**7. Holographic polymer-dispersed liquid crystal**

When monomer concentration is high around 60–70%, nanosized LC droplets are formed and embedded inside the polymer matrix, this kind of polymer-LC composite films is named as HPDLC films. In PDLC films LC droplets are randomly distributed in polymer matrix, whereas in HPDLC films, alternate polymer-rich and LC-rich regions exist. As the size of the LC droplets is much smaller than the wavelength of visible light, composite films, free from scattering effect and with

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

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

[34, 37, 38]. The polymer and LC are chosen such that the refractive index of the

a. Acrylate-based monomer: A wide range of acrylate-based monomers with different functionalities are available such as phthalic diglycol diacrylate (PDDA), 2-ethylhexyl acrylate (EHA), polyurethane acrylate oligomers (PUA), etc. But these multifunctional acrylates carry some drawbacks in HPDLC gratings, such as: (i) Because of early gelation and subsequent vitrification, monomer conversion is low, and polymerization can continue for prolonged periods, even in the dark; (ii) during exposure considerable shrinkage was also observed; (iii) larger interfacial area increases unwanted optical scattering; and (iv) interconnectivity in LC droplets and variety in LC droplet configuration decreases prompt electrical

b. Thiol-ene-based monomer: Thiol-ene-based monomers are composed of multifunctional aliphatic thiols and vinyl monomers comprising ene groups. These monomers have high conversion efficiency, good stability, less shrinkage and more elastic; therefore they are preferably used for constructing HPDLC composite films. Norland optical adhesives are good examples of these kinds of monomers. Earlier these monomers could only be cured by UV light, but now new materials were synthesized, which can be cured under

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

Empty sample cell is prepared by two ITO-coated glass substrate (ITO coating

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

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

polymer (*np*) should match with ordinary refractive index (*no*) of the LC,

<sup>q</sup> � �. The value of *<sup>Δ</sup><sup>n</sup>* may be positive or negative

ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi

*<sup>e</sup>* þ 2*n*<sup>2</sup> *o*

In HPDLC films two types of monomer can be used:

1 <sup>3</sup> *n*<sup>2</sup>

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

average RI, *nav* ¼

typically *ne* ≫ *no* � *np*.

switching.

visible light.

*7.2.4 Sample preparation*

**7.3 Types of HPDLC gratings**

needed [166].

**63**

*7.2.3 Photoinitiator and co-initiators*

the mixture to optimize grating morphologies.

*7.2.2 Monomer unit*

**Figure 46.** *Schematic of HPDLC film.*

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 HPDLC film.
