Articles de revues : « Nematic and Smectic phases »

1

Urban, S., H. Kresse et R. Dąbrowski. « Low Frequency Dielectric Relaxation Process in Liquid Crystals with Nematic and Liquid-Like Smectic Phases ». Zeitschrift für Naturforschung A 52, n^o 5 (1 mai 1997) : 403–8. http://dx.doi.org/10.1515/zna-1997-0505.

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Abstract Results of dielectric studies of the low frequency relaxation process in several substances exhibiting nematic -smectic C polymorphism are presented. They are compared with the data obtained re-cently for substances with nematic-smectic A and nematic-smectic B phase sequences. It was found that the rate of molecular reorientation around the short axes does not change at the transition between the nematic and a liquid-like smectic phase (Sm A, Sm C), whereas it is considerably retarded at that of the nematic-solid-like smectic (Sm B) phase. However, the activation barrier for this motion is markedly lower in the orthogonal smectics (Sm A, Sm B) than in the nematic and Sm C phases. The analysis of numerous experimental data on the activation enthalpy in the nematic phase shows that the Arrhenius equation conforms better with results than the Diogo-Martins approach, if the nematic range exceeds 10 K.

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Pisipati, V. G. K. M., N. V. S. Rao, P. V. Datta Prasad et P. R. Alapati. « Density, Refractive Index and Ultrasonic Velocity Studies Involving N-(p-n-Pentyloxybenzylidene)-p-n-octylaniline ». Zeitschrift für Naturforschung A 40, n^o 5 (1 mai 1985) : 472–75. http://dx.doi.org/10.1515/zna-1985-0509.

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The density, refractive index and ultrasonic velocity dependence on temperature for N-(p-npentyloxybenzylidene)- p-n-octylaniline, 50.8, is presented. The compound 50.8 exhibits smectic-B, smectic-A and nematic phases between the solid and isotropic liquid phases. The nematic-isotropic and smectic A-smectic B phase transformations are found to be first order. The interesting smectic A - nematic transformation is confirmed from the results to be second order, although the thermal expansion coefficient and reported enthalpy data had suggested a weak first order transition. The computed adiabatic compressibility, βad, molar sound velocity, Rn, and molar compressibility Aw are presented.

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Nandi, B., M. Saha et Prabir K. Mukherjee. « Landau Theory of Nematic to Smectic-A Phase Transition ». International Journal of Modern Physics B 12, n^o 02 (20 janvier 1998) : 207–12. http://dx.doi.org/10.1142/s0217979298000156.

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A phenomenological Landau-like theory is presented here, which describes the nematic and smectic-A phase. The problem of the first or second order nature of the nematic to smectic-A phase transition is explored and the stability of the different phases are also calculated. On the basis of this work it is argued that the nematic to smectic-A transition is of first order nature.

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Bailly-Reyre, Aurélien, et Hung T. Diep. « Nematic and Smectic Phases : Dynamics and Phase Transition ». Symmetry 12, n^o 9 (22 septembre 2020) : 1574. http://dx.doi.org/10.3390/sym12091574.

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We study in this paper the dynamics of molecules leading to the formation of nematic and smectic phases using a mobile 6-state Potts spin model with Monte Carlo simulation. Each Potts state represents a molecular orientation. We show that, with the choice of an appropriate microscopic Hamiltonian describing the interaction between individual molecules modeled by 6-state Potts spins, we obtain the structure of the smectic phase by cooling the molecules from the isotropic phase to low temperatures: molecules are ordered in independent equidistant layers. The isotropic-smectic phase transition is found to have a first-order character. The nematic phase is also obtained with the choice of another microscopic Hamiltonian. The isotropic-nematic phase transition is a second-order one. The real-time dynamics of the molecules leading to the liquid-crystal ordering in each case is shown by a video.

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Jiang, Ying, Yuehua Cong et Baoyan Zhang. « Synthesis and characterization of chiral smectic side-chain liquid crystalline elastomers containing nematic and chiral mesogens ». New Journal of Chemistry 40, n^o 11 (2016) : 9352–60. http://dx.doi.org/10.1039/c6nj02001a.

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A novel series of siloxane-based chiral smectic side-chain liquid crystalline elastomers containing nematic and chiral mesogens were fabricated through synthesis involving a one-step hydrosilication reaction via a liquid crystalline crosslinking agent containing smectic and nematic phases.

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Workentin, Mark S., Brian J. Fahie et William J. Leigh. « Photochemical probes of conformational mobility in liquid crystals. The photochemistry of β-phenyl-4-methoxypropiophenone in the nematic and smectic B phases of CCH-n and OS-nm liquid crystals ». Canadian Journal of Chemistry 69, n^o 9 (1 septembre 1991) : 1435–44. http://dx.doi.org/10.1139/v91-212.

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The photochemistry of β-phenyl-4-methoxypropiophenone (1) in the isotropic and liquid crystalline (nematic and smectic B) phases of trans, trans-4′-alkyl-(1,1′-bicyclohexyl)-4-carbonitrile (CCH-n) and trans-4-alkylcyclohexyl-trans-4′-alkylcyclohexylcarboxylate (OS-nm) mesogens has been investigated using nanosecond laser flash photolysis techniques. Solubility limits of 1 as a function of temperature in the smectic phases have been rigorously determined by 2H NMR spectroscopy, using α-, methoxy-, and β-phenyl-deuterated analogs of the ketone. Triplet decay of 1 in the smectic (Crystal B) phase of CCH-4 is multiexponential, suggesting that the ketone is solubilized in a distribution of conformations in the highly-ordered liquid crystalline lattice. The average triplet lifetime of 1 at various temperatures throughout the smectic phase is about a factor of three longer than that in homologous nematic and isotropic phases at the same temperature. The differences in the Arrhenius parameters for triplet decay in the smectic and nematic/isotropic phases are very small, however. The present results for 1 in the smectic phase of the CCH-n mesogens correct the interpretation of previously reported data obtained with higher concentration mixtures, and afford an accurate indication of the true effects of smectic B liquid crystalline order on the conformation-dependent triplet state behavior of this molecule. In OS-nm liquid crystals, triplet decay follows clean first order kinetics in all phases, but the variations in Arrhenius dependence with solvent phase are similar to those observed in the CCH-n liquid crystals. Key words: smectic, liquid crystals, 2H NMR, nanosecond laser flash photolysis, intramolecular triplet quenching, photochemistry.

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Subala, S. Sandy, B. Syama Sundar et S. Sreehari Sastry. « Synthesis and Characterization of Nonsymmetric Liquid Crystal Dimer Containing Biphenyl and Azobenzene Moiety ». Journal of Chemistry 2013 (2013) : 1–6. http://dx.doi.org/10.1155/2013/939406.

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Calamitic liquid crystalline dimer containing azobenzene moiety and a decyloxy biphenyl linked by flexible spacers {4-[7-(4′-decyloxy-biphenyl-4-yloxy)-alkyloxy]-phenyl}-(4-decyl-phenyl)-diazene has been synthesized and characterized by spectroscopic methods. The transition temperatures and phase behaviours were studied by Differential Scanning Calorimeter (DSC) and Polarizing Optical Microscope (POM). The synthesized compounds exhibited enantiotropic liquid crystal phase with higher spacer display nematic and smectic C phases while lower spacer shows nematic and smectic A phases.

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Shrivastav, Gaurav P. « Self-Assembly of an Equimolar Mixture of Liquid Crystals and Magnetic Nanoparticles ». Crystals 11, n^o 7 (19 juillet 2021) : 834. http://dx.doi.org/10.3390/cryst11070834.

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We studied the equilibrium self-assembly of an equimolar mixture of uniaxial liquid crystals (LCs) and magnetic nanoparticles (MNPs) using molecular dynamics simulations. The LCs are modeled by ellipsoids interacting via Gay–Berne potential, and MNPs are represented by dipolar soft spheres (DSS). We found that the LCs show isotropic, nematic, and smectic phases when the mixture is compressed at a fixed temperature. The DSS form chain-like structures, which remain randomly oriented at low densities where the LCs are in the isotropic phase. At intermediate and high densities, the DSS chains align along the nematic and smectic directors of LCs. We found that the DSS inside a chain follow a ferromagnetic ordering. However, the mixture does not show a significant macroscopic magnetization. The extent of nematic order in the DSS remains very similar to the LCs in intermediate densities. At high densities, the DSS have a lower extent of nematic order than the LCs. The structure of the LC–DSS mixture was further analyzed via projected pair correlation functions for distances parallel and perpendicular to directors in the nematic and smectic phases.

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Pocock, Emily E., Richard J. Mandle et John W. Goodby. « Experimental and Computational Study of a Liquid Crystalline Dimesogen Exhibiting Nematic, Twist-Bend Nematic, Intercalated Smectic, and Soft Crystalline Mesophases ». Molecules 26, n^o 3 (20 janvier 2021) : 532. http://dx.doi.org/10.3390/molecules26030532.

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Liquid crystalline dimers and dimesogens have attracted significant attention due to their tendency to exhibit twist-bend modulated nematic (NTB) phases. While the features that give rise to NTB phase formation are now somewhat understood, a comparable structure–property relationship governing the formation of layered (smectic) phases from the NTB phase is absent. In this present work, we find that by selecting mesogenic units with differing polarities and aspect ratios and selecting an appropriately bent central spacer we obtain a material that exhibits both NTB and intercalated smectic phases. The higher temperature smectic phase is assigned as SmCA based on its optical textures and X-ray scattering patterns. A detailed study of the lower temperature smectic ‘’X’’ phase by optical microscopy and SAXS/WAXS demonstrates this phase to be smectic, with an in-plane orthorhombic or monoclinic packing and long (>100 nm) out of plane correlation lengths. This phase, which has been observed in a handful of materials to date, is a soft-crystal phase with an anticlinic layer organisation. We suggest that mismatching the polarities, conjugation and aspect ratios of mesogenic units is a useful method for generating smectic forming dimesogens.

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Oswald, Patrick, John Bechhoefer et Francisco Melo. « Pattern Formation During the Growth of Liquid Crystal Phases ». MRS Bulletin 16, n^o 1 (janvier 1991) : 38–45. http://dx.doi.org/10.1557/s0883769400057894.

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Liquid crystals, discovered just a century ago, have wide application to electrooptic displays and thermography. Their physical properties have also made them fascinating materials for more fundamental research.The name “liquid crystals” is actually a misnomer for what are more properly termed “mesophases,” that is, phases having symmetries intermediate between ordinary solids and liquids. There are three major classes of liquid crystals: nematics, smectics, and columnar mesophases. In nematics, although there is no correlation between positions of the rodlike molecules, the molecules tend to lie parallel along a common axis, labeled by a unit vector (or director) n. Smectics are more ordered. The molecules are also rodlike and are in layers. Different subtypes of smectics (labeled, for historical reasons, smectic A, smectic B,…) have layers that are more or less organized. In the smectic A phase, the layers are fluid and can glide easily over each other. In the smectic B phase, the layers have hexagonal ordering and strong interlayer corrélations. Indeed, the smectic B phase is more a highly anisotropic plastic crystal than it is a liquid crystal. Finally, columnar mesophases are obtained with disklike molecules. These molecules can stack up in columns which are themselves organized in a two-dimensional array. There is no positional correlation between molecules in one column and molecules in the other columns.

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Heinemann, S., H. Kresse, S. Urban et R. Dabrowski. « Dielectric Investigations in Smectic B-Phases ». Zeitschrift für Naturforschung A 51, n^o 12 (1 décembre 1996) : 1203–8. http://dx.doi.org/10.1515/zna-1996-1210.

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Abstract Several liquid crystalline substances exhibiting SB -phases have been investigated by dielectric and calorimetric measurements. Special attention is taken to the low frequency relaxation process at the phase transition nematic-SB . From the large change of relaxation frequencies, crystallinity of the SB -phases has been concluded. Nevertheless, motion is still present and not frozen.

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Urban, Stanisław, Bo Gestbloma, Roman Dąbrowskib et Jerzy Dziaduszekb. « Study of High Frequency Relaxation Processes in Three Substances with the -CN Groups at the Lateral Positions ». Zeitschrift für Naturforschung A 55, n^o 8 (1 août 2000) : 733–40. http://dx.doi.org/10.1515/zna-2000-0810.

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Abstract Three liquid crystalline (LC) substances with the cyano-groups attached at the lateral positions to the molecular cores were studied with the aid of dielectric spectroscopy methods. The high frequency relaxation process connected with the molecular reorientations around the long molecular axes was studied in the isotropic, nematic and several smectic phases. The dielectric spectra are rather complex, indicating a contribution from several molecular processes to the main relaxation process. The dielectric time changes smoothly at the phase transitions between "liquid-like" phases (isotropic -nematic -smectic A -smectic C), and becomes shorter at the transition to the "solid-like" smectic G phase with a low-ering of the activation barrier. This indicates that the molecules perform broad angle librational motions rather than overall rotational motions in this phase. In case of a substance having two cyano groups at-tached to the benzene ring, a pronounced jump-wise change of the relaxation time was observed at the isotropic -smectic A transition.

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Inoue, Shinobu, Shoji Nishiguchi, Satoshi Murakami, Yoshio Aso, Tetsuo Otsubo, Volkmar Vill, Akira Mori et Seiji Ujiie. « Hexakis(terthiophenylthio)benzene as a Mew Class Liquid Crystalline Molecule ». Journal of Chemical Research 23, n^o 10 (octobre 1999) : 596–97. http://dx.doi.org/10.1177/174751989902301005.

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A disk-like molecule, in which six α-linked terthiophenes are connected to a central benzene core via a thioether linkage, shows a sequence of calamitic liquid crystalline mesomorphism involving smectic C, smectic A and nematic phases.

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CLIMENT-EZQUERRA, BLANCA, et FRANCISCO GUILLÉN-GONZÁLEZ. « A review of mathematical analysis of nematic and smectic-A liquid crystal models ». European Journal of Applied Mathematics 25, n^o 1 (7 octobre 2013) : 133–53. http://dx.doi.org/10.1017/s0956792513000338.

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We review the mathematical analysis of some uniaxial, liquid crystal phases. Firstly, we state the models for the two different studied phases: nematic and smectic-A liquid crystals. The spatial and temporal profiles of the liquid crystal configurations will be described by means of strongly nonlinear parabolic partial differential systems, which are presented at the same time. Then we will state some results about existence, regularity, time-periodicity and stability of solutions at infinite time for both models. It is our aim to show that, although nematic and smectic-A phases have different physical properties and are modelled by different nonlinear parabolic problems, there exists a common mathematical machinery to rewrite the models and obtain analytical results.

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Ma, Qing Lan, Bao Gai Zhai et Yuan Ming Huang. « One-Step Synthesis of a Liquid Crystal TB4A at Room Temperature ». Solid State Phenomena 181-182 (novembre 2011) : 168–72. http://dx.doi.org/10.4028/www.scientific.net/ssp.181-182.168.

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A rod-like liquid crystal (LC) N,N-(terephthalylidene) bis (4-n-butylaniline) (TB4A) was synthesized by one step at room temperature. The synthesized TB4A were investigated with nuclear magnetic resonance, fourier transform infrared spectrometer and ultraviolet-visible spectrophotometry, differential scanning calorimetry and polarizing optical microscopy. Our results demonstrated that the TB4A can form nematic, smectic A, smectic C, smectic F and smectic G LC phases.

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Tan, Shuai, Bingzhuo Wei, Ting Liang, Xiaohui Yang et Yong Wu. « Anhydrous proton conduction in liquid crystals containing benzimidazole moieties ». RSC Advances 6, n^o 40 (2016) : 34038–42. http://dx.doi.org/10.1039/c6ra03375j.

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Würflinger, Albert, M. Massalska-Arodż, S. Urbanb, W. Weissflog et S. Ernst. « Thermodynamic Measurements on the Binary System Bis{[2,2-di(n- hexyloxycarbonyl)ethenyl]phenyl} Biphenyl-4,4'-dicarboxylate and 4-n-Octyloxyphenyl 4-n-Pentyloxybenzoate at Elevated Pressures ». Zeitschrift für Naturforschung A 56, n^o 9-10 (1 octobre 2001) : 658–62. http://dx.doi.org/10.1515/zna-2001-0909.

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AbstractThermodynamic measurements have been carried out on the binary system of a swallow-tailed liquid crystal A = bis{[2,2-di(n-hexyloxycarbonyl)ethenyl]phenyl} biphenyl-4,4'-dicarboxylate (4DS6) and B = 4-n-octyloxyphenyl 4-rc-pentyloxybenzoate (5O/O8). A filled smectic A phase is induced in the concentration range 0.11 < xA < 0.75. The maximum of the SmA-N transition temperature (365 K) is observed for xA= 0.33. Dilatometric measurements show that the packing density in the filled phases is increased. High-pressure DTA is employed in order to investigate the coexistence range of the liquid crystalline phases and the slopes of the transition lines. The slopes of the melting curves for the mixtures are significantly smaller than those for the pure components. The slopes for the smectic - nematic - isotropic transitions are considerably larger. The coexistence range for the (smectic + nematic) phases does not seem to be pressure-limited, rather it increases with pressure.

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Izzo, Dora, et Mário J. de Oliveira. « Landau theory for uniaxial nematic, biaxial nematic, uniaxial smectic-A, and biaxial smectic-A phases ». Liquid Crystals 43, n^o 9 (31 mars 2016) : 1230–36. http://dx.doi.org/10.1080/02678292.2016.1164253.

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Izzo, Dora, et Mário J. De Oliveira. « Landau theory for isotropic, nematic, smectic-A, and smectic-C phases ». Liquid Crystals 47, n^o 1 (20 juin 2019) : 99–105. http://dx.doi.org/10.1080/02678292.2019.1631968.

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Mirzaei, Javad, Martin Urbanski, Heinz-S. Kitzerow et Torsten Hegmann. « Hydrophobic gold nanoparticles via silane conjugation : chemically and thermally robust nanoparticles as dopants for nematic liquid crystals ». Philosophical Transactions of the Royal Society A : Mathematical, Physical and Engineering Sciences 371, n^o 1988 (13 avril 2013) : 20120256. http://dx.doi.org/10.1098/rsta.2012.0256.

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We examine for the first time how chemically and thermally stable gold nanoparticles (NPs), prepared by a silane conjugation approach, affect both the thermal and the electro-optical properties of a nematic liquid crystal (LC), when doped at concentrations ranging from 0.25 to 7.5 wt%. We find that the octadecylsilane-conjugated gold NPs stabilize both the enantiotropic nematic and the monotropic smectic-A phases of the LC host with a maximum stabilization of 2 ° C for the nematic and 3.5 ° C for the smectic-A phases for the mixture containing 1 wt% of the silanized particles. The same mixture shows the lowest values for the Fréedericksz transition threshold voltage and the highest value for the dielectric anisotropy. Generally, all NP-containing mixtures, except mixtures with NP concentrations exceeding 5 wt%, reduce the threshold voltage, increase the dielectric anisotropy and reduce both rise and decay time; the latter particularly at temperatures at least 10 ° C below the isotropic–nematic phase transition on cooling.

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DONG, RONALD Y. « RECENT DEVELOPMENTS IN BIAXIAL LIQUID CRYSTALS : AN NMR PERSPECTIVE ». International Journal of Modern Physics B 24, n^o 24 (30 septembre 2010) : 4641–82. http://dx.doi.org/10.1142/s0217979210056487.

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A survey of recent studies of biaxial liquid crystals (LCs), whose nematic and/or smectic-A phases do not possess optical uniaxiality (viz., more than one optical axis exists), is given in this review. In particular, we emphasize on how Nuclear Magnetic Resonance (NMR) spectroscopy can help to advance the understanding of phase biaxiality in general, and to examine recent debates on the existence of biaxial nematic phase reported in low molecular mass bent-core or V-shaped mesogens. A general discussion of orientational order parameters is detailed, particularly in smectic-C (SmC) and biaxial nematic phases. How these orientational order parameters can be determined by various techniques such as NMR, IR absorbance and Raman scattering studies, will be mentioned. Recent X-ray observations of SmC clusters in the nematic phase of V-shaped mesogens are highlighted and contrasted with probable theory. Moreover, deuterium and carbon-13 NMR techniques are briefly reviewed, and their possible utilization to identify phase biaxiality in these biaxial LC systems is explored.

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Kaur, Supreet, Vidhika Punjani, Golam Mohiuddin et Santanu Kumar Pal. « Orthogonal smectic and nematic ordering in three-ring polar bent-core molecules with anti-parallel arrangement ». New Journal of Chemistry 41, n^o 13 (2017) : 5403–11. http://dx.doi.org/10.1039/c7nj00272f.

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Bagdinova, Anna N., Evgeny I. Demikhov, Nataliya G. Borisenko, Sergei M. Tolokonnikov, Gennadii V. Mishakov et Andrei V. Sharkov. « Periodic structures on liquid-phase smectic A, nematic and isotropic free surfaces ». Beilstein Journal of Nanotechnology 9 (30 janvier 2018) : 342–52. http://dx.doi.org/10.3762/bjnano.9.34.

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The free boundary of smectic A (SmA), nematic and isotropic liquid phases were studied using a polarized optical microscope, an interferometric surface structure analyzer (ISSA), an atomic force microscope (AFM) and a scanning near-field optical microscope (SNOM). Images of the SmA phase free surface obtained by the polarized microscope and ISSA are in good correlation and show a well-known focal domain structure. The new periodic stripe structure was observed by scanning near-field optical microscopy on the surface of the smectic A, nematic and isotropic phases. The properties of this periodic structure are similar to the charged liquid helium surface and can be explained by nonlinear electrostatic instabilities previously described.

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Bogdanov, Alexey V., Gleb I. Proniuk et Andrey Kh Vorobiev. « Magnetic field effects in nematic and smectic liquid crystals probed by time resolved observation of orientation relaxation of the spin probe ». Physical Chemistry Chemical Physics 20, n^o 27 (2018) : 18340–47. http://dx.doi.org/10.1039/c8cp01753k.

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Kapernaum, Nadia, Friederike Knecht, C. Scott Hartley, Jeffrey C. Roberts, Robert P. Lemieux et Frank Giesselmann. « Formation of smectic phases in binary liquid crystal mixtures with a huge length ratio ». Beilstein Journal of Organic Chemistry 8 (19 juillet 2012) : 1118–25. http://dx.doi.org/10.3762/bjoc.8.124.

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A system of two liquid-crystalline phenylpyrimidines differing strongly in molecular length was studied. The phase diagram of these two chemically similar mesogens, with a length ratio of 2, was investigated, and detailed X-ray diffraction and electrooptical measurements were performed. The phase diagram revealed a destabilization of the nematic phase, which is present in the pure short compound, while the smectic state was stabilized. The short compound forms smectic A and smectic C phases, whereas the longer compound forms a broad smectic C phase and a narrow higher-ordered smectic phase. Nevertheless, in the mixtures, the smectic C phase is destabilized and disappears rapidly, whereas smectic A is the only stable phase observed over a broad concentration range. In addition, the smectic translational order parameters as well as the tilt angles of the mixtures are reduced. The higher-ordered smectic phase of the longer mesogen was identified as a smectic F phase.

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Kasch, N., I. Dierking, M. Turner, P. Romero-Hasler et E. A. Soto-Bustamante. « Liquid crystalline textures and polymer morphologies resulting from electropolymerisation in liquid crystal phases ». Journal of Materials Chemistry C 3, n^o 31 (2015) : 8018–23. http://dx.doi.org/10.1039/c5tc01639h.

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A small fraction of an acrylate liquid crystalline monomer (≤5%) is mixed into nematic and smectic liquid crystalline phases, and polymerised through the application of a voltage (electropolymerisation).

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Kuijk, Anke, Thomas Troppenz, Laura Filion, Arnout Imhof, René van Roij, Marjolein Dijkstra et Alfons van Blaaderen. « Effect of external electric fields on the phase behavior of colloidal silica rods ». Soft Matter 10, n^o 33 (2014) : 6249–55. http://dx.doi.org/10.1039/c4sm00957f.

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Pusiol, D. J., R. Humpfer et F. Noack. « Nitrogen Nuclear Quadrupole Resonance Dips in the Proton Spin Relaxation Dispersion of Nematic and Smectic Thermotropic Liquid Crystals ». Zeitschrift für Naturforschung A 47, n^o 11 (1 novembre 1992) : 1105–14. http://dx.doi.org/10.1515/zna-1992-1102.

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Abstract The Larmor frequency dependence of the proton spin relaxation time, obtained by means of the fast field-cycling NMR technique, has been used to study the 14N quadrupole coupling constant K and its asymmetry parameter η in the nematic and smectic phases of some liquid crystalline azoxybenzenes (PAA, BAB, HAB, HpAB), cyanobiphenyls (8CB, 9CB, 11CB) and oxycyanobiphenyls (9 OCB). Due to fast molecular reorientations, the effective quadrupole coupling constants are relatively small, whereas surprisingly the asymmetry parameters are rather large. The temperature dependence of both K and η within the mesophases, as well as their discontinuities at the different mesophase transitions, can be interpreted by the anisotropy of molecular rotations. It is found that temperature effects are significantly more pronounced for the (biaxial) smectic-C phase of the heptyloxyazoxybenzene (HpAB) than for the (uniaxial) smectic-A phase of the various investigated cyano- and oxycyanobiphenyls. As a rule, η turned out smaller in the smectic than in the nematic state, whereas K has similar values in both phases

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Shanker, Govindaswamy, Marko Prehm et Carsten Tschierske. « Liquid-crystalline heterodimesogens and ABA-heterotrimesogens comprising a bent 3,5-diphenyl-1,2,4-oxadiazole central unit ». Beilstein Journal of Organic Chemistry 8 (30 mars 2012) : 472–85. http://dx.doi.org/10.3762/bjoc.8.54.

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Three new types of terminally connected ABA-heterotrimesogens and heterodimesogens, composed of a bent 3,5-diphenyl-1,2,4-oxadiazole central unit and one or two rod-shaped 4-cyanobiphenyl cores or one 2-phenyl-1,3,4-thiadiazole core, connected by flexible spacers, have been synthesized, and their mesomorphic behavior was studied by optical polarizing microscopy (PM), differential scanning calorimetry (DSC) and X-ray diffraction (XRD). All dimesogens exhibit broad ranges of cybotactic nematic phases (NcybA and NcybC), in some cases accompanied by additional mesophases (CybA or SmC) at lower temperature. The combination of the 3,5-diphenyl-1,2,4-oxadiazole unit with one cyanobiphenyl core leads to the removal of tilted smectic and cybotactic nematic phases (SmC, NcybC), which are replaced by the nontilted CybA phases and nematic phases composed of SmA-type clusters (NcybA). The orthogonal cybotactic nematic phases of bent-core mesogens are of special interest for achieving biaxial nematic phases of the orthorhombic type. The orthogonal (NcybA) and skewed (NcybC) cybotactic nematic phases were distinguished by XRD and optical observations.

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Sigaud, G., M. F. Achard, M. Mauzac, H. Richard, H. T. Nguyen, F. Hardouin et H. Gasparoux. « Phase separation in nematic and smectic a phases ». Phase Transitions 33, n^o 1-4 (avril 1991) : 249–51. http://dx.doi.org/10.1080/01411599108207735.

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Kishikawa, Keiki, Takahiro Inoue, Naoshi Hasegawa, Masahiro Takahashi, Michinari Kohri, Tatsuo Taniguchi et Shigeo Kohmoto. « Achiral straight-rod liquid crystals indicating local biaxiality and ferroelectric switching behavior in the smectic A and nematic phases ». Journal of Materials Chemistry C 3, n^o 15 (2015) : 3574–81. http://dx.doi.org/10.1039/c5tc00389j.

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Suh, Ahram, Hyungju Ahn, Tae Joo Shin et Dong Ki Yoon. « Controllable liquid crystal defect arrays induced by an in-plane electric field and their lithographic applications ». Journal of Materials Chemistry C 7, n^o 6 (2019) : 1713–19. http://dx.doi.org/10.1039/c8tc06042h.

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Walker, Rebecca, Damian Pociecha, Alfonso Martinez-Felipe, John MD Storey, Ewa Gorecka et Corrie T. Imrie. « Twist-Bend Nematogenic Supramolecular Dimers and Trimers Formed by Hydrogen Bonding ». Crystals 10, n^o 3 (6 mars 2020) : 175. http://dx.doi.org/10.3390/cryst10030175.

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A selection of novel non-symmetric supramolecular liquid crystal dimers and trimers formed by hydrogen-bonding have been prepared and their phase behaviour characterised by polarised optical microscopy, X-ray diffraction and temperature-dependent Fourier-transform infrared spectroscopy. We mix the bent twist-bend nematogen 4-[6-(4’-cyanobiphenyl-4-yl) hexyloxy]-benzoic acid (CB6OBA) with a series of small stilbazole-based compounds 4-[(E)-2-(n-alkoxyphenyl)ethenyl]pyridines (nOS) of varying terminal chain length (n) to obtain the CB6OBA:nOS complexes. Complexes with n ≤ 7 exhibit nematic and twist-bend nematic behaviour, followed on cooling by a smectic C phase for n = 4–7, and finally, a hexatic-type smectic X phase for n = 3–7. Mixtures with n = 8–10 exhibit a smectic A phase below the conventional nematic phase, and on further cooling, a biaxial smectic Ab phase and the same hexatic-type SmX phase. Supramolecular trimers, CB6OBA:CB6OS and CB6OBA:1OB6OS, formed between CB6OBA and dimeric stilbazoles [(E)-2-(4-{[6-(4’-methoxy[1,1’-biphenyl]-4-yl)hexyl]oxy}phenyl)ethenyl]pyridine (1OB6OS) or 4-[(E)-4’-(6-{4-[(E)-2-(pyridin-4-yl)ethenyl]phenoxy}hexyl)[1,1’-biphenyl]-4- carbonitrile (CB6OS), exhibit nematic and twist-bend nematic phases, and are the first hydrogen-bonded trimers consisting of unlike donor and acceptor fragments to do so.

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Urban, Stanisław, et Albert Würflinger. « High Pressure Dielectric Studies of a Substance with the Smectic A1 Phase ». Zeitschrift für Naturforschung A 54, n^o 8-9 (1 septembre 1999) : 455–58. http://dx.doi.org/10.1515/zna-1999-8-901.

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The results of dielectric studies of 5-n-hexyl-2-(4'-isothiocyanato)-l,3-dioxane (6DBT) in the smectic A, phase at pressures up to 150 MPa and temperatures up to 340 K are presented. The low frequen-cy relaxation time τ|| (p, T) yields the activation volume Δ# V|| = RT(∂|| In τ||/p)T and activation enthalpy Δ# H|| = R(∂In τ||/∂T-1)p. The calculated values of these parameters are compared with those ob-tained recently for n-octyl-cyanobiphenyl (8CB) forming the smectic Ad phase. In the case of 6DBT both these quantities are practically constant, whereas those for 8CB decrease, when going away from the phase transitions isotropic -smectic A1 (6DBT) or nematic -smectic Ad (8CB). These differences support our earlier conclusion that increase of pressure leads to a breaking of the antiparallel associa-tions of cyanobiphenyl molecules in the smectic as well as in the nematic phases.

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Latha, D. Madhavi, V. G. K. M. Pisipati, M. Ramakrishna Nanachara Rao et P. V. Datta Prasad. « Experimental Investigations on Ultrasonic Velocity and Behavior of Thermodynamic Parameters of Four Members of nO.m Series ». Solid State Phenomena 181-182 (novembre 2011) : 43–46. http://dx.doi.org/10.4028/www.scientific.net/ssp.181-182.43.

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Ultrasonic velocity measurements are carried out on fournOmcompounds viz., N-(p-n-methoxy and ethoxy benzylidene)-p-n-dodecyl and tetradecyl anilines, 1O.12, 1O.14, 2O.12 and 2O.14. The first three compounds exhibit mono variant nematic phase while the last compound exhibits nematic and smectic-A phases with variable thermal ranges. The temperature dependence of sound velocity, U shows minima at the isotropic-nematic and nematic-smectic-A transitions. In conjunction with the density data of these compounds, the thermo acoustic parameters such as adiabatic compressibility (βad), the molar sound velocity (Rao’s number) (Rn), molar compressibility (W), molecular free length (Lf) and available volume (Va) are estimated. In addition the intermolecular free length (Lf) is estimated in three different ways employing density and ultrasonic velocity data. The results are compared with the data available in literature of other compounds.

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Urban, Stanisław, Joanna Czub et Bo Gestblom. « Comparison of Dielectric Properties of Three Alkyl and Alkoxy Azoxybenzenes (nAOBs and nOAOBs, n = 5, 6, 7) in the Isotropic and Liquid Crystalline Phases ». Zeitschrift für Naturforschung A 59, n^o 10 (1 octobre 2004) : 674–82. http://dx.doi.org/10.1515/zna-2004-1009.

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Results of dielectric studies of three 4,4’-di-n-alkyl-azoxybenzenes (nAOB) and 4,4’-di-n-alkoxyazoxybenzenes (nOAOB), n = 5, 6 and 7, are presented. The nAOB compounds exhibit positive, and the nOAOBs negative dielectric anisotropy in the nematic phase. In the smectic A phase of 7AOB the anisotropy changes its sign, whereas in the smectic C phase of 7OAOB it persists negative; moreover, in the latter case the perpendicular component of the permittivity increases with respect to the value of the nematic phase. This may be due to parallel dipole-dipole correlation of the perpendicular components involved by the layer arrangements in both smectic phases. For all substances a systematic decrease of the permittivity components with chain length was observed. Different types of smectic order in the 7th members of both series are discussed in relation to the mean-field theory developed recently by Govind and Madhusudana. The dielectric relaxation times and activation enthalpies characterizing the molecular rotation around the principal inertia axes are given.

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Koßmehl, Gerhard, et Barbara Hirsch. « Liquid Crystalline Compounds in the Thiophene Series, Part 11+ Liquid Crystalline Thiophene Carboxylic Esters ». Zeitschrift für Naturforschung B 50, n^o 8 (1 août 1995) : 1265–74. http://dx.doi.org/10.1515/znb-1995-0822.

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The new series of ten 4,4'-bis(5-alkyl-2-thienylcarbonyloxy)azobenzenes (2a -2j) and ten 2,5-bis(5-alkyl-2-thienylcarbonyloxy)toluenes (3a -3j) were prepared. These, as yet unknown esters were characterized in relation to their structures by elemental analyses, IR, UV, 1H NMR spectra and MS. The azobenzene-esters 2a -2j which form liquid crystalline phases were studied by microscopy under polarized light and DSC (differential scanning calorimetry). Through the observation of the optical textures nematic phases were monitored. Only 2j has an additional smectic phase; 2h and 2i form monotropic smectic phases during cooling. The toluene-esters 3a -3j do not form liquid crystalline phases.

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Pidwani, D. T., G. N. Bhola et Umed C. Bhoya. « Mesomorphism Dependence on Molecular Flexibility at Constant Rigidity ». International Letters of Chemistry, Physics and Astronomy 50 (mai 2015) : 1–8. http://dx.doi.org/10.18052/www.scipress.com/ilcpa.50.1.

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A Novel homologous series of high transition temperature RO-C6H4-CH=CH-COO-C6H4-N=CH-CH=CH-C6H5 has been synthesis and studied with a view to understand establish and evaluate the relation between liquid crystal properties and the molecular structure of a substance. A series consists of thirteen homologues. All the homologues are mesogenic. Smectic mesophase commences from C12 to C18 homologues. Nematic mesophase appears from C1 to C18 homologues. The textures of a nematic phases are of threaded or schlieren and that of the smectic phase are focal conic fan shaped of smectic-A or C type. Transition temperatures and textures were determined by an optical polarizing microscopy equipped with a heating stage. The analytical and spectral data confirms the molecular structures of homologues. Thermal stability for smectic and nematic are 118.5 0C and 224.77 °C, whose total mesophase length ranger from 73.0 °C to 111 °C. The liquid crystal properties from homologue to homologue varies in the same series. Some liquid crystal properties of present series are compared with the structurally similar known homologous series. Present novel homologous series is partly smectogenic and fully nematogenic and of higher ordered melting type and high thermal stability

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Pidwani, D. T., G. N. Bhola et Umed C. Bhoya. « Mesomorphism Dependence on Molecular Flexibility at Constant Rigidity ». International Letters of Chemistry, Physics and Astronomy 50 (3 mai 2015) : 1–8. http://dx.doi.org/10.56431/p-sboc5i.

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A Novel homologous series of high transition temperature RO-C6H4-CH=CH-COO-C6H4-N=CH-CH=CH-C6H5 has been synthesis and studied with a view to understand establish and evaluate the relation between liquid crystal properties and the molecular structure of a substance. A series consists of thirteen homologues. All the homologues are mesogenic. Smectic mesophase commences from C12 to C18 homologues. Nematic mesophase appears from C1 to C18 homologues. The textures of a nematic phases are of threaded or schlieren and that of the smectic phase are focal conic fan shaped of smectic-A or C type. Transition temperatures and textures were determined by an optical polarizing microscopy equipped with a heating stage. The analytical and spectral data confirms the molecular structures of homologues. Thermal stability for smectic and nematic are 118.5 0C and 224.77 °C, whose total mesophase length ranger from 73.0 °C to 111 °C. The liquid crystal properties from homologue to homologue varies in the same series. Some liquid crystal properties of present series are compared with the structurally similar known homologous series. Present novel homologous series is partly smectogenic and fully nematogenic and of higher ordered melting type and high thermal stability

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Markwick, Phineus, Stanislaw Urban et Albert Würflinger. « High Pressure Dielectric Studies of 8CB in the Isotropic, Nematic, and Smectic A Phases ». Zeitschrift für Naturforschung A 54, n^o 5 (1 mai 1999) : 275–80. http://dx.doi.org/10.1515/zna-1999-0501.

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The static and complex permittivity of 4-n-octyl-4'-cyanobiphenyl (8CB) has been measured for the isotropic, nematic and smectic A phases as functions of temperature and pressure. The ranges of 297 - 361 K, 0.1 - 220 MPa, and 0.1 - 13 MHz, were covered. Only the parallel component of the complex permittivity, ɛ*( f ) = ɛ' ( f ) - iɛ"(f), was measured. The relaxation times T||(p, T) as well as Tis(p, T) were analysed at constant temperature, pressure and volumes yielding the activation volume, Δ ≠V(T), activation enthalpy Δ≠H(p), and activation energy Δ≠U(V), respectively. All activation parameters calculated for the smectic A phase of 8CB are smaller than those obtained for the nematic phase. The activation energy constitutes approximately half of the activation enthalpy value in all three phases studied. The pressure study allowed to calculate the pressure dependence of the retardation factor g|| (p, T), from which the nematic potential q(p,T) can be derived. Using the relationships between g\\ and q/ RT proposed by Kalmykov and Coffey, the order parameter (P2(p, T)) was calculated as a function of pressure.

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Dunmur, D. A., S. P. Farnan et C. I. Murray. « Dielectric relaxation in coexisting nematic and smectic B phases ». Liquid Crystals 19, n^o 6 (décembre 1995) : 779–83. http://dx.doi.org/10.1080/02678299508031099.

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Hussain, Zakir, Henning Hopf et S. Holger Eichhorn. « Perhydroazulene-based liquid-crystalline materials with smectic phases ». Beilstein Journal of Organic Chemistry 8 (16 mars 2012) : 403–10. http://dx.doi.org/10.3762/bjoc.8.44.

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New liquid-crystalline materials with a perhydroazulene core were synthesized and the stereochemistry of these compounds was investigated. The mesomorphic properties of the new LC compounds were investigated by differential scanning colorimetry, polarizing optical microscopy and X-ray diffraction. We report here on the LC properties of nonchiral materials, which predominantly exhibit smectic phases and display nematic phases only within narrow temperature ranges. The dependence of the mesogenic behavior of the new materials on the stereochemistry of the core system was also investigated. All newly synthesized compounds were fully characterized by the usual spectroscopic and analytical methods.

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Rao, N. V. S., et V. G. K. M. Pisipati. « Smectic C to Nematic Phase Transition Studies in NOBA ». Zeitschrift für Naturforschung A 40, n^o 5 (1 mai 1985) : 466–68. http://dx.doi.org/10.1515/zna-1985-0507.

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The variation of density ϱ with temperature of p-n-nonyloxy benzoic acid in its isotropic, nematic and smectic-C phases is presented. The density jumps and the computed thermal expansion coefficient maxima suggest the order of the N - SC and N - I transitions to be first order. The pressure dependence of the transition temperatures as computed from the Clausius- Clapeyron equation and the presence of pretransitional effects at the N - SC and N - I transitions in the nematic phase are discussed.

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Cuetos, Alejandro, Matthew Dennison, Andrew Masters et Alessandro Patti. « Phase behaviour of hard board-like particles ». Soft Matter 13, n^o 27 (2017) : 4720–32. http://dx.doi.org/10.1039/c7sm00726d.

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We examine the phase behaviour of colloidal suspensions of hard board-like particles (HBPs) as a function of their shape anisotropy, and observe a fascinating spectrum of nematic, smectic, and columnar liquid-crystalline phases, whose formation is entirely driven by excluded volume effects.

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Funahashi, Masahiro. « Chiral Liquid Crystalline Electronic Systems ». Symmetry 13, n^o 4 (13 avril 2021) : 672. http://dx.doi.org/10.3390/sym13040672.

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Liquid crystals bearing extended π-conjugated units function as organic semiconductors and liquid crystalline semiconductors have been studied for their applications in light-emitting diodes, field-effect transistors, and solar cells. However, studies on electronic functionalities in chiral liquid crystal phases have been limited so far. Electronic charge carrier transport has been confirmed in chiral nematic and chiral smectic C phases. In the chiral nematic phase, consisting of molecules bearing extended π-conjugated units, circularly polarized photoluminescence has been observed within the wavelength range of reflection band. Recently, circularly polarized electroluminescence has been confirmed from devices based on active layers of chiral conjugated polymers with twisted structures induced by the molecular chirality. The chiral smectic C phase of oligothiophene derivatives is ferroelectric and indicates a bulk photovoltaic effect, which is driven by spontaneous polarization. This bulk photovoltaic effect has also been observed in achiral polar liquid crystal phases in which extended π-conjugated units are properly assembled. In this manuscript, optical and electronic functions of these chiral π-conjugated liquid crystalline semiconductors are reviewed.

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Fernández-Rico, Carla, Massimiliano Chiappini, Taiki Yanagishima, Heidi de Sousa, Dirk G. A. L. Aarts, Marjolein Dijkstra et Roel P. A. Dullens. « Shaping colloidal bananas to reveal biaxial, splay-bend nematic, and smectic phases ». Science 369, n^o 6506 (20 août 2020) : 950–55. http://dx.doi.org/10.1126/science.abb4536.

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Understanding the impact of curvature on the self-assembly of elongated microscopic building blocks, such as molecules and proteins, is key to engineering functional materials with predesigned structure. We develop model “banana-shaped” colloidal particles with tunable dimensions and curvature, whose structure and dynamics are accessible at the particle level. By heating initially straight rods made of SU-8 photoresist, we induce a controllable shape deformation that causes the rods to buckle into banana-shaped particles. We elucidate the phase behavior of differently curved colloidal bananas using confocal microscopy. Although highly curved bananas only form isotropic phases, less curved bananas exhibit very rich phase behavior, including biaxial nematic phases, polar and antipolar smectic-like phases, and even the long-predicted, elusive splay-bend nematic phase.

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Śliwa, Izabela, et A. V. Zakharov. « Structural, Optical and Dynamic Properties of Thin Smectic Films ». Crystals 10, n^o 4 (20 avril 2020) : 321. http://dx.doi.org/10.3390/cryst10040321.

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The problem of predicting structural and dynamic behavior associated with thin smectic films, both deposited on a solid surface or stretched over an opening, when the temperature is slowly increased above the bulk transition temperature towards either the nematic or isotropic phases, remains an interesting one in the physics of condensed matter. A useful route in studies of structural and optical properties of thin smectic films is provided by a combination of statistical–mechanical theories, hydrodynamics of liquid crystal phases, and optical and calorimetric techniques. We believe that this review shows some useful routes not only for the further examining of the validity of a theoretical description of thin smectic films, both deposited on a solid surface or stretched over an opening, but also for analyzing their structural, optical, and dynamic properties.

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Burnell, E. Elliott, Ronald Y. Dong, Adrian C. J. Weber, Xuan Yang et Anand Yethiraj. « Separation of nematic and smectic-A potential effects on solute ordering in a series of binary 6OCB–8OCB mixtures ». Canadian Journal of Chemistry 89, n^o 7 (juillet 2011) : 900–908. http://dx.doi.org/10.1139/v11-027.

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1H NMR and optical microscopy have been used to study four solutes dissolved in samples of a binary mixture of 4-n-hexyloxy-4′-cyanobiphenyl (6OCB) and 4-n-octyloxy-4′-cyanobiphenyl (8OCB) that are close to the region of the phase diagram where the smectic-A (SmA) and re-entrant nematic (RN) phases exist. Optical microscopy clearly indicates that one of the four studied samples shows the phase sequence of isotropic–nematic–SmA–RN. The derived solute order parameters were interpreted by means of two Maier–Saupe mechanisms in conjunction with the Kobayashi–McMillan theory in the case of the SmA phase. The novel feature of this study is that the nematic potential is extrapolated from the nematic to the SmA phase based on concentrations of the 6OCB–8OCB mixtures. The derived smectic order parameters for each of the studied solutes clearly show a maximum absolute magnitude near the centre of the SmA temperature range. The different partitioning of these solutes in the binary mixture is also discussed.

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Aouini, Abir, Maurizio Nobili, Edouard Chauveau, Philippe Dieudonné-George, Gauthier Damême, Daniel Stoenescu, Ivan Dozov et Christophe Blanc. « Chemical-Physical Characterization of a Binary Mixture of a Twist Bend Nematic Liquid Crystal with a Smectogen ». Crystals 10, n^o 12 (4 décembre 2020) : 1110. http://dx.doi.org/10.3390/cryst10121110.

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Nematic twist-bend phases (NTB) are new types of nematic liquid crystalline phases with attractive properties for future electro-optic applications. However, most of these states are monotropic or are stable only in a narrow high temperature range. They are often destabilized under moderate cooling, and only a few single compounds have shown to give room temperature NTB phases. Mixtures of twist-bend nematic liquid crystals with simple nematogens have shown to strongly lower the nematic to NTB phase transition temperature. Here, we examined the behaviour of new types of mixtures with the dimeric liquid crystal [4′,4′-(heptane-1,7-diyl)bis(([1′,1″-biphenyl]4″-carbo-nitrile))] (CB7CB). This now well-known twist-bend nematic liquid crystal presents a nematic twist-bend phase below T ≈ 104 °C. Mixtures with other monomeric alkyl or alkoxy -biphenylcarbonitriles liquid crystals that display a smectic A (SmA) phase also strongly reduce this temperature. The most interesting smectogen is 4′-Octyl-4-biphenylcarbonitrile (8CB), for which a long-term metastable NTB phase is found at room and lower temperatures. This paper presents the complete phase diagram of the corresponding binary system and a detailed investigation of its thermal, optical, dielectric, and elastic properties.

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Hare, Sean M., Beatrice Lunsford-Poe, MinSu Kim et Francesca Serra. « Chiral Liquid Crystal Lenses Confined in Microchannels ». Materials 13, n^o 17 (26 août 2020) : 3761. http://dx.doi.org/10.3390/ma13173761.

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It is known that the liquid crystalline smectic-A phase has geometric defects, called focal conic domains, which can be used as gradient-index microlenses. Cholesteric (chiral nematic) phases also have topological defects with a central symmetry and a singularity at their center. We explore a weakly chiral system in which both types of defects can be present in the same material at different temperatures, and with this strategy we create lenses whose focal length is tunable with temperature. We measure the focal length of the tunable lenses, and we investigate the behavior of the defects near the phase transition. We identify the experimental conditions that make the simultaneous presence of the smectic focal conic domains and the circular cholesteric domains possible, such as the concentration of chiral dopant and the rate of heating and cooling. The transformation of focal conic domains into circular cholesteric domains is a new example of memory at the phase transition between smectic-A and nematic liquid crystals.

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