Liquid Crystalline Textures Research Articles

An Optically Active Diarylethene Having Cholesterol Units: A Dopant for Photoswitching of Liquid Crystal Phases

Abstract A diarylethene1 having cholesterol units was synthesized in an attempt to use it as a dopant to control the phase of liquid crystals. By the addition of the diarylethene, 4-cyano-4′-pentylbiphenyl (K-15)(Tokyo kasei) nematic liquid crystal showed cholesteric liquid crystalline texture. Upon irradiation with UV light, the cholesteric phase was decomposed. The cholesteric phase was recovered by visible light irradiation. The reversible photoinduced-phase change was observed by alternate irradiation with UV and visible light.

Studies on the phase transition and thermal stability of Xydar and Zenite series liquid crystalline polymers

AbstractFour commercially available main‐chain LCPs, Xydar and Zenite series, are characterized by FTIR spectroscopy, DSC, polarized light microscopy (PLM), TGA, TGAFTIR, and elemental analysis. The FTIR spectrum of Zenite 8000B is found to be more similar to that of Vectra A950 than those of the other three LCPs. Xydar SRT900 and 1000 have almost the same FTIR spectra. Zenite LCPs have broader crystal‐mesophase transition (TCM) and less clear liquid crystalline textures at TCM than those of Xydar LCPs. Xydar SRT1000 and Zenite 8000B have lower TCM than Xydar SRT900 and Zenite 6000, respectively. For all the four LCPs, the glass transitions can not be easily observed by DSC without the aid of annealing and the isotropic phase does not appear before thermal degradation. Xydar SRT1000 and Zenite 8000B have lower thermal stabilities as well as lower TCMS than Xydar SRT900 and Zenite 6000, respectively. It is interesting to find that Ea (Apparent activation energy of thermal degradation) curv of Xydar SRT900 and SRT1000 have the similar shapes as those of Zenite 6000 and 8000B, respectively. For the first time, we have found that there is a minor degradation maximum for Zenite 8000 at about 2.5% weight loss in N2. For all the four LCPs, the Ea values in an air atmosphere begin to decrease at a temperature close to the deflection points on the first derivative curves. The first stage of the thermal degradation in an air atmosphere has similar mechanisms as that in N2.

Crystalline Morphology Developing from Cholesteric Mesophase in Cyanoethyl Chitosan Solutions

Two completely different crystals can grow from cyanoethyl chitosan/formic acid solutions in which the anisotropic liquid crystalline (LC) and isotropic phases coexisted. One was normal spherulites developing from isotropic region, and the other was cholesteric-like crystallites developing from mesophase region. The latter had similar optical characters as fingerprint texture. This implies that the crystal structure in solution cast film from cholesteric LC phase remained the main structure characters of cholesteric LC texture. A model is presented to explain the crystallization process from these liquid crystalline polymer solutions.

Fine structure in cholesteric fingerprint texture observed by scanning electron microscopy

The fine structure in some cholesteric fingerprint textures of cyanoethyl chitosan (CNCS)/acrylic acid solution and photo-solidified CNCS/polyacrylic acid composite film was studied using scanning electron microscopy (SEM), small angle light scattering (SALS) and polarizing optical microscopy (POM). Permanganic etching was developed to reveal the cholesteric liquid crystalline textures in the composite films. A schematic model was presented to explain this kind of fine structure. The directors within each cholesteric molecular layer did not orient in nematic-like order, but varied their orientation by an ill-defined period.

Equilibrium nanostructure of primary soot particles

This paper addresses the origin of the characteristic shell/core nanostructure of primary soot particles, in which a zone of concentrically oriented graphene layers surrounds a disordered core. In situ microscopy is used to examine the liquid crystalline optical textures on the external surfaces of droplets and pools of molten mesophase pitch, taken as a model system for investigating the self-organization of high-molecular weight polycyclic aromatic hydrocarbons in condensed phases. A general thermodynamic theory is then developed that governs the equilibrium configuration of ensembles of disk-like polyaromatic compounds with and without chemical reactivity at edge sites. The experiments and theory indicate that the shell/core nanostructure is the equilibrium configuration of nearly planar polyaromatic clusters under the high-temperature reactive conditions of soot formation. The boundary between the concentric shell and the disordered core is predicted to be the point where orientational elastic strain overcomes the free energy difference between bulk isotropic and ordered phases. The theory also yields a quantitative estimate of the size of the core regions, 4–6 nm, in agreement with the observed core diameters in many mature soot particles. Implications for the soot formation process are discussed.

Brownian motion of suspended particles in an anisotropic medium

Some experimental results in the case of anisotropic media are at variance with the Einstein-Stokes formula of Brownian motion and the Singwi-Sjolander model. The disagreement comes from the anchor effect. The concept of “pseudoparticle” is introduced in order to retain the Singwi-Sjolander formalism. Mossbauer spectroscopy can be used to estimate the thickness of the anchored liquid-crystalline molecular textures forming on the surface of Brownian particles.

Synthesis, Characterization of An Unsymmetrical Cu(II) Complex Liquid Crystal of a Schiff Base Containing a Terminal Double Bond

The unsymmetrical Schiff base ligand 4-undecenyloxy-salicylidene-4-dodecyloxyaniline (Im-11–12) containing a terminal double bond derived from 4-undecenyloxysalicylaldehyde and 4-dodecyloxyaniline and its Cu(II) complex [Cu(Im-11–12)2] were synthesized and characterized by elemental analyses, molar conductivity, IR, electronic spectra and 1H NMR. The Schiff base functions as a uninegative bidentate ligand and bonds through oxygen and imino-nitrogen. The complex is a non-electrolyte and has a square planar geometry. The mesogenic behavior of the ligand and the complex was determined by a hot-stage polarized microscope and DSC. The transition processes of the ligand and the complex were tentatively proposed to include K→S→I, but their liquid crystalline texture structures were different.

Syntheses and characterization of poly(ethylene terephthalate) modified withp-acetoxybenzoic acid

Though the structure and properties of a copolyester of 40 mole % of polyethylene terephthalate (PET) and 60 mole % p-hydroxybenzoic acid (PHB) (PET/60PHB) and their blends have been well documented, no work has been reported in an open literature on the systematic investigation of the PET copolymers modified with broad range of p-acetoxybenzoic acid (PABA) composition as yet. In this study, several PETA-x copolyesters having various PABA compositions from 10 to 70 mole % were prepared by the melt reaction of PABA and PET without a catalyst, where x denotes the mole % of PABA. And the modified polyesters obtained were characterized by 1H-NMR spectrophotometry, X-ray diffraction pattern, polarizing microscopy, thermal analysis, and rheometry. The anisotropic phase appeared when x is above 50 mole % of PABA, and especially for the x's of 60 and 70 mole %, the nematic liquid crystalline texture appeared clearly on the whole matrix. As the mole % of PABA increased, melting temperature, heat of fusion, crystalline temperature, degree of crystallinity, and the glass transition temperature of the modified PET were decreased, but the thermal stabilities of those copolyesters were increased. The dependence of melt viscosity on the shear rate for PETA-50 ∼ 70 followed the typical rheological behavior of liquid crystalline polymers. Finally, it was concluded that the PETA-x copolyesters having compositions of higher than 50 mole % of PABA exhibit the behavior of thermotropic liquid crystalline polymers. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 73: 1707–1719, 1999

Rheo-dielectric behavior of low molecular weight liquid crystal 2. Behavior of 8CB in nematic and smectic states

Rheo-dielectric behavior was examined for 4−4′−n-octyl-cyanobiphenyl (8CB) having large dipoles parallel to its principal axis (in the direction of the C≡N bond). In the quiescent state at all temperatures (T) examined, orientational fluctuation of the 8CB molecules was observed as dielectric dispersions at characteristic frequencies ωc>106 s−1. In the isotropic state at high T, no detectable changes of the complex dielectric constant ɛ*(ω) were found under slow flow at shear rates ˙γ≫ωc. In the nematic state at intermediate T, the terminal relaxation intensity of ɛ*(ω) was decreased under such slow flow. In the smectic state at lower T, the flow effect became much less significant. These results were related to the flow-induced changes of the liquid crystalline textures in the nematic and smectic states, and the differences of the rheo-dielectric behavior in these states are discussed in relation to a difference of the symmetry of molecular arrangements in the nematic and smectic textures.

Liquid crystals and flow elongation in a spider's silk production line

Our observations on whole mounted major ampullate silk glands suggested that the thread is drawn from a hyperbolic die using a pre–orientated lyotropic liquid crystalline feedstock. Polarizing microscopy of the gland's duct revealed two liquid crystalline optical textures: a curved pattern in the feedstock within the ampulla of the gland and, later in the secretory pathway, the cellular texture previously identified in synthetic nematic liquid crystals. The behaviour of droplet inclusions within the silk feedstock indicated that elongational flow at a low shear rate occurs in the gland's duct and may be important in producing an axial molecular orientation before the final thread is drawn. Our observations suggested that the structure of the spider's silk production pathway and the liquid crystalline feedstock are both involved in defining the exceptional mechanical properties of spider dragline silk.

Synthesis and Characterization of Poly(Propylene Imine)-Based Liquid Crystalline Dendrimers

Poly(propylene imine)-based liquid crystalline dendrimers (PPILCDs) with a relatively flexible dendritic scaffold were successfully synthesized by reaction of poly(propylene imine) dendrimers with 4′-cyanobiphenyl hydrogen glutarate in the presence of condensing agent, diphenyl(2,3-dihydro-2-thioxo-3-benzoxazolyl)phosphonate. The structures of PPILCDs were characterized by IR, 1H NMR and MALDI-TOF mass spectroscopies and elemental analysis. These LCDs did not show a thermotropic liquid crystalline nature, but exhibited the liquid crystalline textures in the samples sheared at 70 °C. Furthermore, they showed a lyotropic liquid crystalline property in the 80 wt% DMF solutions.

Crystallization morphology of a thermotropic liquid crystalline polymide

AbstractCrystallization morphology of an aromatic thermotropic liquid crystalline polyimide was studied by means of polarized light microscopy. The polyimide was synthesized from 1, 2, 4, 5‐benzenetetracarboxylic dianhydride (PMDA) and 1, 3‐bis[4'(4′ aminophenoxy) cumyl] benzene (BACB), which exhibited three exothermic peaks at onset temperatures of 292, 279, and 250°C in the DSC cooling curve. The results of polarized light microscopy revealed that the polyimide quenched from 350°C in air exhibited fine structure, frozen liquid crystalline texture. The liquid crystalline texture disappeared when the sample was heated to 298°C. However, the polymer melt still exhibited, to some degrees, birefringence until the temperature reached 340°C, where the polyimide was in the truly isotropic state. Isothermal crystallization experiments were carried out at both the isotropic temperature range and the non‐isotropic temperature range. Two types of negative spherulites with lamellar structure and positive needle‐like crystals formed from the isotropic melt have been observed. Interestingly, with a further decrease of the crystallization temperature to the liquid crystalline state temperature, the whole field was covered by the liquid crystalline texture. However, if the sample was kept at 286°C for a long time, and then reheated to 305°C to melt the liquid crystalline phase, negative spherulites with loose structures formed from the liquid crystalline state could be observed. Surprisingly, if the sample was kept at 305°C for a period of time, further crystallization could be observed using the spherulites formed at 286°C as nuclei. Composite spherulites were developed if the low‐high temperature crystallization process was repeated.

Novel Liquid Crystalline Oxadiazole with High Electron Mobility

Very high electron mobility is reported for the liquid-crystalline oxadiazole compound hexyloxyphenyl hexyloxybiphenyl oxadiazole (HOBP-OXD). The synthesis, phase-transition behavior, and electron transport properties of this compound are detailed. HOBP-OXD is found to exhibit one nematic and two smectic phases. A jump in electron mobility is observed at the phase transition between the two smectic phases, leading to the conclusion that liquid-crystalline textures govern the electron mobility through molecular order.

Thickness control and defects in oriented mesoporous silica films

By using a surfactant-based synthesis strategy, we have earlier demonstrated that the polymerization and growth of silicate micellar assemblies at the air–water interface, under quiescent and dilute acidic aqueous conditions, yields free-standing sub-micron thickness hexagonal mesoporous silica films in which the channels are oriented parallel to the film surface. TEM imaging studies of these thin films showed that microscopic defects pervade the channel structure with topologies resembling those found in lyotropic liquid crystals. This suggested that the mesoporous silica film evolved from silicification of a surface lyotropic silicate mesophase. Herein it is demonstrated that film growth, defect structure, extent of polymerization, and mesoporosity sensitively depend on the choice of synthesis acidity, temperature and mixing, and in the case of supported films, on the choice of substrate. In particular, a ten-fold increase in the thickness of the film can be obtained by simply lowering the acidity and moving to ambient temperature conditions whilst an alteration in mixing conditions can change the film from a discrete to a continuous morphology. Combined PXRD, TEM and nitrogen adsorption studies show that the silica films are hexagonal, oriented and mesoporous. Furthermore, the observation of a focal conic fan-type texture in the free-standing films shows that defect controlled director fields, that exist in a precursor hexagonal lyotropic silicate mesophase, are preserved in the channel structure of the mesoporous silica phase. Proof-of-existence of liquid crystalline texture in such free-standing mesoporous silica films, provides direct evidence that film growth evolves from the cooperative assembly and organization of silicate micellar species at the air–water interface.

Tri-substituted s-Triazines as Mesogenic Cores

Two series of equivalently- and inequivalently-tri-substituted s-triazines (as ESTs and ISTs, respectively) were synthesized and characterized in this study. More than one mesogenic biphenyl substituent were required to display mesomorphic properties, and two or three biphenyl substituents resulted in liquid crystalline textures under polarizing light. The mesophase transition temperatures for both IST and EST series are quite close despite their molecular weight difference. With longer sidechains, s-triazine for IST series formed a columnar hexagonal disordered mesophase (D hd); while for the analogous compound in EST series, a perfectly aligned layered structure was found to be co-existed with the D hd phase. With shorter sidechains, the s-triazines exhibited nematic mesomorphism.

Morphological Development in Alkyl-Substituted Semiflexible Polymers

Nonsymmetrically substituted semiflexible polymers were synthesized by reacting (hydroxypropyl)cellulose (HPC) with hexyl, octyl, dodecyl, and octadecyl isocyanates. X-ray diffraction studies indicate that the polymer chains are assembled in regular, layered arrays. Polymers with six and eight carbons in the side chains (C6HPC and C8HPC) showed spherulitic structures typical of common crystalline polymers when examined by a polarized optical microscope (POM), whereas liquid crystalline (LC) textures were observed for C12HPC and C18HPC. When the polymers were slowly cooled from the melt, crystalline (C) textures could be obtained for C12HPC and C18HPC at preselected temperatures slightly higher than the corresponding (isotropic → LC) transition temperatures determined by DSC cooling scans. Upon further cooling, liquid crystalline structures appeared at temperatures 5−6 °C lower. The unusual POM observations are in contrast to conventional thermal transitions reported for liquid crystalline polymers, where the isotropic → LC transition takes place at a temperature higher than that of the LC → C transition, but are consistent with Keller's studies on another liquid crystalline polymer.

Multicritical phenomena in flow of viscoelastic liquids. 3. Liquid-crystalline polymers. The Akay-Leslie anisotropic liquid. Viscometric functions

The advances in determination of the rheological material functions of liquid-crystalline polymers (LCP) in a wide range of concentrations or temperatures are briefly examined. Special attention is focused on a discussion of the concentration region of the existence of liquid-crystalline order and textures initiated by shear flow. The phenomenological rheological equation for an anisotropic liquid proposed by Akay and Leslie (A-L model) was selected for describing the flow resistance of LCP, and the problem of simple shear flow of a liquid was solved. It was shown that the A-L model can describe all of the basic characteristics of the viscometric functions of LCP, including the phenomenon of negative values of the first normal stress difference for some range of concentrations and shear rates. The available data on the initial viscosity of lyotropic LCP were classified based on the types of cohesion (dimensionality) of the system, characterized by the exponent of the concentration dependence of the viscosity. It was found that each type of cohesion has its own kind of viscometric functions and that the A-L rheological model can reflect their basic characteristics. It was shown that by varying the initial ordering parameter of the A-L model, linear (or almost linear) functions N1(q) and σ12(q) can be described in the region of low shear rates. It was hypothesized that region 1 of the flow curve (Onogi and Asada) inevitably exists in the continuous liquid crystalline phase, which could be outside of the region of measurements accessible to rheometry.

Evolution of structure and properties of a liquid crystalline epoxy during curing

The evolution of structure, and thermal and dynamic mechanical properties of a liquid crystalline epoxy during curing has been studied with differential scanning calorimetry (DSC), polarized optical microscopy, x-ray scattering, and dynamic mechanical analysis. The liquid crystalline epoxy was the diglycidyl ether of 4,4′-dihydroxy-α-methylstilbene (DGEDHMS). Two curing agents were used in this study: a di-functional amine, the aniline adduct of DGEDHMS, and a tetra-functional sulfonamido amine, sulfanilamide. The effects of curing agent, cure time, and cure temperature have been investigated. Isothermal curing of the liquid crystalline epoxy with the di-functional amine and the tetra-functional sulfonamido amine causes an increase in the mesophase stability of the liquid crystalline epoxy resin. The curing also leads to various liquid crystalline textures, depending on the curing agent and cure temperature. These textures coarsen during the isothermal curing. Moreover, curing with both curing agents results in a layered structure with mesogenic units aligned perpendicular to the layer surfaces. The layer thickness decreases with cure temperature for the systems cured with the tetra-functional curing agent. The glass transition temperature of the cured networks rises with increasing cure temperature due to the increased crosslink density. The shear modulus of the cured networks shows a strong temperature dependence. However, it does not change appreciably with cure temperature. © 1997 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 35: 2363–2378, 1997

Effects of the Degree of Substitution on the Properties and Structures of Side Chain Liquid Crystalline Polysiloxanes

Poly[6-(4-cyanobiphenyl-4'-oxy)-hexylmetylsiloxanes] (PCS-n) with various degrees of substitution (DS) were prepared, and the effects of the degree of substitution on the phase transition, structures, and liquid crystallization of PCS-n were investigated. PCS-n showed S Ad in the temperature range from glass transition temperature (T s ) to clearing temperature (T i ) in spite of various DS. T 2 and increased linearly with DS, and the temperature range of S A phase between T g and T i expanded with DS. The layer spacing d L of the S A structure decreased with increasing DS, since the non-mesogenic flexible backbones were squeezed between smectic layers. d L of the sample with lower DS highly decreased with increasing temperature, due to entropy relaxation of backbones between the S A layers. DS remarkably influenced the liquid crystallization of PCS-n. PCS-n with higher DS exhibited faster growth rates of liquid-crystalline textures compared with lower DS.

Free-standing mesoporous silica films; morphogenesis of channel and surface patterns

Extraordinary channel patterns have been observed in an oriented mesoporous silica film that is synthesized at the boundary between air and water. The basic channel designs comprise concentric circles, herringbones, fingerprints and hairpins. It is suggested that these patterns are spatio-temporal silicified recordings of the polymerization and growth of a silicate liquid-crystal seed emerging in two spatial dimensions at the air/water interface. The origin of the channel patterns may be from defects, such as disclinations and dislocations, that spontaneously form in a surface confined precursor silicate liquid-crystal film to give a liquid-crystalline texture to the resulting mesoporous silica film. The reaction–diffusion processes and defects that are believed to contribute to the curved three-dimensional morphologies and channel patterns of mesoporous silica free to grow in solution, may also occur in the surface version, except that the isotropy of solution is replaced by the anisotropy of the air/water interface together with proximity effects from neighboring growth centers. Polymerization induced radial stresses produced in the as-synthesized mesoporous silica film before drying, are relieved through warping of the film into micrometer-scale hillock-shaped protuberances. These mounds are arranged into patterns that appear to reflect the channel designs within the film. A linear radial stress model is found to successfully account for the warp patterns. A possible connection between the two-dimensional channel patterns in mesoporous silica films and the two-dimensional stripe domain patterns found in modulated phases is discussed.