Smectic Regions Research Articles

Localized Liquid Secretion from a Photopatterned Liquid-Crystal Polymer Skin

Liquid-releasing artificial skins are made from films made of a smectic liquid-crystal polymer network (LCN) photopolymerized in the presence of a photoactive azobenzene chromophore and a liquid-crystal porogen. The nonreactive porogen phase separates during the polymerization process, and the polymer forms a spongy polymer network filled with liquid. The liquid is excreted from the film when exposed to UV light upon conversion of trans-azobenzene to its cis isomer. Here, localized liquid secretion at preset positions at the polymer film is described. The design principle is based on creating a hybrid molecular architecture with both smectic and nonordered isotropic alignments in a continuous LCN coating. This coating is fabricated by a maskwise photopolymerization of the monomer mixture in the smectic phase, followed by a flood exposure at an elevated temperature above the isotropic state of the unpolymerized region. The smectic regions that polymerized during the maskwise exposure are not affected by the heating needed for the second polymerization step of the isotropic area. Upon activation under light illumination, the embedded liquid is exclusively released from the area with the smectic alignment. This approach reveals films of which the excretion process is reversible. The secreted liquid is reabsorbed spontaneously when azobenzene takes its initial trans-form. This process occurs thermally in time or can be accelerated by light irradiation with visible light.

Monodomain alignment of the smectic-A liquid crystalline phase from the isotropic phase

The liquid crystal alignment method described here provides uniform orientation of otherwise difficult-to-align smectic-A liquid crystal materials lacking a nematic phase. The smectic-A phase is grown in the presence of a 10–20 K/mm temperature gradient from an air bubble located within a cell by a photolithographically defined channel in the cell substrates. We obtain uniform layer alignment in millimeter-wide smectic regions at growth rates below about 0.05 μm/s even though there is a tendency for spontaneous nucleation of focal-conic defects at higher growth rates once the width of the smectic-A region exceeds the critical value of about 20 μm.

P‐163: Mono‐Domain Alignment of the SmC Liquid Crystalline Phase for Analog Display Applications

AbstractWe propose a novel liquid‐crystal alignment method that provides uniform orientation of otherwise difficult‐to‐align smectic materials lacking a nematic phase. The smectic‐A phase is grown in the presence of a 10–20 K/mm temperature gradient from an air bubble located within a cell by a photolithographically defined channel in the cell substrates. We obtain uniform layer alignment in millimeter‐wide smectic regions at growth rates below about 0.05 μm/s even though there is a tendency for growth instability. As many smectic materials that have attractive features for analog display applications, the proposed alignment method can play an important role in designing and development of analog display devices.

Smectic order induced at homeotropically aligned nematic surfaces: A neutron reflection study

Neutron reflection was used to measure the buildup of layers at a solid surface as the smectic phase is approached from higher temperatures in a nematic liquid crystal. The liquid crystal was 4-octyl-4'-cyanobiphenyl (8CB), and the solid was silicon with one of five different surface treatments that induce homeotropic alignment: (i) silicon oxide; (ii) a cetyltrimethylammonium bromide coating; (iii) an octadecyltrichlorosilane monolayer; (iv) an n-n-dimethyl-n-octadecyl-3- aminopropyltrimethyloxysilyl chloride monolayer; and (v) a lecithin coating. The development of surface smectic layers in the nematic phase of 8CB was followed by measuring specular reflectivity and monitoring the pseudo-Bragg peak from the layers. The scattering data were processed to remove the scattering from short-ranged smecticlike fluctuations in the bulk nematic phase from the specular reflection. The pseudo-Bragg peak at scattering vector Q approximately 0.2 A(-1) therefore corresponded to the formation of long-range smectic layers at the surface. The amplitude of the smectic density wave decayed with increasing distance from the surface, and the characteristic thickness of this smectic region diverged as the transition temperature was approached. It was found that the characteristic thickness for some of the surface treatments was greater than the correlation length in the bulk nematic. The different surfaces gave different values of the smectic order parameter at the surface. This suggests that the interaction with the surface is significantly different from a "hard wall" which would give the same values of the smectic order parameter and penetration depths similar to the bulk correlation length. Comparison of the different surfaces also suggested that the strength and range of the surface smectic ordering may be varied independently.

Shear-induced smectic ordering and crystallisation of isotactic polypropylene

Shear-induced smectic ordering and crystallisation of isotactic polypropylene (iPP) has been studied with in-situ small- and wide-angle X-ray scattering. Shear-induced smectic bundles with a periodicity of about 4 nm have been observed at temperatures below as well as above the melting point. This applies to iPP of different molecular weight and from different sources. An increase in the average molecular weight leads to a larger periodicity of the smectic layers. The smectic layers assemble in a fibrillar morphology with a length and a width up to 200 and 10 microm, respectively. After crystallisation, the smectic bundles show upon heating higher melting temperatures than their crystalline counterparts. In agreement with this behaviour, the correlation length along the smectic layer normal is of the order of tens of nanometers, much larger than the crystal thickness. We present an anisotropic drop model of smectic domains forming a conserved system in which the smectic layers can rotate. On the basis of this model we can explain the relative orientation of the smectic layers, the crystalline lamellae and the long axis of the drop, as well as the reversibility of the smectic periodicity during cooling and heating. In the supercooled melt, the smectic ordering is followed by crystallisation; during this process crystals grow epitaxially on the surface of the smectic bundles. This leads to a new picture of the shish-kebab structure in which smectic bundles rather than extended-chain crystals play the role of the shish. The crystallisation process of the smectic regions themselves indicates that the 'mesophase' previously reported in fast-quenched iPP, is a metastable state formed during the transition from the high-temperature smectic phase to a crystal. Moreover smectic domains rather than alpha crystallites form the nuclei for crystallisation of the beta phase. The high-temperature smectic phase presents an ideal model system to study the coupling between density and conformational ordering under shear flow. Our results support a nucleation and growth process for polymer crystallisation, in which smectic bundles or other mesophases are the primary nuclei.

Theory of nematic–smectic phase separation in thin twisted liquid crystal cells

Recently it has been shown experimentally by the authors that a highly twisted thin nematic cell at low temperatures can separate into a smectic A region in the middle of the cell surrounded by twisted nematic layers at the boundaries. In this case the twist is expelled into the nematic layers and the nematic–smectic A transition temperature is strongly depressed. We present a thermodynamic theory of such a phase transition in a twisted nematic cell, taking into account that the smectic A slab inside the nematic cell can be stable only if the decrease of free energy in the smectic region overcomes the increase in distortion energy of the twist deformation in the nematic layers plus the energy of the nematic–smectic A interface. In such a system the equilibrium thickness of the smectic A slab corresponds to the minimum of the total free energy of the whole cell, which includes all the bulk and surface contributions. Existing experimental data are at least qualitatively explained by the results of the present theory. This opens a unique possibility to study the properties of the nematic–smectic interface which is perpendicular to the smectic layers.

The cholesteric-smectic A transition

Instead of studying the cholesteric-smectic A transition from the analogy with superconductors, we present a direct investigation of the properties of this transition. The transition is of first order and a very simple relationship between the four basic lengths (correlation length, penetration length, layer spacing of the smectic structure, and pitch of the cholesteric structure) exists at the transition temperature. We calculate the wall energy separating a cholesteric region from a smectic region in two different configurations. For one of them, we recover the definition of the type I (positive wall energy) and type II (negative wall energy) smectics A. We do not find a specific value of the Ginzburg-Landau parameter kc which gives the distinction between the two types of smectics; kc is a function of the product of the perpendicular correlation length and the wave vector of the smectic density modulation. We recover the well known value from the superconductor analogy, kc 1/tick 2, only if this product is large and the rotation of the director inside the wall is small. For the second configuration, the wall energy is always positive and there is no analogy with superconductors. Finally, we discuss briefly two different possibilities of mixed phases.

Phase Diagram and Evolution of Mesophase Structure in Mixtures of a Low Molar Mass Smectic Liquid Crystal and a Nematic Side-on Side-Chain Liquid Crystalline Polymer

Miscibility phase diagrams in mixtures of side-on side-chain liquid crystalline polymers (s-SCLCP) and low molar mass smectic liquid crystals (K36) have been investigated by means of polarized optical microscopy. K36 is a smectic liquid crystal that exhibits a smectic−isotropic transition at 58 °C and a smectic-to-crystal transition (crystallization) at 27 °C. The nematic/smectic phase diagram of the s-SCLCP/K36 has been analyzed by combining the Flory−Huggins (FH) free energy for isotropic demixing and the Maier−Saupe−McMillan (MSM) free energy for smectic ordering of the mesogens. This combined FH/MSM theory compares reasonably well with the observed nematic/smectic phase diagrams composed of isotropic + nematic, smectic + nematic, smectic + isotropic, pure nematic, and pure smectic regions. Several temperature quench experiments have been conducted to confirm various predicted coexistence regions. However, at present, the combined theory does not include any crystallization processes occurring at a lower temperature.

Smectic Order in Double-Twist Cylinders

I propose a double-twist texture with local smectic order, which may have been seen in recent experiments. As in the Renn-Lubensky TGB phase, the smectic order is broken only through a lattice of screw dislocations. A melted lattice of screw dislocations can produce a double-twist texture as can an unmelted lattice. In the latter case I show that geometry only allows for certain angles between smectic regions. I discuss the possibility of connecting these double-twist tubes together to form a smectic blue phase.

An investigation of the smectic‐isotropic transition in a side‐chain liquid crystal polymer by synchrotron radiation x‐ray diffraction

AbstractIn this work we have used synchrotron x‐radiation diffraction to follow in real‐time the isotropic‐smoetic phase transition of a side‐chain liquid crystalline polymer. The analysis of the x‐ray data indicated that the transition occurs within a very narrow biphasic region. As the transition takes place, the size of the smectic regions, as indicated by the width of the diffraction peaks, changed only very slightly before impingement, suggesting a two‐dimensional growth pattern. A thermal investigation of the polymer paralleling the x‐ray experiments was undertaken and a kinetic analysis applied to the resulting data. Qualitative agreement with a disk‐like growth of the smectic regions was found. We have shown that the overall kinetics of the isotropic‐smectic transition of a side‐chain polymer can be studied by x‐ray diffraction permitting the evaluation of several structural parameters. © 1993 John Wiley & Sons, Inc.

Detailed study of a two component smectic 4TPB–8OCB system with a nematic gap. Phase diagram and viscosity study

Abstract The phase diagram of a two component system composed of two smectic compounds: 4-octyloxy-4′-cyanobiphenyl (8OCB) and 4-isothiocyanatophenyl 4-butylbenzoate (4TPB) was investigated. Three Miesowicz viscosity coefficients η1, η2, η3 and the refractive indices at different temperatures as well as the enthalpies of the phase transitions were measured. It was stated, that the properties of the induced nematic phase, for example, the nematic phase existing between two smectic regions, are the same as the ones observed in the case of low viscosity nematic mesogens.

Effect of properties of the smectic Adphase on the induction of a nematic phase in binary mixtures of smectics A1and Ad

Abstract The phase diagrams of two series of binary mixtures composed of 5-trans-n-butyl-2-(4-isothiocyanatophenyl)-1,3-dioxane (smectic A1) and 4-(trans-4′-n-alkyl-cyclohexyl)-1-(2-cyano-ethenyl)benzene or 4-cyanophenyl-4′-n-alkoxybenzoates (both smectic Ad) are determined. It is shown that the smectic layer spacing ratio and smectic phase transition enthalpy are the factors influencing the width and the position of the nematic gap created between the two smectic regions. It is found that their influence on this ability is in the opposite direction.

Dislocation-loop melting theory of phase diagrams with nematic regions surrounded by smectic regions.

Dislocation-loop melting theory of phase diagrams with nematic regions surrounded by smectic regions.

Binary Smectic Systems Including Trinuclear Pyrimidines with and without Nematic Gap

Phase diagrams of binary mixtures are presented in which one component has been selected from among four trinuclear isomeric pyrimidines forming a nematic or smectic A, phase, and the second one from cyanocompounds forming an Ad smectic phase. Depending on the molecular structure of pyrimidine, mixtures were obtained that show or show not a nematic gap between the A1 and Ad smectic regions. In the latter case it was found that the relation TSA → N= f(x) has two independent branches in different ranges of the concentration x. The measured spacing of the smectic layers in such mixtures points to the possibility of the pyrimidine monolayer being transformed into a bilayer.

Creation of a nematic phase in mixtures of smectic A1phases

Abstract Phase diagrams were determined for binary mixtures consisting of two 5-n-alkyl-2-(4'-isothiocyanatophenyl)-1,3-dioxane compounds (n-DBT) or 4'-isothio-cyanatophenyl 4-(trans-4'-n-decylcyclohexyl)benzoate and n-DBT. All compounds investigated have monolayer smectic A phases. A nematic phase in the upper temperature range and a nematic gap between two smectic regions also were observed, with the smectic layer spacing ratio, d/d', of 1.23 and 1.87 respectively. The variation of the enthalpy of transition with mixture composition in relation to changes of layer spacing ratio are also discussed for these systems.

Gas chromatographic retention behavior of polycyclic aromatic compounds on smectic liquid-crystalline polysiloxane stationary phases

Factors affecting gas chromatographic retention on smectic liquid-crystalline phenyl and biphenyl carboxylate ester polysiloxane stationary phases were studied for neutral polycyclic aromatic hydrocarbons (PAHs) and slightly polar sulfur heterocycles. Retention was greatly affected by the molecular geometry of the solutes in addition to other contributions such as solute vapor pressure and polarity. The major geometrical factor was the length-to-breadth ( L/B) ratio of the solutes, while of less importance, but still significant, was the solute molecular shape. The effect of solute ( L/B) on selectivity was larger in the smectic region than in the nematic region. Furthermore, the selectivity of the smectic phase was generally better than that of the nematic phase. Compounds with at least four aromatic rings annelated in a straight line were retained much longer, and compounds with aromatic rings annelated in a curved or arc-like arrangement were retained much less, than predicted by the ( L/B) ratios for these compounds. Arc-like molecules with groups attached to the outer curved side were, however, retained longer than predicted. Lower chromatographic efficiencies were measured for the arc-like molecules as compared to the latter type of molecules. The latter appear to penetrate the laminar arrangement of the polymer liquid-crystalline side groups more easily, thus leading to better solute diffusion and improved mass transfer.