7Li NMR investigation of Li-Li pair ordering in the paraelectric phase of weakly substitutionally disordered K(1-x)Li(x)TaO3.

Breaking of the average cubic symmetry in Li-doped potassium tantalate was observed with quadrupole-perturbed 7Li NMR at temperatures (150-400 K) far above the nominal glass transition temperature (≈50 K for Li concentration x=0.03). The observed spectrum consists of contributions from both isolated Li ions (i.e., with no nearest-neighbor Li) and from Li-Li pairs. The isolated Li ions move among six equivalent off-center sites in a potential having cubic symmetry. These have zero average electric field gradient and, hence, exhibit no quadrupole splitting. In addition, very low intensity, but well resolved, quadrupole satellites having a temperature-dependent splitting were observed. This splitting indicates that the various Li-Li pair configurations are not all equally probable. These are the first direct observations of biased Li ion ordering that persists in the paraelectric phase at temperatures high above the glass phase.

Micropatterning of light-sensitive liquid-crystal elastomers.

We demonstrate that photoisomerizable liquid-crystal elastomer soft films can be used as tunable holographic gratings. Optomechanical mechanism of imprinting one-dimensional grating structure into the soft matrix by two-beam uv laser interference can be clearly resolved from the time dependence of the reading beam diffraction patterns. We analyze the observed response in terms of cis-trans isomerization-controlled modulation of the grating profile. The grating period can be tuned reversibly by stretching or contraction of the specimen, either thermomechanically or by applying external stress. Temperature-induced modifications of the grating parameters in the vicinity of the nematic-paranematic phase transition are also examined.

Phase separation in nematic microemulsions probed by one-dimensional spectroscopic deuteron magnetic resonance microimaging.

We present a study of a phase-transition-driven separation in microemulsions of nanosized lyotropic inverse micelles and thermotropic liquid crystal pentylcyanobiphenyl (5CB) with 5%, 8%, and 15% micelle concentration. Using deuteron nuclear magnetic resonance (DNMR) microimaging in combination with conventional microscopy as well as ac calorimetry, we demonstrate a phase separation scenario in which micelles are expelled from the nematic phase during the I-N conversion. Due to a difference in density the micelle-rich isotropiclike phase spatially separates from the micelle-free nematic phase. A relatively sharp interface, formed between the two phases, can be controllably shifted by temperature-induced conversion between the phases. Once expelled, micelles do not remix into the nematic phase, whereas in the isotropic state their remixing takes place over several days. Temperature dependence of the linewidth of isotropic spectral component has been analyzed in terms of molecular reorientations mediated by translational displacements, assuming isotropically distributed directors of nanosized nematic domains. With our results, the existence of the proposed transparent nematic state cannot be completely ruled out. However, if present, the nematic order in such a phase must be extremely low.

Criticality controlled by cross-linking density in liquid single-crystal elastomers.

A high-resolution calorimetry and deuteron-nuclear magnetic resonance study of a paranematic-nematic phase transition was performed on liquid single-crystal elastomers. We show that density variations of both rodlike and pointlike cross-links strongly affect the mean value and the dispersion of local mechanical fields. The system exhibits an inherent weakly disordered orientational state composed of regions with the temperature profile of the nematic order parameter ranging from first order to supercritical. On increasing the cross-linking density the predominantly first order thermodynamic response transforms into a predominantly supercritical one.

Calorimetric study of phase transitions in a liquid-crystal-based microemulsion.

A lyotropic inverse micelle phase composed of water, thermotropic liquid-crystal octylcyanobiphenyl (8CB), and surfactant (DDAB) was studied by using high-resolution calorimetry on several mixtures with 3%, 8%, and 15% micelle concentration. Calorimetric results show strong depression of the isotropic to nematic (I-N) phase-transition temperature. Broad heat-capacity anomalies show the existence of a wide coexistence range of isotropic, nematic, and smectic-A phases, which mimics the behavior of a new nearly stable thermodynamic phase. An observation of the rather sharp almost bulklike nematic to smectic-A (N-A) transition at low-temperatures indicates that our heat capacity results are consistent with the phase separation scenario in which significant number of micelles is expelled during I-N conversion leaving almost pure nematic phase at lower temperatures. It was found that micelles get almost completely remixed on heating the mixture back to the isotropic phase.

Unusual dynamic behavior in the isotropic phase of banana mesogens detected by 2H NMR line width and T2 measurements.

In this work the first experimental observation of a peculiar behavior in the isotropic phase of liquid crystals by means of 2H NMR is reported. In particular, two five-ring banana-shaped mesogens, the 1,3-phenylenebis{4,4'-(11-undecenyloxy)benzoyloxy}benzoate (Pbis11BB) and its 4-chloro homologue (ClPbis11BB), selectively deuterium labeled on their central rings, are the subject of our investigation. The dynamic behavior of the two liquid crystals was studied in their isotropic phases and in the nematic phase of ClPbis11BB by means of 2H NMR line width and spin-spin relaxation time (T2) analysis. The results obtained reveal that the unusual line broadening observed in the 2H NMR spectra in the isotropic phase, even far above the isotropic phase-mesophase transition, has a homogeneous nature, thus indicating the presence of reorientational motions much slower than in conventional isotropic liquid-crystalline phases.