Pentaerythritol Derived Tetrapode Exhibiting a Nematic‐Like Mesophase at Ambient Temperatures
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Abstract:
The nematic liquid-crystalline phase exhibits average orientational order, with no positional organisation. So-called modulated nematic phases exhibit this same orientational order with an additional spatially periodic modulation of the nematic director, the most common of which is the twist-bend nematic phase. We report a pentaerythritol derived tetrapode which exhibits a nematic-like mesophase at ambient temperature, and we denote this new mesophase 'NX ' to indicate a nematic phase of unknown structure. X-ray scattering experiments refute the possibility of positional order, yet optical textures are consistent with a periodic structure. We suggest that the mesophase exhibited by this material is a new type of nematic-like mesophase with some form of modulated structure. We find the NX phase to exhibit an electrooptic response consistent with a nematic-like phase.Keywords:
Pentaerythritol
Biaxial nematic
The nematic liquid-crystalline phase exhibits average orientational order, with no positional organisation. So-called modulated nematic phases exhibit this same orientational order with an additional spatially periodic modulation of the nematic director, the most common of which is the twist-bend nematic phase. We report a pentaerythritol derived tetrapode which exhibits a nematic-like mesophase at ambient temperature, and we denote this new mesophase 'NX ' to indicate a nematic phase of unknown structure. X-ray scattering experiments refute the possibility of positional order, yet optical textures are consistent with a periodic structure. We suggest that the mesophase exhibited by this material is a new type of nematic-like mesophase with some form of modulated structure. We find the NX phase to exhibit an electrooptic response consistent with a nematic-like phase.
Pentaerythritol
Biaxial nematic
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A series of new rod-shaped liquid crystal, 6-methoxy-2-(4-alkyloxybenzylidenamino) benzothiazoles possessingeven numbers of carbon atoms at the alkyloxy chain (C n H 2n+1 O-, n = 10, 12, 14, 16, 18) are prepared and characterized. The phase behaviour of these new compounds was studied by differential scanning calorimetry and polarising optical microscopy. All members exhibited mesophase. Influence of alkyl chain length on the mesomorphic properties was studied. Enantiotropic nematic phase was identified in all the members except for n = 18. Lower member (n = 10) exhibited monotropic (metastable) smectic A phase. The enantiotropic smectic A phase was observed from n -dodecyloxy derivatives onward to the last homologous synthesized.
Homologous series
Metastability
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Ten homologues of a new class of asymmetric bent-shaped compounds have been synthesized. Their mesophase behaviour was investigated by polarizing microscopy, X-ray diffraction, dielectric and electro-optical measurements. The long-chain members of the series C8–C16 exhibit the switchable antiferroelectric SmCP phase. The short-chain members C4–C8 form a nematic phase. The homologues C5 and C6 show a transition from the nematic into a low-temperature phase which is indicated by a sharp peak in the DSC trace. Whereas the textures point to a nematic–smectic transition, the X-ray studies definitely prove a structure without any long range (or quasi long range) positional order in the low-temperature phase. A molecular model is described which exhibits structural features similar to those of a NCol phase, which can be regarded as the precursor of the following columnar phase (B1 phase).
Antiferroelectricity
Biaxial nematic
Columnar phase
Polarized light microscopy
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Abstract Orientational order parameters have been determined for the mixture of hexyloxy cyanobiphenyl (60CB) and octyloxy cyanobiphenyl (80CB) for three different compositions containing molar concentration 0.289, 0.337 and 0.401 of 60CB over the complete mesomorphic ranges. Both x-ray diffraction and refractive index measurements on aligned samples have been used for order parameter calculations. The experimental order parameter values show a small enhancement with decreasing temperature in the smectic phase, but decrease on entering the re-entrant nematic phase. Smectic fluctuations were observed even in the nematic phase of the mixtures not having any smectic or re-entrant nematic phase. Previous order parameter values obtained from NMR studies are much smaller than ours. However, since this NMR measurement used a rather small molecule (p-xylene) as a probe, the ordering of these probe molecules may be less than that of the liquid crystalline mesophase studied.
Biaxial nematic
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Nematic phases are some of the most common phases among the lyotropic liquid crystalline structures. They have been widely investigated during last decades. In early studies, two uniaxial nematic phases (discotic, ND, and calamitic, NC) were identified. After the discovery of the third one, named biaxial nematic phase (NB) in 1980, however, some controversies in the stability of biaxial nematic phases began and still continue in the literature. From the theoretical point of view, the existence of a biaxial nematic phase is well established. This review aims to bring information about the historical development of those phases considering the early studies and then summarize the recent studies on how to stabilize different nematic phases from the experimental conditions, especially, choosing the suitable constituents of lyotropic mixtures.
Lyotropic
Biaxial nematic
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This article gives an overview on recent developments concerning the twist-bend nematic phase. The twist-bend nematic phase has been discussed as the missing link between the uniaxial nematic mesophase (N) and the helical chiral nematic phase (N*). After an introduction discussing the key physical properties of the NTB phase and the methods used to identify the twist-bend nematic mesophase this review focuses on structure property relationships and molecular features that govern the incidence of this phase.
Biaxial nematic
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To elucidate induced smectic A and smectic B phases in binary nematic liquid crystal mixtures, a generalized thermodynamic model has been developed in the framework of a combined Flory–Huggins free energy for isotropic mixing, Maier–Saupe free energy for orientational ordering, McMillan free energy for smectic ordering, Chandrasekhar–Clark free energy for hexagonal ordering, and phase field free energy for crystal solidification. Although nematic constituents have no smectic phase, the complexation between these constituent liquid crystal molecules in their mixture resulted in a more stable ordered phase such as smectic A or B phases. Various phase transitions of crystal–smectic, smectic–nematic, and nematic–isotropic phases have been determined by minimizing the above combined free energies with respect to each order parameter of these mesophases. By changing the strengths of anisotropic interaction and hexagonal interaction parameters, the present model captures the induced smectic A or smectic B phases of the binary nematic mixtures. Of particular importance is the fact that the calculated phase diagrams show remarkable agreement with the experimental phase diagrams of binary nematic liquid crystal mixtures involving induced smectic A or induced smectic B phase.
Biaxial nematic
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Liquid crystal hybrid dimers involving bent-core units and rod-like units were designed, synthesized and investigated by a combination of polarizing optical microscopy (POM), differential scanning calorimetry (DSC) and X-ray diffraction studies. All the four unsymmetrical dimers 1–4 exhibited stable nematic phases over a wide temperature range. Moreover, dimer 1 exhibited the biaxial smectic A (SmA) phase and dimer 2 exhibited a highly ordered mesophase below the nematic (N) phase. Two rod-like and bent-core nematogenic units were covalently linked in dimers 3 and 4 to furnish exclusively the nematic phase. The observation of the biaxial SmA phase underlying the nematic phase in one of the bent-core–rod dimers 1 would be a significant step forward towards achieving the biaxial nematic phase and resolving controversies related to the presence of cybotactic groups.
Biaxial nematic
Mesogen
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The transitional properties of three methylene-linked liquid crystal dimers are reported, namely, 1,5-bis(4-cyanoanilinebenzylidene-4′-yl)pentane (CN-5-CN), 1,5-bis(4-methoxyanilinebenzylidene-4′-yl)pentane (1O-5-O1), and 1,5-bis(4-ethoxyanilinebenzylidene-4′-yl)pentane (2O-5-O2). Each dimer exhibits two monotropic mesophases. The higher temperature mesophase is a normal nematic phase while the lower temperature phase is assigned as a twist-bend nematic phase. The assignment of the twist-bend nematic phase was based on the strong similarities in the optical textures observed to those reported recently for a structurally similar dimer. The complete miscibility of the mesophases exhibited by CN-5-CN and 1O-5-O1 has been established. The analogous hexamethylene-linked dimers exhibit only the normal nematic phase as do the corresponding ether-linked dimers. A review of the literature reveals another five methylene-linked odd-membered dimers that exhibit a nematic–nematic transition and, in each, the lower temperature nematic phase exhibits similar properties to those reported for the twist-bend nematic phase. The formation of this new nematic phase has been attributed to a negative bend elastic constant which results from the bent geometry of methylene-linked odd-membered dimers.
Biaxial nematic
Methylene
Mesogen
Pentane
Miscibility
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Abstract Mesomorphic phase transitions of 4,4′-bis(ω-hydroxyalkoxy)-azoxybenzenes (number of carbons in the alkoxy group n = 2,3,6,8,11 and 12) have been studied by differential scanning calorimetry (DSC) and polarized optical microscopy. For the n = 8 compound, an optically isotropic (01) mesophase reminiscent of the smectic D phase was observed in the temperature range of 4 0 M 1 1 K between smectic C (below 400 K) and nematic (above 41 1 K) phases. On the other hand, the 0 1 mesophase was not observed in the other homologues used here; the n = 2,3 and 6 compounds had only a nematic phase and the n= 11 and 12 compounds had only a smectic C phase. This preliminary work points out that the thermal and optical properties of the 01 phase for the n = 8 compound are very similar to those of the smectic D phase.
Polarized light microscopy
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