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Final Project: Polymer-Dispersed Liquid Crystals and Their Applications

Liquid Crystals






Ordering of molecules in different phases
Nematic liquid crystal

Order parameter




Ordering parameter temperature dependence.

For more information on liquid crystals, see references [1] and [2].

Polymer-Dispersed Liquid Crystals


SEM image of PDLC sample.





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 [5]. Depending on the rate of the melting/hardening of polymer, we get different size of LC droplets in the PDLC.

Typical recipes for PDLC preparation are given in the table below [3].

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 [6]. The authors have shown that the surface energy of droplets in nemetic emulsions is only dependent on the radius of the droplet: <math>f_s=C+D/R</math> 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 [3]:


From this expression we can obtain a differential equation (see details in [6]) in cylindrical coordinates:


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


Order parameter

Order parameter in PDLC (<math>S_E</math>), 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 <math>S_E</math> 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 [3]) 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 <math>S_E</math> depends on the radius of particles for different values of applied electric voltage.

<math>S_E</math> 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, <math>S_E</math> reaches unity faster for grater radii of droplets. In other words, bigger droplets of LC need less applied voltage to be completely ordered.



Infrared spectroscopy method sample image


  • 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).
(a) No field applied.
(b) 3.5V applied
(c) 7V applied


"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) [8]. 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 droplets are oriented in all possible directions (see below). As electric filed is applied, we can see molecules orienting in-line with the field [7].

Random orientation of molecules when electric field is zero





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

State-of-the-art "smart" glass


  • reduced driving voltage from 90V to 20V

What else can we control besides transparency?


Projection displays


PDLC projection displays are particularly well suited for use in light projection devices where we desire to display bright colored and high intensity images [4]. In this case, there are several advantages compared to diplays made of other materials.



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 [4].

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)




[1] Liquid Crystal

[2] Introduction to Liquid Crystals

[3] Nonlinear Optical Properties of Liquid Crystals and Polymer Dispersed Liquid Crystals by Francesco Simoni

[4] Liquid Crystals: Application and Uses by Birendra Bahadur

[5] Preparation of Polymer-Dispersed Crystals


[7] Droplet configuration

[8] Smart Glass

[9] PDLC Products

[10] Switchable Windows

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