Polymer-Dispersed Liquid Crystals

Ilija Zeljkovic - Final Wiki Entry for APPHY 225, Fall 2008

Final Project: Polymer-Dispersed Liquid Crystals and Their Applications

Liquid Crystals

History

The study of liquid crystals (LCs) began in 1888 when an Austrian botanist Friedrich Reinitzer experimented on a material known as cholesteryl benzoate. He observed that the aforementioned substance had two distinct melting points. In his experiments, Reinitzer increased the temperature of a solid sample and watched the crystal change into a hazy liquid at 145.5°C. As he increased the temperature further, the material changed again into a clear, transparent liquid at about 178.5°C. Because of this early work, Reinitzer is often credited with discovering a new phase of matter - the liquid crystal phase . Friedrich Richard Reinitzer (February 25, 1857- February 16, 1927)- "founding father" of liquid crystals

Basics

Liquid crystal is a substance that exhibits properties that are between those of a conventional liquid and a solid. The main characteristic of the LC state is the tendency of molecules to point along a common axis, called the director. As we know, molecules in the liquid phase do not exhibit any intrinsic order. Also, in the solid state, molecules are highly ordered and have little to no translational freedom. Therefore, we see that the characteristic orientational order of the liquid crystal state is somewhere between solid and liquid phases, as seen below. Ordering of molecules in different phases Nematic liquid crystal

Order parameter

In order to describe how ordered a certain liquid crystal is, we use a quantity called order parameter (S). Based on the second Lagandre polynomial, S is given by the following equation:

$S = \left \langle \frac{3 \cos^2 \theta}{2} - \frac{1}{2} \right \rangle$

where $\theta$ is the angle between the common direction and the orientation of that particular molecule. The closer S is to unity, the more ordered (and closer to solid structure) the liquid crystal is. When S drops to zero, that means we have transitioned to a liquid state. Typical values of S are between 0.3 to 0.8. This value decreases as you increase the temperature, which makes sense since thermodynamic entropy increases. Order parameter dependence on temperature is shown in the image below. As we can see, for small temperatures we approach (but never quite reach) the value of one. On the other hand, as we increase the temperature, we'll have a "breakdown point" at which we reach complete anisotropy. Increasing temperature above this critical point will not change anything. Ordering parameter temperature dependence.

For more information on liquid crystals, see references  and .

Polymer-Dispersed Liquid Crystals

Polymer-Dispersed Liquid Crystal (PDLC) consists of liquid crystals dissolved or dispersed into a liquid polymer, followed by solidification of the polymer. The resulting material is a sort of "swiss cheese" polymer with liquid crystal droplets filling in the holes of a polymer matrix. Photograph of PDLC sample taken using Scanning Electron Microscopy (SEM) is shown below .

Preparation

The two most common methods in PDLC production are encapsulation and phase separation. Each of these methods produces a different PDLC depending on the size of the LC droplets achieved, type of polymer used, and external variables such as temperature .

Encapsulation

Encapsulation is the oldest technique used to construct PDLCs. Using this procedure, liquid crystal and polymer are mixed with water. As the mixture is heated and water evaporates, we achieve PDLC configuration of LC droplets surrounded by polymer. However, this method produces LC droplets of different sizes, and sometimes different droplets even stick together.

Phase Separation

Phase separation is the more commonly used method in which we first form a homogeneous mixture of LC and polymer, and then separate the phases in one of the following ways:

• Polymerization-Induced Phase Separation (PIPS) happens when prepolymer (polymer not yet undergone polymerization) is used to make the mixture with liquid crystal. As we induce the polymerization of the prepolymer in the mixture by using a curing agent, we will see LC droplets separating away from the mixture. Depending on the temperature of the process, we will get different configurations of PDLC.
• Thermally-Induced Phase Separation (TIPS) is used when the polymer has a melting temperature below its decomposition temperature. Taking advantage of this fact, a homogeneous mixture of liquid crystal and a melted polymer is formed. This mixture is cooled at a specific rate which induces phase separation - LC droplets form as the polymer hardens. We can see an example of LC droplet formation below . Depending on the rate of the melting/hardening of polymer, we get different size of LC droplets in the PDLC.
• Solvent-Induced Phase Separation (SIPS) requires both the liquid crystal and polymer to be dissolved in a solvent. The solvent is then evaporated 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. Similarly to other phase separation techniques, the rate of the evaporation determines the size of droplets.

Typical recipes for PDLC preparation are given in the table below . Proportions of constituents for typical PDLC preparation.

LC droplets configurations

The size and the configuration of the liquid crystal droplets are affected by the solidification conditions (changing temperature, pressure, applied field and other external variables). Furthermore, this affects the final applicability of PDLC. There is a lot of research currently going on, exploring this topic. We will briefly explain several most probable LC configurations and how we theoretically explain this.

Director configuration of large droplets has been studied many years ago. The most important work on this topic was published in 1969 . The authors have shown that the surface energy of droplets in nemetic emulsions is only dependent on the radius of the droplet: $f_s=C+D/R$ where C and D are constants and R is the radius of the droplet. In the approximation of only one elastic constant, neglecting surface-induced charge and anisotropy of the order parameter, free elastic energy density can be written as seen in :

$F_K=\frac{K}{2}((\nabla n)^2+(\nabla \times n)^2)$ where n is the director vector.

From this expression we can obtain a differential equation (see details in ) in cylindrical coordinates:

$\nabla^2 \theta_n - \frac{1}{r^2}cos \theta_n sin \theta_n=0$

Using energy minimization method, we discover that there are several different stable configurations.

• Radial configuration is produced when the LC molecules are anchored with their long axes perpendicular to the droplet walls. This and all the following arrangements are shown in the images below, as presented in . There is only one point defect in the center because we can orient the molecule in any direction and still be perpendicular to the surface. In the equations above, this is achieved for $n_r=-sin\theta$, $n_\phi=0$ and $n_z=cos\theta$.
• Axial configuration is very similar to radial configuration and also occurs when the molecules are oriented perpendicular to the droplet wall. However, it only occurs when there is weak surface anchoring, creating a line defect that runs around the equator of the spherical droplet. 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. In the equations above, this is achieved for $n_x=cos\phi$, $n_y=sin\phi$ and $n_z=0$.
• Bipolar configuration is created 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. In the equations above, this is achieved for $n_r=cos\theta$, $n_\phi=0$ and $n_z=sin\theta$.

Order parameter

Order parameter in PDLC ($S_E$), similarly to the way we defined it for LC, is a measure of how exactly the droplets of LC point along the same direction. For example, when $S_E$ is equal to unity, it means the droplets are all pointing along the same direction, whereas if it is zero the droplets are pointing in random directions. The exact expression for order parameter can be derived (see p. 185-190 of ) after pages of tedious algebra. The important part to take out of it is that it depends on several factors including droplet elipticity, elastic constant and the radius of droplets. In the graph below we can see how $S_E$ depends on the radius of particles for different values of applied electric voltage. $S_E$ dependence on applied voltage. Different curves represent different values of droplet radii, keeping all other factors constant.

What is interesting to see is that there is complete disorder for zero voltage, and order parameter eventually reaches one for all presented radii. However, $S_E$ reaches unity faster for grater radii of droplets. In other words, bigger droplets of LC need less applied voltage to be completely ordered.

Imaging

In order to characterize our PDLC we can use infrared spectroscopy, a method used for various polymer systems . This technique unveils information about the sample, such as concentration of the constituents and its spatial distribution. You can see a sample image obtained using infrared spectroscopy below. Red curve in the spectroscopy image represents the distribution of polymer, whereas blue one corresponds to liquid crystal droplets. The green curve shows its use in exploring interfaces through relative absorbances. 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. Infrared spectroscopy method sample image

Because the infrared spectroscopy images depend on refractive indices, they can also be used to study the effect of an applied electric field. 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).

• Figure (a) shows absorbance at the interface regions which is due to the refractive index mismatch between the liquid crystal and the polymer matrix that causes the incident radiation to be refracted. On the other hand, high absorbing regions within some of the liquid crystal droplets is due to coalescencing of two or more droplets .
• Figure (b) shows the same sample scanned over the same region with applied voltage of 3.5V creating an electric field. The image shows no absorbence. The interface still causes a small amount of scattering, however (dark blue).
• Higher applied voltage in Figure (c) decreases the amount of scattering but does not totally eliminate it. (a) No field applied. (b) 3.5V applied

Applications

"Switchable" glass

How does it work?

"Switchable" glass or "smart" glass is a type of glass that changes properties such as transparency and heat permittivity as voltage is applied . Since it is made of PDLC, it consists of tiny liquid crystal droplets dispersed in a polymer. These tiny droplets (a few microns in diameter) are responsible for the unique behavior of the material. When there is no electric voltage applied, the electric field is zero, and the LC droplets of PDLC are oriented in all possible directions (see below). As electric filed is applied, we can see molecules orienting in-line with the field . Random orientation of molecules when electric field is zero When electric field is applied, all the molecules point in the same direction

To form a commercial "smart" window, the liquid mix of polymer and liquid crystals is placed between two layers of glass or plastic that include a thin layer of a transparent and conductive material. This structure is in effect a capacitor and electrodes from a power supply are attached to the outside of the conductive material surrounding the LC mix. As we mentioned before, with no applied voltage, the liquid crystals are randomly arranged in the droplets, and the light scatters as it passes through. This results in the translucent, "milky white" appearance (OFF state). But when we apply the voltage, the LC droplets align, allowing the light to pass through. This results in a transparent state of the glass (ON state). The amount of light that passes through can be controlled by the value of applied voltage. If lower voltage is applied, there will be more LC droplets out of alignment, resulting in more light scattering and lower transparency. Conversely, as we increase the voltage, more droplets will align, thus resulting in less scattering and more transparency. An example can be seen below. The transparent state of "smart" window (left) and the opaque state (right)

Problems

There are still several potential problems in the way of wide-spread use of "smart" glass:

• high level of haze in transparent state
• high driving voltage around 100 V
• short time of operation

State-of-the-art "smart" glass

State-of-the-art technology today is on the way to solve at least some of the problems described above. One of the front-runners in this field is Scienstry, the company that developed the latest generation of "smart" glass - NPD-LCD Film . They utilized a novel non-linear polymer system that has been successfully used to solve all long lasting problems in switchable window industry, such as high haze in clear state, high driving voltage and short operational life time. The NPD-LCD technology has:

• theoretically and practically blocked all chemical factors that can cause the film to deteriorate.
• reduced haze in transparent state from 10% to 4%.
• reduced driving voltage from 90V to 20V.
• extended operational life time to 50 times longer (> 100 millions times of on-off switching).

Miscellaneous

Nowadays, we can control not only the transparency, but also the amount of heat passing through. This feature is achieved by using various special inter-layers beside the LC mix and the outside conductor material. Most of the devices offered today operate in on or off states only, even though the technology to provide for variable levels of transparency is easily applied. This technology has been used in interior and exterior settings for privacy control such as conference rooms, intensive-care areas, or bathroom shower doors.

Projection displays

Introduction

PDLC projection displays are particularly well suited for use in light projection devices where we desire to display bright colored and high intensity images . In this case, there are several advantages compared to diplays made of other materials.

• brightness of the image is not reduced by light polarizers as is the case in the conventional LC shutters where up to 70% of light may be lost
• light is highly collimated, with contrast ratios of about 100:1
• it is possible to have full-color projection without the compensating filters and shutters can be adjusted to give real time projection

Setup

For completeness, the setup of typical projection display utilizing PDLC technology is shown below. Schematic of method to achieve full color projection using PDLC shutters

Large-scale flexible displays

Large-scale flexible displays are typically used in big outdoor signs and billboards. An example of billboard we see all the time.

Schematic of the typical display is shown below . Schematic of layers of the typical large-scale flexible display.

The advantages of these displays are the following:

• unlimited size and flexible substrates
• they do not require special "alignment" layers to close the cell unlike many other LC displays
• many coating and laminating methods available (TIPS, SIPS and PIPS discussed in the PDLC phase separation section are all used)