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

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.     

Shape minimisation problems in liquid crystals

ABSTRACT

We consider a class of liquid crystal free-boundary problems for which both the equilibrium shape and internal configuration of a system must simultaneously be determined, for example in films with air–liquid or fluid–liquid crystal interfaces and elastomers. We develop a finite element algorithm to solve such problems with dynamic mesh control, achieved by supplementing the free energy with an auxiliary functional that promotes mesh quality and is minimised in the null space of the energy. We apply this algorithm to a flexible capacitor, as well as to determine the shape of liquid crystal tactoids as a function of the surface tension and elastic constants. These are compared with theoretical predictions and experimental observations of tactoids from the literature.     

Streamlined ellipsometry procedure for characterizing physical aging rates of thin polymer films

Existing studies in the research literature showing conflicting changes in physical aging rates with decreasing film thickness in nanoconfined polymer films highlight the need for a single experimental technique to efficiently characterize physical aging rates in thin polymer films of varying chemical structure. To that end, we have developed a streamlined ellipsometry procedure to measure the structural relaxation of thin glassy polymer films. We evaluate different methods of calculating a physical aging rate β from the measured thickness h(t) and index of refraction n(t) data. We present extensive measurements of β as a function of aging temperature and aging time for polystyrene (PS) films supported on silicon, and determine that the physical aging rate β can be easily and reliably determined from β = −1/h₀ dh/d(log t), where h₀ is the initial measure of the film thickness at an aging time of 10 min. We have also carried out oxygen permeation studies on poly(methyl methacrylate) (PMMA) films from 800 μm down to 190 nm in thickness, and find no change in the permeability with film thickness or physical aging at room temperature for up to 65 days, which suggests that gas permeation may be insensitive to physical aging in such low free volume polymers. © 2009 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 47: 2509–2519, 2009     

Structural variety of CdS in films synthesized by vapor deposition polymerization

Poly(p-xylylene)—CdS (PPX—CdS) nanocomposite films with different thicknesses (～0.2, ～0.5, ～1, and ～1.5 μm) and concentrations of CdS from 5 to 90 vol.%, as well as single-component CdS and PPX films with various thicknesses were studied by X-ray diffraction. The films were synthesized on polished silicon or quartz substrates by low-temperature vapor deposition method. It was shown that CdS nanoparticles in PPX—CdS films, depending on their thickness and CdS content, can have an X-ray amorphous structure, a defect crystal structure (RCP structure), or a wurtzite-type crystal structure. Similar structures were observed for single-component CdS films of the corresponding thickness. The sizes of the coherent scattering regions of CdS were determined for some nanocomposite and single-component films. Poly(p-xylylene) in the studied nanocomposite films was characterized by an X-ray amorphous structure.

     

Some Physical Properties of Binary Liquid Systems: (2-Butanone + n-Propionic Acid or n-Butyric Acid)

Abstract

Density, viscosity and surface tension of two binary liquid systems: 2-butanone + n-propionic acid, 2-butanone + n-butyric acid have been determined at 20, 30 and 40°C, over the whole compositional range. The excess values of molar volume, viscosity, Gibbs free energy for the activation of flow and surface tension were evaluated. These excess values were fitted to a Redlich-Kister type of equation. The Grunberg-Nissan parameter, d, was also calculated. The binary viscosity data were fitted to the models of McAllister, Heric, Auslander and Teja and Rice. Surface tension data were fitted to the models of Zihao and Jufu, Rice and Teja, and an empirical two-constant model proposed in this study.     

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-surface effects on the response of nematic liquid crystal films to a laser pulse

Abstract

The dynamic response of 5CB films with a free surface to a laser pulse is investigated. A magnetic field above the Fréedericksz transition is applied initially to induce a starting angle on the molecular orientation. A single 1·06 μm laser pulse with e ^?1 pulse width 0·2 ms is incident normally upon the films. When the laser pulse polarization is perpendicular to the magnetic field, only a thermal effect is involved. While it is parallel to the magnetic field, both molecular orientation and thermal effects are involved. The results from films with a free surface are compared with those from films sandwiched between two glass substrates. The free surface effect, beam size effect, and thermal effect are discussed by comparing with theoretical analysis.     

Gas flow-field induced director alignment in polymer dispersed liquid crystal microdroplets deposited on a glass substrate

Abstract

Polymer dispersed liquid crystal thin films have been deposited on glass substrates by the processes of polymerization and solvent evaporation induced phase separation. The electron and the optical polarization microscopies of the films reveal that PDLC microdroplets formed during the process of phase separation near the top surface of the film remain exposed and respond to shear stress due to air or gas flow on the surface. Optical response of the film to an air flow-induced shear stress input on the free surface has been measured. Director orientation in the droplets changes with the applied shear stress leading to time varying transmitted light intensity. Director dynamics of the droplet for an applied step shear stress has been discussed from free energy considerations. Results on the measurement of light transmission as a function of the gas flow parameter unambiguously demonstrate the potential of these systems for use as boundary layer and gas flow sensors.     

A study of the surface anchoring at a polymer/liquid crystal interface in the neighbourhood of the glass transition

Abstract

Recently, we have observed dramatic changes in the electro-optic properties of PDLC films composed of droplets of the liquid crystal E7 in a matrix of polyvinylformal (PVFM) in the vicinity of the glass transition temperature of the matrix. One plausible explanation for these changes is a decrease in the surface anchoring strength on passing through the glass transition. For this reason, we undertook a study of the surface anchoring strength between E7 and a thin film of PVFM using the technique of Yokoyama and van Sprang. To mimic the properties of the matrix more closely, we dissolved 20 per cent E7 liquid crystal into the PVFM film. The surface anchoring for this film was compared with that of a pure PVFM film. From these data, we observed no obvious changes in the surface anchoring on passing through the glass transition of the matrix. However, the incorporation of liquid crystal into the matrix did cause a decrease in the surface anchoring strength. We discuss the impact of these results on the interpretation of the PDLC electro-optic properties.     

Influence of the surface topography on the surface transitions in nematics

Abstract

In quasi-homeotropic oriented liquid crystal cells, obtained by oblique SiO deposition on glass and surfactant added to the nematic, a continuous tilt angle versus temperature change followed by an abrupt surface transition was observed. At room temperature, the sign of the tilt angle measured from the normal to the surface depends on the SiO deposition angle. A model for the temperature dependence of the tilt angle is proposed.     

A NEW METHOD TO PREPARE UNIFORM PARTICLES; PRECIPITATION FROM LYOTROPIC LIQUID CRYSTALS

Basic copper sulfate panicles of uniform size and shape were prepared at room temperature by precipitation from a lyotropic liquid crystal of water and Tween 80, a nonionic surfactant.

The size (< 2.5 μm) of the panicles depended on the pH of the aqueous copper salt solution and the precipitation time.

The formation of the basic precipitates at room temperature was due to the presence of polyethylene oxide chains in the aqueous layers, while the narrow size distribution was due to immobilization of panicle nuclei by the rigidity of the liquid crystal.     

Optical and dielectric properties of antiferroelectric liquid crystals and their surface effects

Abstract

A detailed study of the dielectric and optical properties of the ferroelectric liquid crystal material (R)-4′-(3-methoxycarbonyl-2-propoxycarbonyl)phenyl 4-(4-(n-octyloxy)phenyl)benzoate (3MC2PCPOPB) has been carried out. It has been found that an anomalous temperature dependence of the dielectric constant in 3MC2PCPOPB is due to the antiferroelectric and ferrielectric properties. A T(temperature)-E(electric field) phase diagram has been obtained on the basis of the apparent tilt angle measurements. In a thin cell (< 3 μm), both ferroelectric and antiferroelectric domains are simultaneously observed over a wide temperature range, and the complete antiferroelectric phase does not appear even at low temperature. A characteristic texture in which boundary focal conics are aligned parallel to a smectic layer has been observed. The movement of the zig-zag defect line caused by the application of the voltage is also observed.     

Alignment and ordering mechanisms at a liquid crystal–solid interface

Abstract

The role of surface coupling agents on the aligning and ordering mechanisms at a liquid crystal–solid interface are examined with deuterium nuclear magnetic resonance. The cylindrical channels of alumina membranes 0·2 μm in diameter are chemically modified using an aliphatic acid (C _n H_2n+1 COOH) as a surface coupling agent and filled with the liquid crystal compound 4′-pentyl-4-cyanobiphenyl deuteriated in the α position of the hydrocarbon chain (5CB-αd ₂). The preferred anchoring direction at the cavity wall and its strength are found to depend on the length of the aliphatic chain of the surface coupling agent which determine the nematic director field in the pores. The planar polar configuration with homeotropic anchoring conditions is stable for agents with n ≥7 while chain lengths n ≤6 support a uniform axial configuration with planar anchoring at the cavity wall. The pretransitional orientational ordering at the cavity wall above the clearing temperature is strongly reflected in the spectra. The radical changes in the quadrupole splitting as the length of the aliphatic chain of the surface coupling agent is varied indicates strong coupling between the 5CB molecules and the n = 15 surface, while shorter chain lengths reveal substantially reduced degrees of coupling.     

Spontaneously modulated chiral nematic structures of flexible bent-core liquid crystal dimers

ABSTRACT

We report the induction of spontaneously undulated chiral nematic structures of liquid crystal (LC) dimers with rigid aromatic molecular arms linked by flexible chains with an odd number of carbons. When a small amount of chiral dopants (CD) are added to the dimers, we find the formation of different stripe textures on cooling 4–10 μm films in the nematic phase. The temperature where the stripes form depends on the film thickness and the direction of the stripes depends on the CD concentrations. We show that the experimentally observed stripes are due to undulation instabilities that spontaneously form as a result of the anomalously small bend elastic constant that prefers director bend instead of twist deformation, the opposite of the situation in usual cholesteric LCs.     

Study of the molecular tilt angle and the order parameter of a ferroelectric liquid crystal mixture using IR spectroscopy

Abstract

We report measurements of the molecular tilt angle of a ferroelectric liquid crystal mixture (ZLI 3654), using infra-red spectroscopy, and of the optical tilt angle using polarizing microscopy. The molecular tilt angle is found to be approximately the same for different molecular vibrations. The second rank orientational order parameter for the axis of molecular vibrations of different groups of the molecule for homogeneously aligned samples have been determined as a function of temperature. The order parameter depends slightly on the sample thickness and is about 8 per cent higher for a lower thickness (～ 7 μm) compared to a higher thickness (～ 12 μm). The order parameter does not change significantly at the S_C?–S_A transition. The order parameter of S_C? is found to be unaffected by the application of a constant electric field. The order parameter for the alkyl chain is found to be about three times lower than for the molecular core.     

High on-state clarity polymer dispersed liquid crystal films

Abstract

Results are presented which show that the on-state clarity of a UV cured polymer dispersed liquid crystal (PDLC) film depends on the refractive index of the final polymer in the PDLC film, the ordinary refractive index of the liquid crystal, the solubility of the liquid crystal in the prepolymer and the rate at which the film is cured. Liquid crystal mixtures for use in PDLC films are chosen such that the ordinary refractive index of the liquid crystal is equal to the refractive index of the polymer matrix. It has been shown previously that a large quantity of liquid crystal remains dissolved in the polymer matrix, thus increasing the mismatch between the refractive index of the polymer and the ordinary refractive index of the liquid crystal and therefore reducing the on-state clarity. For liquid crystal mixtures which have high solubility in the prepolymer (>60 per cent) the mismatch in the refractive indices can be very large and the on-state clarity of the resulting film can be very poor (T _on<70 per cent). Results are presented which show that it is possible to increase the on-state clarity of such films by increasing the rate at which these films cure. If the liquid crystal is less soluble in the prepolymer (<45 per cent), a PDLC film formed from such a liquid crystal/ prepolymer system often has very good on-state clarity (T _on>75 per cent) be it cured slowly or quickly. Results are also presented which show that in order to achieve a true measure of on-state clarity it is necessary to use a small collection angle (<3°) in the detecting optics. If larger collection angles are used, the photodetector collects light which is scattered out of the specular beam, thus leading to a false measure of on-state clarity.     

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.     

Optical and electrooptical properties of homeoplanar layers of cholesteric liquid crystals

Abstract

A wedge shaped layer of a cholesteric liquid crystal, with the director surface orientations first planar and the other homeotropic, shows two distinctive textures depending on the relation between the local thickness, d, and the equilibrium pitch, P ₀. If d/P ₀ < 1, the texture does not show any domains; the director distribution is reminiscent of a corkscrew. If d/P ₀ > 1, there are linear periodic domains. The domain direction rotates as the thickness of the layer increases. The voltage dependence of light transmission of the homeoplanar cholesteric layer placed between crossed polarizers is less pronounced and more linear than the corresponding dependence for the twisted nematic effect.     

Above,below, and in‐between the two glass transitions of ultrathin free‐standing polystyrene films: Thermal expansion coefficient and physical aging

In previous work we observed two simultaneous transitions in high molecular weight (MW) free‐standing polystyrene films that were interpreted as two thickness‐dependent reduced glass transition temperatures (T_gs). The weaker lower transition agreed well with the MW‐dependent T_g(h) previously reported, while the much stronger upper transition matched the MW‐independent T_g(h) previously observed in low‐MW free‐standing films. Here, we investigate the nature of these two transitions by inspecting the temperature dependence of the films' thermal coefficient of expansion (TCE) and present physical aging measurements using ellipsometry both below and in‐between the two transitions. TCE values indicate approximately 80 to 90% of the film solidifies at the upper transition, while only 10 to 20% remains mobile to lower temperatures, freezing out at the lower transition. Physical aging is observed at a temperature below the upper transition, but above the lower transition, indicative of the upper transition being an actual glass transition associated with the α‐relaxation. © 2014 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2015 , 53, 64–75