Recent Developments in Nematogenic Bent-Core Mesogens: an X-ray Diffraction Perspective

Bent-core mesogens are attracting great attention because of their potentially biaxial and polar nematic phase. In this paper, we review the results of our recent X-ray diffraction studies on the nematic phase of oxadiazole-based mesogens. The evolution of the smallangle diffraction signal with temperature points to the existence of nano-sized cybotactic clusters, characterized by short-range smectic-like fluctuations. However, in contrast to classical cybotaxis of calamitic nematics, these fluctuations do not have a pretransitional nature. They explain most of the extraordinary properties of bent-core mesogens and, in particular, are responsible for the unique ferroelectric response exhibited by one of our compounds

Structure of the nematic mesophase with cybotactic groups in liquid-crystalline poly(urethane-ester)s

The results of a structural analysis by x-ray diffraction on fiber samples of the nematic mesophase with cybotactic groups of poly(urethane-ester) TDI-C2C6, TDI-C6C4, TDI-C6C6, TDI-C6C8, and TDI-C6C10 as prepared from various mesogenic alkylene di[4-(omega-hydroxyalkyloxy-4-oxybenzoyl)oxybenzoate]s (CmCn; m = 2 or 6 and n = 4, 6, 8, or 10) and 2,4-toluenediisocyanate (TDI) are reported. Evidence is provided for the existence of two different molecular orderings within the nematic phases with cybotactic groups, to which different internal degrees of correlation correspond. The average dimension of the cybotactic clusters along the director is comprised between similar to 260 and similar to 560 Angstrom, whereas the average dimension perpendicular to the director ranges from similar to 130 to similar to 270 Angstrom. Accordingly, the cybotactic clusters present an asymmetrical ellipsoidal-like shape. On the other hand, the correlation length within the nematic liquid crystal surrounding the cybotactic clusters and along the director is between 38 and 45 Angstrom. These values are of the order of the molecular length. These findings highlight the multidomain nature of the mesophase consisting of large and asymmetric clusters surrounded by a conventional nematic endowed with a very low correlation degree

Light-induced molecular adsorption and reorientation at polyvinylcinnamate-fluorinated/liquid-crystal interface

We have carried out a detailed experimental study, by means of x-ray reflectometry (XRR) and half-leaky guided mode (HLGM) optical characterization, of the light-induced molecular adsorption and reorientation at the polyvinylcinnamate-fluorinated (PVCN-F)/liquid-crystal (LC) interface of a LC cell doped with the azodye methyl red (MR). The XRR data allowed characterizing the microscopic structure of the adsorbed dye layer both before irradiation (dark adsorption) and after irradiation (light-induced adsorption). The HLGM optical characterization has made possible the experimental determination of the nematic director profile in the LC cell and evaluation of the effects of light-induced adsorption on the LC anchoring conditions. The experimental findings have confirmed the formation of a dark-adsorbed layer and are in agreement with the absorption model previously proposed to account for the complex phenomenology related to light-induced anchoring and reorientation in dye-doped liquid crystals

Hybrid liquid-crystalline block copolymers

Invited Articl

An X-ray study of a smectic C and smectic A liquid crystal polyacrylate

An X-ray study was performed on powder specimens and on stretched oriented fibres of a smectic polyacrylate. The X-ray diffraction patterns showed the existence of monolayer smectic A and bilayer smectic C mesophases that coexisted over a well defined range of temperature. In the latter mesophase the mesogenic groups were titled with respect to the smectic layer normal by an angle $\beta\approx 30^\circ$. The electron density profile $\rho(z)$ along the direction normal to the smectic layers in the smectic C mesophase was determined by Fourier inversion and indicated the presence of a minor interdigitation of the terminal substituents. The phase problem related to the Fourier inversion was solved by using a pattern recognition approach based on the histogram of the electron density map. The positional smectic layer order parameters were also calculated for the first five harmonics of the positional distribution function of the smectic layers and their values suggested a well ordered layer structure

Synthesis and liquid-crystalline properties of polyacrylates containing prochiral sulfide substituents

The synthesis and thermal behaviour of a new series of side-chain liquid-crystalline polyacrylates 1a-h containing prochiral alkyl sulfide substituents with a variable number n of carbon atoms are described. No liquid-crystalline behaviour was exhibited in the melt by the relevant monomers and intermediate compounds. In contrast, all polymers displayed thermotropic properties. The lower homologues 1a-f(n less than or equal to 6) displayed smectic B1 and smectic A1 mesophases, while the higher homologue 1h (n = 10) formed a bilayer, partly interdigitated smectic C mesophase. A special position was held by 1g with a sulfide substituent of intermediate length (n = 8), for which monolayer and bilayer smectic mesophases coexisted over a wide temperature range

Hybrid liquid-crystalline block copolymers. 2. X-Ray diffraction analysis of polyester-polymethacrylate block copolymers

The molecular organization in the smectic mesophases of a series of main-chain/side-chain block copolymers was studied by X-ray diffraction on oriented specimens. The main-chain and the side-chain blocks were phase-separated and gave rise to two coexisting smectic domains with different layer periodicities. The orientation of the smectic planes in the distinct domains with respect to the fiber axis was very different in the various samples and depended on the relative length of the blocks. In the presence of relatively short side-chain blocks, the orientation of the main-chain block dominated and the smectic planes of the side-chain block were forced to be parallel to those of the main-chain block and perpendicular to the fiber axis. In contrast, when the side-chain block was sufficiently long it imposed its own orientation to the main-chain block. In this case, the smectic planes of the main-chain block were oriented parallel to those of the side-chain block and to the fiber axis