Nuno A. Vaz

A Light Control Film Composed of Liquid Crystal Droplets Dispersed in a UV-Curable Polymer

Abstract This paper describes a new class of light control films consisting of submicron liquid crystal droplets dispersed in ultraviolct-cured polymer matrices. These films, which can respond optically to both applied electric fields and temperature changes, are potentially useful for displays and light shutters. The optical performance of these films depends on a variety of structural, electro-optical and thermal properties. This report describes scanning electron microscope studies of film structure, measurements of voltage dependent film transmittance and light scattering, and calorimetric studies which indicate that microdroplet formation in the films occurs as a result of phase separation which takes place during the cure process.

A Light Control Film Composed of Liquid Crystal Droplets Dispersed in an Epoxy Matrix

Abstract This paper describes light control films consisting of submicron liquid crystal droplets dispersed in epoxy matrices. These films, which can respond optically to both applied electric fields and temperature changes, are potentially useful for displays and light shutters. The optical performance of these films depends on a variety of structural, electrooptical and thermal properties. This paper describes scanning electron microscope studies of film structure, measurements of voltage dependent film transmittance, contrast ratio and light scattering, and calorimetric studies of the cure process which governs microdroplet formation in the films.

The relationship between formation kinetics and microdroplet size of epoxy-based polymer-dispersed liquid crystals

Abstract Polymer films containing dispersions of liquid crystal microdroplets have considerable potential for use in displays and other light control devices. These polymer-dispersed liquid crystal (PDLC) films operate by electric field control of light scattering, rather than by polarization control as in the case of twisted nematic systems. The scattering characteristics of the PDLC films are determined by the refractive indices of the polymer and liquid crystal and by the size of the microdroplets. We have found that it is possible to regulate the microdroplet size by controlling the droplet formation rate (i.e. the cure kinetics of the film). Using calorimetry and scanning electron microscopy, we determined the influence of cure kinetics on microdroplet size for epoxy-based PDLCs. We found that droplet size increased with increasing cure time constant. However, the relationship changed as cure temperature was varied, perhaps as a result of competing cure processes. We also determined the phase behavio...

Dual frequency addressing of polymer-dispersed liquid-crystal films

We report the feasibility of using dual frequency addressing (DFA) to switch polymer‐dispersed liquid crystal (PDLC) films between their on‐ and off‐states. In this scheme, the on‐state is activated with an applied voltage of low frequency while the off‐state is activated with a high‐frequency voltage. We find that DFA increases the forward‐scattering efficiency of PDLC films in the off‐state without decreasing their on‐state transmittance. Consequently, DFA can be used to improve the contrast ratio of PDLC films in projection displays and similar devices. We also find that the addressing frequency required to activate the off‐state in a PDLC film has the same exponential temperature dependence observed in conventional liquid‐crystal devices not employing microdispersed liquid crystals; this limits the use of DFA in PDLC films to applications which do not require operation over a wide temperature range.

Refractive indices of polymer‐dispersed liquid‐crystal film materials: Epoxy‐based systems

Polymer‐dispersed liquid crystal (PDLC) films are potentially useful in applications requiring electrically controllable light transmission. In these applications, both a high on‐state transmittance and a strong off‐state attenuation are often needed over a wide operating temperature range. These transmittance characteristics depend strongly on the refractive indices of the materials in the PDLC films. We have measured the temperature dependent refractive indices of typical PDLC film materials and the temperature dependent electro‐optic transmittance of a PDLC film composed of liquid crystal microdroplets dispersed in an epoxy matrix. We show that our refractive index measurements can account for all the features in the measured transmittance characteristics and discuss several methods for controlling refractive indices to optimize electro‐optic transmittance over an extended temperature range. We have also measured the room temperature refractive indices of mixtures of epoxy resins and hardeners as a fun...

Contrast ratios of polymer-dispersed liquid crystal films.

Contrast ratio is an important measure of the performance of an electrooptic display. From measurements of film brightness and transmittance, we determined contrast ratios of thin polymer films containing microdroplets of liquid crystalline material. Contrast ratios based on brightness were typically ~3 for all samples studied, whereas contrast ratios based on transmittance varied from ~20 to 200 depending on the sample. We explain these differences by analyzing the relations between the illumination geometries in the two measurements and the voltage-dependent multiple scattering of light which controls the electrooptic performance of the films. Effects of the spectral content of the light source and the response of the human eye on contrast ratio were also determined. The results presented here demonstrate the importance of standardizing procedures for measuring contrast ratios of these new liquid crystal films.

Polymer-Dispersed Liquid Crystal Films: Materials And Applications

Typical applications of polymer-dispersed of liquid crystal (PDLC) films include large and flexible information &splays and windows with electronically controlled transmission. Whenever these applications are intended for outdoor usage as, for example, when the films are to be applied to automobile sunroofs, selection criteria for the materials and choice of preparation method become important in order to satisfy the environmental temperature and humidity extremes. This paper reviews and compares different technologies for preparing PDLC films, giving special emphasis to their microstructure and ability to exhibit good electro-optic performance over an extended temperature range.

Temperature-Dependent Electro-Optic Performance Of Polymer-Dispersed Liquid Crystal Films

Polymer-dispersed liquid crystal (PDLC) films, consisting of micrometer-sized liquid crystal (LC) droplets dispersed in a polymer matrix, have considerable promise for a variety of electro-optic applications. VVe report the first measurements of the electro-optic performance of PDLC films over an extended temperature range. These properties are strongly sensitive to the liquid crystal microdroplet size. Calorimetric studies have shown that the droplet size is controlled by the rate at which the film is cured while the operating temperature range is determined, not by cure rate, but by the choice of liquid crystal and polymer matrix material. We have measured transmittance vs voltage and response time characteristics of PDLC films for temperatures from -10°C to 60°C. For thin (≈ 17.5 μm) films, transmittance of 70% or greater is achieved over this entire temperature range for driving voltages below 60 Vrms. Off-state transmittance of collimated light) is typically about 17o. The threshold voltage, at which transmittance begins to increase from its off-state value, and the voltage needed to achieve maximum transmittance both decrease with increasing temperature. Response times are rapid compared to those of conventional nematic liquid crystal devices. At a 60 Vrms driving voltage, rise time decreases from about 250 μs to 500 μs as temperature increases from -10°C to 60°C; even shorter rise times are measured at higher driving voltages. Decay times are nearly independent of voltage at temperatures above 0°C; at subzero temperatures, decay time increases with increasing voltage. This behavior can be explained in terms of molecular reorientation within the PDLC film. Decay times at -10°C range from 300 ms to 600 ms for voltages between 60 and 120 Vrms ; at temperatures above 15°C, decay times are below 50 ms for all voltages in this range. These results suggest that PDLC films of 12-25µm thickness are promising systems for future automotive displays.

The Interfacial Free Energy of Nematogen Droplets in an Isotropic Matrix: Determination of its Temperature Dependence from Coalescence Kinetics

Abstract We have determined the surface free energy, [sgrave], at the interface between a nematogen and an isotropic liquid by measurement of the time constant for coalescence of nematogen droplets imbedded in the matrix. The basis for this technique is an analysis developed by Frenkel in 1945. The major conclusions of the present work are four-fold: 1) the magnitude of the interfacial free energy is on the order of 10–2 to 10–1 dyne/cm; 2) the temperature dependence of [sgrave] for nematic droplets shows a greater negative slope than for isotropic droplets; 3) the accuracy of the method is insufficient to detect a discontinuity in [sgrave] at the nematic to isotropic transition temperature; and 4) the interfacial free energy decreases toward zero as the temperature increases toward that at which the droplets dissolve in the matrix. This last behavior was expected by analogy with the Eotvos expression for ordinary liquid/vapor surface free energies. The droplet coalescence technique shows promise as a mea...

Polymer-dispersed liquid crystal films formed by e-beam cure

Polymer-dispersed liquid crystal (PDLC) films are useful in electro-optic applications because they can be switched electrically between opaque and transparent states. We have prepared PDLC films using electron-beam radiation (e-beam cure). The resulting films exhibit promising mechanical, electro-optic, and thermal response. Compared with the ultraviolet cure process, e-beam cure has the advantage of not requiring photoinitiators. In addition, e-beam cure is characterized by a fast cure rate. The e-beam cure method may, therefore, be a good candidate for production of PDLC ifims.