Influence of the molecular length on the occurrence of incommensurate low ordered smectic phases for non-symmetric dimesogenic compounds

Systematic physical chemistry studies are in progress concerning the occurrence of incommensurate low ordered smectic phases (Sic) in non-symmetric dimesogens with molecular parameters varying from those of the standard compound KI-5(4). In the present study, the selected molecules possess the same cholesteryl unit and the same spacer length, but the terminal chain of the aromatic mesogenic part is varied. Up to a critical molecular length, a competition between two incommensuratelengths is observed and the dimesogeniccompounds respond to the frustration connected to this competition with the formation either of an incommensurate phase Sic or of a two dimensional modulated phase. For longer dimesogens, a commensurate lock-in of the two characteristic lengths systematically occurs.     

The helical structure of highly ordered smectic phases

Optical studies of smectic phases have been performed in homogeneously oriented samples of chiral 4-(2′-methylbutyl) phenyl-4′-n-octylbiphenyl-4-carboxylate (CE8). The helix structure has been found in smectic phases C, I and J, but not in the smectic G phase. Two chiral phases have been found between S_I* and S_G phases. Up to now one of them has not been observed. The pitch of the helix has been measured in all of the twisted smectic phases, including the S_J* phase. The existence of the helix in this phase suggests that the correlations between smectic layers are not very strong.     

The helical structure of highly ordered smectic phases

Optical studies of smectic phases have been performed in homogeneously oriented samples of chiral 4-(2'-methylbutyl) phenyl-4'-n-octylbiphenyl-4-carboxylate (CE8). The helix structure has been found in smectic phases C, I and J, but not in the smectic G phase. Two chiral phases have been found between S_I* and S_G phases. Up to now one of them has not been observed. The pitch of the helix has been measured in all of the twisted smectic phases, including the S_J* phase. The existence of the helix in this phase suggests that the correlations between smectic layers are not very strong.     

Polymorphic smectic A phases in perfluoroalkylated mesogenic dimers

New homologous symmetric dimers are presented in which two-ring monomers are connected by a flexible spacer and perfluorinated chains are introduced in terminal positions. The mesophase behaviour depends on the length and structure of the spacer and on the length of the terminal chains. The most interesting finding is the occurrence of dimorphic SmA phases in the longer chain homologues with odd-numbered spacer. The transition between the dimorphic SmA phases is accompanied by a relatively high enthalpy. Considering the results of XRD measurements and computer simulations, structural models are proposed, based on different conformations of the molecules.     

Polymorphic smectic A phases in perfluoroalkylated mesogenic dimers

New homologous symmetric dimers are presented in which two-ring monomers are connected by a flexible spacer and perfluorinated chains are introduced in terminal positions. The mesophase behaviour depends on the length and structure of the spacer and on the length of the terminal chains. The most interesting finding is the occurrence of dimorphic SmA phases in the longer chain homologues with odd-numbered spacer. The transition between the dimorphic SmA phases is accompanied by a relatively high enthalpy. Considering the results of XRD measurements and computer simulations, structural models are proposed, based on different conformations of the molecules.     

Brownian reorientation in smectic phases

The rotation of molecules around their long axes is influenced by the in-plane order in smectic phases. Assuming a Brownian reorientation process, the effect of polar and quadrupolar order on the dielectric spectrum is investigated. In the case of quadrupolar order two modes should be visible, while only one mode arises if the order is polar.     

Brownian reorientation in smectic phases

The rotation of molecules around their long axes is influenced by the in-plane order in smectic phases. Assuming a Brownian reorientation process, the effect of polar and quadrupolar order on the dielectric spectrum is investigated. In the case of quadrupolar order two modes should be visible, while only one mode arises if the order is polar.     

Induced smectic C* phases

Induced ferroelectric S*_C phases are formed by non-chiral S_C host phases doped with chiral dipolar guest molecules. In those mixtures the spontaneous polarization P_s and the tilt angle Θ has been investigated as a function of the mole fraction x_G of the chiral dopant. In most cases the reduced polarization P₀ = P_S/ sin Θ has been found to depend linearly on x_G. The polarization power which is defined by δ_P=(∂P₀/∂x_G)ΔT is discussed in terms of the molecular structure of the chiral dopants. There are systems in which P₀(x_G) deviates positively from linearity. This behaviour can be understood by considering a local field correction to P₀. By assuming a local field of Lorentz type a theoretical relation for P₀(x_G) has been derived which explains the experimental results. The effect of a local field is considerable if the transverse dipole moment and the polarizability of the chiral dopant are large.     

Different kinds of smectic B phases

Smectic B phases are divided into three subgroups, B₁, B₂, and B₃. The characteristics of these subgroups are as follows. B₁: optically uniaxial, molecules perpendicular to smectic planes, and order within the layers. B₂: optically biaxial, molecules tilted with respect to planes, order within the layers, and possibly also order between the layers. B₃: optically uniaxial, molecules apparently tilted with respect to planes, and layers probably free to rotate on each other. The model of de Gennes and Sarma is discussed with reference to the three subgroups.     

Phase transitions between smectic A phases as well as smectic C* phases and undulated structures in terminal non-polar compounds

Two types of smectic A and smectic C phases, respectively, have been proved by X-ray and DSC methods for compounds that are terminal non-polar, but consist of tuning-fork shaped molecules. The structural models of the phases are discussed on the basis of steric interaction and of a dense packing. The X-ray patterns of oriented samples point to an undulation of the smectic layers in the case of the smectic A and smectic B phase.     

The effect of molecular chirality on the incidence of twisted smectic A phases

Four chiral chloro-esters were synthesized in order to examine systematically the effect that molecular chirality has on the temperature range and incidence of the twisted smectic A phase or the twist grain boundary phase as it has become known. We find that when the motion of the chiral centre is restricted by rotational hindrance, thereby increasing the chirality of the system, the twisted S_A phase range is increased. At low chirality the twisted S_A phase disappears altogether. Furthermore, it is shown that the transition temperatures of the chiral compounds are lower than their racemic analogues.     

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.     

Twisted smectic A phases in side chain liquid crystal polymers

The syntheses of two side chain liquid crystal polymers, a polyacrylate and a polymethacrylate, are reported. In each of the polymers the liquid-crystalline side group carries an asymmetric carbon atom, thereby making some of the liquid crystal phases formed by the polymers optically active and chiral. For the chiral polyacrylate smectic A and chiral ferroelectric smectic C phases are observed, however for the chiral polymethacrylate a cholesteric phase is detected above the smectic A phase. It is found that the smectic A to cholesteric phase transition is mediated by the formation of an intermediary twisted smectic A phase. This intermediary phase is a liquid-crystalline analogue of the Abrikosov flux phase found in Type II superconductors.     

Twisted smectic A phases in side chain liquid crystal polymers

Abstract

The syntheses of two side chain liquid crystal polymers, a polyacrylate and a polymethacrylate, are reported. In each of the polymers the liquid-crystalline side group carries an asymmetric carbon atom, thereby making some of the liquid crystal phases formed by the polymers optically active and chiral. For the chiral polyacrylate smectic A and chiral ferroelectric smectic C phases are observed, however for the chiral polymethacrylate a cholesteric phase is detected above the smectic A phase. It is found that the smectic A to cholesteric phase transition is mediated by the formation of an intermediary twisted smectic A phase. This intermediary phase is a liquid-crystalline analogue of the Abrikosov flux phase found in Type II superconductors.     

Filled smectic A phases in binary liquid-crystalline systems of terminal-nonpolar compounds

We present five phase diagrams of binary systems in which one component is a bi-swallow tailed compound and the second component is a compound with pronounced nematogenic properties. The common feature of these binary systems is the induction of smectic A phases which have been investigated using X-ray. The smectic A induction seems largely to be the result of specific steric interactions between the components.     

Reactivity within smectic B liquid crystalline phases

ASE quaternization reaction is used as reactivity probe within Sm B solvents. The aim of the present study was that of investigating, by means of many physical techniques (DSC calorimetry, Optical Microscopy, X-ray diffraction. Kinetic measurements and Linear Dichroism), the profound differences which occur in a reaction carried out in “internally non-homogeneous” smectic phases with respect to “internally homogeneous” media, such as isotropic, nematic or cholesteric solvents.The Linear Dichroism technique proved particularly valuable in this regard because it displayed multisite distribution of the solute reactant molecules within the SmB solution. This observation provides a unifying framework explaining the experimental data from all the different physical techniques employed.     

New switchable smectic phases in banana-shaped compounds

A new series of achiral compounds with banana-shaped molecules ('E _n ') has been synthesized and studied by the classical techniques (optical microscopy, differential scanning calorimetry, X-ray diffraction, miscibility studies and electro-optic investigations). The short homologues (E₇-E₁₀) give switchable smectic mesophases. Nevertheless their textures clearly differ from those of the B2 mesophase. In addition, the E₁₀ compound displays a smectic polymorphism involving a new bilayered structure.     

Creation of a nematic phase in mixtures of smectic A1 phases

Abstract

Phase diagrams were determined for binary mixtures consisting of two 5-n-alkyl-2-(4'-isothiocyanatophenyl)-1,3-dioxane compounds (n-DBT) or 4'-isothio-cyanatophenyl 4-(trans-4'-n-decylcyclohexyl)benzoate and n-DBT. All compounds investigated have monolayer smectic A phases. A nematic phase in the upper temperature range and a nematic gap between two smectic regions also were observed, with the smectic layer spacing ratio, d/d', of 1.23 and 1.87 respectively. The variation of the enthalpy of transition with mixture composition in relation to changes of layer spacing ratio are also discussed for these systems.     

Thermodynamic transition properties of highly ordered smectic phases

A series of polyethers have been synthesized from 1-(4-hydroxy-4-biphenyl)-2-(4-hydroxyphenyl)propane and , -dibromoalkanes having different numbers of methylene units [TPPs]. Both odd- and even-numbered TPPs [TPP(n=odd)s and TPP(n=even)s) exhibit multiple transitions during cooling and heating and they show little supercooling dependence, indicating close-to-equilibrium nature of these transitions. Combining the structural characterization obtainedvia wide angle X-ray diffraction powder and fiber patterns at different temperatures and the morphological observations from microscopy techniques, not only the nematic liquid crystalline phase but also highly ordered smecticF, smectic crystalG andH phases have been identified. The phase diagrams for both TPP(n=odd)s and TPP(n=even)s have been constructed [1–3]. Thermodynamic properties (enthalpy and entropy changes) during these transitions are studied based on differential scanning calorimetry experiments. The contributions of the mesogenic groups and methylene units to each ordering process can be separated and they indicate the characteristics of these processes thereby providing estimations of the transition types.This work was supported by the SZDC's Presidential Young Investigator Award from the National Science Foundation (DMR-9175538). The generous donation of a DSC-7 from Perkin Elmer Inc. is also greatly appreciated.     

Stability of smectic phases in the Gay-Berne model

We present a detailed computer simulation study of the phase behavior of the Gay-Berne liquid crystal model with molecular anisotropy parameter kappa=4.4. According to previous investigations: (i) this model exhibits isotropic (I), smectic-A (Sm-A), and smectic-B (Sm-B) phases at low pressures, with an additional nematic (N) phase between the I and Sm-A phases at sufficiently high pressures; (ii) the range of stability of the Sm-A phase turns out to be essentially constant when varying the pressure, whereas other investigations seem to suggest a pressure-dependent Sm-A range; and (iii) the range of stability of the Sm-B phase remains unknown, as its stability with respect to the crystal phase has not been previously considered. The results reported here do show that the Sm-A phase is stable over a limited pressure range, and so it does not extend to arbitrarily low or high pressures. This is in keeping with previous investigations of the effect of molecular elongation on the phase behavior of Gay-Berne models. A detailed study of the melting transition at various pressures shows that the low-temperature crystalline phase melts into an isotropic liquid at very low pressures, and into a nematic liquid at very high pressures. At intermediate pressures, the crystal melts into a Sm-A liquid and no intermediate Sm-B phase is observed. On the basis of this and previous investigations, the reported Sm-B phase for Gay-Berne models appears to be a molecular solid rather than a smectic liquid phase.