Calorimetric study of the nematic to smectic-A and smectic-A to smectic-C phase transitions in liquid-crystal-aerosil dispersions

A high-resolution calorimetric study has been carried out on nanocolloidal dispersions of aerosils in the liquid crystal 4-n-pentylphenylthiol-4'-n-octyloxybenzoate (8S5) as a function of aerosil concentration and temperature spanning the smectic-C to nematic phases. Over this temperature range, this liquid crystal possesses two continuous XY phase transitions: a fluctuation-dominated nematic to smectic-A transition with alpha approximately alphaXY=-0.013 and a mean-field smectic-A to smectic-C transition. The effective critical character of the N-SmA transition remains unchanged over the entire range of the introduced quenched random disorder while the peak height and enthalpy can be well described by considering a cutoff length scale to the quasicritical fluctuations. The robust nature of the N-SmA transition in this system contrasts with cyanobiphenyl-aerosil systems and may be due to the mesogens being nonpolar and having a long nematic range. The character of the SmA-SmC transition changes gradually with increasing disorder but remains mean field like. The heat capacity maximum at the SmA-SmC transition scales as rho with an apparent evolution from tricritical to a simple mean-field step behavior. These results may be generally understood as a stiffening of the liquid crystal (both the nematic elasticity as well as the smectic layer compression modulus B) with silica density.

First-order isotropic-smectic-A transition in liquid-crystal-aerosil gels

The short-range order which remains when the isotropic to smectic- A transition is perturbed by a gel of silica nanoparticles (aerosils) has been studied using high-resolution synchrotron x-ray diffraction. The gels have been created in situ in decylcyanobiphenyl, which has a strongly first-order isotropic to smectic- A transition. The effects are determined by detailed analysis of the temperature and gel density dependence of the smectic structure factor. In previous studies of the continuous nematic to smectic- A transition in a variety of thermotropic liquid crystals the aerosil gel appeared to pin, at random, the phase of the smectic density modulation. For the isotropic to smectic- A transition the same gel perturbation yields different results. The smectic correlation length decreases more slowly with increasing random-field variance in good quantitative agreement with the effect of a random pinning field at a transition from a uniform phase directly to a phase with one-dimensional translational order. We thus compare the influence of random fields on a freezing transition with and without an intervening orientationally ordered phase.

High-resolution x-ray study of the nematic-smectic-A and smectic-A-smectic-C transitions in liquid-crystal-aerosil gels

The effects of dispersed aerosil nanoparticles on two of the phase transitions of the thermotropic liquid-crystal material 4-n-pentylphenylthiol-4(')-n-octyloxybenzoate (8;S5) have been studied using high-resolution x-ray diffraction techniques. The aerosils hydrogen bond together to form a gel which imposes a weak quenched disorder on the liquid crystal. The smectic-A fluctuations are well characterized by a two-component line shape representing thermal and random-field contributions. An elaboration on this line shape is required to describe the fluctuations in the smectic-C phase; specifically the effect of the tilt on the wave-vector dependence of the thermal fluctuations must be explicitly taken into account. Both the magnitude and the temperature dependence of the smectic-C tilt order parameter are observed to be unaffected by the disorder. This may be a consequence of the large bare smectic correlation length in the direction of modulation for this transition. These results show that the understanding developed for the nematic to smectic-A transition for octylcyanobiphenyl and octyloxycyanobiphenyl liquid crystals with quenched disorder can be extended to quite different materials and transitions.

Velocity of an electric-field-induced synclinic solitary wave invading the anticlinic liquid crystal phase

The electric-field dependence of the velocity of synclinic fingers invading the anticlinic phase is determined by a time-of-flight technique. The time delay for a rapid increase in the transmitted optical intensity through the sample is measured between two points as a function of their separation along the trajectory of the solitary wave. The data are quantitatively consistent with the rapid velocities deduced from a previous measurement [Liq. Cryst. 27, 249 (2000)], demonstrating that the previous data were not affected by multiple nucleation sites occurring at higher fields.

Freedericksz transition in an anticlinic liquid crystal

The Freedericksz geometry is used to show experimentally that a very-long-pitch, surface stabilized, anticlinic liquid crystal undergoes a two-step electric-field-induced transition to the synclinic phase. The liquid crystal remains undistorted below the threshold field E(th). For E>E(th), a Freedericksz transition occurs, wherein molecules in adjacent smectic layers undergo unequal azimuthal rotations about the layer normal, resulting in a nonzero polarization that couples to the applied field. Measurements of E(th) as a function of temperature are reported. Related quasielastic light scattering measurements demonstrate that acoustic Goldstone mode fluctuations are quenched by a dc electric field E>E(th). At high fields a transition to the synclinic phase occurs via solitary waves.

Structural study of the smectic-I to smectic-F transition in freely suspended films

The smectic-I (S(I)) to smectic-F (S(F)) phase transition in terephthal-bis-(4n)-decylaniline (TB10A) has been examined for a possible continuous transition via the intermediate smectic-L (S(L)) phase. X-ray diffraction measurements of thick, single-domain, freely suspended films are used to classify the phases and to determine the hexatic order parameter C6 and its harmonics. Instead of the continuous transition suggested in the literature we find a first-order S(I)-->S(F) transition with a discontinuous change in the direction of the bond-orientational order relative to the molecular tilt. The tilt of the molecular form factor in the hexatic phases is inconsistent with the tilt estimated from published layer spacing measurements, suggesting that the hexatic phases of TB10A must have the molecular cores oriented at an angle relative to the tails. This result taken together with published results on the S(C) phase suggests that the S(C)-->S(I)-->S(F) transitions are driven by changes in the conformation of the hydrocarbon tails. Examination of the harmonic scaling relation between the hexatic order parameters shows mean-field behavior in the S(I) phase. This result will make binary mixtures of TB10A with other materials a practical system for the study of the crossover from mean field to the XY behavior seen in other hexatic systems.

Electric field-induced acoustic-optic mode coupling in an anticlinic liquid crystal

A dc electric field was applied perpendicular to the tilt plane of a pitch-compensated (unwound helix) anticlinic liquid crystal. By means of quasielastic light scattering, the field was found to couple the acoustic and optic Goldstone modes, resulting in an increase of the relaxation time tau(beta) of the acousticlike eigenmode. Elastic constants were estimated from the relaxation time data.

Bend elasticity of mixtures of V-shaped molecules in ordinary nematogens

Freedericksz measurements were performed on mixtures of a nematic mesogen and a V-shaped molecule. The bend elastic constant was found to decrease significantly with increasing concentration of the V-shaped molecule. The results are discussed theoretically, taking into account the detailed structure of the molecules.

Ring-pattern dynamics in smectic-C* and smectic-C*A freely suspended liquid crystal films

Ring patterns of concentric 2pi solitons in molecular orientation form in freely suspended chiral smectic-C films in response to an in-plane rotating electric field. We present measurements of the driven dynamics of ring formation under conditions of synchronous winding and of the zero-field relaxation of ring patterns, and propose a simple model which enables their quantitative description in low polarization DOBAMBC. In smectic-C*A TFMHPOBC we observe an odd-even layer number effect, with odd layer number films exhibiting order of magnitude slower relaxation rates than even layer films. We show that this rate difference is due to a much larger spontaneous polarization in odd layer number films.

Freedericksz transition in an anticlinic liquid crystal

It is shown experimentally that a very-long-pitch, surface-stabilized, anticlinic liquid crystal undergoes a two-step electric-field-induced transition to the synclinic phase. The liquid crystal remains undistorted below a threshold field E(th). For E>E(th), a Freedericksz transition occurs, wherein molecules in adjacent smectic layers undergo unequal azimuthal rotations about the layer normal. At higher fields a transition to the synclinic phase occurs via solitary waves.