Surface tension measurements in freely suspended bubbles of thermotropic smectic liquid crystals

We present an experimental technique for the inflation of freely suspended films of thermotropic smectic liquid crystals to macroscopic bubbles. These bubbles have many properties in common with well known lyotropic soap bubbles, but on the other hand represent a completely different class of material with different physical properties and new phenomena. We describe the generation of such smectic bubbles and compare their physical properties with those of lyotropic soap bubbles. The measurement of inner pressure versus curvature is used for the determination of surface tensions of the smectic materials.     

IR dichroism study of orientational ordering of liquid-crystalline acrylates

Orientational ordering of four liquid-crystalline acrylates induced by the surface of KBr plates over a wide temperature range was studied by the IR dichroism technique. IR spectra of homogeneously aligned samples were used to calculate the angles between the direction of the transition moment for a series of vibrations of the mesogenic fragment and its long axis. A method for calculation of the homeotropic orientation parameter of molecules was proposed. The orientation parameters of homogeneously and homeotropically aligned samples in the nematic, smectic (A), and chiral smectic (I andH) phases were calculated. Thein situ photopolymerization of acrylates in the smectic phases occurs with retention of the orientational ordering in the polymer films formed. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 2, pp. 319–323, February, 1999.     

Polystyrene–Polyperfluorooctylethyl acrylate Diblock Copolymers: The Effect of Dilution of the Fluorinated Mesogenic Chains on Bulk and Surface Properties

Summary : Three smectic poly(styrene-b-perfluorooctylethyl acrylate) block copolymers (S-b-AF8) with different degrees of polymerization (n, m) of the relative blocks were synthesized by atom transfer radical polymerization (S, n = 25; AF8 m = 2, 6, 23). The mesophase structure and transition temperatures were investigated by DSC and WAXD. The block copolymer having the shortest fluorinated block was blended with a thermoplastic elastomer SEBS in different proportions, in order to look at the effect of a further dilution of the perfluorinated groups on non-wetting properties. Thin films of the block copolymers as well as the blends exhibited large contact angles with both water and n-hexadecane, which resulted in low solid surface tensions. XPS findings at different photoemission angles confirmed the effective surface segregation of the mesogenic chains of the fluorinated polymer block.     

Strain‐induced compression of smectic layers in free‐standing liquid crystalline elastomer films

The deformation of oriented smectic liquid crystal elastomer films with smectic layers parallel to the film surface was studied using optical reflectometry and small angle X‐ray diffraction. Reflectometry data show that in the chosen material, in‐plane strain causes a change in the optical thickness of the free‐standing films. Small angle X‐ray scattering was used to explore the molecular origin of this effect. The X‐ray scattering data confirm that the change in optical thickness originates from the compression of the individual smectic layers. The measured Poisson ratio in the smectic A and C* phases is close to ½, in contrast to the smectic elastomers investigated earlier by Nishikawa et. al. [Macromol. Chem. Phys. 200, 312 (1999)]. In this unique material, the molecular lattice dimensions can be reversibly controlled by macroscopic stretching of the oriented samples.     

Ionic conductivity of conjugated water-soluble rigid-rod polymers

Poly[(1,7-dihydrobenzo[1,2-d:4,5-d′] diimidazole-2,6-diyl)-2-(2-sulfo)-p-phenylene], a conjugated rigid-rod polymer, was derivatized with pendants of propane-sulfonated ionomers. The derivatized rigid-rod polymer was soluble in aprotic solvents as well as in water for isotropic solutions that were processed into isotropic films. Direct-current electrical conductivity σ of the films was measured using the four-probe technique. Room-temperature σ as high as 2.9 × 10^?4S/cm was achieved on pristine isotropic films without using dopants. When the rigid-rod polymer concentration exceeded 25 wt %, the isotropic solution could be transformed into a liquid-crystalline solution that allowed deformations to be applied to produce anisotropic films. Significant increase in σ was obtained in a sheared film along both the parallel direction (∥) and the transverse direction (⊥) with a σ_∥/σ_⊥ = 5. Additionally, enhanced σ was realized in films heat-treated at about 100°C, in the derivatized polymer with higher molecular weight from dialysis, and in substituting the sulfonated ion Na⁺ by H⁺ in the pendants of the polymers. Constant-voltage measurements were applied to the polymers to monitor the σ stability for ascertaining the nature of the conductivity. No electronic contribution in σ was detected. Instead, a monotonically decreasing σ was consistently observed indicative of ionic conductivity. © 1993 John Wiley & Sons, Inc.     

Influence of the order parameter and of the director orientation on the surface tension of free nematic films

Abstract

A radial hydrodynamic flow in the nematic phase of free, suspended cylindrical films of 4-n-heptyl- and decyloxybenzoic acid and in 4,4′-di-n-heptyl-oxyazoxybenzene has been observed. The flow starts about 7°C before the phase transition into the smectic C phase. Under the same experimental conditions such a hydrodynamic flow is not established in free nematic films of 4-n-heptyl-and octyloxy-4′-cyanobiphenyl, 4,4′-dimethoxyazoxybenzene and N-(4-ethoxybenzylidene)-4′-n-butylaniline after the completion of the transition from the isotropic liquid to the nematic phase. The observed hydrodynamic flow is explained by a non-linear temperature dependence of the surface tension.     

Influence of substrate nature and growth conditions on the morphology of thin DMOAP films

We report some preliminary results on the morphology of thin N,N -dimethyl-n-octadecyl-3-aminopropyltrimethoxysilyl chloride (DMOAP) films. When deposited on a glass substrate, DMOAP forms a mono- or multi-layer structure parallel to the substrate. The surface topography of the film is probed by atomic force microscopy. In general, the free surface of such a film is not flat and smooth. Islands and holes are formed on the free surface of the films when a sufficiently flat substrate is used. The thin film surface topography depends strongly on the nature of the bare substrate, the curing conditions, and the immersion time of the substrate in the DMOAP solution. The film is always rougher than the bare substrate used. Annealing roughens the surface of the alkoxysilane thin films deposited on a glass substrate. For films on glass plates covered with an indium tin oxide layer, annealing has minor effects. The surface topography affects the microstructure of homeotropic smectic samples.     

Interface induced crystal structures of dioctyl-terthiophene thin films

Temperature dependent structural and morphological investigations on semiconducting dioctyl-terthiophene (DOTT) thin films prepared on silica surfaces reveals the coexistence of surface induce order and distinct crystalline/liquid crystalline bulk polymorphs. X-ray diffraction and scanning force microscopy measurements indicate that at room temperature two polymorphs are present: the surface induced phase grows directly on the silica interface and the bulk phase on top. At elevated temperatures the long-range order gradually decreases, and the crystal G (340 K), smectic F (348 K), and smectic C (360 K) phases are observed. Indexation of diffraction peaks reveals that an up-right standing conformation of DOTT molecules is present within all phases. A temperature stable interfacial layer close to the silica-DOTT interface acts as template for the formation of the different phases. Rapid cooling of the DOTT sample from the smectic C phase to room temperature results in freezing into a metastable crystalline state with an intermediated unit cell between the room temperature crystalline phase and the smectic C phase. The understanding of such interfacial induced phases in thin semiconducting liquid crystal films allows tuning of crystallographic and therefore physical properties within organic thin films.     

Preparation of ordered and crosslinked films from liquid crystalline vinyl ether monomers

Thermally stable ordered films were prepared by in-situ photopolymerization of an oriented monomer mixture, consisting of mesogenic monofunctional and bifunctional vinyl ethers. Orientation was achieved by a simple surface treatment, using an unidirectionally rubbed polyimide film. The films restored their orientation when cooled down from temperatures of 200°C. Highly ordered densely crosslinked films have been prepared by polymerization of bifunctional mesogenic vinyl ether monomers. Polymerization from various monomer phases resulted in LC polymer network films with different molecular organizations. It was shown that films with nematic, smectic A and smectic B structures were obtained, the latter having a very high degree of orientation. The films were analyzed with small-angle X-ray scattering, polarized light microscopy and infrared- dichroism measurements.     

Orientation and order in thin films of a combined liquid crystalline polymer

The order in thin films of a combined liquid crystalline polymer is studied by X-ray reflection. Films of thicknesses of less than 200 nm on float glass are investigated as a function of temperature. The polymer with mesogenic groups in the main and side-chains exhibits smectic and cholesteric mesophases. Measurements in the smectic phases show a Bragg peak and smectic layers are oriented parallel to the substrate. The sample is thus macroscopically ordered by the influence of substrate and free surface. The film surface is very smooth after spincoating; surface roughness is typically 0.8 nm. First annealing of samples leads to a significant roughening of the free surface; roughness increases to 2.1 nm. Order as a function of film thickness depends on the interaction of the polymer with the substrate and free surface. These interactions give rise to a typical correlation length of perturbations in smectic ordering.     

Behaviour of free-standing smectic A films upon heating

In the framework of a previously proposed microscopic mean-field model for thin smectic A liquid crystal films with two boundary surfaces, the behaviour of free-standing smectic A films upon heating is investigated theoretically. It is shown that the model accounts for both the film rupture and layer-thinning transitions with increasing temperature. A close correlation between the behaviour of the film upon heating and the surface tension is found. The model accounts also for essential features of the layer-thinning transitions for thick and thin free-standing smectic A films of various liquid crystal compounds.     

Behaviour of free-standing smectic A films upon heating

In the framework of a previously proposed microscopic mean-field model for thin smectic A liquid crystal films with two boundary surfaces, the behaviour of free-standing smectic A films upon heating is investigated theoretically. It is shown that the model accounts for both the film rupture and layer-thinning transitions with increasing temperature. A close correlation between the behaviour of the film upon heating and the surface tension is found. The model accounts also for essential features of the layer-thinning transitions for thick and thin free-standing smectic A films of various liquid crystal compounds.     

Atomic Force Microscopy of freely suspended liquid crystal films transferred to octadecyltriethoxysilane self-assembled monolayers

Thin freely suspended films of 4-(2-methylbutyl)phenyl 4'-octylbiphenyl-4-carboxylate (8SI) in the smectic C phase were transferred to self-assembled monolayer coated glass and imaged by atomic force microscopy. Upon rapid cooling to room temperature, the smectic C quenches into a lamellar phase, exhibiting islands and single smectic layer steps. The surface of such films can be imaged with molecular resolution, revealing a partially disordered lattice of fixed molecular positions. Over a period of several days these films undergo a transition to a crystal phase having a different film morphology.     

The influence of a bulk twist on the surface memorization in the liquid crystal HOBA

The surface-induced memorization of a smectic C liquid crystal texture in the temperature range of the nematic phase of 4-n-heptyloxybenzoic acid oriented by obliquely evaporated SiO on ITO was removed by imposing a bulk twist (Prevention of surface memorization was not observed when we used a simple ITO coating as the orienting surface.) A twist angle value (Ω_c ≈ 70°) above which the surface memorization is prevented was found. Using microtextural polarization analysis of the smectic C texture, a qualitative explanation of the phenomenon is suggested. The conditions for preventing the surface memorization were deduced in terms of the balance between the surface and bulk torques. These conditions provide surface anchoring breaking, so removing the surface memory effect.     

Surface tension of some liquid crystals in the cyanobiphenyl series

The temperature dependence of the surface tension was measured by the pendant drop method for four compounds from the homologous series of alkylcyanobiphenyls (nCB), in the nematic liquid crystal and isotropic phases. For 8CB (octylcyanobiphenyl) the temperature dependence was also measured in the smectic range. Not very close to the isotropic transition temperature, and with the exception of 8CB, the surface tension decreases with increase in temperature in the nematic range. A downward jump at the transition temperature was observed for all liquid crystals studied. The shape of the drop in the smectic A phase of 8CB gives indications of stratification in a system of terraces.     

Morphology of side chain liquid crystalline block copolymer thin films and effects of thermal annealing

The self-assembly behavior of siloxane based side chain liquid crystalline block copolymer thin films are investigated via grazing incidence small angle X-ray scattering and atomic force microscopy. The as-spun films displayed polystyrene cylinders perpendicular to the substrate and the cylinders reoriented parallel to the surface after thermal annealing. The morphology observed in the as-spun films is resultant from the orientation of the smectic LC mesophase relative to the substrate. Annealing above both the polystyrene glass transition temperature and the smectic to isotropic transition temperature eliminates the influence of the LC phase, leading to a reorientation of the morphology that minimizes the interfacial energy of the system. © 2007 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 45: 3263–3266, 2007     

PDLC composites based on polyvinyl boric acid matrix – a promising pathway towards biomedical engineering

ABSTRACT

Polymer dispersed liquid crystal (PDLC) systems based on a smectic liquid crystal embedded in polyvinylalcohol-boric acid (PVAB) as biocompatible carrying matrix were prepared and characterised. The smectic liquid crystal contains biologically friendly structural blocks and was designed to have a direct isotropic–smectic transition and a mesophase stability range at human body temperature. The resulted PDLCs were characterised from morphological and thermotropic aspects by polarised light microscopy (POM), differential scanning calorimetry (DSC), scanning electron microscopy (SEM) and Raman microspectroscopy, and their surface properties were determined by contact angle measurements and surface energy calculations.

It was concluded that the electron-deficient PVAB matrix constrains the ester liquid crystal to grow as spherical droplets with planar anchoring. The droplet diameter was comprised in the range 4–11 µm, with a predominant droplet population around 7 µm and a narrower polydispersity as the amount of the liquid crystal in the polymeric matrix increases. The resulted PDLC films exhibited versatile morphology and surface properties which allow targeting of their application.     

Free-standing smectic films at high temperature

Optical reflectivity studies on free-standing liquid crystal films above the bulk smectic temperature range have revealed different melting phenomena. Our measurements are performed on tilted smectic phases (smectic C*, smectic C) using optical microscopy in polarized light in order to visualize the changes of the film structure. We observe the formation of twodimensional defect structures from string-like lines in very thick (about 1000 layers) as well as in thin (about 20 layers) films. In thick films these structures nucleate around the temperature of the bulk smectic-cholesteric phase transition, while in thin films the formation of the defects occurs well above this temperature and just before the thinning transitions. In thick and intermediate thickness films, cholesteric or nematic droplets and a ‘quasi-smectic’ structure are observed. The films exhibiting the ‘quasi-smectic’ structure definitely exist at higher temperatures than the smectic films with the same thickness.     

Selective reflection and textural studies of free-standing films of a chiral liquid crystal

Abstract

We present a detailed study of selective reflection from free-standing films of chiral smectic phases. Free-standing films are drawn of the chiral compound 2-(4-hexyloxyphenyl)-5-[4-(1-chloro-3-methylbutanoyloxy)phenyl]pyrimidine (2f-6) possessing a chiral smectic C and the recently discovered chiral smectic M phase, both phases showing the phenomenon of selective reflection of visible light in a certain temperature range. With linearly polarized incident light two selective reflection maxima are observed which are caused by the presence of π-walls in the film. The wavelength of the selective reflection maxima is independent of the film thickness, whereas the intensity decreases with decreasing film thickness. The dependence of the reflection maxima on temperature and enantiomeric excess is studied.     

Free-standing smectic films at high temperature

Optical reflectivity studies on free-standing liquid crystal films above the bulk smectic temperature range have revealed different melting phenomena. Our measurements are performed on tilted smectic phases (smectic C*, smectic C) using optical microscopy in polarized light in order to visualize the changes of the film structure. We observe the formation of twodimensional defect structures from string-like lines in very thick (about 1000 layers) as well as in thin (about 20 layers) films. In thick films these structures nucleate around the temperature of the bulk smectic-cholesteric phase transition, while in thin films the formation of the defects occurs well above this temperature and just before the thinning transitions. In thick and intermediate thickness films, cholesteric or nematic droplets and a 'quasi-smectic' structure are observed. The films exhibiting the 'quasi-smectic' structure definitely exist at higher temperatures than the smectic films with the same thickness.