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.     

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.     

New mesogenic compounds with trans-stilbene oxide as the central chiral core

Several derivatives with the trans-stilbene oxide moiety as the central chiral core were synthesized. With the appropriate substituents, nematic and smectic C phases were observed. One derivative was obtained with an ee of 47 per cent by asymmetric synthesis. The chemical stability of these epoxides is low and they rearrange to give substituted phenyl benzylketones which are also liquid-crystalline. The structure of the two solid phases present in all the derivatives and of the smectic C phases was investigated by X-ray diffraction.     

Synthesis and characterization of halogen-containing ferroelectric liquid crystals and side chain liquid crystalline polymers

A new series of ferroelectric liquid crystals and side chain liquid crystalline polymers based on halogen-containing chiral centres has been synthesized. Chemical structures were analysed by NMR. Liquid crystal phases were characterized by differential scanning calorimetry, optical polarizing microscopy, and X-ray diffractometry. The behaviour of the liquid crystalline phases was investigated as a function of spacer units and differing terminal asymmetric moieties. It was found that phase transition temperatures decreased with increasing length of the oligooxyethylene spacer unit. Differing terminal asymmetric moieties led to differing mesophase phenomena. Furthermore, a wide temperature range (including room temperature) of a chiral smectic C phase was achieved.     

Synthesis and characterization of halogen-containing ferroelectric liquid crystals and side chain liquid crystalline polymers

A new series of ferroelectric liquid crystals and side chain liquid crystalline polymers based on halogen-containing chiral centres has been synthesized. Chemical structures were analysed by NMR. Liquid crystal phases were characterized by differential scanning calorimetry, optical polarizing microscopy, and X-ray diffractometry. The behaviour of the liquid crystalline phases was investigated as a function of spacer units and differing terminal asymmetric moieties. It was found that phase transition temperatures decreased with increasing length of the oligooxyethylene spacer unit. Differing terminal asymmetric moieties led to differing mesophase phenomena. Furthermore, a wide temperature range (including room temperature) of a chiral smectic C phase was achieved.     

Long‐range effects of chirality in aromatic poly(isocyanide)s

The preparation of optically active atropoisomeric polymers which present chiral backbones, thanks to induction during their synthesis from stereogenic centers, located far away from the skeleton is possible, thanks principally to semirigid conformations of the promesogenic spacers between them. The result is that chiral “information” can be passed as far as 21 Å from the asymmetric center to the carbon atom that forms the polymeric chain in poly(isocyanide)s. The sense of chiral induction in these conformationally rigid polymers parallels the helical sense of the cholesteric phases, as well as to the helical senses of chiral smectic C phases, induced by the monomers in nematic and smectic C phases, respectively. All these phenomena obey the odd–even rules proposed for chiral sense changes in these liquid crystalline phases. Noncovalent interactions play an important part in the induction process, in which steric arguments can be used to justify the inductions observed. The methodology can be used to prepare macromolecules, which display switching behavior upon thermal or electrochemical stimulus. © 2006 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 44: 3161–3174, 2006     

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.     

Optical activity studies in the pretransitional isotropic region of strongly chiral propiolate ester liquid crystals

Optical rotation and circular dichroism measurements performed in the pretransitional isotropic region above the chiral smectic phases TGBA*, TGBC*, S*_C, and S*_A exhibited by 1-alkylalkyl 4'-(4'-n-alkoxyphenylpropioloyloxy)biphenyl-4-carboxylates are reported. These results revealed a high degree of increasing chiral ordering on cooling in the isotropic phase, particularly in the temperature region where a broad DSC peak appears. However, the behaviour of the pretransitional optical activity was unlike that commonly observed for chiral nematic or blue phase compounds and did not follow a simple Landau-de Gennes temperature dependence. Characteristic features of the pretransitional optical activity may aid in understanding the local structure of chiral smectic phases. Electric fields did not seem to influence the results significantly, but strong electric field-induced optical rotation behaviour was demonstrated for a mixture constituted of the laterally fluorinated propiolate ester component and a nematic with a large positive dielectric anisotropy. These field-induced results could be fitted to expressions from the existing theory of pretransitional optical activity. Circular dichroism studies at low temperatures were performed for two compounds in dilute solution and were found to exhibit very large changes in their extinction coefficient at wavelengths in the ultraviolet region, which may indicate strong chiral interactions even in dilute isotropic solutions of these materials.     

Properties of new ferroelectric materials

A homologous series of chiral 4-(3-methylpentyl)benzenethio-4′-n-alkoxy-benzoates has been studied. These thioesters display a ferroelectric, chiral smectic C phase in addition to cholesteric and smectic A phases. A comparison is made between the thioester series and a phenylbenzoate, having the same molecular end group. The effect of the different central linkage on the transition temperature, and on the physical and ferroelectric liquid crystal (FLC) properties has been investigated. Several mixtures, containing these thioester components, were calculated and formulated to obtain room temperature chiral smectic C phases. Spontaneous polarization P_s values and electro-optical response times are correlated with chemical structures. Although these thioesters have very low P _s values, they are useful components for FLC mixtures because of their convenient chiral smectic C temperature ranges and their low viscosities.     

A chiral material with a new phase sequence: twist grain boundary smectic C phase-smectic blue phases

A new phase sequence: twist grain boundary smectic C (TGBC) to smectic blue phases (BP_Sm) is observed in a chiral compound (S)- or (R)-1-methyloctyl 3'-fluoro-4'-(3-fluoro-4-hexadecyloxybenzoyloxy)tolane-4-carboxylate. It is the first time that a TGBC phase has been found to occur under smectic blue phases in the absence of the twist grain boundary smectic A (TGBA) phase. These phases are characterized by polarizing optical microscopy, differential scanning calorimetry and X-ray scattering.     

Density studies on various smectic liquid crystals

Density measurements as a function of temperature for four homologues of the 5-n-alkyl-2-(4-n-alkyloxy-phenyl)-pyrimidines (PYP nOm) which exhibit nematic, smectic A and smectic C phases are reported. Furthermore 1-butyl-c-4-(4'-octyl-biphenyl-4-yl)-r-cyclo-hexan- carbonitrile (NCB84) is studied; this has additionally a smectic G phase. From these data the thermal expansion coefficients are calculated. Comparing PYP 907 and PYP 709, differing in their exchanged alkyl chains, we observe in the smectic A and the smectic C phase a distinctly lower density for PYP 709 whereas their densities nearly agree in the isotropic phase. The pyrimidines PYP 709 and PYP 808 exhibit a continuous volume change on crossing the smectic A-smectic C transition which differs dramatically from PYP 909 which shows a small volume jump. Furthermore a binary mixture of PYP 708 and PYP 706 is analysed which shows only a nematic and a smectic C phase. The associated phase transition is probably first order revealing nearly no pretransitional behaviour. The smectic A-smectic C transition of NCB84 seems to be second order exhibiting a continuous change of volume across the transition whereas the smectic C-smectic G transition shows a volume discontinuity and is first order. In order to induce ferroelectric smectic C* phases all smectic C materials were doped with a chiral pyrimidine dopant. Astonishingly the thermal expansion coefficient across the smectic A-smectic C* transition is influenced by the dopant in a very different way.     

Influence of hydrogen bonding on phase abundance in ferroelectric liquid crystals

The synthesis and characterization of five hydrogen-bonded ferroelectric liquid crystal complexes (HBFLCs) prepared from mesogenic p-n-alkoxy benzoic acids and non-mesogenic propionic/butyric acids with different chiral centres are reported. Complementary intermolecular hydrogen bonding is confirmed through IR study. HBFLCs are found to exhibit chiral nematic (N*), smectic C* (SmC*) and smectic G* (monotropic) phases in their cooling profiles during polarizing thermal microscopy and differential scanning calorimetry. Phase coexistence regions are observed above the IN* transition. The chiral nematic to smectic C* transition is found to be of first order. The temperature variation of spontaneous polarization exhibited by these HBFLC complexes in their SmC* phase is presented. The effect of non-covalent interaction imparted by the soft hydrogen bonding in these LC complexes on enhanced or induced thermal stability of tilted LC phases is discussed.     

Untwisting of the helical superstructure in the cholesteric and chiral smectic C*phases of cross-linked liquid-crystalline polymers by strain

It is possible to untwist reversibly the helical superstructure of elastomers with cholesteric and chiral smectic C*phases by using strain. In that way a cholesteric structure can be transformed into a nematic structure and a chiral smectic C*into a smectic C structure. The latter case is especially interesting because a structure without a macroscopic polarization (chiral smectic C*) is transformed into one with a macroscopic polarization (smectic C like arrangement).     

Liquid crystallinity of newly synthesized glucose derivatives with mesogenic side chains

Synthesis of glucose derivatives by direct esterification of the five available sites on D-(+)-glucose with side chains containing a biphenyl mesogenic moiety, a pentyl spacer and an alkyl tail is reported for the first time. Liquid crystalline phase behaviour of these glucose derivatives was studied by optical microscopy, thermal and X-ray diffraction methods. A layered arrangement of the smectic A (SmA) type was commonly observed in the above materials. An increase in the length of the alkyl tail results in a change of the phase structure from smectic Ai(SmAi), intercalated layer phase, to smectic As(SmAs), segregated bilayer phase. From the transition temperature and enthalpy observations, the SmAs phase has higher order than the SmAi phase. Cholesteric and chiral smectic C phases were also observed in addition to the SmA for some of the glucose derivatives, demonstrating a potential for preparing chiral liquid crystals.     

A chiral material with a new phase sequence: twist grain boundary smectic C phase-smectic blue phases

A new phase sequence: twist grain boundary smectic C (TGBC) to smectic blue phases (BP_Sm) is observed in a chiral compound (S)- or (R)-1-methyloctyl 3'-fluoro-4'-(3-fluoro-4-hexadecyloxybenzoyloxy)tolane-4-carboxylate. It is the first time that a TGBC phase has been found to occur under smectic blue phases in the absence of the twist grain boundary smectic A (TGBA) phase. These phases are characterized by polarizing optical microscopy, differential scanning calorimetry and X-ray scattering.     

Chiral palisade side-chain liquid crystal copolymers: effect of comonomer end group on the nature of the liquid crystalline

Three series of copolymers were prepared from 4-[2-(S)-methyl-3-acryloxy)-4′-methoxy phenyl] benzoate, as the common chiral monomer, and three nonchiral 4-alkyloxy phenyl-4′-(6-acryloxy hexyloxy) benzoates with alkyloxy tail groups containing seven to nine atoms. All of the copolymers exhibited liquid crystalline (LC) behavior over the entire composition ranges studied. It was demonstrated that by changing the length of the alkyloxy unit, significant differences could be induced in the LC phases observed. When the alkyloxy unit was seven atoms long only chiral nematic (N*) phases were detected, whereas lengthening the alkyloxy unit to eight and nine atoms led to the formation of smectic A (S_A) and chiral smectic C (S*_C ) phases in addition to the N* phase. Films of these materials exhibited selective reflection in the visible region as one would expect from the presence of N* and S*_C phases. © 1998 John Wiley & Sons, Ltd.     

Untwisting of the helical superstructure in the cholesteric and chiral smectic C∗phases of cross-linked liquid-crystalline polymers by strain

Abstract

It is possible to untwist reversibly the helical superstructure of elastomers with cholesteric and chiral smectic C?phases by using strain. In that way a cholesteric structure can be transformed into a nematic structure and a chiral smectic C?into a smectic C structure. The latter case is especially interesting because a structure without a macroscopic polarization (chiral smectic C?) is transformed into one with a macroscopic polarization (smectic C like arrangement).     

Influence of hydrogen bonding on phase abundance in ferroelectric liquid crystals

The synthesis and characterization of five hydrogen-bonded ferroelectric liquid crystal complexes (HBFLCs) prepared from mesogenic p-n-alkoxy benzoic acids and non-mesogenic propionic/butyric acids with different chiral centres are reported. Complementary intermolecular hydrogen bonding is confirmed through IR study. HBFLCs are found to exhibit chiral nematic (N*), smectic C* (SmC*) and smectic G* (monotropic) phases in their cooling profiles during polarizing thermal microscopy and differential scanning calorimetry. Phase coexistence regions are observed above the IN* transition. The chiral nematic to smectic C* transition is found to be of first order. The temperature variation of spontaneous polarization exhibited by these HBFLC complexes in their SmC* phase is presented. The effect of non-covalent interaction imparted by the soft hydrogen bonding in these LC complexes on enhanced or induced thermal stability of tilted LC phases is discussed.     

Two unusual mesophases in chiral side chain polymers

Abstract

We have reported in a previous paper the synthesis, characterization, structural and electro-optic properties of a new family of chiral mesomorphic side chain polyacrylates. The most salient finding was that many of these polymers presented two S*_C phases of similar structure, but differing in their switching properties. In addition, two unusual mesophases called U1 and U2 were also discovered in this family. We describe here in more detail the symmetries and molecular organizations for these U1 and U2 phases. We also discuss their relation, on the one hand, to the newly discovered antiferroelectric chiral S*_CA and S*_o phases, and, on the other hand, to the 2-dimensional fluid smectic à and smectic °C phases displayed by strongly polar low molar mass mesogens. The occurrence of chevron-like ordering in the U1 and U2 phases may give a clue to an understanding of this peculiar S*_C polymorphism.