Search

EPR spin probe spectra of the liquid crystal phases exhibited, in the bulk, by the 1",9"-bis(4- cyanobiphenyl-4'-yl)nonane (CB9CB) bent-shape dimer showed that, on cooling from the isotropic phase, this material forms a uniaxial nematic phase with a uniform director macroscopically aligned along the spectrometer field. Upon further cooling, a transition into the twist-bend nematic phase is observed after a biphasic region of approximately 4 K. In this lower temperature nematic phase, the director does not appear to be macroscopically aligned. The non-uniform director distribution is modelled as a collection of monodomains tilted with respect to the magnetic field and orientationally distributed around the tilt direction. [ABSTRACT FROM AUTHOR]

Random disorder effects of an hydrophobic aerosil on the order and dynamics of the 3ß-DOXYL-5a-cholestane spin probe in the liquid crystal 4- n -pentyl-4'-cyanobiphenyl (5CB) were studied using the ESR technique. Increasing the aerosil concentration up to 10 wt% does not change the nematic–isotropic transition temperature but decreases the probe order parameter. The dynamics in the isotropic phase is of Arrhenius type and essentially independent of the concentration. Deviations from this behavior are clearly observed in the nematic phase even at 1 wt% of aerosil and are well represented by a Vogel-Fulcher-Tammann type law, suggesting that a glass-like behavior can be induced by a very small amount of random disorder. PACS numbers: 61.30. Gd, 76.30.-v, 82.70. Gg [ABSTRACT FROM AUTHOR]

We report on a comparison between the random disorder effects of an hydrophobic (R812) or an hydrophilic (A380) aerosil on the order and dynamics of the 3β-DOXYL-5α-cholestane spin probe in the liquid crystal 4-n-pentyl-4′-cyanobiphenyl (5CB) using the ESR technique. Increasing the aerosil concentration up to 3 wt% the probe order parameter is depressed with a larger effect for the hydrophilic aerosil. The dynamics in the isotropic phase is of Arrhenius type and essentially concentration and aerosil-type independent. Deviations from this behavior are observed in the nematic even at 0.3 wt% of hydrophilic aerosil and are well represented by a Vogel–Fulcher–Tammann type law, suggesting that a glass-like state can be induced by a very small amount of random disorder. [Copyright &y& Elsevier]