Light modulation characteristics of a single-polarizer electro-optical cell based on polymer dispersed ferroelectric liquid crystals

Features of the light passing through a single-polarizer electro-optical cell based on a uniaxially oriented film of a polymer dispersed ferroelectric liquid crystal have been studied. The optical response as a function of the applied electric field is shown to depend on the cell geometry. A detailed analysis is presented of the dependence of the maximum light transmission, modulation depth and contrast ratio on the angle between the light polarization and the film orientation direction, on its optical anisotropy and on the molecular tilt angle. An approximate formula is proposed to estimate the highest attainable contrast. Parameters under study have been measured for PDFLC films varying in optical anisotropy value. The experimental results prove to be in good agreement with theoretical estimations.     

Morphology and electro-optical properties of reverse mode polymer dispersed liquid crystals

We have investigated the morphology and electro-optical properties of reverse mode polymer dispersed liquid crystals as a function of liquid crystal loading. Reverse mode shutters have been obtained by a polymerization-induced phase separation of mixtures, consisting of a liquid crystalline monomer and a non-reactive nematic liquid crystal, placed between rough conductive surfaces. Such surfaces are able to keep the photopolymerizable mixtures homeotropically aligned without the use of any aligning polymer substrate. OFF state transmittances are always larger than 80% and the switching fields decrease if the non-reactive liquid crystal percentage is increased. Both rise and decay times are always lower than 10 ms. The electro-optical properties have been related to the sample morphology and a simple mode is proposed.     

Morphology and electro-optical properties of reverse mode polymer dispersed liquid crystals

We have investigated the morphology and electro-optical properties of reverse mode polymer dispersed liquid crystals as a function of liquid crystal loading. Reverse mode shutters have been obtained by a polymerization-induced phase separation of mixtures, consisting of a liquid crystalline monomer and a non-reactive nematic liquid crystal, placed between rough conductive surfaces. Such surfaces are able to keep the photopolymerizable mixtures homeotropically aligned without the use of any aligning polymer substrate. OFF state transmittances are always larger than 80% and the switching fields decrease if the non-reactive liquid crystal percentage is increased. Both rise and decay times are always lower than 10 ms. The electro-optical properties have been related to the sample morphology and a simple mode is proposed.     

Temperature dependence of electro-optical characteristics of polymer dispersed liquid crystal films

Polymer dispersed liquid crystal (PDLC) films consist of microdroplets of a liquid crystal dispersed in a polymer matrix. Their applications are based on the electrically controllable light scattering properties of the liquid crystal droplets. The effects of temperature on the electro-optical properties of PDLC films have been rarely investigated. In this work, we studied the light transmission on varying the temperature and frequency. It was observed that the transmission at a fixed voltage decreased with increasing temperature above 43°C, independent of frequency. We examined possible origins of this unusual dependence of the transmission on the temperature. It was concluded that conductivity effects due to free ions newly created at high temperatures could be responsible for the unusual behaviour observed.     

Temperature dependence of electro-optical characteristics of polymer dispersed liquid crystal films

Polymer dispersed liquid crystal (PDLC) films consist of microdroplets of a liquid crystal dispersed in a polymer matrix. Their applications are based on the electrically controllable light scattering properties of the liquid crystal droplets. The effects of temperature on the electro-optical properties of PDLC films have been rarely investigated. In this work, we studied the light transmission on varying the temperature and frequency. It was observed that the transmission at a fixed voltage decreased with increasing temperature above 43°C, independent of frequency. We examined possible origins of this unusual dependence of the transmission on the temperature. It was concluded that conductivity effects due to free ions newly created at high temperatures could be responsible for the unusual behaviour observed.     

Enhancing the electro-optical properties of ferroelectric liquid crystals by doping ferroelectric nanoparticles

The effects on the physical and electro-optical properties of ferroelectric liquid crystals (FLCs) after the doping of a dilute suspension of ferroelectric nanoparticles (BaTiO₃) have been studied. Due to the permanent electric dipole moments of the ferroelectric nanoparticles, the spontaneous polarisation of FLCs with low doping concentration was about twice that of pure FLCs, in addition to a significant improvement in the dielectric properties, the response time and the V-shaped switching in the chiral smectic C (SmC?) phase. The results obtained point the way to an alternative for improving the applicability of FLCs without resorting to chemical synthesis.     

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.     

The effect of fluorine-substituted acrylate monomers on the electro-optical and morphological properties of polymer dispersed liquid crystals

The effects of fluorinated acrylate monomers on the electro-optical and morphological properties of polymer dispersed liquid crystal (PDLC) films are reported. The partial fluorination of host polymer matrices resulted in improved optical properties and better defined morphologies. An enhancement in contrast ratio was observed for fluorinated systems containing trifluoroethyl acrylate (TFEA) and hexafluoroisopropyl acrylate (HFIPA). Conversely, the incorporation of methyl acrylate (MA), a chemically similar non-fluorinated acrylate, resulted in no appreciable change in contrast ratio and an increase in relaxation time. Scanning electron microscopy morphological studies were conducted to understand further the influence of fluorinated monomers in PDLC systems.     

The effect of fluorine-substituted acrylate monomers on the electro-optical and morphological properties of polymer dispersed liquid crystals

The effects of fluorinated acrylate monomers on the electro-optical and morphological properties of polymer dispersed liquid crystal (PDLC) films are reported. The partial fluorination of host polymer matrices resulted in improved optical properties and better defined morphologies. An enhancement in contrast ratio was observed for fluorinated systems containing trifluoroethyl acrylate (TFEA) and hexafluoroisopropyl acrylate (HFIPA). Conversely, the incorporation of methyl acrylate (MA), a chemically similar non-fluorinated acrylate, resulted in no appreciable change in contrast ratio and an increase in relaxation time. Scanning electron microscopy morphological studies were conducted to understand further the influence of fluorinated monomers in PDLC systems.     

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.     

Thermo-mechanical and electro-optical properties of polymer dispersed liquid crystal films

We have observed a strong dependence of the electro-optical properties of polymer dispersed liquid crystal (PDLC) films on temperature. One plausible explanation for this dependence is the thermal expansion of the films. We investigated the thermal expansion of various PDLC films using a thermal mechanical analyser (TMA). As the temperature increased, the films expanded and their droplet size decreased, and their contrast, response time, and hysteresis changed simultaneously. We observed obvious changes of thermal expansion coefficient of the films. We studied the qualitative relationship between thermo-mechanical properties and electrooptical properties. The thermal expansion of PDLC films could be adjusted by changing the degree of cross-linking of the polymer.     

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.     

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.     

Comparative analysis of basic physical properties of a ferroelectric liquid crystal and a polymer dispersed ferroelectric liquid crystal

A polymer film of polyvinylbutyral with dispersed droplets of a ferroelectric liquid crystalline mixture (FLC309c) has been prepared and characterized. The collective processes have been studied by dielectric relaxation spectroscopy in the frequency range 10 Hz to 13 MHz. In comparison with the FLC309c mixture, the polymer dispersed ferroelectric liquid crystal (PDFLC309c) based on FLC309c exhibits a Goldstone-like mode relaxation with a much higher relaxation frequency, but a smaller dielectric strength than the Goldstone mode observed for the FLC309c mixture. The spontaneous polarization of PDFLC309c decreases by nearly one order of magnitude in comparison with FLC309c, while the tilt angle decreases by 20%. Considering these results, we believe that a non-switching region exists near the polymer boundaries and that significant deformations of the helical structure occur due to stronger anchoring.     

The fabrication of novel optical diffusers based on UV-cured polymer dispersed liquid crystals

Polymer dispersed liquid crystals (PDLCs) with different sizes of the LC droplets are prepared based on the ultraviolet (UV) light curable acrylate monomers/LCs composites to fabricate the optical diffuser films. To acquire light diffusers with high optical performance, the effects of the monomer structure and the UV light intensity on the micro-structure of the PDLC films are studied. Results show that the PDLC films could exhibit a strong light scattering at the premise of maintaining high transmittance in the visible region. As the LC droplets are spherically dispersed in the polymer networks, when the size of LC droplets is about 3.0 μm, the haze can reach 88.5% and the transmittance is nearly 90.0%, which can be used as a bottom diffuser film. While when the size of LC droplets is about 10.0 μm, the haze and transmittance are 39.2% and 90.2%, respectively; hence, it can be a good choice for a top diffuser film. With the advantages of simple preparation, roll-to-roll industrial production and tunable optical properties, it is supported that the films based on UV-cured PDLC films can be applied as outstanding optical diffuser films in the liquid crystal display industry.