very useful to analyse the changes in dynamic and static properties [3][4][5][6][7][8][9][10]. In this work we report a study on 8CB confined in untreated and treated cylindrical pores of Anopore membranes.…”

Influence of the chain length on the nematic-to-isotropic phase transition for the odd members of a highly non-symmetric pyrene-based series of liquid crystal dimers

“…The heat capacity data shown in Figure 1 have been used to study the critical behaviour of the N-I phase transition of the two shortest chain members by fitting the experimental data measured outside the coexistence region to the following standard powerlaw expressions [9,21,22] above and below the transition.…”

Twist, tilt, and orientational order at the nematic to twist-bend nematic phase transition of 1″,9″-bis(4-cyanobiphenyl-4′-yl) nonane: A dielectric,H2NMR, and calorimetric study

“…During the last few years interest in liquid crystal dimers 33 has experienced an extraordinary growth due to the observation 34 of nematic-nematic transitions for relatively simple materials 35 where two mesogenic units are linked by methylene chains 36 having odd numbers of carbon atoms [1][2][3][4][5]. Although the 37 high temperature nematic mesophase of these materials is a 38 conventional nematic phase, its elastic and dielectric properties 39 are far from typical [2,[6][7][8].…”