**4. Applications of liquid crystals**

LC science and applications now permeate almost all segments of the society from display technology to beyond display front. LCD is a well-known and leading application of LC [60] in the information display industry. They are used in smallsized displays such as smart phones, calculators, wearable displays and digital cameras, medium-sized displays such as desktop and laptop computers and largesized displays such as data projectors and direct view TVs. They have the advantages of having high brightness and high resolution and being flat paneled, energy saving, light weight and even flexible in some cases [61]. To select the appropriate LCD for application and to tailor their optical performance, we need to understand broad classification (**Figure 18**) of LCD and their basic mechanism [62]. Till date mainly three types of LCD have been developed: transmissive, reflective and transreflective.

*An Overview of Polymer-Dispersed Liquid Crystal Composite Films and Their Applications DOI: http://dx.doi.org/10.5772/intechopen.91889*

**Figure 18.** *Classification of LCD.*

material (generally PI), followed by baking and rubbing [45, 51]. The thin layer of PI is known for its exceptionally strong and outstanding heat, mechanical and chemical resistivity [52]. The mechanical treatment such as unidirectional rubbing modifies surface topography by breaking the symmetry and creating linear microgrooves on the polymer surface [48, 53, 54]. The rubbing direction on one ITO plate is 0° or 90° with respect to other depending upon the parallel/antiparallel or twisted

*(a) Homogeneous and (b) homeotropic alignment of liquid crystals. [1, PI coated ITO glass plate; 2, LC*

In homeotropic alignment the LC molecule directors are oriented perpendicular to the electrode surface (**Figure 17(b)**). It can be achieved by treating the surface with a surfactant such as hexadecyltrimethylammonium bromide (HTAB), lecithin and polymers [57]. The polar head of a surfactant chemically attaches to the substrate, and the hydrocarbon tail points out, perpendicular to its surface. At this point maximum intermolecular interaction between LC molecules and surfactant

Apart from these two standard alignments, there are many other variations such as hybrid, twisted, supertwisted, fingerprint, multidomain vertically aligned, etc.

LC science and applications now permeate almost all segments of the society from display technology to beyond display front. LCD is a well-known and leading application of LC [60] in the information display industry. They are used in smallsized displays such as smart phones, calculators, wearable displays and digital cameras, medium-sized displays such as desktop and laptop computers and largesized displays such as data projectors and direct view TVs. They have the advantages of having high brightness and high resolution and being flat paneled, energy saving, light weight and even flexible in some cases [61]. To select the appropriate LCD for application and to tailor their optical performance, we need to understand broad classification (**Figure 18**) of LCD and their basic mechanism [62]. Till date mainly three types of LCD have been developed: transmissive, reflective and trans-

promotes perpendicular anchoring of the nematic LC director. However, surfactant-based homeotropic alignment is not stable against humidity and heat

mode, respectively [55, 56].

*Liquid Crystals and Display Technology*

*3.6.2 Homeotropic alignment*

which are employed in various LCD devices.

**4. Applications of liquid crystals**

[58, 59].

**Figure 17.**

*molecules].*

reflective.

**28**

#### **4.1 Transmissive LCD**

A transmissive LCD transmits a backlight for illuminating the LCD panel, which results in high contrast ratio and high brightness. As their viewing angle is limited, they are more suitable for single-viewer applications, such as games and notebook computers. To make them applicable for multiple viewers, such as televisions and desktop computers, a phase compensation film should be introduced in them. They can also be used for projection displays, for which a high-power arc lamp or a lightemitting diode (LED) array is used as a light source. The most common and finest example of transmissive LCD is twisted nematic liquid crystal (TNLC) cells which are extensively used for notebook computers, where viewing angle is not critical. Its operating principle is based on the ability of the nematic LC to rotate the polarization of light beams passing through it [43, 63].

#### *4.1.1 Twisted nematic liquid crystal cell*

It was first invented by Schadt and Helfrich and demonstrated by Fergason in 1971 [64, 65]. It consists of two ITO-coated glass substrates, additionally coated with transparent alignment layers, usually PI. These PI-coated glass plates are rubbed with velvet cloth in one direction; as a result, the LC molecules orient parallel to the rubbing direction. The rubbing directions on two substrates are perpendicular to each other. These glass plates are arranged in such a way that a 90° twist of director from one substrate to the other is formed inside the cell. The cell is kept in between two crossed polarisers in such a way that their polarization is parallel to the rubbing direction of the same glass substrate. In the absence of electric field, the top LC alignment is parallel to the optical axis of the top polarizer, while the bottom LC directors are rotated 90° and parallel to the optical axis of the bottom polarizer (analyzer) as shown in **Figure 19(a)**. When *dΔn* ≫ 0.5λ (the Gooch-Tarry's first minimum condition) is satisfied, the incoming linearly polarized light will follow closely the molecular twist and transmit the crossed analyzer. Here *Δn* is the birefringence of LC, *d* is the cell gap, and λ is the wavelength of the light. This is called the normally white (NW) mode, since light is transmitted without application of any voltage. In the voltage-on state (**Figure 19(b)**), the LC molecules undergo a Freedericksz transition. In this state, the director of the nematic LC is parallel to the field and no longer twisted. When polarized light enters a cell in such a configuration, it is not twisted and is absorbed/blocked by the analyser, resulting in a dark state. Regions where an electric field is applied appear dark against a bright

consume low power, are lighter in weight and have good readability in outdoor environment, but are inapplicable under low or dark ambient conditions.

*An Overview of Polymer-Dispersed Liquid Crystal Composite Films and Their Applications*

In order to overcome the drawbacks and to take advantage of both transmissive as well as reflective LCDs, trans-reflective LCDs have been developed, which use both ambient light and backlight to display images based on availability and necessity. It has a semi-reflective film in the back of LCD screen, the backlight can transmit through it so that it may work as a transmissive mode, but the front light cannot pass through it and get reflected, and it simultaneously works as a reflective mode. Broadly, trans-reflective LCDs are classified into four categories: (a) absorption, (b) scattering, (c) reflection and (d) phase retardation. As the name suggests, the first category absorbs light, and the corresponding device is referred to as guest-host (GH) display. The second one scatters light, and polymer-dispersed liquid crystal (PDLC), polymer-stabilized liquid crystal (PSLC) and LC gels are related technologies. The third category is based on reflection of light. The fourth one modulates the phase of an incident light. Here we will discuss operating princi-

These types of display systems were first introduced by Heilmeier and Zanoni [66]. As the name suggests, in these systems light is absorbed by the guest material,

Polymer-LC composites are thin films prepared from phase-separated high molecular weight polymer and low molecular weight LCs. The polymer, which is homogenously mixed into LC, provides mechanical and structural stabilization to LC devices. Polymer-LC composites have been used in a wide range of applications such as high-definition spatial light modulators; switchable windows; flat-panel large area flexible displays; light valves; color projectors; thermal, optical and strain sensors; bi-stable reflective displays and haze-free normal- and reverse-mode light shutter devices [68–71]. The confinement of LC material in both dense polymer matrix and moderate polymer networks modifies the bulk LC phase. Depending upon the concentration of monomer unit, they are classified as polymer-stabilized liquid crystal or polymer-dispersed liquid crystal or holographic polymer-dispersed liquid crystal (HPDLC). In PSLC, polymer forms a sponge-like structure in a continuous LC medium. The concentration of LC is much higher than the polymer concentration. In PDLC, the LC is in the form of micron- and submicron-sized droplets, which are dispersed in a continuous polymer matrix. The concentration of polymer is comparable to the LC [61]. In HPDLC the polymer concentration is high around 60–70 wt%. As droplet size is much smaller than the visible wavelength,

which are generally dichroic dye molecules, dissolved in a host LC material. Dichroic dye molecules are rod-shaped molecules, which absorb light of certain wavelength more along one axis than the other. Dye molecules get dissolved in LC and orient along with LC molecule. Upon application of external field, the rotation of absorption axis of dye molecule along with LC molecule modulates light transmission. Mainly, there can be three types of GH displays exist: the Heimeier type, the double-layer type and the PDLC type, which uses 1, 2 and zero polarizers,

The R-LCDs are of two types: direct view and projection view.

ple of PSLC, PDLC and holographic PDLC (HPDLC).

**4.3 Trans-reflective LCD**

*DOI: http://dx.doi.org/10.5772/intechopen.91889*

*4.3.1 Guest-host display*

respectively [15, 67].

**31**

*4.3.2 Polymer-liquid crystal composites*

#### **Figure 19.**

*Twisted nematic LCD in (a) OFF state and (b) ON state. [1, unpolarised light; 2, polarizer; 3, PI coated ITO glass plates; 4, LC droplets; 5, polarized light].*

background. Because of the orthogonality of boundary layers, the dark state is achieved at relatively lower voltage. Depending on the field strength, twisted nematic displays can switch between light and dark states, or somewhere in between (greyscale.) How the LC molecules respond to applied field is the important characteristic of this type of display. However, every device has some shortcoming, in TNLC is its narrow viewing angle and poor color production. To overcome these problems, new technologies such as in-plane switching and vertical alignment mode have been introduced [61].
