Dual effects of fullerene doped to holographic polymer dispersed liquid crystals

Doping of conductive fullerene particles to the formulation of conventional holographic polymer dispersed liquid crystal‐induced dual effects of reducing both droplet coalescence and operating voltage. Fullerene induced an induction period which otherwise does not exist, followed by a gradual increase of diffraction efficiency to a saturation value being increased with increasing fullerene content. The increased diffraction efficiency was caused by the decreased droplet coalescence which was due to the hindered migration of LC by the fullerene particles. On the other hand, doped fullerene particles augmented the conductivity of polymer phase and hence the local electrical field imposed on LC droplet, which overcome the threshold for driving and reduced operating voltage and response times. © 2007 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 45: 5590–5596, 2007     

Oriented polymer dispersed discotic liquid crystals

Polymer dispersed discotic liquid crystals (PDDLCs) were prepared using the hexa-n-octanoate of rufigallol (RHO) and three polymer matrices: polystyrene (PS), poly(methyl methacrylate) (PMMA) and poly(ethyl methacrylate) (PEMA). The molecular orientation of RHO in stretched PDDLC films was characterized by means of infrared dichroism. It was found that the stretching of films that contain RHO in both the columnar D1 and crystalline phase can effectively align columns of RHO along the stretching direction, with the short axes of the rigid cores lying in the plane of the film. By contrast with stretched polymer dispersed nematic liquid crystals, no orientation of RHO is induced for films stretched with RHO in the isotropic phase, followed by rapid cooling to room temperature. However, if stretched films are cooled under strain into the columnar D1 phase, orientation of RHO can develop with time.     

Oriented polymer dispersed discotic liquid crystals

Polymer dispersed discotic liquid crystals (PDDLCs) were prepared using the hexa-n-octanoate of rufigallol (RHO) and three polymer matrices: polystyrene (PS), poly(methyl methacrylate) (PMMA) and poly(ethyl methacrylate) (PEMA). The molecular orientation of RHO in stretched PDDLC films was characterized by means of infrared dichroism. It was found that the stretching of films that contain RHO in both the columnar D1 and crystalline phase can effectively align columns of RHO along the stretching direction, with the short axes of the rigid cores lying in the plane of the film. By contrast with stretched polymer dispersed nematic liquid crystals, no orientation of RHO is induced for films stretched with RHO in the isotropic phase, followed by rapid cooling to room temperature. However, if stretched films are cooled under strain into the columnar D1 phase, orientation of RHO can develop with time.     

Putting liquid crystal droplets to work: a short history of polymer dispersed liquid crystals

A Commentary on the paper ”Reorientation dynamics of polymer dispersed nematic liquid crystal films?, by P. Drzaic. First published in Liquid Crystals, 3, 1543‐1559 (1988).     

Putting liquid crystal droplets to work: a short history of polymer dispersed liquid crystals

A Commentary on the paper “Reorientation dynamics of polymer dispersed nematic liquid crystal films„, by P. Drzaic. First published in Liquid Crystals, 3, 1543-1559 (1988).     

Mechanically activated cholesteric polymer dispersed liquid crystals

In this paper we deal with a cellulose derivative cholesteric dispersed liquid crystal (CCDLC) with mechanically tuneable optical properties. The composite is formed with a matrix of acetoxypropylcellulose with embedded micrometric and submicrometric droplets of a cholesteric mixture. Polarizing optical microscopy and atomic force microscopy (AFM) measurements performed on the system are reported and it is shown that the wavelength of the reflected light can be changed by a temperature variation and it is also changeable through a mechanical deformation. The pitch of the deformed droplets can be measured from AFM photographs and compared with the wavelength reflected by the CCDLC composite material.     

Electrooptic properties of polymer dispersed liquid crystals

An investigation of the electrooptic properties of polymer dispersed liquid crystals (PDLC) is presented. These materials are light modulating systems. They show a reversible optical response from an opaque state to a highly transmitting state under the action of an appropriate electric field which aligns the liquid crystal director. The switching voltage required to establish such an electric field has been monitored as a function of (i) the starting materials used for the preparation of the PDLCs, (ii) the ageing (curing time) of the PDLC cells. Other physical properties, such as the electrical resistivity and the dielectric constant of the materials, have been measured. The correlations between these properties have been studied. The PDLC switching voltage appears to be strongly correlated with the resistivity. Our data suggest that ionic impurities play a dominant role with respect to the electrooptic response of PDLC films.     

Electrooptic properties of polymer dispersed liquid crystals

An investigation of the electrooptic properties of polymer dispersed liquid crystals (PDLC) is presented. These materials are light modulating systems. They show a reversible optical response from an opaque state to a highly transmitting state under the action of an appropriate electric field which aligns the liquid crystal director. The switching voltage required to establish such an electric field has been monitored as a function of (i) the starting materials used for the preparation of the PDLCs, (ii) the ageing (curing time) of the PDLC cells. Other physical properties, such as the electrical resistivity and the dielectric constant of the materials, have been measured. The correlations between these properties have been studied. The PDLC switching voltage appears to be strongly correlated with the resistivity. Our data suggest that ionic impurities play a dominant role with respect to the electrooptic response of PDLC films.     

Mechanically activated cholesteric polymer dispersed liquid crystals

In this paper we deal with a cellulose derivative cholesteric dispersed liquid crystal (CCDLC) with mechanically tuneable optical properties. The composite is formed with a matrix of acetoxypropylcellulose with embedded micrometric and submicrometric droplets of a cholesteric mixture. Polarizing optical microscopy and atomic force microscopy (AFM) measurements performed on the system are reported and it is shown that the wavelength of the reflected light can be changed by a temperature variation and it is also changeable through a mechanical deformation. The pitch of the deformed droplets can be measured from AFM photographs and compared with the wavelength reflected by the CCDLC composite material.     

Photochromic reverse mode polymer dispersed liquid crystals

Control of light intensity and colour are two of the major features required in the realization of smart windows. We designed a bi-functional polymer dispersed liquid crystal (PDLC) film in order to satisfy such requirements, i.e. it is able both to modulate the optical transmission, if an external electric field is applied, and to change colour if exposed to sunlight. A monomer/liquid crystal mixture was doped with a small amount of photochromic material and homeotropically aligned by means of rough surfaces. A transparent and pale pink coloured film was achieved after photopolymerization. Such a film changes colour upon exposure for some seconds to sunlight or ultraviolet radiation in a persistent but reversible manner. In addition, the film appears transparent without the application of an electric field (OFF state) and becomes opaque on application of a driving voltage of about 75 V (ON state), and thus the film operates in reverse mode with respect to conventional PDLCs.     

Photochromic reverse mode polymer dispersed liquid crystals

Control of light intensity and colour are two of the major features required in the realization of smart windows. We designed a bi‐functional polymer dispersed liquid crystal (PDLC) film in order to satisfy such requirements, i.e. it is able both to modulate the optical transmission, if an external electric field is applied, and to change colour if exposed to sunlight. A monomer/liquid crystal mixture was doped with a small amount of photochromic material and homeotropically aligned by means of rough surfaces. A transparent and pale pink coloured film was achieved after photopolymerization. Such a film changes colour upon exposure for some seconds to sunlight or ultraviolet radiation in a persistent but reversible manner. In addition, the film appears transparent without the application of an electric field (OFF state) and becomes opaque on application of a driving voltage of about 75?V (ON state), and thus the film operates in reverse mode with respect to conventional PDLCs.     

Effects of prepolymer structure in holographic polymer dispersed liquid crystal

Three different types of polyols—poly(ethylene glycol) (PEG), poly(propylene glycol) (PPG), and poly(tetramethylene glycol)—with different molecular weights (M_n) were incorporated into the prepolymer structure, and the effects were examined with regard to the morphology and performance of holographic polymer dispersed liquid crystal. Among them, PEG showing the lowest glass transition temperature (T_g) exhibited the lowest threshold voltage (V_th), lowest operating voltage (V_op), and lowest diffraction efficiency with slow grating formation owing to its high viscosity of prepolymer mixture. The T_g decreased with increasing M_n because of the decreased crosslink density and hydrogen bonding, which reduced the anchoring strength, V_th and V_op. PPG400 gave V_th and V_op of approximately 7 and 24 V, respectively, with a contrast ratio of approximately 12. © 2012 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2012     

Kinetics of formation of polymer dispersed liquid crystals in a liquid-crystalline polymer matrix

With the use of optical polarization microscopy, the kinetics of phase separation during cooling of molten mixtures of a nematic low-molecular-mass liquid crystal and a liquid crystalline polymer is studied to produce polymer dispersed liquid crystals. The statistical drop-size distribution of a low-molecular-mass liquid crystal is described in the terms of equilibrium thermodynamics of irreversible processes. For a nematic polymer component of a mixture, the analysis of time dependences of the average diameter of drops of a low-molecular-mass liquid crystal makes it possible to reveal two stages in the kinetics of their growth and to describe this process according to the universal law of cluster growth. For a smectic polymer component, the Avrami equation is used to quantitatively describe the kinetics of growth of low-molecular-mass liquid-crystal drops.     

Diffraction grating in a holographic polymer dispersed liquid crystal based on polyurethane acrylate

Diffraction modes of holographic grating were fabricated with polyurethane acrylates of various monomers, and with various film compositions, irradiation intensities, cell gaps and reading angles. An optimum monomer composition, LC content and irradiation intensity were obtained in terms of diffraction efficiency. Of the two types of multifunctional acrylate examined, dipentaerythritol penta-/hexa-acylate (DPHPA) gave better diffraction efficiency than the trimethylolpropane triacrylate (TMPTA) throughout the monomer compositions tested. This was interpreted in terms of high elasticity of the high functionality monomer. The existence of an optimum irradiation intensity at fixed resin composition was interpreted in terms of optimum rate of cure. A monotonic increases of diffraction efficiency with cell gap and interbeam angle were also noted, implying that the grating was formed uniformly as visualized by scanning electron microscopy.     

Shear aligned polymer dispersed ferroelectric liquid crystal devices

Fast switching liquid crystal devices can be produced by forming a dispersion of ferroelectric liquid crystal droplets in a polymer film. Such PDFLCs have been fabricated using a polymerization-induced phase separation technique involving ultraviolet photopolymerization, during which the film was sheared to obtain a uniform orientation of the liquid crystal medium. These birefringence devices show fast response times (sub-millisecond), optimum tilt angle (22.5°), and good contrast (∼ 30:1) at room temperature, using ferroelectric switching. We studied the tilt angles, response times and contrast ratio as a function of voltage and temperature to determine the effects of the preparation parameters on the electro-optic behaviour of these devices. Using a ferroelectric liquid crystal with long helical pitch, such devices appear to be bistable.     

Numerical analysis for a nematic droplet in polymer dispersed liquid crystals

Starting from the Landau-de Gennes free energy expression, the author has numerically analysed the director pattern in a nematic droplet of polymer dispersed liquid crystals. The nematic director has been understood as the eigenvector, which corresponds to the largest eigenvalue of the tensor order parameter. To investigate the droplet structure influence, all equations have been treated on the curvilinear coordinate system which is generated along the droplet boundary. In the case of spherical and spheroidal droplets with normal strong anchoring, the director exhibits an axial configuration and a disclination ring. The ring radius and the capactiance of the system change without hysteresis with the applied voltage.     

Effects of off-state alignment in polymer dispersed liquid crystals

Partial off-state alignment of the liquid crystal in polymer dispersed liquid crystal (PDLC) droplets was obtained by the application of electric or magnetic fields during their formation. Photopolymerization was used to induce phase separation of the liquid droplets from monomer/liquid crystal solutions. Substantial director directionality was retained in these PDLC films after removal of the fields used during their formation. This alignment affected both the off-state and the on-state electro-optic properties of the films. Transverse electrical fields (5 to 60 V across a 15 μm thickness) applied during PDLC formation from a solution of E7 (BDH Ltd) in a monomer resulted in PDLC films with progressively lower off-state scattering and lower threshold voltage. Strong longitudinal magnetic fields (9 to 14 T) applied during PDLC formation with these materials resulted in strong polarization effects in the light scattering off-state. In the infrared region, where there is less light scattering than in the visible region, the longitudinally aligned films shows tunable birefringent electro-optic effects while retaining the fast time response characteristics of PDLC films with small droplet sizes.     

Effects of off-state alignment in polymer dispersed liquid crystals

Partial off-state alignment of the liquid crystal in polymer dispersed liquid crystal (PDLC) droplets was obtained by the application of electric or magnetic fields during their formation. Photopolymerization was used to induce phase separation of the liquid droplets from monomer/liquid crystal solutions. Substantial director directionality was retained in these PDLC films after removal of the fields used during their formation. This alignment affected both the off-state and the on-state electro-optic properties of the films. Transverse electrical fields (5 to 60 V across a 15 μm thickness) applied during PDLC formation from a solution of E7 (BDH Ltd) in a monomer resulted in PDLC films with progressively lower off-state scattering and lower threshold voltage. Strong longitudinal magnetic fields (9 to 14 T) applied during PDLC formation with these materials resulted in strong polarization effects in the light scattering off-state. In the infrared region, where there is less light scattering than in the visible region, the longitudinally aligned films shows tunable birefringent electro-optic effects while retaining the fast time response characteristics of PDLC films with small droplet sizes.