Mesophase formation in a system of top-shaped hard molecules: density functional theory and Monte Carlo simulation

We present the phase diagram of a system of mesogenic top-shaped molecules based on the Parsons-Lee density functional theory and Monte Carlo simulation. The molecules are modeled as a hard spherocylinder with a hard sphere embedded in its center. The stability of five different phases is studied, namely, isotropic, nematic, smectic A, smectic C, and columnar phases. The positionally ordered phases are investigated only for the case of parallel alignment. It is found that the central spherical unit destabilizes the nematic with respect to the isotropic phase, while increasing the length of the cylinder has the opposite effect. Also, the central hard sphere has a strong destabilizing effect on the smectic A phase, due the inefficient packing of the molecules into layers. For large hard sphere units the smectic A phase is completely replaced by a smectic C structure. The columnar phase is first stabilized with increasing diameter of the central unit, but for very large hard sphere units it becomes less stable again. The density functional results are in good agreement with the simulations.     

New biforked mesogen series

Four new series of biforked mesogens derived from 3,4-dialkoxybenzoic acids or 3,4-dialkoxybenzaldehydes have been prepared and studied. These derivatives exhibit an interesting polymorphism in which both columnar and lamellar mesophases exist in the same series and in some cases in the same compound. The short chain derivatives exhibit a nematic and/or a smectic C phase but those with long chains display the hexagonal, rectangular or oblique columnar phases. In some of them, a centred cubic phase is found.     

X-ray studies of liquid-crystalline phases of polycatenar substances Evidence for a rectangular columnar structure

The liquid-crystalline phases of two series of bi-swallow tailed compounds have been investigated using textural observations and X-ray studies. Besides nematic and smectic phases a new phase type has been observed. This is characterized by a rectangular columnar structure.     

Chemical reactivity within a smectic B liquid crystalline phase: A model of enzyme catalysis?

The rearrangement of allyl p-dimethylaminobenzenesulphonate (ASE) to form a zwitterionic product has already been recognized as an effective probe for the study of reactivity within the smectic B phase [4, 5, 19]. We have used deuterium NMR, linear dichroism and X-ray diffraction techniques to investigate the phase diagram of the ASE-OS35 reaction system. The partitioning of the reactant molecules between coexisting smectic, nematic and/or isotropic phases and the structural organization of the smectic catalytic host at different temperatures and reactant guest concentrations have been characterized. On the basis of these measurements, a model of ASE reactivity in smectic solvents has been developed. The reaction takes place provided that coexisting isotropic or nematic phases are present to act as a reservoir for the ASE reactant molecules prior to their entering the smectic phase; they then react and leave the smectic phase as a zwitterionic product. The analogy between this model of reactivity within smectic phases and the Michaelis-Menten enzyme processes is discussed. This relationship opens up the intriguing possibility of designing new experiments with which to investigate further liquid crystalline models of enzyme catalysis.     

Chiral ordering in the nematic and an optically isotropic mesophase of bent-core mesogens with a halogen substituent at the central core

Two new homologous series of bent-core compounds have been synthesized. Their mesophase behaviour has been investigated by polarizing microscopy, differential scanning calorimetry, X-ray diffraction, NMR spectroscopy, and by dielectric and electro-optical measurements. It was found that, with one exception, all the chlorine-substituted compounds form a nematic phase and an optically isotropic 'banana phase'. The latter phase shows spontaneously chiral domains of opposite handedness. This phase may be considered as a type of smectic blue phase. The mesophase behaviour of the homologous bromine-substituted compounds is more complicated. Depending on the chain length, B₆, columnar, nematic or the isotropic 'banana phase' occur.     

Chiral ordering in the nematic and an optically isotropic mesophase of bent-core mesogens with a halogen substituent at the central core

Two new homologous series of bent-core compounds have been synthesized. Their mesophase behaviour has been investigated by polarizing microscopy, differential scanning calorimetry, X-ray diffraction, NMR spectroscopy, and by dielectric and electro-optical measurements. It was found that, with one exception, all the chlorine-substituted compounds form a nematic phase and an optically isotropic ‘banana phase’. The latter phase shows spontaneously chiral domains of opposite handedness. This phase may be considered as a type of smectic blue phase. The mesophase behaviour of the homologous bromine-substituted compounds is more complicated. Depending on the chain length, B₆, columnar, nematic or the isotropic ‘banana phase’ occur.     

Thermotropic and lyotropic liquid crystalline phases of rigid aromatic amphiphiles

Rodlike amphiphilic molecules that contain exclusively aromatic building-blocks and no flexible alkyl chains have been synthesized and their mesomorphic properties investigated. These novel compounds bear diol head groups of different size (2,3-dihydroxypropyloxy or 5,6-dihydroxy-3-oxahexyloxy groups) at one end of a biphenyl unit, various aromatic segments (benzyloxy, 4-, 3-, or 2-methylbenzyloxy, phenoxy groups) at the other, and additional methyl substituents in different positions. They were synthesized by using Suzuki cross-coupling reactions as the key steps. Their thermotropic mesomorphism was investigated by means of polarized light optical microscopy, differential scanning calorimetry, and, for enantiotropic phases, by X-ray scattering. The liquid crystallinity of this class of compounds is influenced by protic solvents, such as water and glycerol. Dependent on the temperature and the solvent content, different SA phases were found. Several mesophases resulting from the frustration of these layer structures (e.g., different columnar phases, optical isotropic mesophases, and nematic phases) were also present. The smectic phases have different degrees of intercalation (SAd, SA2). The columnar phases are supposed to be ribbon structures that result from the collapse of the smectic layers. They occur in some pure compounds or they are induced upon the addition of protic solvents. The particular phase sequences of the different compounds depend mainly on the position of the methyl substituents at the biphenyl cores and are largely determined by the degree of intercalation of the aromatic cores.     

A polycatenar mesogen with a perfluorinated moiety showing continuous phase transformation between a smectic A phase and a structured,fluid, optically isotropic phase

A polycatenar mesogen consisting of a four aromatic ring system with three long alkoxy chains and a bulky perfluorinated substituent has been synthesized and found to show the polymorphism of a cubic phase, a columnar phase and a smectic A phase, including a structured, fluid, optically isotropic phase with the phase transformation between the smectic A phase and the isotropic phase occurring continuously.     

Stability of smectic phases in hard-rod mixtures

Using density-functional theory, we have analyzed the phase behavior of binary mixtures of hard rods of different lengths and diameters. Previous studies have shown a strong tendency of smectic phases of these mixtures to segregate and, in some circumstances, to form microsegregated phases. Our focus in the present work is on the formation of columnar phases which some studies, under some approximations, have shown to become thermodynamically stable prior to crystallization. Specifically we focus on the relative stability between smectic and columnar phases, a question not fully addressed in previous work. Our analysis is based on two complementary perspectives: on the one hand, an extended Onsager theory, which includes the full orientational degrees of freedom but with spatial and orientational correlations being treated in an approximate manner; on the other hand, we formulate a Zwanzig approximation of fundamental-measure theory on hard parallelepipeds, whereby orientations are restricted to be only along three mutually orthogonal axes, but correlations are faithfully represented. In the latter case novel, complete phase diagrams containing regions of stability of liquid-crystalline phases are calculated. Our findings indicate that the restricted-orientation approximation enhances the stability of columnar phases so as to preempt smectic order completely while, in the framework of the extended Onsager model, with full orientational degrees of freedom taken into account, columnar phases may preempt a large region of smectic stability in some mixtures, but some smectic order still persists.     

The homologous series of some new six-ring double-swallow-tailed compounds

Recently we reported three homologous six-ring double-swallow-tailed compounds. For one homologue we observed the unusual phase sequence re-entrant smectic C-D_ob-smectic C-nematic. For this reason, results for a more complete homologous series are now presented. It was found that the phase behaviour can be compared with some series of biforked compounds. The lower members of the series exhibit nematic and smectic C phases, whereas the long-chain homologues form columnar phases. For three members of the series with intermediate lengths of the terminal chains, lamellar (S_c) and columnar (D_ob) phases occur for the same substance.     

Density functional theory of freezing of a system of conjugated oligomers parameterised via Gay–Berne potential

Density functional theory (DFT) of freezing is used to study the isotropic–nematic, isotropic–smectic A and nematic–smectic A phase transitions in a system of large, semi-flexible conjugated oligomers parameterised within Gay–Berne (GB) potential. The pair correlation functions of the isotropic fluid, used as structural inputs in the DFT, are calculated by solving the Percus–Yevick integral equation theory. Large number of spherical harmonic coefficients of each orientation-dependent functions has been considered to ensure the numerical accuracy at different densities and temperatures for the system of these model GB ellipsoids having large aspect ratio (length-to-breadth ratio). We found that the system of GB ellipsoids parameterised for conjugated oligomers shows stable isotropic, nematic and smectic A phases. At low temperatures, on increasing the density, isotropic fluid makes a direct transition to smectic A phase. Nematic phase get stabilised in between the isotropic and smectic A phases on increasing the temperature. Using the transition parameter obtained through the DFT, we have plotted the temperature–density and pressure–temperature phase diagrams which are found to be qualitatively similar to the one obtained in simulations for the systems with low aspect ratio GB particles.     

Induced smectic phases in phase diagrams of binary nematic liquid crystal mixtures

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.     

侧基含纳米构筑单元的甲壳型液晶高分子的多层次自组装

甲壳型液晶高分子可以呈现超分子柱或片层的链构象,因此可以作为超分子液晶基元形成多种液晶相态,如六方柱状相、柱状向列相、六方柱状向列相、近晶相等.将纳米构筑单元,如一维的二联苯、二维的苯并菲、三维的多面体低聚倍半硅氧烷(POSS)等,引入到甲壳型液晶高分子中,所得聚合物可以自组装形成在亚十纳米和近纳米尺度的多级有序结构.这些结构具有尺寸可控及单分散的优点,可望在有机光电、纳米多孔膜以及纳米光刻等领域有着广阔的应用前景.本文主要介绍了将二联苯、偶氮苯、棒状多苯结构、苯并菲和POSS基元引入到甲壳型液晶高分子中制备多级组装结构的相关工作.     

Phospholipids with a stimuli-responsive thermotropic liquid-crystalline moiety

Lyotropic liquid-crystalline phospholipids having an electro- and/or light-responsive mesogenic core were prepared. These LCs show lyotropic smectic A, smectic C, rectangular columnar, and nematic phases. We succeeded in the dynamic control of these LC phases by applying an electric field, which could lead to electrically switchable phospholipid bilayers.     

Liquid Crystalline Organisation and Reaction Rate in the Formation of Liquid Crystal Thermoset Networks from Diepoxides and Dicyanates

Summary: The influence of the type of mesophase on the rate of reaction in amine curing or anionic polymerisation of mesogenic diepoxides, and in the cyclotrimerisation of mesogenic dicyanates has been investigated by isothermal DSC and IR-spectroscopy. Epoxide/amine systems were found to react faster in the nematic than in the isotropic phase. Anionic polymerisation of epoxides is an example where increase of the reaction rate occurs with the transition from a mesophase of higher order (smectic) to phases with lower order (nematic, isotropic). For triaromatic dicyanates with one or three methyl groups at the central ring cyclotrimerisation is faster in the isotropic than in the nematic phase showing an increase of activation energy in the nematic phase with the number of methyl groups. A pronounced rate increase has been observed in the smectic phase as compared to the nematic phase for the unsubstituted triaromatic dicyanate. In this case activation energies in the two phases are comparable with those of non liquid crystalline dicyanates.     

Hydrogen-bonded block mesogens derived from semiperfluorinated benzoic acids and the non-mesogenic 1,2-bis(4-pyridyl)ethylene

Thermal properties of benzoic acids carrying one or two semiperfluorinated alkoxy tails on the aromatic core have been investigated in binary mixtures with the non-liquid crystalline bidirectional trans-1,2-bis(4-pyridyl)ethylene. The hydrogen bonded complexes built from the complementary molecular species show a significantly enhanced mesophase stability compared with the fluorinated acids in their pure states. The mesophase morphologies of the complexes are governed mainly by the number of the partially fluorinated chains grafted to the acid component. Mixed systems comprising the one-chain acids exhibit a smectic C phase followed by a smectic A phase at more elevated temperatures. Incorporation of a second semiperfluorinated chain into the acid leads to the formation of columnar mesophases. These columnar phases of the H-bonded complexes should represent ribbon phases resulting from the collapse of the smectic layers.     

Solidification-induced and shear-induced band texture in thermotropic liquid crystalline polymer films

Two phase diagrams of a six-ring double-swallow-tailed compound are presented where the mixing components involve electron-acceptor compounds. In the mixed phase region, nematic, smectic A and C, cubic and columnar phases are induced. In this way transitions between lamellar, cubic and columnar phases can be realized by variation of the con- centration. The mesophases occurring in these systems have been characterized by X-ray investigations.     

Self-assembly of a coil-rod-coil block molecule complexed with LiCF3SO3

Enantiotropic liquid crystalline phases were induced by the complexation of docosyl 4-(4-oxy-4'-biphenylcarbonyloxy)-4'-biphenyl carboxylate, containing poly(ethylene oxide) of twelve ethylene oxide subunits, with 0.25 mol of LiCF₃SO₃ per ethylene oxide unit. The existence of oblique columnar, rectangular columnar (Col_rec), and smectic A (SmA) mesophases of this complex was demonstrated by a combination of polarized optical microscopy and X-ray diffraction. This complex was observed to transform from the rectangular columnar phase to a lamellar phase on heating. The unusual phase transition behaviour in this complex is believed to originate from the mixing entropy of the added LiCF₃SO₃. Also, the Col_rec-SmA phase transition was accompanied by a change of the packing structure from bilayer to monolayer.     

Structure formation in doped discotic polymers and low molar mass model systems

Doping of low molar mass materials or polymers, possessing disc-like units, with electron acceptors leads to the stabilization of columnar discotic phases or even to the induction of such phases in compounds which either display a nematic discotic phase or only an amorphous phase in the absence of the electron acceptor. The induced columnar phase corresponds frequently to a hexagonally ordered one. We have observed, however, in addition the induction of new columnar phases such as the rectangularly ordered (D^ro) and the columnar nematic phase (N^c). The enhancement of the tendency towards the formation of columnar phases is a consequence of electron acceptor—electron donor complex formation. Using a model proposed by de Jeu to describe the induction of smectic phases by complex formation we are able to account qualitatively for the experimental findings.