LC Related Links

　

[Polymer Dispersed Liquid Crystal (PDLC) mode]

The PDLC display consists of droplets of liquid crystals inside a polymer network shown in figure 1. These droplets are sized from 0.3μm to 3μm, and inside which the molecules align themselves in a bipolar configuration.

Figure 1. Working principle of Polymer Dispersed Liquid Crystal (PDLC) display

In the off state, the droplets are randomly aligned, the refractive index n' seen by the light will be different from n_p, which is the refractive index of the polymer, hence the light is scattered or reflected in a large angle towards the viewer. In the on state, the LC molecules orient uniformly along the direction of the applied field, therefore the no is the refractive index seen by the light, and usually n_p is chosen as n_p ~ n_o, for this index matched situation, light can be transmitted with a very high transmission. Obviously, the black/off state appears milky opaque, it is a poor black state, as a result, the contrast ratio of PDLC is relatively low.

The formation of droplets inside the polymer can be a micro-encapsulation process (NCAP), or a phase separation process. In the first process, an aqueous emulsion consisting of polymer and LC is coated on a substrate then let it dry. The second process is a polymerization induced phase separation: the mixture is prepared by homogeneous solution of pre-polymer LC and curing agent / photo-initiator which catalyzes the polymerization process when the mixture is UV irradiated. Phase separation can also be thermally induced or solvent induced.

The working voltage and response time of the PDLC can be affected by many factors, but mainly the resistive and dielectric properties of LC inside the droplet and the polymer. These properties affect the internal electric field of the droplet which differs from the field outside the droplet. The competition between these two field determines the final director configuration inside the droplet, therefore, the work voltage and response time will be present. Other factors like the size and shape of the droplets, the viscosity of the LC etc.

There are a few factors influencing the contrast ratio of the PDLC display. First of all is the cell gap and the density of the droplets, they are proportional to the contrast ratio. Secondly it is the temperature. As temperature decreases, no decreases and n_e increases, the birefringence becomes larger, so is the scattering. As a consequence, the black/off state is darker, so contrast ratio is higher.

Since no polarizers are not needed, high brightness is obtained. PDLC can be used into different applications. For example, as a light valve, PDLC can be made into a privacy window. PDLC also find applications in project systems.

---

Further Readings and References:

Fergason, J. L. "Polymer encapsulated nematic liquid crystals for display and light control applications". SID Int. Symp.Dig. Technol. 16, 68–70 (1985).

J.W. Doane, N. A. Vaz, B. G.Wu, and S. Zumer, “Field controlled light scattering from nematic microdroplets,” Appl. Phys. Lett., Vol. 48, 269 (1986).

J. W. Doane, A. Golemme, J. L. West, et al, "Polymer Dispersed Liquid Crystals for Display Applications," Mol Cryst Liq Cryst 165, 511 (1988).

J. L. West, "Phase Separation of Liquid Crystals in Polymers," Mol Cryst Liq Cryst 157, 427 (1988).

T. Nagata et al., "Silicon Chip Based Reflective PDLC Light Valve for Projection Display", SID Symposium Digest 29, 37 (1998).