*6.1.1 Phase separation*

The phase separation method starts with homogenous, isotropic solution of LC and polymer or prepolymer, followed by the formation of nucleated LC droplets (phase separation) and then finally solidification of polymer matrix pioneered by Doane and co-workers [122]. The morphology of a phase-separated PDLC film depends on the chemical nature of the LC and polymer constituents and kinetics of the processes driving the phase separation to occur, which ultimately control the electro-optic properties of device. Therefore, to obtain desired electro-optic properties in a device, systematically processed phase separation of LC and polymer is one of the key parameters. This technique is preferred where film moldability is required because during phase separation, polymer plasticization takes place. To induce phase separation in PDLC, generally any of the three routes, namely, polymerization-induced phase separation (PIPS), solutioninduced phase separation (SIPS) and temperature-induced phase separation (TIPS), has been practised. Each method produces PDLC film with different properties and characteristics.

controlling the rate of cooling. Droplet size (droplet numbers) shows

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

optic properties of the PDLC device.

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

encapsulation with proper optical properties [120, 123, 124].

þ *K*<sup>22</sup> *n*

!*:*∇ � *n* ! <sup>2</sup>

where the proportionality constants *K*11*, K*<sup>22</sup> and *K*<sup>33</sup> are associated with splay, twist and bend deformations, respectively. It is usually not possible to pack the nematic director field into curvatures without creating one or more defects. These defects are responsible for the transformation of LC droplet from one configuration to another. Different defect structures are classified on the basis of their twodimensional structure often known as "strength (*s*)" of the defect. "*s*" is defined by the rotation of the nematic director on a closed path around the defect; *s* indicates how many times of 2π the director rotates. Since +*n* and – *n* of the director are equivalent, half-integer values of *s* are allowed. The light scattering and dielectric properties of different droplet configurations can vary considerably, which is an important factor in the PDLC devices. Accordingly, the configuration of confined LCs is an area of both scientific and technological interest [19, 126]. The configuration adopted by the nematic director field within a droplet reflects the subtle interplay of a number of forces, such as shape and size of cavity containing LC

þ *K*<sup>33</sup> *n*

! <sup>2</sup> (23)

! � ∇ � *n*

**6.2 Nematic configurations in PDLC films**

*K*<sup>11</sup> ∇*:n* ! <sup>2</sup>

*6.1.2 Encapsulation method*

[19, 125]

**45**

*<sup>F</sup>* <sup>¼</sup> <sup>1</sup> 2

inverse (direct) relation with rate of cooling. Methodically, it is a simple way of phase separation, but it is less preferred because the properties of TIPS films are difficult to reproduce, often sensitive to the processing history, and homogenous mixture of LC and thermoplastic is obtained at relatively higher temperature than in PIPS and SIPS techniques, which may worsen electro-

In contrast to phase separation method, encapsulation method starts from inhomogeneous solution. LC is dispersed by rapid stirring as an emulsion in an aqueous solution of a film-forming polymer such as PVA. This emulsion is then spin-coated or deposited onto a transparent conductive substrate like ITO-coated glass plate and dried. Thousands of non-uniform, sometimes interconnected LC capsules (droplets), surrounded by a solid layer of polymer are produced. Size of these droplets depends on stirring speed and time. Materials manufactured from this method are also known as nematic curvilinear aligned phase (NCAP). Encapsulation method is beneficial as LC is insoluble in aqueous solution; equilibrium phase separation is easily achieved as well as polymer plasticization is prevented. But it has limitation also; because of water evaporation, there is significant change in volume of film which tends to deform droplet structure, and only few polymers are appropriate for

The nematic material confined in a droplet in a PDLC is in a particular arrangement, called the director configuration. LC droplets are usually spherical because of surface tension, but due to photopolymerization reaction the texture changes significantly to adopt different configurations. When LCs are confined to small cavities, curved surfaces deform the director field, inducing three basic Frank elastic deformations in the director structure, namely, splay, twist and bend. The contribution of each deformation to the overall energy density *F* is given by


controlling the rate of cooling. Droplet size (droplet numbers) shows inverse (direct) relation with rate of cooling. Methodically, it is a simple way of phase separation, but it is less preferred because the properties of TIPS films are difficult to reproduce, often sensitive to the processing history, and homogenous mixture of LC and thermoplastic is obtained at relatively higher temperature than in PIPS and SIPS techniques, which may worsen electrooptic properties of the PDLC device.
