Liquid Crystal Microdroplets Research Articles

Electrooptical Properties of Polymer Films Containing Nematic Liquid Crystal Microdroplets

Polymer films containing nematic liquid crystal microdroplets are useful for light modulation devices. Light scattering in these films is controlled by applied electric fields. These films were produced by cooling solutions of polymer in the isotropic phase of liquid crystal to temperatures below the clearing point (the nematic-phase-to-isotropic-phase transition temperature). In this process, sizes of microdroplets were regulated by controlling the rate of cooling. The relationship between the microdroplet size and electrooptical properties was investigated experimentally and theoretically.

Study of Formation, Phase Behavior, and Microdroplet Size of a Polyurethane-based Polymer-dispersed Liquid Crystal

Using calorimetry and scanning electron microscopy, we have investigated the formation process and phase behavior of a polyurethane-based polymer-dispersed liquid crystal system. We have measured the kinetics and energetics of the cure process during which liquid crystal microdroplets form by phase separation from the matrix as it cross-links. The greatest degree of cure occurs for sample cured at 375 K. For a given cure temperature, the heat of cure decreases more or less linerly with increasing liquid crystal concentration due to a dilution effect. The time constant for the cure process decreases rapidly with increasing temperature but is much less sensitive to liquid crystal content. Samples cured below 375 K are apparently not fully phase separated, but subsequent treatment at higher temperatures evidently increases the degree of cure. The highest nematic-isotropic transition temperatures were achieved for liquid crystal concentrations above 40 volume percent. The nematic-isotropic transition enthalpy, △H NI , is a measure of the amount of liquid crystal contained in the microdroplets. A model has been developed which explains the linear increase of △H NI with increasing liquid crystal concentration. Optimum microdroplet formation occurs at 375 K, but only for liquid crystal concentrations below about 53 volume percent. At higher concentrations a reversed phase ( polymer ball ) morphology is seen. For the lower concentration droplet size increases linearly with LC content. Droplet number density decreases with increasing droplet size in rough agreement with a simple model

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...

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 function of composition. We discuss the problems associated with predicting the refractive indices of such mixtures in terms of either the volume fractions or mole fractions of the mixture components. These considerations are important in matching refractive indices of droplets and matrix materials to maximize on-state transmittance. The refractive indices of epoxy matrix materials increase monotonically with time during their chemical cure. The measured time dependence can be described by a simple model in which the concentrations of the reacting resin and hardener each decay exponentially in time with their own characteristic time constants while the concentration of the cured polymer increases. Finally, we relate the measured rates of index change with temperature to the coefficients of volume expansion of PDLC film materials; the results are used to discuss the mechanical stability of PDLC films.

Field controlled light scattering from nematic microdroplets

The light scattering and electro-optic response of new material with display potential are investigated. The materials consist of microdroplets of nematic liquid crystals which are spontaneously formed in a solid polymer at the time of its polymerization. Droplet size, spacing, and distribution are readily controlled in these materials to allow optimization of displays based upon electrically controlled light scattering from the liquid crystal droplets. Preliminary experimental and theoretical studies of the light scattering properties show these materials to offer new features suitable for many display applications.

Thin thermosensitive film coating

AbstractThin film thermosensitive coatings are obtained, representing liquid crystal microdroplets dispersed in a polymeric matrix. Gelatin is chosen as a polymeric material. The obtaining of microdroplets is carried out by condensation of the solution of liquid crystal in organic solvents at their mixing with gelatin aqueous solution. The obtained coating distinguishes with temperature stability, homogeneity of the colouring, stability of the time indicators.

Research Note: Gamma Rays Modification of Encapsulated Liquid Crystals Temperature Range

Abstract Cholesteric Liquid Crystals have the interesting property of exhibiting reversible changes in their colors over a well defined temperature range.1 They are used in many applications as sensitive temperature indicators. In recent years commercial sheets of encapsulated liquid crystals (ELC) have become available. These sheets contain a carefully controlled layer of microdroplets of liquid crystals suspended in a thick resinous coating and protected from contaminants by a thin transparent polyester sheet on one side and by a black opaque coating on the other side. They do not contaminate easily, are stable over many months and they can be reused many times.