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Final Project: Liquid Crystals and Their Applications

History

Friedrich Richard Reinitzer (February 25, 1857- February 16, 1927)- "founding father" of liquid crystals

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.









Liquid Crystals

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
Ordering parameter temperature dependence.


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:

<math>S = \left \langle \frac{3 \cos^2 \theta}{2} - \frac{1}{2} \right \rangle </math>

where <math>\theta</math> 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 on the upper right. 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.

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.

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.

Phase Separation

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

  • Polymerization-Induced Phase Separation happens when prepolymer (polymer not yet undergone polymerization) is used to make the mixture with liquid crystal. As we induce the polimerization of the prepolymer in the mixture, we will see LC droplets separating away from the mixture. Depending on the curing temperature of the process, we will get different configurations of PDLC.
  • Thermally-Induced Phase Separation 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 awith time below. Depending on the rate of the melting/hardening of polymer, we get different size of LC droplets in the PDLC.
Beginning
Intermediate
End
  • Solvent-Induced Phase Separation 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 od droplets.

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. It is made of polymer dispersed liquid crystal (PDLC). Since it is made of PDLC, it consists of tiny liquid crystal droplets dipersed 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 droplets 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)

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 operating properties of the "smart glass". There is a lot of research currently going on, exploring this topic. We will briefly explain several most important LC 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. 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.
  • 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.
  • 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.
Radial
Axial
Bipolar

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!).

What else can we control besides transparency?

Nowadays, we can control not only the transparency, but also the amount of heat passing through. This feature is achieved by using various special interlayers 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.

Addendum

You can access the applet demonstrating how "switchable" windows work here:

http://www.scienstry.us/demonstrate_principle.htm

References

[1] http://plc.cwru.edu/tutorial/enhanced/files/lc/Intro.htm

[2] http://en.wikipedia.org/wiki/Liquid_crystal

[3] http://www.lci.kent.edu/switch.html

[4] http://en.wikipedia.org/wiki/Smart_glass

[5] http://plc.cwru.edu/tutorial/enhanced/files/pdlc/droplet/droplet.htm

[6] http://www.scienstry.us/products.htm

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