Second-harmonic generation in a novel crosslinked pyroelectric liquid crystal polymer (PLCP)

We report second-harmonic generation in samples with a pyroelectric liquid crystal polymer (PLCP) prepared from the ferroelectric liquid crystalline acrylate monomer 4-[(R)-(-)-2-(10-acryloyloxydecyl)oxy]-3-nitrophenyl 4-[4-(11-acryloyloxyundecyloxy)phenyl]- benzoate (A2c) by in situ photopolymerization. The relation between SHG efficiency, spontaneous polarization and amount of chiral substance in this and previously synthesized PLCP materials is discussed. Electro-optical properties of the monomeric and crosslinked A2c are investigated. In the crosslinked state, the molecular mobility is found to be very low, supporting the fact that the degree of polar order in the polymer is high. In contrast to the case of a poled electret NLO material, the polarization of this material is not 'frozen-in', but an intrinsic thermodynamic property.     

Correlations between surface and bulk liquid crystal alignment observed with optical second-harmonic generation

Abstract

We report a direct experimental observation of surface-induced liquid crystal (LC) monolayer orientation and its relation to the LC bulk alignment. Using optical second-harmonic generation from these LC monolayers, adsorbed on rubbed polymer surfaces, we determined both the polar and azimuthal orientation of the LC molecules. A close correlation between the rubbing-induced azimuthal orientational distribution of the LC monolayers and the resulting LC bulk pretilt angle has been observed. A simple model based on epitaxial growth of the bulk LC on top of the first LC monolayer is proposed to explain this correlation.     

A styrylpyrene-containing liquid crystal polymer for photoinduced crosslinking

Photoresponsive polymers have attracted increasing attention. Herein, we designed and synthesized a photoresponsive liquid crystal polymer (LCP). The LCP was prepared through the copolymerization of a styrylpyrene-containing photoresponsive monomer and a monomer containing mesogen 4-cyanophenyl benzoate. Gel permeation chromatography (GPC) curve and ¹H nuclear magnetic resonance (¹H NMR) spectrum confirmed that the LCP was successfully synthesized. Thermogravimetric analysis and differential scanning calorimetry curves showed that the LCP had high thermal stability, with a decomposition temperature at 5% weight loss of 305°C, and T_g and T_i of 46 and 128°C, respectively. Polarized optical microscopy studies suggested that thermal annealing promoted the orientation of the LCP films. Model compounds containing styrylpyrene units showed photo-controlled dimerization. Furthermore, the dimerization of the styrylpyrene units could control the crosslinking densities in the LCP, thereby regulating the network structures and properties. These properties demonstrate that the LCP is a promising photoresponsive material.     

Volume generation towards dynamic surface morphing in liquid crystal polymer networks

ABSTRACT

Thin coatings based on liquid crystal networks (LCNs) modified with azobenzene moieties are able to create dynamic surface topographies in the micrometre range by exposure with UV light. The surface corrugations can be erased and restored by switching ‘off’ and ‘on’ the UV illumination. Various configurations were presented. The formation of the protrusions was proven to be induced mainly by excessive volume formation when the order in the LCNs is reduced. It is suggested that this extra volume formation can be further enhanced by stimulating the oscillatory dynamics of trans-cis and cis-trans isomerisation. Therefore, dual-wavelength exposure not only exciting the trans state of azobenzene by 365 nm UV light but simultaneously also the cis state by 455 nm blue light was shown to enhance the effect.     

Phase-matched second-harmonic generation in poled polymer waveguide based on a main chain polymer

The electric field-induced dynamic phase-matching of second harmonic generation (SHG) waveguide was demonstrated by using a main chain polyarylamine. The linear and nonlinear optical properties of this polymer are presented. The optimum phase-matching thickness was controlled by applying an electric field to the polymer waveguide. The effective phase-matching thickness variation induced by poling is about 0.025 μm that is six times larger than full width at the half-maximum (FWHM) of phase-matching thickness in conventional slab waveguide. The efficient phase-matched SHG was observed from a taperless slab wave-guide. The optical loss of poled polymer on glass substrate at 632.8 nm was 2.7 dB/cm. © 1995 John Wiley & Sons, Inc.     

Sorption of uranyl cations on a rutile (001) single crystal monitored by surface second-harmonic generation

The rotational anisotropy of second-harmonic generation at the surface of a (001) single-crystal rutile is obtained in the presence of uranyl cations sorbed at the surface from acidic solutions at various concentrations. Surface second-harmonic generation appears to be sensitive to the presence of uranyl cations on the rutile samples. Evolution of the anisotropy pattern with initial uranyl concentration is analyzed through a phenomenological model. The elements obtained for the nonlinear susceptibility tensor Chi(2) for each sample significantly constrain the geometry of the possible sorption complexes between uranyl cations and rutile and lead to the proposition of two sorption sites involving different oxygen atoms of the rutile surface.     

Bent-core liquid crystal-functionalised flexible polymer substrates for liquid crystal alignment

ABSTRACT

A transparent flexible polymer film is chemically functionalised with a bent-core liquid crystal (BCLC) compound for effective alignment of the bulk BCLC sample at the substrate–LC interface. The surface attachment was achieved via a simple procedure which involved pre-treatment of the polymer film (commercial name: over head projector film) using piranha solution followed by chemically attaching the BCLC compound through silane condensation reaction. Surface characterisation of the unmodified and BC-modified flexible films was carried out through X-ray photoelectron spectroscopy (XPS), attenuated total reflectance fourier transform infrared (ATR-FTIR) spectroscopy, contact angle (CA) and atomic force microscopy (AFM) techniques. The BC-modified flexible substrates are analysed for their efficiency to orient the bulk LC sample. Remarkably, the chemically modified polymer substrates are highly efficient in vertically aligning both the BC and rod-like LC samples at the substrate–LC interface, in comparison to their unmodified counterparts. The described method is simple, reproducible, surface modified substrates are highly stable and more importantly reusable. The demonstrated method for the alignment of BCLCs advances a step forward towards the realisation of applications proposed for these fascinating compounds.     

Efficient second-harmonic generation from low-dimensional dye aggregates in thin polymer film

Thiapyrylium dye can aggregate spontaneously in a polymer matrix to form a low-dimensional filament-like ordered structure. Efficient second-harmonic generation from a thin polymer film containing such dye aggregates is observed. Second harmonic signal from a 5 μm polymer film containing 1.5 wt% of dye molecules is comparable to that of a 250 μm thickness of urea. This is the first example of an optically nonlinear organic medium that contains the following desired features for integrated optics: large polarizability, self dipolar alignment, and active in a thin polymer film.     

Greyscale generation for optically driving liquid crystal display

It is very difficult to display greyscale for an optically driving liquid crystal display (ODLCD) because the driving unit and display unit are separate, and there are no electronics in the display part of ODLCD. In this paper, two greyscale generation methods are proposed to achieve this goal. Six and nine grey levels are separately achieved through the two methods.     

An azobenzene-containing side-on liquid crystal polymer

A new azobenzene-containing side-on LC polymer was synthesized. It shows a nematic phase. Its photochemical properties were characterized in solution and in thin film. The N-I phase transition can be induced upon irradiation in the π-π* absorption region of the polymer.     

Copolymerizable initiator for improved polymer dispersed liquid crystal films

The problems of photoinitiator contamination are addressed for the liquid crystal phase in polymer dispersed liquid crystal films formed by photopolymerization induced phase separation of liquid crystal from monomer solutions. Initiator contamination lowers the clearing point of the liquid crystal phase, and decreases the photostability and resistivity of the polymer dispersed liquid crystal. These problems are minimized by replacing the conventional photoinitiators with copolymerizable initiators which become incorporated in the polymer phase as it separates. Copolymerizable photoinitiators are studied and used to form polymer dispersed liquid crystals with higher clearing point liquid crystal phases, higher resistivity, and better photostability than polymer dispersed liquid crystals formed with conventional photoinitiators. These improvements provide very significant advantages for many polymer dispersed liquid crystal applications.     

Copolymerizable initiator for improved polymer dispersed liquid crystal films

Abstract

The problems of photoinitiator contamination are addressed for the liquid crystal phase in polymer dispersed liquid crystal films formed by photopolymerization induced phase separation of liquid crystal from monomer solutions. Initiator contamination lowers the clearing point of the liquid crystal phase, and decreases the photostability and resistivity of the polymer dispersed liquid crystal. These problems are minimized by replacing the conventional photoinitiators with copolymerizable initiators which become incorporated in the polymer phase as it separates. Copolymerizable photoinitiators are studied and used to form polymer dispersed liquid crystals with higher clearing point liquid crystal phases, higher resistivity, and better photostability than polymer dispersed liquid crystals formed with conventional photoinitiators. These improvements provide very significant advantages for many polymer dispersed liquid crystal applications.     

Switchable anti-peeping film for liquid crystal displays from polymer dispersed liquid crystals

Liquid crystals displays (LCDs) currently dominate the display market, wherein a wide viewing angle is considered as one of the most important characteristics. However, for LCDs with wide viewing angles, some private information inevitably becomes more visible; thus, an LCD with a switchable viewing angle has attracted greater interest. Here, we report a novel switchable viewing angle film that can make the viewing angle of an LCD electrically switchable between ±30° and ±60°, i.e. between an anti-peeping mode and a share mode, by 5.0 V is turned on and off, respectively. The response time necessary to change between the modes is in milliseconds. It is believed that it has potential applications in LCDs with high quality.     

A dual frequency addressed polymer stabilized pi-cell liquid crystal device

Dual frequency addressing of nematic pi-cell devices produces submillisecond switching times since the liquid crystal can be driven both parallel and perpendicular to the applied field and there is no kick-back of the director during switching. The nucleation of the V state in devices containing dual frequency liquid crystal materials is much slower than that in conventional pi-cells, however. Polymer stabilization of the V state eliminates the need for nucleation each time the device is used. In this paper we present a polymer stabilized pi-cell containing a dual frequency liquid crystal material, and show that the presence of the polymer network significantly influences the switching of the device. Some optimization of the addressing scheme is required when switching the polymer stabilized device in order to avoid transient formation of the twisted state. Using this optimization, the switching time is under 3 ms across a wide range of addressing voltages.     

Phase grating generation and optical phase conjugation in photorefractive polymer/dissolved liquid crystal composites

The orientational photorefractive properties of photorefractive mesogenic composites have been investigated by means of two-beam coupling and degenerate four-wave mixing. Photorefractive mesogenic composites, consisting of low molar mass liquid crystals, polymer, and photoconductive sensitizer, constitute novel organic materials possessing high performance photoreractivity. The refractive index change was estimated on the basis of the theory of two-beam coupling and four-wave mixing, and large index modulation of over 0.01 was obtained.     

Sandwich structure containing liquid crystal polymer grafted on polymer support

Graft post-polymerization of mesogenic monomers onto fluorine-containing polymer support was initiated by the simultaneous action of vacuum ultraviolet radiation and atomic oxygen. The resultant two-layer structure possesses the combined physical–mechanical properties of the polymer-supporting film and the optical characteristics of the anisotropic liquid crystal layer.     

Studies of polymer ball type polymer dispersed liquid crystal films

'Reverse' or 'polymer ball' polymer dispersed liquid crystal (PDLC) samples were prepared by a photopolymerization induced phase separation method. A detailed study of the effects of the sample preparation parameters, such as curing time, curing intensity, and liquid crystal concentration are reported. It was found that by adjusting these parameters, we were able to change the morphology of these 'polymer ball' PDLC samples and thus change their optical characteristics. Incomplete polymerization of the PDLC samples results in a higher threshold voltage and a lower ON state transmission. When the amount of monomer is too low, the shape of the resulting polymer ball becomes irregular, and the sample has a larger threshold voltage and a larger saturation voltage.     

Polymerization induced phase separation (PIPS) in a polymer dispersed liquid crystal (PDLC) system: A Monte-Carlo simulation approach

The polymerization induced phase separation (PIPS) process in a polymer dispersed liquid crystal (PDLC) system was studied by using Monte-Carlo (MC) simulation methods. In particular, the dependence of the phase separation between liquid crystal and polymer on the parameters, such as temperature γ = ε/kT, polymerization reactivity p and curing time t_c, was examined. It was found that the pair correlation function G(a, t) decreases with the decrease of temperature when the polymerization reactivity p is fixed. Our results also revealed that at a constant temperature, the final value of G(a) first increases with the increasing of p, and finally approaches a constant value. This observation provides us an effective way of controlling the size of liquid crystal droplets as well as their distributions. It was observed that the equilibrium value of G(a) increases as the curing time increases when both temperature and p are kept constant. This is another way of controlling the size of liquid crystal droplets.