From colloids in liquid crystals to colloidal liquid crystals

ABSTRACT

Liquid crystals in combination with nanoparticles are a fascinating topic of research, because of the wealth of aspects and questions to study. These range from simple effects of nanoparticles on phase transitions and phase diagrams, to the tuning of physical properties, adding of novel functionalities, all the way to the formation of spontaneous order by nanoparticles themselves and the possibilities that templating has for future materials design and applications. This article intends to provide a flavour of the multiplicity, variety and diversity that these thermotropic and lyotropic systems have to offer in the area of materials development, which we believe will become increasingly important, especially for switchable non-display applications and nanotechnology. It is not intended to provide a conclusive overview, which would be a presumptuous attempt considering the limited space available, but rather to place our own work into a wider context and to point out some more recent developments and trends in liquid crystal – nanoparticle dispersions.     

纤维素及其衍生物液晶研究新进展

本文全面地综述了纤维素及其衍生物溶致性液晶和热致性液晶的形成, 液晶性与大分子链结构, 以及具有胆甾型液晶相结构的纤维素衍生物复合材料等方面的最新研究进展。     

Functional Columnar Liquid Crystalline Phases From Ionic Complexes of Dendronized Polymers and Sulfate Alkyl Tails

We describe how cationic dendronized polymers of generations 1, and 2 and anionic monoalkyl tails can be combined by supramolecular ionic complexation into comb-like liquid crystalline polymers. The final structures in bulk of these supramolecular complexes were studied by differential scanning calorimetry (DSC), cross-polarized optical microscopy (CPOM), small angle x-rays scattering (SAXS) and transmission electron microscopy (TEM). The combination of these techniques allowed elucidating (i) that these complexes exhibit thermotropic behaviour, (ii) that various liquid crystalline structures in the 3–5 nm length scale can be obtained such as columnar rectangular, columnar tetragonal, columnar hexagonal and lamellar, depending both on alkyl tail length and polymer generation, (iii) that although the alkyl tails represent the majority phase in the columnar phases, they form the cylindric domains, and the dendronized polymers occupy the continuous domains. Therefore, upon selective cleavage of the alkyl tails in the columnar phases, the present self-assembly approach may constitute an efficient strategy towards the formation of porous organic matrices with ultra-dense pore size in the range of 2 to 4 nm.     

The synthesis and thermal,optical and electrical properties of novel aromatic–aliphatic five- and six-membered thermotropic polyimides

Polymers have been prepared by the polycondensation of 4,4′-(butane-1,4-diylbis(oxy)) bis(butane-4,1-diyl) bis(4-aminobenzoate) and 5,10,15,20,25,30,35,40,45,50,55,60-dodecaoxatetrahexacontane-1,64-diyl bis(4-aminobenzoate) (PBBA 1200) with three dianhydrides based on naphthalene, perylene and phthalic moieties, respectively. This has resulted in five novel aliphatic–aromatic polyimides. The polyimides differed in aliphatic chain length and whether the imide ring was five- or six-membered. The chemical structure of the polyimides has been confirmed by ¹H NMR and FTIR spectroscopy and by elemental analysis. The optical and electrical properties of the polyimides have been studied using current–voltage measurements, and the effect of the polyimide structure on thermal and mesomorphic behaviour investigated by differential scanning calorimetry and polarising optical microscopy. Wide-angle X-ray diffraction at different temperatures was employed to confirm the structural properties of the polyimides. All the novel polyimides, with the sole exception of that obtained from PBBA1200 and 3,4,9,10-perylenetetracarboxylic dianhydride, showed liquid crystalline properties. As far as we are aware, this is the first time that six-membered polyimides exhibiting liquid crystalline properties have been reported.     

Mesogenic ketohexose and aldopentose derivatives: anomalous behaviour of the alkyl D-fructopyranosides

The thermotropic and lyotropic behaviour of a number of alkyl ketopyranosides, alkyl ketofuranosides, an alkyl pentopyranoside and an alkyl pentofuranoside were studied. With the exception of the alkyl beta-D-fructopyranosides, all the compounds display the expected smectic A* phases. The three alkyl fructopyranoside homologues studied (octyl, decyl and dodecyl) display a novel, rather viscous mesophase (monotropic), the nature of which is as yet unclear. The unknown phase is not a smectic A phase, because a phase transition from smectic A* to phase X is observed for both the decyl and the dodecyl derivative. The lyotropic behaviour of all the compounds in this study is quite similar to that reported earlier for other monoalkylated monosaccharide derivatives, except that the unknown phase X is again observed for the fructopyranoside derivatives.     

Synthesis and mesomorphic properties of three- and four-ring liquid crystals containing cyclohexyl,phenyl and pyridyl units

Two series of 4-(3-alkoxycarbonylpyridyl-6-ethynyl)phenyl trans-4-n-alkylcyclohexylcarboxylates and 4-(3-alkoxycarbonylpyridyl-6-ethynyl)phenyl 4-(trans-4-n-alkylcyclohexyl)benzoates have been prepared. Their mesmorphic properties were observed and measured by polarizing optical microscopy and differential scanning calorimetry. The relationship between structures and properties is discussed, showing that the ferroelectric phase can be observed when chiral terminal chains exist.     

Synthesis and liquid crystalline properties of mono-, di- and tri-O-alkyl pentaerythritol derivatives bearing tri-, di- or monogalactosyl heads: the effects of curvature of molecular packing on mesophase formation

Self-organisation and self-assembly are critical to the stability of synthetic and biological membranes. Of particular importance is consideration of the packing arrangements of the various molecular species. Both phospho- and glycolipids can pack in ways in which curvature can be introduced into self-organised or self-assembled systems. For instance, it is known that the degree of curvature can affect the structures of any condensed phases that are formed. In this article we report on a systematic study in which we have varied the shapes of glycolipids and examined the condensed phases that they form. In doing so, we have also unified the shape dependency of lyotropic liquid crystals with those of thermotropic liquid crystals. In order to undertake this systematic study a range of different pentaerythritol derivatives was synthesized, which covers combinations of one to three alkyl chains of different lengths (6,7,9,10,11,12,14,16 carbon atoms) and three to one galactosyl heads. Mono- and di-O-galactosyl derivatives were prepared directly by glycosylation of the corresponding alcohols using 2,3,4,6-tetra-O-benzoyl or acetyl-alpha-D-galactopyranosyl trichloroacetimidate or bromide as the donors; the tri-O-galactosyl derivatives were synthesized from O-alkyl-O-benzyl di-O-galactosyl pentaerythritol intermediates, followed by de-O-benzylation and glycosylation steps. All of the fully deprotected products were obtained by standard methods, and their self-organising and self-assembling properties examined.     

Engineered Anisotropic Fluids of Rare-Earth Nanomaterials

The self-assembly of inorganic nanoparticles into well-ordered structures in the absence of solvents is generally hindered by van der Waals forces, leading to random aggregates between them. To address the problem, we functionalized rigid rare-earth (RE) nanoparticles with a layer of flexible polymers by electrostatic complexation. Consequently, an ordered and solvent-free liquid crystal (LC) state of RE nanoparticles was realized. The RE nanomaterials including nanospheres, nanorods, nanodiscs, microprisms, and nanowires all show a typical nematic LC phase with one-dimensional orientational order, while their microstructures strongly depend on the particles’ shape and size. Interestingly, the solvent-free thermotropic LCs possess an extremely wide temperature range from −40 °C to 200 °C. The intrinsic ordering and fluidity endow anisotropic luminescence properties in the system of shearing-aligned RE LCs, offering potential applications in anisotropic optical micro-devices.     

Engineered Anisotropic Fluids of Rare‐Earth Nanomaterials

The self‐assembly of inorganic nanoparticles into well‐ordered structures in the absence of solvents is generally hindered by van der Waals forces, leading to random aggregates between them. To address the problem, we functionalized rigid rare‐earth (RE) nanoparticles with a layer of flexible polymers by electrostatic complexation. Consequently, an ordered and solvent‐free liquid crystal (LC) state of RE nanoparticles was realized. The RE nanomaterials including nanospheres, nanorods, nanodiscs, microprisms, and nanowires all show a typical nematic LC phase with one‐dimensional orientational order, while their microstructures strongly depend on the particles’ shape and size. Interestingly, the solvent‐free thermotropic LCs possess an extremely wide temperature range from ?40 °C to 200 °C. The intrinsic ordering and fluidity endow anisotropic luminescence properties in the system of shearing‐aligned RE LCs, offering potential applications in anisotropic optical micro‐devices.     

Shape dependence in the formation of condensed phases exhibited by disubstituted sucrose esters

We report on the self-organizing properties of sucrose esters that are di-(1',6', 1',6, and 6,6')-substituted with aliphatic chains of identical or different chain lengths and levels of saturation. For the materials possessing two saturated aliphatic chains, the compounds exhibited thermotropic lamellar smectic A phases. A remarkable new phase transition was observed for the di-octadecanoyl homologue in which one smectic A phase transformed into another with a continuous change in layer spacing, but with a discontinuous change in the correlation length. The incorporation of long cis-unsaturated chains led to increased cross-sectional areas of the chains relative to the sucrose head groups and, hence, columnar phases were observed.     

Mesomorphic Behavior of Symmetric Azomethine Dimers Containing Different Chromophore Groups

A series of new azomethine dimers was synthesized by the condensation reaction of flexible bis-benzaldehydes with four aromatic amines containing phenyl, naphthyl, anthracene and pyrene groups. Their right structure was confirmed by FTIR and ¹H-NMR spectroscopy and their thermal properties were investigated by thermogravimetric analysis, differential scanning calorimetry and polarized light optical microscopy. A view on their photophysical behavior was gained by UV-vis and photoluminescence spectroscopy. The dimers containing pyrene and anthracene presented liquid crystalline behavior, while the other dimers were crystalline compounds. Two dimers containing pyrene moieties formed mesomorphic glasses and had intense luminescence, promising properties for applications in building optoelectronic devices.     

Lyotropic and thermotropic behavior of Alkylglucosides and related compounds

 The phase diagram of the binary system composed of octyl-β-D-glucopyranoside and water was investigated and the phase boundaries were determined. Polarising optical microscopy was used to define the different phases, proton and deuterium NMR experiments to define the region of existence of the different phases and to obtain information on axiality and head group solvation. DSC experiments were performed to determine the thermal transitions from solid to thermotropic liquid crystals for octyl-β-D-gluco-pyranoside, the related alkylglucosides or maltosides, and to gain information on the role played by sugar units in the thermodynamics of such phase transitions. Received: 15 May 1997 Accepted: 08 September 1997     

Cubic liquid-crystalline nanoparticles

Cubic liquid-crystalline nanoparticles prepared from aqueous dispersions of cubic lipid-water phases are kinetically stable in the presence of certain dispersing agents. The properties of cubic nanoparticles from monoolein-water and other lipid-water systems have been examined with a variety of experimental techniques. The cubic nanoparticles can be stabilized by polymerization of the reactive lipids in cubic lipid assemblies. Several low-energy-input methods have been developed to facilitate the production and application of cubic nanoparticles. The ability to incorporate and deliver lipophilic, amphiphilic, and water-soluble molecules in a controlled manner and the good biocompatibility of cubic nanoparticles make them excellent candidates for drug-delivery applications.     

Ethanol-induced changes in glycolipids of Saccharomyces cerevisiae

Total glycolipid content of Saccharomyces cerevisiae cells increased in ethanol-treated yeast cells. Sialic acid and hexosamine contents of glycolipids from ethanol-treated cells decreased, whereas those of hexoses increased. Increased sialidase activity in the presence of ethanol may be responsible for the decrease in sialic acid content of glycolipids. The saccharide moieties of glycolipids of S. cerevisiae consisted of fucose, mannose, galactose, and glucose. Ethanol treatment of yeast cells caused an increase in glucose and a decrease in galactose content of glycolipids. The changes in glucose content can be related to changes in β-glucosidase activity under alcohol stress. The content of cerebrosides, sulfatides, and monoglucosyldiglycerides was enhanced following ethanol treatment. An increase in cerebroside as well as in sulfatide content during alcohol stress might play an important role in stabilizing the membrane both physically and structurally. Such variations in glycolipid content and composition of S. cerevisiae cells may represent an adaptive response to ethanol stress.     

Synthesis of a Monophosphoryl Derivative of Escherichia coli Lipid A and Its Efficient Coupling to a Tumor‐Associated Carbohydrate Antigen

Monophosphoryl lipid A is a safe and potent immunostimulant and vaccine adjuvant, which is potentially useful for the development of effective carbohydrate‐based conjugate vaccines. This paper presents a convergent and efficient synthesis of a monophosphoryl derivative of E. coli lipid A that has an alkyne functionality at the reducing end, which is suitable for coupling with various molecules. The coupling of this derivative to an N‐modified analogue of tumor‐associated antigen GM3 through click chemistry is also presented.     

Development and Application of Liquid Crystals as Stimuli-Responsive Sensors

This focused review presents various approaches or formats in which liquid crystals (LCs) have been used as stimuli-responsive sensors. In these sensors, the LC molecules adopt some well-defined arrangement based on the sensor composition and the chemistry of the system. The sensor usually consists of a molecule or functionality in the system that engages in some form of specific interaction with the analyte of interest. The presence of analyte brings about the specific interaction, which then triggers an orientational transition of the LC molecules, which is optically discernible via a polarized optical image that shows up as dark or bright, depending on the orientation of the LC molecules in the system (usually a homeotropic or planar arrangement). The various applications of LCs as biosensors for glucose, protein and peptide detection, biomarkers, drug molecules and metabolites are extensively reviewed. The review also presents applications of LC-based sensors in the detection of heavy metals, anionic species, gases, volatile organic compounds (VOCs), toxic substances and in pH monitoring. Additionally discussed are the various ways in which LCs have been used in the field of material science. Specific attention has been given to the sensing mechanism of each sensor and it is important to note that in all cases, LC-based sensing involves some form of orientational transition of the LC molecules in the presence of a given analyte. Finally, the review concludes by giving future perspectives on LC-based sensors.