*7.2.1 LC material*

The optical and dielectric anisotropy of nematic LC make them suitable candidate for HPDLC films. Similar to the PDLC composite films, the operation of these composite films is based on birefringence property of LC. Nematic LCs are

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

optically uniaxial materials, i.e. they have two direction-dependent refractive indices, (ordinary RI, *no*, and extraordinary RI, *ne*) with birefringence *Δn* ¼ *ne* � *no* and average RI, *nav* ¼ ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 1 <sup>3</sup> *n*<sup>2</sup> *<sup>e</sup>* þ 2*n*<sup>2</sup> *o* <sup>q</sup> � �. The value of *<sup>Δ</sup><sup>n</sup>* may be positive or negative [34, 37, 38]. The polymer and LC are chosen such that the refractive index of the polymer (*np*) should match with ordinary refractive index (*no*) of the LC, typically *ne* ≫ *no* � *np*.

## *7.2.2 Monomer unit*

faster response time, have been obtained. Similar to the PDLC composite films, no surface alignment layer is needed in HPDLC films. **Figure 46** shows schematic of

When a mixture of LC, monomer and photoinitiator (PhI) is exposed under the

standing wave, formed from the interference of two or multiple coherent laser beams, it generates periodic dark and bright fringes. These periodic fringes regulate polymerization and hence phase separation process. High polymerization rate in the bright region (diffusion of monomer from dark to bright region) and low polymerization rate in the dark region (diffusion of LC from bright to dark region) create Bragg grating with alternate polymer-rich and LC-rich regions. Similar to the PDLC, the polymer and LC material are chosen such that the RI of the polymer should match with the ordinary RI of LC. Upon application of electric field, HPDLC film becomes optically transparent/homogenous, and the grating is in its OFF state. When the electric field is removed, LC molecules return to their original random state, and grating is in its ON state. This HPDLC grating reflects light of a particular wavelength and transmits light of all other wavelengths [163]. Morphology and diffraction properties of grating depends upon writing set-up, materials, diffusion rate, curing conditions and phase separation process [164, 165]. The particular wavelength that is reflected is a function of the refractive index difference and the width of the layers in the grating. When a voltage is applied, the liquid crystals align with the field, and their new refractive index matches that of the polymer, causing

HPDLC film.

**Figure 46.**

**7.1 Operating principle**

*Schematic of HPDLC film.*

*Liquid Crystals and Display Technology*

the grating to become transparent.

*7.2.1 LC material*

**62**

**7.2 Material used and sample preparation of HPDLC films**

photoinitiator and dopant (if any) in a desired ratio.

HPDLC composite films are also prepared by mixing LC, monomer,

The optical and dielectric anisotropy of nematic LC make them suitable candi-

date for HPDLC films. Similar to the PDLC composite films, the operation of these composite films is based on birefringence property of LC. Nematic LCs are In HPDLC films two types of monomer can be used:


#### *7.2.3 Photoinitiator and co-initiators*

Along with LC material and monomer, to induce photopolymerization, PhI and co-initiators are also required. Choice of PhI depends on the wavelength of laser beam employed for writing. Sometimes chain extenders are also incorporated into the mixture to optimize grating morphologies.

#### *7.2.4 Sample preparation*

Empty sample cell is prepared by two ITO-coated glass substrate (ITO coating facing each other) separated by suitable spacers. Mixture of LC, monomer, photoinitiator and dopant (if any) is stirred for homogenization. Mixture is filled into ITO cell and then exposed under suitable light depending upon the PhI. Generally, for curing sample cell is placed under the interference pattern formed by coherent laser beams. Samples are again placed under UV lamp for postcuring if needed [166].

#### **7.3 Types of HPDLC gratings**

Different writing set-ups produce different types of HPDLC gratings, named as transmission grating and reflection grating. If the writing beams are incident on

**Figure 47.** *Types of HPDLC grating: (a) transmission grating and (b) reflection grating.*

the same side of sample cell, transmission grating will be formed, with grating planes perpendicular to the sample cell surface as shown in **Figure 47(a)**. If the writing beams are incident from both sides of the samples, reflection grating will be formed, with grating planes parallel to the sample cell surface as shown in **Figure 47(b)**.

#### **7.4 HPDLC grating parameters**

#### *7.4.1 Grating period/grating pitch*

The grating period depends on the writing wavelength and intersection beam angle:

$$A = \frac{\lambda}{2\overline{n}\ \sin\left(\theta\_i/2\right)}\tag{30}$$

**7.5 HPDLC morphology**

ogies can be categorized into three types.

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

*7.5.1 LC droplet-like morphology*

*7.5.2 Polymer scaffolding morphology*

**Figure 48.**

**Figure 49.**

**65**

Different materials and varying curing conditions produce different types of polymer-LC morphologies in HPDLC composite films. Mainly, HPDLC morphol-

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

Generally, this type of morphology has been found in transmission or reflection grating (**Figure 48**). These types of gratings are made up of acrylate- or thiol-enebased monomers. During PIPS process, because of fast curing process (intense curing light and small curing time) and high effective functionality of monomer, LC molecules get diffuse and configure themselves into distinct and elongated LC droplets. These LC droplets, embedded into the polymer matrix, are distinguished in SEM image (**Figure 48(b)**). Here, light scattering is more because the size of the

In transmission gratings prepared from acrylate-based material systems under slow curing process, polymer scaffolding morphology can be obtained (**Figure 49**). Here instead of small LC droplets, large LC domains or LC layer is obtained. If the curing process is relatively slow as compared to that in "LC droplet-like morphology", transverse polymer filaments are obtained in polymer-rich region, in between

LC droplets is comparable to the wavelength of the visible light [164, 168].

*LC droplet-like morphology: (a) schematic and (b) SEM image of HPDLC film [164, 168].*

*Polymer scaffolding morphology: (a) schematic and (b) SEM image of HPDLC film [164, 169].*

Here, *Λ* is the grating period, *λ* is the writing wavelength and *n* is the average refractive index of the material mixture. A grating with varied period can be obtained by inserting a refractive cylindrical lens in conventional double interference optical path. It varies the angle of incidence of one of the two interference laser beams [167].

#### *7.4.2 Cook-Klein parameter*

This parameter depends on the grating period and thickness of grating:

$$Q = 2\pi \frac{\lambda d}{\overline{n}\Lambda^2} \tag{31}$$

Here, λ is the wavelength of incident light, and *d* is the thickness of the grating.

If the value of Q < 1, then it is a Raman-Nath-type grating. It is thin grating and multiple diffraction orders can be found.

If the value of Q > 1, then it is Bragg-type grating. It is thick or volume grating, and only zero order or first order of diffraction can be found. Since the optical losses are low in Bragg-type-grating, it is preferred for practical applications.

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