The Synthesis, Polymerization and Second Order Non-Linear Optical Properties of Novel, Conjugated, Unsymmetrical Diacetylenes Showing Liquid Crystal Behaviour

A number of novel, conjugated, unsymmetrically disubstituted diphenyl-diacetylenes have been synthesized and polymerized in the liquid-crystalline phase to yield liquid-crystalline polymers. The efficiency of second harmonic generation is reported for both monomers and polymers.     

Discotic charge transfer twins Structure and mesophase behaviour of covalently linked triphenylenes and trinitrofluorenones

Discotic charge transfer twins, a novel class of discotic liquid-crystalline compounds were studied. These compounds consist of triphenylene units (as donors) which are chemically linked via flexible spacers of various lengths to trinitrofluorenone units (acting as acceptor). They display a liquid-crystalline phase over a wide temperature range extending up to 240-260°C. Based on X-ray analysis a structural model is proposed for the liquid-crystalline phase: the molecules are arranged in columns in such a way that mixed stacks occur, the intercolumnar packing possesses an orthorhombic symmetry. The neighbouring columns are connected along specific directions via flexible spacers which give rise to highly anisotropic structural properties of this columnar liquid-crystalline phase.     

Induction of smectic mesomorphism by mixing a non-liquid-crystalline methacrylate with a nematic liquid-crystalline compound having a cyano-containing mesogen

The structure and phase diagram of a binary mixture of a non-liquid-crystalline, mono-functional (meth)acrylate monomer and a liquid-crystalline compound having a cyano group-containing mesogen were examined. The monomers had mesogenic units such as biphenylene and phenyl benzoate, but did not show liquid-crystallinity. The liquid-crystalline compounds possessed a cyanobiphenyl or a cyanophenyl benzoate mesogenic unit. The liquid-crystalline 4-cyanophenyl 4'-n-octyloxybenzoate did not show a smectic A phase, while the binary mixture of the 4-butoxy-4'-(ω-methacryloyloxyhexyloxy)biphenyl monomer with 4-cyanophenyl 4'-n-octyloxybenzoate showed the induction of a smectic A phase in a nearly equimolar composition range. However, the binary mixture of the monomer, containing a phenyl benzoate group, and the liquid-crystalline compound did not induce a smectic phase.     

Curing kinetics of liquid-crystalline epoxy resins

By endcapping mesogenic rigid rod molecules with reactive epoxy groups a novel class of liquid-crystalline thermoset has been obtained. In fact is has been shown that the nematic molecular arrangement is sustained over the crosslinking reaction of liquid-crystalline epoxy resins when the curing reaction is carried out in the thermal stability range of the liquid-crystalline phase. Calorimetric analysis was used in characterizing the isothermal cure. An unsophisticated model is proposed for evaluating the activation energies of the crosslinking reaction. For liquid-crystalline epoxy resins lower activation energies result with respect to the cure reactions for non liquid-crystalline epoxy resins.     

Hydrogen bonding and the self-assembly of supramolecular liquid-crystalline materials

Novel supramolecular liquid-crystalline materials have been obtained by the hydrogen bond formation between different and independent molecules. The self-assembly of carboxylic acid and pyridine moieties that function as H-bond donors and acceptors, respectively, results in the formation of mesogenic complex structures. A wide variety of liquid-crystalline low molecular weight complexes have been prepared using this approach. This concept has been extended to the polymeric systems. Hydrogen-bonded liquid-crystalline polymers that exhibit mesophases over wide temperature ranges, ferroelectricity or photo-responsibility have been prepared by the molecular association. Moreover, liquid-crystalline polymer networks that show reversible smectic-isotropic phase transitions have been formed by the hydrogen bonds. The dynamics of the hydrogen bonding may contribute to the induction of the mesomorphism of the networks.     

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.     

Self-cleaning resins

This paper introduces a fundamentally new concept in adsorbents, whereby the sorption of an aqueous solute by a cross-linked polymer is controlled by a gel to liquid-crystalline phase transition. To demonstrate proof of principle, a bilayer forming surfactant, N,N-dioctadecyl,N,N-dimethylammonium bromide (DODAB) has been immobilized onto a cation exchange resin, Dowex 50WX2, and its thermotropic phase behavior and solute-adsorption properties have been investigated. Examination by a combination of differential scanning calorimetry and X-ray scattering has confirmed the retention of a gel to liquid-crystalline phase transition of the surfactant, occurring between 296 and 318 K. Adsorption measurements that were made for 4-chlorotetrahydropyran, 1,2-dichloroethane, and benzyl alcohol have also confirmed uptake by the resin in the liquid-crystalline phase and release in the gel phase.     

A mixed fullerene-ferrocene thermotropic liquid crystal: synthesis, liquid-crystalline properties, supramolecular organization and photoinduced electron transfer

Grafting of a ferrocene-containing liquid-crystalline malonate derivative to C60 led to the mixed fullerene-ferrocene material 1 which gave rise to a smectic A phase. Cholesterol was used as liquid-crystalline promoter. X-ray diffraction experiments and volumetric measurements indicated that 1 is organized in double layered structures. The corresponding supramolecular organization within the mesomorphic lamellar phase is characterized by a microsegregation of the different units (ferrocene, fullerene, and cholesterol) in distinct sublayers. In such a smectic A phase, C60 imposes the arrangement of the other molecular moieties. Photophysical studies revealed that electron transfer occurs from the donor ferrocene to the electron accepting fullerene. The formation of a long-lived radical pair, with lifetimes of the order of several hundred nanoseconds, was confirmed by time-resolved spectrometry, especially in the near infrared region, in which the radical anion of the fullerene moiety displays its characteristic fingerprint absorption.     

Cubic, sponge, and lamellar phases in the glyceryl monooleyl ether-propylene glycol-water system

The phase behavior of 1-glyceryl monooleyl ether (GME) in mixtures of propylene glycol (PG) and water was investigated by visual inspection, polarization microscopy, small-angle X-ray diffraction, and conductance measurements. A phase diagram, based on over 200 samples of the ternary system GME-PG-water, was constructed at 20 degrees C. Without PG, GME forms a reverse micellar phase with up to 10 wt % water and a reverse hexagonal liquid-crystalline phase between 10 and 25 wt % water, a phase that can coexist with excess water. If PG is added in amounts exceeding about 10 wt %, then cubic and lamellar liquid-crystalline phases start to form. A cubic phase, belonging to space group Pn3m, can coexist with excess PG-water mixtures. If even more PG is added, then the cubic phase is transformed into a sponge phase. A lamellar phase forms at water contents between 10 and 15 wt % and with widely differing PG/GME weight ratios. We postulate that the phase behavior is caused by the fact that PG makes the interfacial region between self-assembled GME and PG-water less negatively curved, which in turn allows for the formation of the new phases. The phase behavior obtained for the GME system shows a striking similarity with the phase behavior of the corresponding system in which the GME has been replaced by the ester, 1-glycerol monooleate (GMO), differing only in one extra carbonyl oxygen. The major difference is the lower amount of water present in the GME phases, an effect that is mainly due to the more hydrophobic character of GME compared to that of GMO.     

Invite Article: The Liquid-Crystalline Phases of Double-Stranded Nucleic Acids in Vitro and in Vivo

This article describes the state of and progress in experimental studies of liquid crystals of naturally occurring nucleic acids and synthetic polynucleotides. The areas considered in this review include: (i) the liquid-crystalline phase of nucleic acids in aqueous salt solutions, (ii) the liquid-crystalline phase of nucleic acids in aqueous polymer solutions, (iii) the liquid-crystalline phase of nucleic acids in living systems. Some unsolved problems which are of interest from both a physicochemical and a biological point of view are discussed.     

X-ray investigation of combined main-chain/side-chain liquid-crystalline polymers

Structural investigations on six combined liquid-crystalline polymers using the X-ray technique are described. The measurements have allowed a characterization of the liquid-crystalline phases and have led to our first ideas about the phase structures. All mesogenic groups are arranged parallel to each other, the shortest segment in the main-chain or in the side-chain determines the smectic layer spacing. A modified designation for liquid-crystalline phases of high polymers is proposed.     

Synthesis and characterization of graft copolymers containing poly(p-phenylene) main chains and mesogen-jacketed liquid-crystalline polystyrene side chains

A series of novel comblike mesogen-jacketed liquid-crystalline graft copolymers, poly(p-phenylene)-g-poly{2,5-bis[(4-methoxyphenyl)oxycarbonyl]styrene} (PPP-g-PMPCS) copolymers, have been designed and successfully synthesized by a Yamamoto coupling reaction and subsequent atom transfer radical polymerization (ATRP). ¹H NMR spectroscopy, ultraviolet–visible spectra, and gel permeation chromatography (GPC) have been used to confirm the molecular structure of the macroinitiator and the copolymers. A study of the polymerization kinetics of ATRP has shown that the molecular weight of the copolymer increases linearly with the conversion of the monomer, whereas the polydispersity remains narrow (≤1.28), indicating that the ATRP of 2,5-bis[(4-methoxyphenyl)oxycarbonyl]styrene is well controlled. Thermogravimetric analysis and differential scanning calorimetry (DSC) measurements have indicated that the PPP-g-PMPCS copolymers have better thermal stabilities than the macroinitiator, and their thermal stabilities increase with increasing molecular weight. The liquid-crystalline behavior has been examined with polarized optical microscopy, DSC, one-dimensional wide-angle X-ray diffraction (1D WAXD), and two-dimensional wide-angle X-ray diffraction (2D WAXD). The results show that all the comblike copolymers exhibit obvious liquid-crystalline behaviors, even though the GPC molecular weight of the segments of poly{2,5-bis[(4-methoxyphenyl)oxycarbonyl]styrene} (PMPCS) have been determined to be far less than the critical value of linear PMPCS. Moreover, 1D WAXD measurements show that the temperature at which the comblike mesogen-jacketed liquid-crystalline copolymers can transform into a liquid-crystalline phase is low; about 20 °C in comparison with the linear ones. 2D WAXD analysis has revealed that these comblike copolymers should be assigned to a hexatic columnar nematic (Φ_HN) phase. © 2007 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 45: 2543–2555, 2007     

Synthesis and characterization of liquid-crystalline side group polymers with benzylideneaniline as mesogenic moiety

Two new thermotropic liquid-crystalline side group polymers were synthesized, characterized and compared with the liquid-crystalline monomeric analogues. Some packing features of these polymeric liquid crystals are discussed.

Investigations were carried out by differential calorimetry (D.S.C.), polarization microscopic observation and X-ray diffraction on non-aligned and magnetic field-aligned samples in the wide and small angle region.

The synthesized polymers contain as mesogenic moiety a benzylideneaniline group which is attached in the 4 position via a hexamethylene spacer to a poly-methacrylate backbone. The benzylideneaniline group is substituted in 4′ position with an ethoxy or butoxy group (PEt or PBu). The monomeric analogues are denoted MEt and MBu. The two polymers show a phase sequence crystalline-smectic A-nematic-isotropic. The liquid-crystalline temperature range is observed between 90 and 150°C. The monomeric MEt exhibits only a monotropic nematic, MBu an enantiotropic nematic and a smectic A phase.     

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.     

Columnar Phases from Covalent and Hydrogen-Bonded Liquid-Crystalline Ferrocene Derivatives

The synthesis and liquid-crystalline properties of tetracatenar covalent and H-bonded bis-ferrocene derivatives 1 and 2 , respectively, are reported. Both compounds gave rise to enantiotropic columnar liquid-crystalline behavior with a hexagonal molecular organization. To explore the possibility to obtain also calamitic liquid-crystalline phases from H-bonded ferrocene-containing liquid crystals, a rod-shaped ferrocene mesogen 3 was synthesized, which gave rise to enantiotropic smectic C and smectic A phases. For the first time, a rational synthetic design at the ferrocene level led to ferrocene-based liquid-crystals with columnar behavior and to H-bonded metallomesogens.     

Isotropic to smectic-C phase transition in liquid-crystalline elastomers

A phenomenological model is developed to describe the isotropic-smectic-C phase transition in liquid-crystalline side-chain elastomers. We analyze the influence of external mechanical stress on the isotropic-smectic-C phase transition. While this phase transition is first order in low-molecular-weight materials, we show here that the order of this transition does not change in liquid-crystalline elastomers. The temperature dependence of the heat capacity and the nonlinear dielectric effect in the isotropic phase above the isotropic-smectic-C phase transition in liquid crystalline elastomers are calculated. The theoretical results are found to be in good agreement with experiment.     

The 2012 Luckhurst–Samulski Prize

Abstract

This article describes the state of and progress in experimental studies of liquid crystals of naturally occurring nucleic acids and synthetic polynucleotides. The areas considered in this review include: (i) the liquid-crystalline phase of nucleic acids in aqueous salt solutions, (ii) the liquid-crystalline phase of nucleic acids in aqueous polymer solutions, (iii) the liquid-crystalline phase of nucleic acids in living systems. Some unsolved problems which are of interest from both a physicochemical and a biological point of view are discussed.     

Hybrid thermotropic liquid-crystalline block copolymers

A new approach to the preparation of two classes of block copolymers containing liquid-crystalline segments is reported. Copolymers 1 and 2 are constituted by polytetrahydrofuran and side-chain liquid-crystalline polymethacrylate blocks, whereas block copolymers 3 consist of polystyrene and main-chain liquid-crystalline polyester blocks. The synthetic procedures leading to copolymers 1-3 are extremely versatile and can be used to prepare a great variety of block copolymer architectures. In both copolymer classes the chemically different blocks are strongly segregated in the solid and melt phases and undergo individual phase transitions. The mesophase transition temperatures of the liquid-crystalline blocks are very similar to those of the corresponding homopolymers, and their enthalpies are directly proportional to the content of the liquid-crystalline block.     

New stereoregular liquid-crystalline phenylcyclosiloxanes

Representatives of stereoregular phenylcyclotetra(hexa)siloxanes with mesogenic cyanobiphenylyl groups cis-oriented relatively to the siloxane ring have been synthesized for the first time. The ability of the compounds to transit to the liquid-crystalline state was confirmed by the thermooptical, X-ray diffraction, and calorimetric methods. The temperatures and enthalpies of the phase transitions were determined, and their reversibility was shown. The bilayered molecular packing with antiparallel arrangement of molecules is the most probable for the smectic A phase of the cyclosiloxanes studied. Taking into account differences in the optical textures and interlayer distances, we cannot exclude completely the possibility of formation of liquid-crystalline phases of other types.     

Influence of Perfluoroarene–Arene Interactions on the Phase Behavior of Liquid Crystalline and Polymeric Materials

A stabilization of the liquid-crystalline mesophase and thus an enlarged temperature range of the mesogenic phase is achieved by adding perfluorotriphenylene to a chiral liquid-crystalline triphenylene. This mesophase is based on 1:1 perfluoroarene–arene interactions (see picture). In a polymer with triphenylenes as mesogens in the side chains, the addition of perfluorotriphenylene led to crystallization.