We present a calorimetric study of the phase behavior of octylcyanobiphenyl (8CB) liquid crystal confined to a controlled-pore glass (CPG). We used CPG matrices with characteristic void diameters ranging from 400 to 20 nm. In bulk we obtain weakly first-order isotropic to nematic (I-N) phase transition and nearly continuous character of the nematic to smectic-A (N-SmA) phase transition. In all CPG matrices the I-N transition remains weakly first order, while the N-SmA one becomes progressively suppressed with decreasing CPG pore radius. With decreased pore diameters both phase transition temperatures monotonously decrease following similar trends, but increasing the stability range of the N phase. The heat-capacity response at the weakly first order I-N and continuous N-SmA phase transitions gradually approaches the tricritical-like and three-dimensional XY behavior, respectively. The main observed features were explained using a bicomponent single pore type phenomenological model.

Calorimetric study of octylcyanobiphenyl liquid crystal confined to a controlled-pore glass.

We study the influence of the dispersed aerosil particle concentration on the soft and Goldstone mode dynamics across the smectic-A-smectic-C* phase transition using dielectric spectroscopy. We use CE8 liquid crystal filled with aerosils of concentrations x=m(s)/(m(s)+m(LC))=0.025, 0.05, 0.1, and 0.15, where m(s) and m(LC) stand for masses of aerosil and liquid crystal, respectively. In dispersions with x 0 the degeneracy of the modes at the SmA-SmC* transition temperature is lifted. With increasing x the modes' intensities gradually decrease and the critical effective coefficient gamma tentatively approaches the value characterizing the three-dimensional XY universality class. For x=0.15, a drastic change in behavior is observed. The Goldstone mode is completely suppressed and the f(s)(T) dependence begins to deviate from the Arrhenius behavior. A simple phenomenological approach is used in the explanation of results.

http://samo.kralj.fnm.uni-mb.si/clanki/a2c4.pdf

Effect of dispersed silica particles on the smectic-A-smectic-C* phase transition.

We propose a Landau--de Gennes variational theory fit to simultaneously describe isotropic, nematic, smectic-$A$, and smectic-$C$ phases of a liquid crystal. The unified description allows us to deal with systems in which one, or all, of the order parameters develop because of the influence of defects, external fields and/or boundary conditions. We derive the complete phase diagram of the system, that is, we characterize how the homogeneous minimizers depend on the value of the constitutive parameters. The coupling between the nematic order tensor and the complex smectic order parameter generates an elastic potential which is a nonconvex function of the gradient of the smectic order parameter. This lack of convexity yields in turn a loss of regularity of the free-energy minimizers. We then consider the effect on an infinitesimal second-order regularization term in the free-energy functional, which fixes the optimal number of defects in the singular configurations.

Landau - de Gennes theory of isotropic-nematic-smectic liquid crystal transitions

The structural and thermodynamic phase behavior of the thermotropic liquid crystal dodecylcyanobiphenyl (12CB) confined in controlled-pore glasses (CPGs) has been studied by means of high-resolution calorimetry and small-angle X-ray scattering. Non-treated CPG matrices have been used with a characteristic pore size R ranging from nanometre to micrometre scale. Even in the case of the strongest confinement the isotropic-to-smectic-A (I-SmA) transition was pronounced. Both the smectic pretransitional ordering and the broadening of the specific heat capacity anomaly were relatively weak. This signals the secondary importance of the surface interaction potential and of the disorder upon phase behavior. We claim that the surface interaction is dominated by the coupling between surface-frozen and volume liquid crystal molecules. This coupling is weak due to the flexible long alkyl chains which dominantly influence the phase behavior of 12CB. The depression of the phase transition temperature Tc is dominated by smectic elastic compressibility distortions. The observed anomalous Tc(R) dependence is explained by allowing surface rearrangement processes. In severely-confined samples the 1st order I-SmA phase transition tends to be replaced by a 2nd order one. This phenomenon is attributed to a dimensional-crossover which, in 12CB samples, is not masked by surface interactions as it is in case of similar liquid crystals with shorter alkyl chains. The observed behavior is explained using scaling and dimensional analysis within the Landau–Ginzburg–de Gennes mesoscopic approach.

Dimensional crossover and scaling behavior of a smectic liquid crystal confined to controlled-pore glass matrices

Thermal study of octylcyanobiphenyl liquid crystal confined to controlled-pore glass ☆

The covariant form of the Landau–de Gennes free energy is used to study the chevron structure formed by cooling from the Sm-A to the nonchiral Sm-C phase in a surface-stabilized cell with planar boundary conditions. We show that the chevron is the thermodynamic equilibrium structure. The chevron structure is studied depending on the liquid-crystal elastic properties, temperature, and the surface orientational anchoring strength. We show that the bistability of the chevron structure results from the continuity of the molecular director over the chevron tip of finite width, and is strongly dependent on the surface orientational anchoring. We estimate analytically the threshold temperature for the chevron formation and show that above this temperature the bookshelf geometry is stable. We show that the energy of the chevron interface follows a power-law dependence on reduced temperature with the exponent of 3/2.

http://samo.kralj.fnm.uni-mb.si/clanki/a96c3.pdf

N. Vaupotic

Papers

Landau-de Gennes theory of the chevron structure in a smectic liquid crystal