Thermodynamically Anchoring-Frustrated Surface to Trigger Bulk Discontinuous Orientational Transition.

Surface-specific liquid crystal (LC) nanostructures provide a unique platform for studying surface-wetting phenomena and also for technological applications. The most important studies on LC properties are related to bulk alignment, surface anchoring, and so on. Here, we study an LC system with a nematic liquid crystal (NLC) on a perfluoropolymer-coated substrate, in which a discontinuous bulk orientational transition has recently been found. Using free-energy analysis based on experimental results of the newly-conducted grazing-incidence X-ray diffraction (GI-XRD) measurements, we have confirmed a thermodynamic growth process of smectic liquid crystalline wetting nanosheets on the surface and successfully explained that a frustrated surface of planar and vertical anchoring states accompanied by an elastic energy cost kinetically triggers the bulk reorientation in the first-order manner. This interfacial bottom-up process may offer a general insight into how interfacial hierarchical molecular architectures alter the bulk properties of matter thermodynamically.

Observation of two isotropic-nematic phase transitions near a surface.

Using specified conditions, we succeeded in observing the isotropic-nematic (Iso-N) liquid crystal phase transition at surfaces followed by that in bulk for the first time. An additional heat anomaly peak was found at a higher temperature side of a main phase transition peak using highly sensitive differential scanning calorimetry (HS-DSC). The peak is pronounced particularly in the cooling process, since the transition starts at surfaces on cooling. The temperature dependence of retardation allows us to safely conclude that the higher temperature peak that appeared in HS-DSC is attributed to the Iso-N transition at surfaces. These measurements also indicate that the surface transition is of first order. These behaviors were theoretically explained by generalized Maier-Saupe theory.

Stepwise heat-capacity change at an orientation transition in liquid crystals.

During a phase transition in a bulk material, heat is exchanged with matter to balance the changes in the internal energy and the entropy of the system. Here we report on the thermal detection of a surface-mediated anchoring transition, a spontaneous and discontinuous orientation change between planar (P) and homeotropic (H) alignments within a single nematic phase by changing temperature. In this case a stepwise change in the heat flow, similar to a glass transition, is observed by means of high-resolution differential scanning calorimetry. We found that the jump in the specific heat does not depend on the sample volume, although the contribution of molecules in the vicinity of surfaces, which trigger the transition, becomes less with increasing the sample volume. This means that different molecular orientations, H and P, with respect to surfaces have different thermodynamic free energies. We also address why the anchoring transition occurs by means of grazing-incidence x-ray diffraction measurements, which clearly reveal the formation of quasismectic layers parallel to surfaces in the nematic phase.

Local orientational analysis of helical filaments and nematic director in a nanoscale phase separation composed of rod-like and bent-core liquid Crystals using small- and wide-angle X-ray microbeam scattering.

We analyzed the local nanostructure in binary mixtures of rod- and bent-shaped molecules, n-pentyl-4-cyanobiphenyl (5CB) and 1,3-phenylene bis[4-(4-n-octyloxyphenyliminomethyl) benzoates] (P-8-OPIMB), respectively, using small- and wide-angle X-ray microbeam and macrobeam scattering. From the orientational X-ray scattering patterns, we concluded that the nematic director of 5CB is almost parallel to the smectic layers dominated by bent-core molecules in Bx. Moreover, we observed oriented small-angle diffraction peaks (about 300 Å), which is close to the spacing of 5-7 layers, and also consistent with the width of a helical nanofilament textures as observed by freeze-fracture transmission electron microscopy. The kinetics in B4 was also discussed based on the contact experimental method.

Critical anomalies in thermal diffusivity of liquid-crystalline terephthal-bis-(4-n-butylaniline).

A temperature wave method has been applied to observe the thermal diffusivity through the isotropic (Iso)-nematic (N)-smectic Sm-A-Sm-C-Sm-B-crystals VI-VII-VIII phase transitions of terephthal-bis-(4-n-butylaniline) (TBBA). Critical anomalies have been found in the N-Sm-A and Sm-C-Sm-B phase transitions as diplike shaped, consistent with the predictions based on the dissipative couplings between the order parameter and the conserved free-energy density. Singular points with a gap have been observed at the Sm-B-crystal VI, crystals VI-VII, and crystals VII-VIII phase transitions, which show polymorphic behaviors on heating and cooling. The second-order Sm-A-Sm-C phase transition emerged as a singular temperature dependence. In all the phases thermal diffusivity decreases with increasing temperature except for Sm-C, where thermal diffusivity increases with increasing temperature. The origin of the anomaly in the thermal diffusivity in Sm-C is discussed based on the parametric analysis of dynamic critical behavior in the Sm-A-Sm-C phase transitions together with the tilt angle change obtained by use of simultaneous measurements of x-ray diffraction and differential scanning calorimetry.

Isotropic-nematic transition at the surface of a liquid crystal embedded in an aerosil network.

We reexamined the isotropic-nematic (Iso-N) phase transitions of 4-n-heptyl cyanobiphenyl (7CB) embedded in aerosils of silica nanoparticle dispersions using highly sensitive differential scanning calorimetry (HS-DSC), polarizing optical microscopy (POM), and retardation measurements. We found a simple and very profound relationship between the calorimetric and optical measurements; in addition to double DSC peaks, which have been observed previously, a two-step change of the retardation was clearly observed by varying temperature. From our analysis, there is no doubt that the Iso-N liquid-crystal phase transition certainly occurs in two steps, i.e., the bulk transition takes place at first and then the surface transition takes place upon cooling. We should note that the surface transition takes place below the bulk transition temperature, the opposite of what was recently observed in a polymer-dispersed liquid-crystal system.

Two transitions between isotropic and nematic phases in confined liquid crystals.

The molecular mean-field theory for the nematic-isotropic (N-Iso) phase transition in the vicinity of the surface is derived. We have shown that the nematic order parameter in liquid crystal near the surface is generally different from that in the bulk. It is never equal to zero if the anisotropic interaction with the surface is present. At the same time, transition from the phase with large nematic order parameter at the surface to the phase with small nematic order parameter at the surface is possible on heating. This surface transition always happens at higher temperatures than N-Iso phase transition related to the bulk. The theoretical prediction was well reproduced experimentally.