Penetration by water of polyhydroxy amphiphiles in the crystalline versus the liquid crystalline states

The penetration of water in contact preparations with polyhydroxy amphiphiles in different aggregation states is compared. The onset of penetration is found at much higher temperatures when the samples are in the solid state than when they are in a super-cooled liquid-crystalline state in the case of the smectic A_d and smectic B₂phases, but not the columnar hexagonal (D_hd) mesophase. The enhanced accessibility of the bilayer smectic phases can be explained by assuming that the molecular arrangement in the layers is similar to that found in the lamellar lyotropic phase, where the polar groups are on the outside and the (partially intercalated) alkyl chains are in the core of the layers.     

Synthesis of azobenzene-containing liquid crystalline gelator for use in liquid crystal gels

A liquid crystalline gelator containing the azobenzene chromophore was synthesized for the first time; it was used to form self- assembled network in nematic liquid crystals resulting in liquid crystal gels with distinct features.     

Effect of stoichiometry on liquid crystalline supramolecular polymers formed with complementary nucleobase pair interactions

We report herein studies on the liquid crystalline behavior of a series of supramolecular materials that contain different ratios of two complementary symmetrically-substituted alkoxy-bis(phenylethynyl)benzene AA- and BB-type monomers. One monomer has thymine units placed at either end of the rigid mesogenic core, while the other has N⁶-(4-methoxybenzoyl)-adenine units placed on the ends. Differential scanning calorimetric and polarized optical microscopy studies have been carried out on these systems. These studies show that the material's behavior is strongly dependent on its thermal history. As a result, the materials can exhibit, on heating, either a liquid crystalline phase, a crystalline phase, or the coexistence of crystalline and liquid crystalline regions. © 2006 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 44: 5049–5059, 2006     

Lyotropic liquid crystalline phases formed by phyosterol ethoxylates in room-temperature ionic liquids

The phase behaviors of four phytosterol ethoxylates surfactants (BPS-n, n = 5, 10, 20, and 30) with different oxyethylene units in room temperature ionic liquid, 1-butyl-3-methylimidazolium tetrafluoroborate ([Bmim]BF₄), have been studied. The polarized optical microscopy and small-angle X-ray scattering techniques are used to characterize the phase structures of these binary systems at 25 °C. The structure and ordering of the liquid crystalline (LC) phases in such BPS-n/[Bmim]BF₄ systems are found to be influenced by BPS-n concentration and the temperature. Due to the bulky and rigid cholesterol group, the phytosterol ethoxylates surfactants exhibit different properties and interaction mechanism from the conventional C_nEO_m type nonionic surfactant systems. The rheological measurements indicate a highly viscoelastic nature of these lyotropic LC phases and disclose a lamellar phase characteristic with a rather strong rigidity at high surfactant concentrations. The control experiment with Brij 97(polyoxyethylene (10) oleyl ether)/[Bmim]BF₄ system and the FTIR measurements help to recognize that the solvophobic interaction combining with the hydrogen bonding are the main driving forces for the LC phases formation.     

Morphology control of liquid crystalline composite gels based on molecular self-assembling kinetics

Liquid crystalline composite gels consisting of a low molecular mass gelator and a low molecular mass liquid crystal were prepared by two types of gelation method (continuous cooling and isothermal gelating), which provide different molecular self-assembling kinetics of the low molecular mass gelator as gelation proceeds. Optical microscopy and atomic force microscopy revealed that numerous fine strands of the one-dimensionally assembled low molecular mass gelators were formed in the composite gels for both the continuous cooling method and the isothermal gelating method. However, the thinner strands were more homogeneously dispersed in the isothermal gelation product at an appropriate temperature, than in the continuous cooling process. This difference in dispersion state of the strands was shown (by polarizing optical microscopy) to have a significant influence on the molecular alignment of the low molecular mass liquid crystal in the liquid crystalline composite gel. The electro-optical response and light scattering-transmitting switching, of the liquid crystalline composite gel in an applied electric field was extremely dependent on the morphology of the gelators. High contrast light switching was achieved for the composite prepared by isothermal gelation. The response time of electro-optical switching was less than 100 µs under 30 V_rms.     

Microstructure and stability of a lamellar liquid crystalline and gel phase formed by a polyglycerol ester mixture in dilute aqueous solution

The self-assembly behavior of a commercial mixture of polyglycerol fatty acid esters (PGE) and water is investigated as a function of temperature and surfactant content. The phase diagram of this pseudo-binary mixture was characterized using a combination of cross-polarized light and freeze-fracture electron microscopy (cryo-SEM), X-ray diffraction (XRD), small-angle neutron scattering (SANS), and differential scanning calorimetry (DSC). Our experiments show that the morphology of the supramolecular aggregates is lamellar and present in the form of a continuous or dispersed phase (multilamellar vesicles) depending on the water content of the system. Under the effect of temperature, the short- and long-range order of the bimolecular layers successively changes from a biphasic surfactant dispersion to a lamellar liquid-crystalline (Lalpha) and a stable lamellar gel phase (Lbeta) upon cooling; this transition is found to be irreversible. Formation of the lamellar aggregates can be related to the average molecular structure and shape factor of PGE. The stability of the resulting gel phase (Lbeta) appears to be due to the presence of small amounts of unreacted ionic co-surfactant, namely, fatty acid soaps, in this per se nonionic commercial mixture.     

Morphology control of liquid crystalline composite gels based on molecular self-assembling kinetics

Liquid crystalline composite gels consisting of a low molecular mass gelator and a low molecular mass liquid crystal were prepared by two types of gelation method (continuous cooling and isothermal gelating), which provide different molecular self-assembling kinetics of the low molecular mass gelator as gelation proceeds. Optical microscopy and atomic force microscopy revealed that numerous fine strands of the one-dimensionally assembled low molecular mass gelators were formed in the composite gels for both the continuous cooling method and the isothermal gelating method. However, the thinner strands were more homogeneously dispersed in the isothermal gelation product at an appropriate temperature, than in the continuous cooling process. This difference in dispersion state of the strands was shown (by polarizing optical microscopy) to have a significant influence on the molecular alignment of the low molecular mass liquid crystal in the liquid crystalline composite gel. The electro-optical response and light scattering–transmitting switching, of the liquid crystalline composite gel in an applied electric field was extremely dependent on the morphology of the gelators. High contrast light switching was achieved for the composite prepared by isothermal gelation. The response time of electro-optical switching was less than 100?µs under 30?V_rms.     

Effect of hydrophobicity on the stability of the wetting films of water formed on gold surfaces

We have developed a methodology that can be used to determine disjoining pressures (Π) in both stable and unstable wetting films from the spatial and temporal profiles of dynamic wetting films. The results show that wetting films drain initially by the capillary pressure created by the changes in curvature at the air/water interface and subsequently by the disjoining pressure created by surface forces. The drainage rate of the film formed on a gold surface with a receding contact angle (θ(r)) of 17° decreases with film thickness due to a corresponding increase in positive Π, resulting in the formation of a stable film. The wetting film formed on a hydrophobic gold with θ(r)=81° drains much faster due to the presence of negative Π in the film, resulting in film rupture. Analysis of the experimental data using the Frumkin-Derjaguin isotherm suggests that short-range hydrophobic forces are responsible for film rupture and long-range hydrophobic forces accelerate film thinning.     

Effect of ionic aggregates of sulphonate groups on the liquid crystalline behaviours of liquid crystalline elastomers

A series of siloxane-based liquid crystalline elastomers containing biphenyl benzoate mesogenic units and ionic Brilliant Yellow moieties was synthesized. The chemical structures and liquid crystalline properties of the samples were characterized by FTIR, ¹H NMR, DSC, POM and XRD. The effective crosslink density of the ionic elastomers was determined by swelling experiments in organic/buffer mixtures. All the polymers displayed a smectic mesophase. It was shown that the glass transition and melting point temperatures of the polymers increased slightly with increasing content of ionic and mesogenic groups in the polymers, while the liquid crystal mesophase region decrease slightly.     

Effect of silica colloids on the rheology of viscoelastic gels formed by the surfactant cetyl trimethylammonium tosylate

The effects of the addition of submicrometer-sized colloidal silica spheres on the linear and nonlinear rheology of semidilute solutions of a viscoelastic gel are studied. For a 1.4 wt% solution of the surfactant CTAT, a peak in the zero-shear rate viscosity eta(0) is observed at approximately equal weight percents of silica and CTAT. This peak shifts to lower silica concentrations on increasing either the CTAT concentration or the surface charge on silica and disappears when the CTAT concentration is increased to 2.6 wt%. The increases in eta(0) and the high frequency plateau modulus G(0) on the introduction of SiO(2) are explained by considering the increasingly entangled wormlike micelles that are formed due to the enhanced screening of the electrostatic interactions. The observed decrease in the values of G(0) and eta(0) at higher concentrations of silica particles is explained in terms of the formation of surfactant bilayers due to the adsorption of the positively charged cetyl trimethylammonium to the negatively charged silica.     

Study of the liquid crystalline phases of benzopurpurin in water

We report the NMR study of the liquid crystalline phases of aqueous solutions of the dye benzopurpurin (BP-4B). Upon changing dye concentration, the system exhibits a phase transition between two ordered phases at about 3·5 wt %. The structure of these phases was not determined but the evidence suggests that they consist of columns or helically twisted columns of stacked dye molecules which are randomly oriented, similar to nematic phases.     

Synthesis and characterisation of main-chain hydrogen-bonded supramolecular liquid crystalline complexes formed by azo-containing compounds

A series of azopyridine-containing hydrogen bonding acceptors (4a-c) with flexible spacers of oligo(methylene) were synthesised. Hydrogen-bonded polymeric complexes 4/5 and trimeric complexes 4/6₂ , where 5 and 6 are aromatic dicarboxylic acids and monocarboxylic acids, respectively, were prepared and their liquid crystallinity was examined using differential scanning calorimetry and polarising optical microscopy. The study showed that most of the complexes displayed reversible thermotropic nematic phase. The isotropic to nematic phase transition temperatures of polymeric complexes 4/5 and trimeric complexes 4/6₂ in general decreased with the increase in length of spacers and terminal groups in the corresponding proton acceptors 4 and the proton donors 5 and 6, respectively. Hydrogen bonding interactions in complexes 4/5 and 4/6₂ were studied by X-ray photoelectron spectroscopy and Fourier transform infrared spectroscopy.     

A new type of square columnar liquid crystalline phases formed by facial amphiphilic triblock molecules

A series of three novel liquid crystalline amphiphilic molecules is reported which are composed of three incompatible molecular parts, a rigid terphenyl core, two lipophilic decyloxy chains in the terminal 4- and 4' '-positions, and a polar group in the lateral 2'-position. The polar group comprises a polyether chain, an amide group, and a polyhydroxyalkyl end group (1-acylamino-1-deoxy-d-sorbitol derivatives). The self-organization of these compounds was studied by polarized light microscopy, differential scanning calorimetry, and different X-ray diffraction techniques. These investigations confirm a novel liquid crystalline phase with a square 2D-lattice (square columnar mesophase, plane group p4mm). This structure is built up by a set of three distinct columns, namely columns containing the polar lateral groups, columns incorporating the alkyl chains, and ribbons of the rodlike terphenyl units. The calamitic cores form walls bounding square-shaped channels occupied by the microsegregated polar lateral chains. The lipophilic columns containing alkyl chains are at the corners interconnecting the aromatic rods end-to-end.     

Hexagonal liquid crystalline phases formed in ternary systems of Brij 97-water-ionic liquids

Phase diagrams of two ionic liquids: hydrophobic 1-butyl-3-methylimidazolium hexafluorophosphate (bmim-PF(6)) and relatively hydrophilic 1-butyl-3-methylimidazolium tetrafluoroborate (bmim-BF(4)) in aqueous solutions of Brij 97 were determined at 25 degrees C. Two hexagonal liquid crystalline phases formed in bmim-PF(6)- and bmim-BF(4)-containing ternary systems were investigated by means of small-angle X-ray scattering (SAXS) and rheological techniques, with comparison of composition and temperature effects. From analysis of the SAXS data, bmim-PF(6) is dominantly penetrated between the oxyethylene chains of surfactant molecules, whereas bmim-BF(4) is mainly located in the water layer of hexagonal phases. The strength of the network of hexagonal phase formed in the Brij 97/water/bmim-BF(4) system is appreciably stronger than that of the Brij 97/water/bmim-PF(6) system, indicated by the smaller area of the surfactant molecule at the interface and the higher moduli (G', G' '). Temperature has a converse effect on the lattice parameters of the two hexagonal phases.     

Study on formation of gels formed by polymer and zirconium acetate

The gel system used in the preparation of dispersed particle gel for water shutoff treatments, which is composed of polyacrylamides and zirconium acetate, was investigated. The gelation process, the effects of various parameters on the gelation properties, the thermal stability, and the microstructure were addressed. The cross-linking reaction process is divided into three successive steps: induction, rapid cross-linking, and stabilization. High polymer and crosslinker concentrations reduce gelation time and increase gel strength. In addition, adding salts to the brine or increasing the temperature also decrease gelation time and increase gel strength. The optimum pH for the gel system is 7.49. In field applications, this gel system is recommended to be used within 130 °C using differential scanning calorimetry. The gel formed in a three-dimensional network structure was confirmed through environmental scanning electron microscopy.     

Effect of substituents on the curing of liquid crystalline epoxy resin

The synthesis of an aromatic ester based liquid crystalline epoxy resin (LCE) with a substituent in the mesogenic central group is described. Chlorine and methyl groups were introduced as substituents. The curing behaviors of three epoxy resins were investigated using diaminodiphenyl ester as the curing agent. The curing rate and heat of curing of LCE were measured with dynamic and isothermal DSC. The chlorine substituent accelerated the curing of LCE, while the methyl substituent decelerated the curing of LCE. The heat of curing of substituted LCE was diminished compared to LCE with no substituent. Glass transition temperature and elastic modulus of LCE decreased with increasing the size of the substituent. Three liquid crystalline epoxy resins based on aromatic ester mesogenic groups formed a liquid crystalline phase after curing, and the liquid crystalline phase was stable up to the decomposition temperature. © 1998 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 36: 911–917, 1998     

Studies on thermotropic liquid crystalline polymer. XI. Effect of amide group on thermotropic liquid crystalline polymer properties

A series of poly(azomethine)s containing amide, ether, or ester groups was prepared by the condensation of dialdehydes with various diamines. The thermotropic liquid crystalline properties were examined by DSC and microscopic observations. The effects of the number and position of amide groups, which are attached to the rigid segment, on the thermotropic liquid crystalline properties of the homo-and copoly(amide-azomethine-ether)s were also investigated in this study. The copolymerization took place by changing the amount of amide group to obtain copoly(amide-azomethine) ( P13 and P14 ) which exhibit thermotropic liquid crystalline properties. The poly(azomethine)s containing ether or ester groups also exhibited thermotropic liquid crystalline properties. © 1993 John Wiley & Sons, Inc.     

A Study on the Stability and Enzymatic Activity of Yeast Alcohol Dehydrogenase in Presence of the Self-Assembling Block Copolymer Poloxamer 407

Yeast alcohol dehydrogenase (ADH) is an enzyme widely studied for biotechnological applications due to its involvement in fermentation industry, and various attempts to improve its catalytic properties and its thermal stability have been carried out. In this paper, the influence of a block copolymer (Poloxamer 407) on ADH enzymatic activity and thermal behaviour has been studied in order to get new insights about the use of poloxamers in formulation of sustained release systems for therapeutic proteins. Poloxamer 407 has the ability to form micelles and gel due to its self-assembling and thermoresponsive properties. The effect of the copolymer towards thermal stress and pH changes, which often reduce enzymes activity it has been investigated by means of enzymatic assays and differential scanning calorimetry. Results showed that at pH?9.1 and 7.3, the Poloxamer in the form of unimeric, micellar and gel state is able to effectively preserve the enzyme from thermoinactivation. In addition by calorimetric data Poloxamer 407 has showed an effect in preserving ADH from aggregation at pH?7.3. In conclusion, Poloxamer 407 seems to be very effective in protecting ADH from stress related events, like alkaline inactivation and aggregation.     

Minimization of spatial inhomogeneity in polystyrene gels formed by free-radical mechanism

The spatial inhomogeneity in polystyrene (PS) gels has been investigated with the static light scattering technique. PS gels were prepared starting from styrene monomer and ethylene glycol dimethacrylate crosslinker in a homogeneous solution. The gel synthesis parameters varied were the crosslinker concentration, the primary chains length and the quality of the polymerization solvent. The gels were characterized by elasticity tests as well as by light scattering measurements at a gel state just after their preparation. The degree of spatial gel inhomogeneity decreased with decreasing crosslinker content, with decreasing primary chain length or, with increasing quality of the polymerization solvent. It was shown that the gel synthesis parameters varied mainly affect the distance between the pendant vinyl groups locating on the same macromolecule during the gel formation process. Increasing the distance between the pendant vinyl groups reduces the rate of the multiple crosslinking reactions so that the resulting PS gels exhibit a lesser degree of inhomogeneity.