Quantification of fluorine density in the outermost atomic layer

The outermost atomic layer of perfluorinated thiol monolayers on gold and poly(tetrafluoroethylene) (PTFE) is analyzed by low-energy ion scattering. Absolute quantification of fluorine density in this layer was achieved after calibrating the fluorine signal with a freshly cleaved LiF(100) single crystal. The fluorine density of monolayers of a C8F17-thiol on gold was 1.48 x 10(15) F atoms/cm2, whereas for PTFE a value of 1.24 x 1015 F atoms/cm2 was observed. This difference was explained by the different tilt angles of the thiol on gold and PTFE chains with respect to the surface normal. Both a configurational and a molecular interpretation on the perfluorinated thiol monolayer on gold are given.     

Scanning tunnelling microscopic study of the interfacial order in a ferroelectric liquid crystal

We present scanning tunnelling microscopy images of a ferroelectric liquid crystal in its chiral smectic C phase. These images show evidence of pronounced interaction between the adjacent lateral dipoles located near the chiral centres of neighbouring molecules. The two dimensional structure at the graphite—liquid crystal interface is found to be highly ordered. Features of this crystal-like order are the pronounced dimer arrangement within a layer and a kinked conformation in which the aromatic core is tilted with respect to the chains. The core axis is tilted at 30° with respect to the layer normal in the plane of the interface. This tilt is very similar to the tilt angle of the bulk material.     

Scanning tunnelling microscopic study of the interfacial order in a ferroelectric liquid crystal

Abstract

We present scanning tunnelling microscopy images of a ferroelectric liquid crystal in its chiral smectic C phase. These images show evidence of pronounced interaction between the adjacent lateral dipoles located near the chiral centres of neighbouring molecules. The two dimensional structure at the graphite—liquid crystal interface is found to be highly ordered. Features of this crystal-like order are the pronounced dimer arrangement within a layer and a kinked conformation in which the aromatic core is tilted with respect to the chains. The core axis is tilted at 30° with respect to the layer normal in the plane of the interface. This tilt is very similar to the tilt angle of the bulk material.     

Novel UV‐induced photografting process for preparing poly(tetrafluoroethylene)‐based proton‐conducting membranes

A novel process comprising the UV‐induced photografting of styrene into poly(tetrafluoroethylene) (PTFE) films and subsequent sulfonation has been developed for preparing proton‐conducting membranes. Although under UV irradiation the initial radicals were mainly generated on the surface of the PTFE films by the action of photosensitizers such as xanthone and benzoyl peroxide, the graft chains were readily propagated into the PTFE films. The sulfonation of the grafted films was performed in a chlorosulfonic acid solution. Fourier transform infrared and scanning electron microscopy were used to characterize the grafted and sulfonated membranes. With a view to use in fuel cells, the proton conductivity, water uptake, and mechanical properties of the prepared membranes were measured. Even through the degree of grafting was lower than 10%, the proton conductivity in the thickness direction of the newly prepared membranes could reach a value similar to that of a Nafion membrane. In comparison with γ‐ray radiation grafting, UV‐induced photografting is very simple and safe and is less damaging to the membranes because significant degradation of the PTFE main chains can be avoided. © 2007 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 45: 2624–2637, 2007     

Influence of 4-cyano-4'-biphenylcarboxylic acid on the orientational ordering of cyanobiphenyl liquid crystals at chemically functionalized surfaces

We report two methods that involve tailoring of the chemical composition of the nematic liquid crystal 4-cyano-4'-pentylbiphenyl to achieve control over the orientational ordering of the liquid crystal on chemically functionalized surfaces. The first method involves the direct addition of 4-cyano-4'-biphenylcarboxylic acid to 4-cyano-4'-pentylbiphenyl. The second method involves exposure of 4-cyano-4'-pentylbiphenyl to ultraviolet light and photochemical generation of a range of products, including 4-cyano-4'-biphenylcarboxylic acid. The addition of the acid or exposure to ultraviolet light accelerated the rate at which the liquid crystal exhibited an orientational transition from planar to perpendicular (homeotropic) alignment on surfaces presenting ammonium groups. The appearance of the homeotropic orientation of the UV-treated 4-cyano-4'-pentylbiphenyl on ammonium-terminated surfaces was dependent on the thickness of the film of liquid crystal (13-50 mum), consistent with a dipolar coupling between the liquid crystal and the electric field associated with an electrical double layer generated at the ammonium surface. Although the addition of 4-cyano-4'-biphenylcarboxylic acid or UV treatment of the liquid crystal also promoted homeotropic orientations on surfaces presenting hydroxyl groups, the orientations of the UV-treated liquid crystal on the hydroxyl-terminated surface did not change with thickness of the film of liquid crystal in the manner observed on the ammonium-terminated surfaces. The latter result indicates that the mechanism leading to homeotropic anchoring on hydroxyl-terminated surfaces is distinct from that on ammonium-terminated surfaces. Measurements performed using polarization modulation infrared reflection-absorption spectroscopy suggest that hydrogen bonding between the 4-cyano-4'-biphenylcarboxylic acid and the hydroxyl-terminated surface is responsible for the homeotropic anchoring on the surface. Finally, the orientation of the liquid crystal on methyl-terminated surfaces was not influenced by the addition of 4-cyano-4'-biphenylcarboxylic acid nor UV treatment. These results illustrate how the chemical composition of liquid crystals can be manipulated to achieve control over their ordering on surfaces that possess chemical functionality relevant to the development of liquid crystal-based sensors and diagnostic tools. We illustrate the utility of this approach by using the tailored liquid crystal to amplify and optically transduce the presence of proteins arrayed on ammonium-terminated surfaces.     

Relationship between alignment layer polymer surface structures and liquid crystal display parameters

The polar alignment layer (AL) surface provided relatively small liquid crystal (LC) pretilt angles while polyimides with long alkyl side chains gave relatively large LC pretilt angles. The results suggest that LC pretilt angles, in addition to an anchoring effect, are greatly affected by both electronic and steric interactions between LC molecules and a polyimide alignment layer surface. Rubbing with a cotton cloth induces functional groups, side chains, and repeat units at the surface of a liquid crystal polyimide AL to re-orient. It was discovered that rubbing induced polar functional groups and repeat units to re-orient out-of-the-plane of the surface, and it made non-polar aliphatic side chains partially re-orient inwards, toward the bulk of the film.     

About the microstructure of PCVD prepared crystal mats of statistical oligo-vinylidene-fluoride-trifluoroethylene in relation to other fluorinated polymers

The room temperature phase of the physical/chemical vapor-deposited statistical co-oligomer VDF/TrFE(70/30) has been characterized by different experimental methods such as small-angle x-ray scattering, wide-angle x-ray diffraction, size-exclusion chromatography, infrared absorption and optical refractometry. The characterization of the elastic properties was carried out using high-performance Brillouin spectroscopy in connection with special scattering geometries. The co-oligomer VDF/TrFE(70/30) was obtained by cracking the parent statistical copolymer and subsequently vapor depositing the shortened chains on highly oriented PTFE substrates (PIA-technique). The room temperature phase of the resulting oriented waxy crystal mats of VDF/TrFE(70/30) is predominantly ferroelectric. The physical properties are very different to those of similarly PIA-prepared n-alkanes, perfluoroalkanes, and blockfluoroalkanes. The microstructure of VDF/TrFE(70/30) is interpreted in terms of partially crystalline nano-sized structures giving rise to a marked freezing process below room temperature. © 1995 John wiley & Sons, Inc.     

Large-scale periodic pattern in homeotropic liquid crystals induced by electric field

In-plane periodic modulation of the director field of an initially homeotropic liquid crystal layer with negative dielectric anisotropy is studied in external electric field with time varying amplitude envelope. Periodically turning a low frequency electric field on and off the cell, an in-plane periodic modulation of the director develops with a length scale that is orders of magnitude larger than the cell gap and can be as large as couple of centimetres. Doping the liquid crystal layer with low concentration of dichroic dye allows easy mapping of the director. We show that the periodic pattern is a periodic arrangement of +1 and ?1 point defects, so called ‘umbilics’. We argue that the in-plane direction of the director is governed by the local flow that accompanies the turn on and off of the electric field. The finite size and the shape of the cell, incompressibility constraint and the nature of the surface alignment material, all together with the flow, are ultimately responsible for the development of the observed periodic director modulation.     

Accordian-type Laser-Raman scattering by polymers

All the improvements of the independent-rod model of longitudinal accordian-type acoustic mode (LAM) oscillations have assumed that the oscillation energy is retained either on the isolated macromolecule oscillating in a vacuum or in a narrow cylinder containing the straight sections of the macromolecules in the crystal lattice and their straight continuations through the amorphous layers. According to such concepts, concentration of the oscillation energy in gauche defects or amorphous layers occurs, respectively, whenever the axial elastic modulus of the straight sections (crystal lattice) is very much larger than that of the kinked sections (amorphous layers). The effect is enhanced by low crystallinity. Actually such behavior has never been observed. To agree with experimental data the model has to be modified in such a manner that the oscillation amplitude in the amorphous layer steadily decreases with increasing distance from the boundary between the two phases. The necessary large damping of the LAM oscillation in the kinked sections results from true damping in the viscoelastic amorphous component and energy transfer to adjacent chains which turns out to be just as easy as energy conduction along the kinked chain. Such a transfer is equivalent to radiation of the oscillation energy in all directions in the kinked phase. As a consequence of damping, the coupling of chains in adjacent crystals becomes so small that it may be completely neglected. Such a model explains in a satisfactory manner the observed accordion Laser-Raman spectra of the semicrystalline polymers and the infrared absorption of paraffins in the liquid state.     

Ionic liquid crystals based on viologen dimers: tuning the mesomorphism by varying the conformational freedom of the ionic layer

We investigated the liquid crystal behaviour of newly synthesised bistriflimide salts of symmetric viologen dimers. A smectic A phase was observed for intermediate spacer lengths and for relatively long lateral alkyl chains. The systems were characterised by thermal analysis, polarised optical microscopy, X-ray scattering and solid-state NMR. An intermediate ordered smectic phase was also exhibited by the compounds (except for systems with very short lateral chains) consisting of molten layers of alkyl chains and partially ordered ionic layers. These results, relating to the mesomorphic behaviour of viologen salts, are qualitatively compared to those of the more common imidazolium salts, highlighting the importance of the conformational degrees of freedom of the anions and of the cationic core. It appears that fine tuning of the conformational degrees of freedom of the ionic layer is an important component of mesophase stabilisation.     

Orientational order of a nematic polymer grafted on polytetrafluoroethylene

The orientation behavior of a nematic side‐chain polymer grafted on polytetrafluoroethylene (PTFE) is studied. The sandwich‐like composite was prepared by the graft post‐polymerization of 4‐cyan‐4′‐(biphenyloxy)butyl acrylate on a PTFE film pretreated by vacuum ultraviolet irradiation (VUV). The oriented liquid crystal layer was prepared by mechanical stretching of the composite film. Orientation degree was measured by means of FT‐IR spectroscopy. The orientation function increases with the increasing strain and attains the upper limit value at the strain Δl/l > 1.0. The temperature dependence of the orientation degree is completely reversible under cooling and heating. Copyright © 2000 John Wiley & Sons, Ltd.     

成像级CdSe中红外波片的表面处理工艺研究

本文利用改进的垂直无籽晶气相升华法生长出尺寸达Φ30×40 mm的优质硒化镉(CdSe)单晶体。解理晶体,通过X射线衍射仪测试精确的获得(001)晶面。然后定向切割、研磨、抛光,获得了尺寸为20×20×3mm~3的CdSe中红外波片初胚。以弱碱性溶液与刚玉粉的混合液作为抛光液,利用化学机械抛光法对CdSe中红外波片进行表面抛光处理。结果显示,抛光处理有效的减少了波片表面的损伤层、划痕及结构缺陷,晶片表面的粗糙度降低,在2~20μm波段透过率较高(达到70%),满足中红外波片的应用需求。     

Encapsulation of ionic liquids within polymer shells via vapor phase deposition

We demonstrate the use of vapor phase deposition to completely encapsulate ionic liquid (IL) droplets within robust polymer shells. The IL droplets were first rolled into liquid marbles using poly(tetrafluoroethylene) (PTFE) particles because the marble structure facilitates polymerization onto the entire surface area of the IL. Polymer shells composed of 1H,1H,2H,2H-perfluorodecyl acrylate cross-linked with ethylene glycol diacrylate (P(PFDA-co-EGDA)) were found to be stronger than the respective homopolymers. Fourier transform infrared spectroscopy showed that the PTFE particles become incorporated into the polymer shells. The integration of the particles increased the rigidity of the polymer shells and enabled the pure IL to be recovered or replaced with other fluids. Our encapsulation technique can be used to form polymer shells onto dozens of droplets at once and can be extended to encapsulate any low vapor pressure liquid that is stable under vacuum conditions.     

Surface Modification of Poly(tetrafluoroethylene) Films by Plasma Pre-Activation and Plasma Polymerization of Glycidyl Methacrylate

Surface modification of poly(tetrafuoroethylene) (PTFE) film by plasma polymerzation and deposition of glycidyl methacrylate (GMA), in the presence and absence of Ar or O₂ plasma pre-activation, was carried out to enhance the adhesion with polyimides (PI) film in the presence of an epoxy adhesive. For deposition carried out at low RF power, a high epoxide concentration was preserved in the plasma-polymerized GMA (pp-GMA) layer on PTFE (pp-GMA-PTFE). However, high adhesion strength of the PI/pp-GMA-PTFE laminate was obtained only in the presence of O₂ plasma pre-activation of the PTFE substrates prior to plasma polymerization and deposition of GMA. In the absence of any plasma pre-activation or in the presence of Ar plasma pre-activation, the deposited pp-GMA layer on the PTFE surface could be readily removed by solvent extraction. The adhesion enhancement of the PI/pp-GMA-PTFE laminates in the presence of O₂ plasma pre-activation was attributed to the preservation of the epoxide functional groups in the pp-GMA layer, the curing of the GMA chains into the matrix of the epoxy adhesive, and the covalent bonding of the pp-GMA layer on the PTFE surface.     

Infrared intensity and morphology of l-monolaurin-water systems

The infrared spectra for some metastable states in 1-monolaurin water systems were observed at room temperature, where the relative intensity of bands due to paraffin chains changed considerably, especially in the CH2 rockings, which disappear in some cases. It is considered that the spectral changes result from the morphology change on going from the crystal to the liquid crystal, smectic B phase, so-called gel phase, which consists of the lipid bilayers with ordered paraffin chains alternating with water layers. The model for explaining the intensity change is proposed on the basis of the interaction among oscillating dipoles.     

Donor-acceptor-donor-type liquid crystal with a pyridazine core

[structure: see text] A new liquid crystalline material having an ethylenedioxythiophene-pyridazine-ethylenedioxythiophene (EDOT-PDZ-EDOT) core with two peripheral long alkyl chains was prepared. The designated donor-acceptor-donor (D-A-D)-type core structure induced a distinct smectic liquid crystalline phase due to the strong intermolecular interaction. The photophysical property and the layer structure of the liquid crystal were investigated by differential scanning calorimetry, polarized light microscopy, X-ray diffraction, and cyclic voltammetry.     

Determination of the molecular orientation of very thin films on solid substrates: surface liquid crystal layers and rubbed polyimide films

The molecular orientation of very thin films on solid substrates can be determined quantitatively by measuring the polarized infrared (IR) absorption spectra of samples as a function of angle of incidence. The quantitative molecular orientation is derived by fitting the incident angle dependence and the dichroic ratio with theoretical calculations. We applied this method to a technologically important system: liquid crystal (LC)/rubbed polyimide film. To understand the alignment mechanism of LC molecules in contact with rubbed polyimide films, we have quantitatively determined the molecular orientation of rubbed polyimide films and a surface LC layer in contact with a rubbed polyimide film. In this paper two relations are discussed: (1) correlation between the inclination angle of polyimide backbone structures in rubbed films and the pretilt angle of bulk LC in contact with them, and (2) relation among the molecular orientation of a rubbed polyimide film and those of surface and bulk LC layers in contact with it.     

Highly birefringent nematic and chiral nematic liquid crystals

We report a simple interference method to determine the dispersion of the extraordinary refractive index and birefringence of highly conjugated and coloured nematic liquid crystals used as light-emitting materials in organic electroluminescent devices. The measurements are made in the nematic glass phase at room temperature. The birefringence is highly dispersive and values up to 1.1 are obtained. Chiral groups are incorporated into the end chains giving a chiral nematic liquid crystal with a very wide stopband in the visible region. The Berreman matrix method is used to simulate transmission through the chiral nematic liquid crystal cell using the refractive index parameters obtained experimentally. Excellent agreement between theory and experiment is found.     

Surface damages of P.T.F.E. by sputtering: an infrared study

Sputter cleaning of bulk PTFE is studied in the infrared range by Attenuated Total Reflectance. The I.R. spectra displays that sputtering produces a fine powder by breaking long molecular chains of PTFE, with roughly the same spectra as the bulk sample, but with sharpened and shifted absorptions. This phenomenon is discussed and powder formation during sputter cleaning could explain an adhesion weaker than the expected one, when copper films are deposited by ion plating on this polymer.     

Mixtures of liquid-crystalline polymers

A density-functional expansion method is used to derive the free energy of a polymer mixture. The expression obtained includes the entropy of mixing, the entropy of configuration of the chains and the interactions (both isotropic and anisotropic ones). The chains are modelled as interacting elastic lines (bend curvature). The method is very general, and we only focus our attention on binary mixtures. The phase diagram and the order parameters are calculated. We show some results for two types of mixtures: a nematic polymer in a non-mesomorphic particle (polymer or solvent) and in another nematic liquid crystal (small-molecule or polymer). We discuss the influence of the molecular weights, the persistence length and the interactions on the phase separation.