Phase transitions in liquid crystals

Landau theory of the nematic-isotropic phase transition

This is a review of recent progress in our knowledge about the behaviour of synthethic and biological macromolecules and larger biological particles in dilute solution and in the liquid crystalline state, when exposed to strong magnetic fields. Emphasis is on the observation of magnetic orientation as resulting mostly from an anisotropic molecular diamagnetic susceptibility, and on its interpretation in terms of molecular structure, flexibility and intermolecular orientational correlation. Some biological applications of magnetic fields are briefly discussed, including the separation of macromolecules in inhomogeneous fields, the field-induced modification of cellular growth and the field sensitivity of animals.

Biomolecules and Polymers in High Steady Magnetic Fields

An ac calorimetric technique has been used to study the nematic---smectic-$A$ ($N\ensuremath{-}\mathrm{Sm}A$) and nematic-isotropic ($N\ensuremath{-}I$) transitions in octyloxycyanobiphenyl (8OCB) along a series of isobars between 1 atm and 2 kbar. The excess heat capacity $\ensuremath{\Delta}{C}_{p}(\mathrm{NA})$ associated with the essentially second-order $N\ensuremath{-}\mathrm{Sm}A$ transition is found to diminish rapidly with pressure and is not observed for $pg1.5$ kbar. The variation in $\ensuremath{\Delta}{C}_{p}(\mathrm{NA})$ with reduced temperature is not consistent with a logarithmic singularity but can be characterized by a critical exponent $\ensuremath{\alpha}={\ensuremath{\alpha}}^{\ensuremath{'}}=0.25\ifmmode\pm\else\textpm\fi{}0.05$ for all the pressures studied. The excess heat capacity at the $N\ensuremath{-}I$ transition is almost independent of pressure; these data suggest but do not clearly establish quasitricritical behavior. Details of the experimental method and the fitting procedures are given.

Critical heat capacity near the nematic-smectic-A transition in octyloxycyanobiphenyl in the range 1-2000 bar

This paper reports a novel liquid crystal phase having the characteristics of a twist-bend nematic phase formed by a non-symmetric ether-linked liquid crystal dimer. The dimer 1''-(2',4-difluorobiphenyl-4'-yloxy)-9''-(4-cyanobiphenyl-4'-yloxy) nonane (FFO9OCB) exhibits two liquid-crystalline phases on cooling at a sufficiently high rate from the isotropic phase. The high temperature mesophase has been reported in the literature as nematic and confirmed in this study. The other mesophase is metastable and can be supercooled giving rise to a glassy state. Its identification and characterization are based on optical textures, broadband dielectric spectroscopy, calorimetry, measurements of both splay and bend elastic constants in the nematic phase and miscibility studies. It is concluded that the low temperature mesophase exhibits the characteristics of a twist-bend nematic phase. Dielectric measurements enable us to obtain the static permittivity and information about the molecular dynamics in the isotropic phase, in the nematic mesophase and across the isotropic-to-nematic phase transition. Two orientations, parallel and perpendicular to the director, have been investigated. In the high temperature nematic mesophase, the dielectric anisotropy is found to be positive. Measurements of the parallel component of the dielectric permittivity are well-explained by the molecular theory of dielectric relaxation in nematic dimers (M. Stocchero, A. Ferrarini, G. J. Moro, D. A. Dunmur and G. R. Luckhurst, J. Chem. Phys., 2004, 121, 8079). The dimer is modelled as a mixture of cis and trans conformers and the model allows an estimate of their relative populations at each temperature. The nematic-to-isotropic phase transition has been exhaustively studied from the accurate evolution of the heat capacity and the static dielectric permittivity data. It has been concluded that the transition is first order in nature, but close to tricritical. The nature of the nematic-to-the novel liquid crystal phase transition is difficult to analyze to the same extent because of insufficient precision. Only observations at cooling rates of 10 K min(-1) or higher were possible because on heating from the glassy state, the twist-bend nematic mesophase crystallizes at temperatures far below the nematic-nematic phase transition.

Dielectric, calorimetric and mesophase properties of 1′′-(2′,4-difluorobiphenyl-4′-yloxy)-9′′-(4-cyanobiphenyl-4′-yloxy) nonane: an odd liquid crystal dimer with a monotropic mesophase having the characteristics of a twist-bend nematic phase

J. R. Shane

Papers

Tricritical Exponents for the Isotropic-nematic Transition: An Experimental Verification