Influence of rubbing conditions of polyimide alignment layer on optical anisotropy of immobilized liquid crystal film

The relationship between the molecular orientation of a rubbed polyimide film (alignment layer) and that of mesogens in a photopolymerized liquid crystal (LC) coated on the film has been investigated using optical measurements. LC monomers were deposited on the alignment layer and were aligned in one direction. The LC monomers were subsequently photocured. Alignment layers under various rubbing conditions were prepared. It was found that the inclination angle of the refractive index ellipsoid and the optical retardation of photopolymerized LC films are strongly related to the optical anisotropy of the rubbed polyimide film. The photopolymerized LC film exhibited high optical anisotropy when alignment layers with an inclination angle of the refractive index ellipsoid smaller than 6° were used.     

THE SPATIAL DISTRIBUTION OF LIGHT EMISSION FROM LIQUID PHASE BIO- AND CHEMILUMINESCENCE: VARIATIONS WITH CONTAINER TYPES, TURBIDITY AND CONTAINER FROSTING

Abstract— The spatial distribution of emitted light from liquid phase bioluminescence and chemiluminescent sources in three common types of containers has been evaluated. These sources, while theoretically isotropic, exhibit considerable anisotropy due to reflection and refraction effects at the interfaces between the solution, the container and the surrounding air. This anisotropy represents a considerable systematic error (>25%) in some values of quantum yields reported in the literature. The degree of asymmetry in spatial distribution depends on the solution volume, the refractive index, and the degree of light scattering in the solution. Quantitative evaluation of the projected image of these sources using a video camera system indicates that a major contribution to this asymmetry is due to reflection at the meniscus. Since container frosting removes the variability due to scattering, volume changes, refractive index differences and even, to some extent, use of different types of containers, it is recommended that quantum yields and comparative measurements be determined using frosted containers and point source geometry. The container of choice is a frosted, 1 ± 7.5 cm test tube.     

Optical anisotropy of polymeric films measured by waveguide propagation mode determination

Extruded thin films of a liquid-crystalline charge-conjugated rigid-rod polymer poly(p-phenylenebenzobisthiazole), PBT, and a semicrystalline thermoplastic polyethylene-terephthalate (Mylar) were fabricated and examined for film thickness, refractive index, and linear attenuation coefficient. Optical waveguide modes were successfully induced on the polymeric films using a prism coupler at λ = 633 and 1300 nm. Highly consistent thickness values were obtained for the polymeric films. In addition, the anisotropic nature of the optical properties was determined using TE and TM propagation modes. A refractive index as high as 2.3 was observed on PBT film. The refractive index data suggested that the PBT and Mylar films were optically anisotropic with refractive indices n? (out-of-plane) invariably smaller than n∥ (in-plane). Large anisotropy was also discovered in the linear attenuation coefficient α, with α_∥/α_? ≈ 50 for the Mylar films. Complementary polarimetric and spectroscopic interference measurements were also applied to investigate the optical anisotropy of the extruded polymeric films. © 1992 John Wiley & Sons, Inc.     

Flexoelectricity of lipid bilayers

Abstract

The direct flexoeffect in single lipid bilayers in the form of black lipid membranes has been investigated experimentally by the oscillating pressure technique in the regime of voltage measurement. Black lipid membranes of various composition have been studied in order to check the effect of lipid surface charge on the curvature-electric response and its frequency dependence; these include egg yolk lecithin (low negative charge); egg yolk lecithin plus phosphatidyl serine (high negative charge); egg yolk lecithin with surface adsorbed ions of uranyl acetate (high positive charge). An increase of the response has been found by increasing the surface charge and a reversal of the sign of the flexoelectric coefficient from positive to negative has been obtained by changing the sign of the surface charge from negative to positive. These results underline the leading role of the contribution of the surface charge to the flexoelectricity of lyotropics. Their theoretical interpretation provides further insight into the molecular mechanism of this phenomenon.     

Synthesis and Characterization of Highly Oriented Sol-Gel (Pb, La) TiO3 Thin Film Optical Waveguides

The preparation and the optical characteristics of highly oriented PLT thin films are investigated. PLT films fabricated on MgO(100) and c-plane sapphire substrates have highly grown in (100) and (111) orientations, respectively. PLT films with high La content have a near cubic structure and weak anisotropy of refractive indices. The optical propagation losses of PLT films decrease as the La content of the films increases due to a complex interaction of surface roughness reduction and a reduction in the anisotropy refractive index. However, optical scattering in thicker sol-gel PLT thin film waveguides occurs by the internal scattering mechanism from the defects and the interfaces rather than by the surface scattering mechanism.     

Optical anisotropy in solution-cast film of cellulose triacetate

The out-of-plane birefringence and its wavelength dispersion are studied employing solution-cast films of cellulose triacetate (CTA). In solution-cast process, CTA molecules are induced to align in the film plane. Although refractive index is the lowest in the oriented direction for the CTA films stretched more than 110 %, refractive index is found to be the lowest in the normal direction for the unstretched cast film. Attenuated total reflection measurements reveal that in-plane alignment of the acetyl group which provides strong polarizability anisotropy is responsible for the phenomenon. Furthermore, the out-of-plane birefringence is found to increase with increasing wavelength, i.e. extraordinary wavelength dispersion, whereas a stretched CTA film shows ordinary wavelength dispersion. The level of the out-of-plane birefringence in cast films depends on the preparation conditions, which is predictable considering the evaporation rate. Moreover, it is demonstrated for the first time that the out-of-plane birefringence and its wavelength dispersion can be modified by addition of a certain plasticizer such as tricresyl phosphate (TCP). During the evaporation, TCP molecules orient in the film plane accompanying the orientation of CTA chains by intermolecular orientation correlation, called nematic interaction. This technique will widen the scope of material design of retardation films because there are numerous liquid compounds having strong polarizability anisotropy.     

DEPTH RESOLUTION IN PHOTOELECTRON MICROSCOPY OF ORGANIC SURFACES. THE PHOTOELECTRIC EFFECT OF PHTHALOCYANINE THIN FILMS

Abstract— The application of photoelectron microscopy as a general method of imaging organic and biological surfaces requires a knowledge of the photoelectric effect of very thin organic films. In this study, low magnification images of a 7 Å thick pattern of copper phthalocyanine were obtained, demonstrating that it is possible to visualize a monolayer of organic compound in photoelectron microscopy. Relative photoelectron currents were measured for a series of copper phthalocyanine films ranging in thickness up to 1900 Å. The relative photoelectron currents were independent of temperature (90–298°K), suggesting that electron-electron and not electron-phonon scattering is the dominant mechanism. The photoelectric properties measured are determined primarily by the large organic ring structure and not the central metal atom, as evidenced by the fact that substitution of metal-free phthalocyanine for copper phthalocyanine did not substantially alter the values of observed photoelectron currents. An analysis of the data indicates the depth resolution is 15 ű 5 Å, and equals the electron mean free path. This very good depth resolution is a result of the low kinetic energy associated with electrons released by irradiation near the photoemission threshold.     

Interaction of the antimalarial agents halofantrine and lumefantrine with lipid bilayers

The effects of antimalarial drugs halofantrine and lumefantrine on the fluoresence anisotropy of diphenylhexatriene (DPH)-containing phospholipid vesicles have been examined. Lumefantrine increases DPH anisotropy, indicating a condensing effect on bilayers of dipalmitoylphosphatidyl choline (DPPC), dioleoylphosphatidylcholine (DOPC), egg lecithin and mouse erythrocyte membranes (including membranes isolated from plasmodial-infected mice). Its condensing effect is more pronounced in bilayers of lower microviscosity. In contrast, increases or decreases in DPH anisotropy are observed with halofantrine, depending on the nature of the lipid. Decreases in anisotropy, which reflect a perturbing effect, are observed in bilayers of high microviscosity (for example, gel state of DPPC bilayers). Increases in anisotropy are observed in bilayers of low microviscosity (such as DOPC and egg lecithin bilayers). The perturbing effect of halofantrine is further confirmed by the increases in permeability of calcein-containing DPPC vesicles in the presence of the drug. However the perturbative effects of halofantrine are observed to the same magnitude in uninfected and plasmodial-infected erythrocyte membranes, and may not be relevant to the antimalarial action of the drug. In contrast, the condensing effect of lumefantrine is significantly greater in infected erythrocyte membranes and may contribute to its antimalarial action.     

Water in polymer membranes. Part III : Water sorption and pore volume in cellulose acetate films

Changes in waveguide properties of several cellulose acetate membranes and one polyimide membrane were measured as a function of their exposure to varying levels of relative humidity. The volume fraction of water in the films and the occupied pore volumes were determined from refractive index and thickness changes. The dependence of the refractive index on water absorption is related to a competition between two processes: one of filling pores with no film expansion and one of “free expansion” where the film expands to completely accommodate the added water volume. The term “pore” is taken to mean a volume with molecular and not macroscopic dimensions. The hydration properties of these dense cellulose acetate membranes were affected by degree of acetylation, casting temperatures and annealing treatments. Annealing CA398 membranes at 180°C decreased film water concentration by reducing the amount of free expansion. Annealed CA398 membranes that were tested in a reverse osmosis cell were found to have high salt rejection compared to unannealed films. The hydration characteristics of a polyimide membrane are compared to cellulose acetate membranes.     

Anisotropy in Langmuir layers of a bent-core liquid crystal

Langmuir layers of a symmetric bent-core molecule with hydrocarbon end chains and two chlorine atoms substituted on the central phenyl ring of the bent core were characterized by a combination of surface pressure isotherms, Brewster angle microscopy, and surface potential measurements. These layers were found to be optically anisotropic, in contrast to Langmuir layers of similar molecules with different substitutions on the core. After compression, the orientation of the optical axis was essentially uniform over the film. Upon decompression, the film broke into uniform islands or domains. Measuring domain reflectivity while changing the domain orientation allowed the determination of the tilt angle with respect to both domain features and the film normal, as well as the refractive index anisotropy. The tilt angle, near 90 degrees, suggests that the bent-core molecules lie quite flat on the surface.     

Optical properties of polybutadiene in the bulk and near a gold interface

We have investigated thick films from polybutadiene on gold-coated glass using surface plasmon resonance / leaky optical waveguide spectroscopy with the aim of investigating differences in bulk and interphase properties in a single measurement. A broad range of molar masses was studied. Drying under ambient conditions leads to an exponential decay of the film thickness. Subsequent vacuum drying does not result in any further changes in the bulk part of the film but at the polymer-solid interface, indicating the absence of residual solvent. For all molar masses studied, the surface plasmon resonance is observed at angles which are incompatible with the properties of the bulk part of the film. A polymer interphase is thus present next to the gold layer which has a refractive index lower than in the bulk. Using transversal magnetic- and transversal electric polarized light, an optical anisotropy is found in the interphase which is attributed to segment alignment along the interface with gold.     

Chain orientation and anisotropies in optical and dielectric properties in thin films of stiff polyimides

Thin films of poly(p-phenylene biphenyltetracarboximide) (BPDA-PDA), prepared by thermal imidization of the precursor poly(amic acid) on substrates, have been investigated by optical waveguide, ultraviolet-visible (UV-VIS), infrared (IR), and dielectric spectroscopies. The polyimide films exhibit an extraordinarily large anisotropy in the refractive indices with the in-plane index n_∥ = 1.806 and the out-of-plane index n_⊥ = 1.589 at 1064 nm wavelength. No discernible effect of the film thickness on this optical anisotropy is found between films of ca. 2.1 and ca. 7.8 μm thickness. This large birefringence is attributed to the preferential orientation of the biphenyltetracarboximide moieties with their planes parallel to the film surface, coupled with the strong preference of BPDA-PDA chains to align along the film plane. The frequency dispersion of the in-plane refractive index n_∥ is consistent with the results calculated by the Lorentz–Lorenz equation from the UV-visible spectrum exhibiting several absorption bands in the 170–500 nm region. The contribution from the IR absorption in the range 7000–400 cm,^?1 computed by the Spitzer-Kleinmann dispersion relations from the measured spectra, adds ca. 0.046 to the in-plane refractive index n_∥. Tilt-angle–dependent polarized IR results indicate nearly the same increase for the out-of-plane index n_⊥. Application of the Maxwell relation then leads to the out-of-plane dielectric constant ε^⊥ ? 2.7 at 1.2 × 10¹³ Hz, as compared with the measured value of ca. 3.0 at 10⁶ Hz. Assuming this small difference to remain the same for the in-plane dielectric constants ε_∥, we obtain a very large anisotropy in the dielectric properties of these polyimide films with the estimated in-plane dielectric constant ε_∥ ? 3.4 at 1.2 × 10¹³ Hz, and ε_∥ ? 3.7 at 10⁶ Hz. © 1992 John Wiley & Sons, Inc.     

PRIMARY PRODUCTS OF LIQUID WATER PHOTOLYSIS AT 1236, 1470 AND 1849 Å

Abstract— The yield of the primary products of the liquid water photolysis at 1236 and 1470 Å is reported. It was found that besides the dissociation of the excited water molecules into H and OH radicals probably e⁻_aq is also formed. The H and OH radicals were scavenged by means of formate, and the e⁻_aq together with a part of H₂O* by adding carbon dioxide. The quantum yields determined at 1236 Å, are: Φ(H, OH) = 1.03 & 0.02, 0.06 <Φ( e⁻_aq , H₂O*) < 0.12 and at 1470 Å,: Φ(H, OH)=0.72±0.02, 0.037 < Φ( e⁻_aq , H₂O*) <0.075. The quantum yield of high purity water at 1849 Å in the absence of any scavengers is Φ(H, OH)=0.022. Previously published data by us for 1849 Å are also given: Φ(H, OH)=0.33 ± 0.01 and 0.02 < Φ ( e⁻_aq H₂O*) < 0.04. Reaction mechanisms are proposed.     

New organo-soluble polyimides for optical,microelectronic and fiber applications

A novel class of organo-soluble polyimides has been developed based on monomers containing twisted-biphenyl structures. Thus, 2, 2'-disubstituted-4, 4'-diaminobiphenyls and 2, 2'-disubstituted-4, 4', 5, 5'-biphenyltetracarboxylic dianhydrides have been prepared and polymerized in refluxing phenolic solvents. The polyimides, some of which are soluble in ketone and ether solvents, can be readily solution cast into colorless thin films. The rigid polymer chains align parallel to the substrate surface (in-plane orientation) during the casting process, which results in anisotropy in the film properties. The anisotropy in the refractive index has been used in the development of compensation layers for liquid crystal displays. The polymers have also been dry jet-wet spun from phenolic solutions to afford fibers that display tensile strengths of 3.5 GPa and moduli of 130 GPa.     

OPTICAL SPECTROSCOPY OF BILAYER MEMBRANES

Abstract— The absorption and fluorescence spectroscopy of natural and model bilayer lipid membranes is reviewed. Basic structural features of biological membranes and the relative advantages of black lipid membranes (BLM) and of liposomes are discussed. Theoretical considerations show that the wavelengths of absorption maxima are affected by the refractive index and dielectric constant of the medium surrounding the chromophore. Techniques of obtaining photoelectric action spectra, direct absorption spectra, and reflection spectra of BLM are described. Polarized spectra can give information about the orientation of membrane constituents and show, for example, that the porphyrin ring of chlorophyll in BLM is tilted at 45 ± 5° to the membrane surface. Absorption maxima of chlorophyll in BLM are compared with solution spectra of various chlorophyll adducts and aggregates. It is concluded that chlorophyll in BLM exists largely as solvated monomer and dimer (or oligomer), depending on concentration, and is not coordinated with water. From the theory of fluorescence spectroscopy it follows that aggregation and the polarity of the environment affect the fluorescence yield and lifetime of a membrane component, and also the wavelength of its emission maximum. The microviscosity of the membrane matrix affects the anisotropy of fluorescence. Techniques of steady-state fluorescence spectroscopy and of fluorescence lifetime measurements are reviewed. Examples of the use of fluorescent probes in membrane studies are given. Certain probes such as anilinonaphthalene sulfonate (ANS) preferentially bind to membrane proteins. The location of a probe in a particular membrane region can be pinpointed from its fluorescence yield and emission maximum. The orientation of the hydrocarbon chains of membrane lipids has been found, from fluorescence polarization of certain probes, to be normal to the membrane surface as postulated a priori on the basis of the lipid bilayer model. Anisotropy of fluorescence shows that elongated probe molecules rotate rapidly about their long axes when surrounded by phospholipids but become immobilized when bound to proteins. Changes in intensity and anisotropy of fluorescence as function of temperature have demonstrated the existence of phase transitions and phase equilibria of membrane lipids. Excimer fluorescence has been used as a measure of the available lipid core volume of membranes. Mechanisms of energy transfer between membrane components are reviewed. The theoretical dependence of energy transfer on distance and orientation for several rigid and fluid membrane models is discussed in terms of the structural information it can provide. Fluorescence sensitization resulting from energy transfer within and across bilayer membranes has been demonstrated in various systems. Quantitative measurement of energy transfer efficiency in BLM has shown that such transfer is about five times more efficient than in solutions at comparable donor-acceptor distances. Lipid membranes can be viewed as structures which maintain their components at high concentrations, in a reactive state, and at favourable orientations.     

THE SOLID STATE LUMINESCENCE OF ESTROGENS

Abstract. The luminescent behavior of certain estrogens in the solid state is discussed. Spectra of solid films of estradiol and estriol are red shifted compared to that in liquid solutions with estriol shifted to a lesser extent. The red shifts are attributed to intermolecular hydrogen bonding, this conclusion being based on IR spectra and X-ray data. The calculated distance over which hydrogen bonding takes place is 2.755 Å in estradiol and 2.638 Å in estriol. The increased shift in the spectrum of estradiol is believed to be due to an additional hydrogen bond with a water molecule present in the solid film (O—O distance 2.793 Å). In estrone the weak fluorescence present in liquid solution is absent in solid film, in contrast to 3-desoxyestrone where fluorescence is preserved. Out of three possible forms of crystalline estrone only two can form hydrogen bonds, based on O-O distance calculations. In equilenin, which is highly fluorescent in liquid solution, no fluorescence is detected in solid film. This again is attributed to hydrogen bonding between the hydroxyl group of the β-naphtholic chromophore and the carbonyl group in the C(17) position. Dihydroequilenin which lacks the carbonyl group and equilenin dissolved in a host crystal retain their fluorescence.     

Quasi-isotropic analysis of anisotropic thin films on optical waveguides

Thin films assembled on a substrate are often anisotropic. Nevertheless, because of experimental limitations, sufficient parameters to characterize the anisotropy, even in the simplest (and perhaps most common) case of uniaxial thin films, which are birefringent, are not usually available. This paper examines the consequences of treating them as isotropic thin films, with particular reference to their characterization via perturbation of the propagation constants (effective refractive indices) of optical waveguides. It is shown that the refractive index and geometrical thickness of a thin film thus calculated are often unrealistic (especially when the thin film is positively birefringent), but the mass per unit area may be quite precise, depending on the sign and magnitude of the birefringence.     

Quantitative characterization of the optical properties of absorbing polymer films: Comparative investigation of the internal reflection intensity analysis method

Nondestructive, three‐dimensional refractive‐index measurements are used for the determination of both the crystallinity and orientation in thin polymer films. The prism wave‐guide coupler is particularly suited for three‐dimensional isotropic and anisotropic thin‐film studies because of the quantitative character of the information obtained and the ease of data acquisition. It has been limited, however, to determining only the refractive index of transparent or weakly absorbing thin‐film samples. On the basis of thin‐film optics, this study develops a new internal reflection intensity analysis (IRIA) method, which uses the intensity information rather than the conventional mode angle values to acquire both the refractive index and the extinction coefficient over a range of transparent to highly absorbing polymer films. Therefore, the IRIA method overcomes the limitations of this prism wave‐guide coupler technique, which can only measure the refractive index of a weakly absorbing sample. With a Metricon PC‐2010 as the skeletal framework, a prototype instrument has been developed to apply and test the IRIA method. A study comparing both the refractive index and extinction coefficient obtained with ellipsometry, ultraviolet–visible/near‐infrared reflectometry, and IRIA for solvent blue 59 dyed polystyrene films confirms that the IRIA method is effective for obtaining the three‐dimensional refractive indices and extinction coefficients of polymer films. In addition, the refractive index and extinction coefficient spectrum (400–800 nm) of solvent blue 59 have been determined with the effective media theory. Furthermore, the three‐dimensional complex refractive indices of highly absorbing black electrical tape, inaccessible to other optical measurement because of its surface character, has been determined by the IRIA method. © 2003 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 41: 842–855, 2003     

RELATION BETWEEN THE OPTICAL ANISO-TROPY INDEX AND THE POLARIZABILITY ELLIPSOID OF ATOM GROUPS

The optical anisotropy index is a new conception which has been proposed by the author of this paper on the basis of the Gladstone refractivity (i.e. specific energy by Larsen). The conception is very useful for studying the structural-optical mineralogy. The polarizability ellipsoid of atom groups in minerals may be obtained from the optical anisotropy index; conversely, when the crystal structure, orientation, and the polarizability ellipsoid of atom groups are known, the optical anisotropy index of the minerals may be calculated, and then three principal refractive indices, and the optical axis angles may be calculated, too. The calculated values of optical properties are in very good agreement with the experimental values. This calculation method is much simpler thanBragg's for the carbonates, and the precision of calculated values is much higher than that of Bragg's. The principle of calculation method is suitable for all the minerals containing some strong anisotropic atom groups, besides carbonat     

Optical switching of azobenzene sidechain molecules observed by surface plasmon spectroscopy

The use of surface plasmons is reported for the optical characterization of thin organic films, in particular thin polymer films at a metal dielectric interface. The basic properties of PSP are presented, including excitation and the technique of surface plasmon microscopy, which allows to obtain an image. As an example studies are presented on the trans-cis photoisomerization of azobenzene sidechain polymers in ultra thin films, prepared by the Langmuir-Blodgett-Kuhn technique. The transition from the trans to the cis state induces a change in the refractive index of the film. These changes were detected and their kinetics measured by a surface plasmon spectroscopy technique. A refractive index pattern could be written into the films and the areas of different refractive indices have been detected by surface plasmon microscopy.