Introduction to Polymer-Dispersed Liquid Crystals Polymer-dispersed liquid crystals (PDLCs) are a relatively new class of materials that hold promise for many applications ranging from switchable windows to projection displays. These materials, which are simply a combined application of polymers and liquid crystals, are the focus of extensive research in the display industry. PDLCs consist of liquid crystal droplets that are dispersed in a solid polymer matrix. The resulting material is a sort of "swiss cheese" polymer with liquid crystal droplets filling in the holes. These tiny droplets (a few microns across for practical applications) are responsible for the unique behavior of the material. By changing the orientation of the liquid crystal molecules with an electric field, it is possible to vary the intensity of transmitted light. The following sections explain the principle of operation of PDLC devices as well as the techniques used to manufacture them. Preparation of Polymer-Dispersed Liquid Crystals Polymer-dispersed liquid crystals have been prepared in several different ways including: encapsulation (emulsification) and phase separation; the latter process has become the primary method of manufacture. Each method produces PDLCs with different properties and characteristics. Among the factors influencing the properties of the PDLC material are the size and morphology (shape) of the droplets, the types of polymer and liquid crystal used, and cooling and heating rates in production. The relationship between the method of production and these factors is explained below. The work discussed here has nematic liquid crystals but studies of droplet dispersions containing chiral nematics have also been reported.
Encapsulation Early attempts to produce PDLCs were made with a technique known as microencapsulation. In this method, a liquid crystal is mixed with a polymer dissolved in water. When the water is evaporated, the liquid crystal is surrounded by a layer of polymers. Thousands of these tiny "capsules" are produced and distributed through the bulk polymer. Droplets produced with this method tend to be non-uniform in size and can even be interconnected with each other. Materials manufactured by encapsulation are referred to as NCAP or nematic curvilinear aligned phase. Phase Separation In order to obtain PDLCs by phase separation, a homogeneous mixture of polymer (or prepolymer) and liquid crystal is first produced. The liquid crystal droplets are then formed by the separation of the two phases. The separation can take place in one of the following three ways: Polymerization-Induced Phase Separation Polymerization-induced phase separation, or PIPS, occurs when a liquid crystal is mixed with a solution that has not yet undergone polymerization (a prepolymer). Once a homogeneous solution is formed, the polymerization reaction is initiated. As the reaction progresses, the liquid crystal molecules come out of solution and begin to form droplets. The droplets grow until the polymer binder becomes solid enough that the molecules are trapped and can no longer move easily. The two main factors that influence the size of liquid droplets in PIPS are the cure temperature and the type and proportions of materials used. The cure temperature affects the speed of the polymerization as well as the diffusion rate and solubility of the liquid crystal in the polymer. These factors can greatly influence the size of the liquid crystal droplets which translates into different macroscopic optical properties. Thermally-Induced Phase Separation Thermally-induced phase separation, or TIPS, can be used when the polymer binder has a melting temperature below its decomposition temperature. In this method, a homogeneous mixture of liquid crystal and a melted polymer is formed. The solution is cooled at a specific rate to induce phase separation. Liquid crystal droplets begin to form as the polymer hardens. The droplets
continue to grow until the glass transition temperature of the polymer is crossed. Droplet size is affected the most by the cooling rate of the polymer melt/liquid crystal solution. Fast cooling rates tend to produce small droplets because there is not sufficient time for large particles to form. Therefore, droplet size and cooling rate are related inversely. The following pictures are of the TIPS process taken at three different times. Note that the droplets grow in size as time passes. Solvent-Induced Phase Separation The third common type of phase separation is called solvent-induced phased separation, or SIPS. This process requires both the liquid crystal and polymer to be dissolved in a solvent. The solvent is then removed (typically by evaporation) at a controlled rate to begin the phase separation. Droplets start growing as the polymer and liquid crystal come out of solution and stop when all of the solvent has been removed. The main factor affecting droplet size in SIPS is the rate of solvent removal. Like TIPS, droplet size increases as the rate of solvent removal decreases. Imaging of PDLCs Infrared (IR) spectroscopy can be used to chemically characterize polymeric systems. It provides information on the relative concentration of various species in a sample and their spatial distribution. It allows the difference in the chemical state and interactions at the interface from either phase bulk to be studied. These features are illustrated in the following figure.
Use of spectroscopy in determining the distribution of relative concentration is illustrated by the red and blue curves which correspond, respectively, to the matrix and the liquid crystal droplets. Here the absorption at the wavenumber (frequency divided by the velocity of the radiation) of the characteristic OH peak is chosen to represent polymer concentration with blue representing the least concentration (in droplets) and red the greatest concentration (matrix). The green curve shows its use in exploring interfaces through relative absorbance. At the interface, intensity of transmitted IR radiation is lowered because of scattering and refraction. This leads to an apparent absorbance that can be used as a measure of concentration. The loss in intensity depends on the refractive index ratio and refractive index varies dependent on the frequency of radiation. Because the IR images depend on refractive indices, they can also be used to study the effect of an applied electric field. First, the IR images below were taken at non-absorbing regions. This means that neither component, liquid crystal nor the matrix, normally absorb at this wavenumber (2600 cm -1 ). As shown in figure (a) the interface shows absorbance at this wavenumber, which is due to the refractive index mismatch between the liquid crystal and the matrix that causes the incident radiation to be refracted. The high absorbing regions within some of the liquid crystal droplets is due to coalescence (liquid crystal molecules are still coming together to form larger droplets). Refractive indices are matched due to the applied field as seen in figure (b), since there is no longer absorbance. The interface still causes a small amount of scattering, however. This is partly because of imperfect matching between the liquid crystal and matrix indices, which could be due to a predominance of one of the components of the liquid crystal at the boundary. Higher potential difference decreases the amount of scattering but does not totally eliminate it as shown in figure(c). This residual scattering could be due to liquid crystal molecules near the boundary not switching with the bulk. Images and spectra courtesy of R. Bhargava, Macromolecular Science Department, CWRU.
absorbanc e a) no electric field applied b) 3.5 V rms field applied c) 7 V rms field applied Droplet Configuration The configuration of the liquid crystal droplets in a polymer matrix is the focus of much current research. Many different configurations have been observed and they depend on factors such as droplet size and shape, surface anchoring, and applied fields. This section will describe some of the most common configurations. The radial configuration occurs when the liquid crystal molecules are anchored with their long axes perpendicular to the droplet walls. This arrangement is shown in the diagram below. Note the point defect in the center of the droplet. The axial configuration of the liquid crystal droplets also occurs when the molecules are oriented perpendicular to the droplet wall, but only when there is weak surface anchoring. This configuration creates a line defect that runs around the equator of the spherical droplet, as seen in the diagram below. When an electric field is applied to a radial droplet, the molecules adopt the axial configuration. The radial configuration is returned when the field is removed. The bipolar configuration is obtained by tangential anchoring of the liquid crystal molecules. This creates two point defects at the poles of the droplet and is shown in the diagram below. Radial Bipolar Axial
In a typical PDLC sample, there are many droplets with different configurations and orientations. When an electric field is applied, however, the molecules within the droplets align along the field and have corresponding optical properties. In the following diagram, the director orientation is represented by the black lines on the droplet. No Electric Field Electric Field Applied Applications of PDLCs Polymer-dispersed liquid crystals hold potential for a variety of electro-optic applications ranging from displays to light shutters. Below, we illustrate their applications as electro-optic light shutters in the construction of privacy windows. PDLC windows are based on the ability of the nematic director of the liquid crystal droplets to align under an electric field as discussed in the previous section. In a typical application, a thin PDLC film (about 25 microns thick) is deposited between clear plastic covers. The plastic substrates are coated with a very thin layer of a conducting material known as indium tin oxide (ITO). Transmission of light through a PDLC window depends primarily on scattering which in turn depends on the difference in refractive index between droplets and their environment. In the case of high droplet density, the environment consists mainly of other droplets, which makes the relative orientation of their directors an important factor. The droplets are anisotropic with the index of refraction parallel to the director different from that perpendicular to it. In the field OFF, the random array of droplet orientation provides significant differences in indices and hence strong scattering. In this state, the cell appears opaque. When a voltage is applied, however, the director of the individual droplets align with the field. There is now little difference in refractive index for neighboring droplets, and the cell appears transparent.