Octyl-cyanobiphenyl

Pentyl-4 -cyanobiphenyl and 4-octyl-4 -cyanobiphenyl liquid crystals (LCs) confined in molecular sieves of MCM-41 and cloverite types are studied in a wide temperature range by dielectric spectroscopy, thermal analysis and in-situ FTIR spectroscopy. The phase transitions of the bulk LCs cannot be detected when confined in MCM-41 sieve. A relaxational process occurs due to the molecular motions in the layer at the pore walls the temperature dependence of the characteristic frequency obeys a Vogel-Fulcher-Tamman law associated to a glassy state. In the cloverite cages, the LCs keep the phase transitions of the bulk but shifted. Interactions between Lewis and Brdnsted sites and the LC molecules are monitored by IR spectroscopy. DTA measurements put also in evidence strong guest-host interactions. 

FIGURE 1.3. Orientational order parameter for the nematic and smectic A phases of the 4-octyl-4 -cyanobiphenyl (8CB) [7]. 

This section describes both dynamics studies at the molecular level and also cooperative bulk macroscopic properties as sensed by diffusion studies. The measurements of the proton spin-lattice relaxation time of liquid crystal 4- -octyl-4 -cyanobiphenyl (8CB) confined in randomly oriented untreated porous glass have been presented. The studies are in agreement with the model of mutually independent pores with nematic director parallel to the pore axis in each segment. The local translational diffusion of molecules within the cavities is found to be nearly as fast as in bulk. Orientational relaxation of a model discotic liquid crystal, consisting of dislike molecules... 

In the following we consider the experimental data obtained for two liquid crystals belonging to the same family, i.e. 4-heptyl-4 -cyanobiphenyl (7CB) and 4-octyl-4 -cyanobiphenyl (8CB) in the bulk phase and in confined samples. 

Some very popular deuterated probes are the -cyanobiphenyls, where n stands for the length of their carbon chain [3-5,34,40], The H-NMR studies of side-chain LCEs presented in this chapter have, in most cases, been conducted with alpha-deuterated octyl-cyanobiphenyl (aD2-8CB) as a NMR probe (Fig. 7). The alpha... 

As one example. Figure 10.4 shows results obtained with octyl-cyanobiphenyl (8CB, Hi7C8(C6H4)2CN). At high temperature, 8CB is an isotropic liquid. When it is cooled slowly to 40.5 °C, the material becomes a nematic liquid crystal, where the molecules align and show a preferred orientation. Cooling further to 33.5 C, 8CB undergoes a phase transition from smectic to nematic, where in addition to the orientational order the molecules form a layered structure. 8CB forms solid crystals when the temperature is reduced below 21.5 °C. When confined between two... 

The nematic LC is the original eutectic mixture of 35% (weight %) 4 (m-octyl oxy)-4-cyanobiphenyl (80CB) and 65% 4/-(n-pentyl)-4-cyanobiphenyl (5CB). The sample is doped with perylene and 2% (mol %) of NIAC, which is acrylate monomer containing the easily reduced l,4 5,8-naphthalenediimide moiety (see Scheme 3). Finally, 0.5% (mol %) of benzoin methyl ether (BME) is added to photoinitiate polymerization of the NIAC. 

If the ratio f lf2 I is greater than unity the torques induced by the symmetric and antisymmetric strain rates respectively will never cancel out and the antisymmetric pressure will never vanish. This means that the director continues rotating for ever. The liquid crystal is said to be flow unstable and complicated flow patterns arise. TTiey have been studied comprehensively both experimentally and theoretically [30]. Some nematic liquid crystals are flow stable whereas others are not. For example, 4-n-pentyl-4 -cyanobiphenyl (5CB) is flow stable whereas 4-n-octyl-4 -cyanobiphenyl (8CB) is flow unstable. The only difference between this two substances is the length of the hydrocarbon chain attached to the cyanobiphenyl skeleton. Nematic liquid crystals that are flow stable usually become flow unstable close to the nematic-smectic A transition. The reason for this is that there is an emergent layer structure in the fluid that is incommensurate with the strain rate field. 

A typical measnrement with the nematic liquid crystal 4 — n — octyl — 4— cyanobiphenyl (8CB) is presented in Fig. 4.3. It is obvious that the thickness of the snrface induced ordered region and the amount of order increase close to the phase transition. This increase is due to the growing nematic correlation... 

Table 4.2. Comparison of anchoring energy coefficients of 4-n-pentyI-4 -cyanobiphenyl (5CB) at T = 32° C and 4-n-octyl-4-cyanobiphenyl (8CB) at T = 37° C on rubbed Nylon and photosensitive poly-(vinyl)-cinnamate. Different anchoring coefficients on rubbed Nylon are achieved by different rubbing strengths.

Fig. 4.3. The temperature dependence of the ellipsometric coefficient at the Brewster angle pb for A — n — octyl — 4 — cyanobiphenyl (8CB). Phase transition temperature Tjvz is at 315.3 K. The solid line is a fit to (4.5) and (4.6) with...

Four liquid crystalline compounds were studied 4-/i-pentyl-4 -cyanobi-phenyl (5CB), 4-/z-octyl-4 -cyanobiphenyl (8CB), 4-/i-hexyloxy-4 -n-pentyl-a-cyanostilbene (HOPCS) and MBBA. All the compounds have the nematic phase in convenient temperature range, and, in addition, 8CB has the smectic A phase. Both 5CB and 8CB have positive dielectric... 

Abstract Surface-induced ordering of 4-n-octyl-4 -cyanobiphenyl (8CB) near the isotropic-nematic phase transition was investigated using temperature-controlled atomic force microscopy (AIM). The glass surfaces in contact with liquid crystal were modified by an adsorbed silane surfactant, a deposited 8CB mono-layer, or a deposited 8CB trilayer. 

In this paper, we explore the effect of different surface treatments upon surface-induced ordering of the liquid crystal 4-n-octyl-4 -cyanobiphenyl (8CB). Cyanobiphenyls have been intensively studied [11-13]... 

The electrochemical polymerization of 4-cyano-4 -[8-(3-thienyl)octyloxy]-biphenyl, a thiophene monomer containing a p-cyanobiphenyl group which is electronically and sterically decoupled from the thiophene ring by means of an octyl spacer, leads to a highly conducting and electroactive conjugated polymer... 

Daoudi et al. [83] applied the thermodynamic approach equal Gibbs energy analysis to the pressure-composition phase diagrams of binary mixtures exhibiting reentrant phase behavior. Three different solution models are tested. The experimental data for the 4-n-hexyloxy- and 4-n-octyl-oxy-4 -cyanobiphenyl system are successfully described by the regular solution hypothesis. 

The variety of data on this homologuous series allows for a comparison of the reliability of different methods. Moreover, the octyl homologue (8-CB) [165-180] exhibits a nematic phase followed by a SmA phase and is therefore particularly interesting for the study of the critical divergence of the Kjj and K22 elastic coefficients on approaching the transition from the nematic to the SmA phase. Elastic coefficient measurements show that short-range pretransitional smectic-Uke order is found more or less in all n-CB homologues [98]. In Fig. 2a-c the elastic constants of the cyanobiphenyl series, as determined by Karat and Madhusu-... 

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