*6.1.2 Encapsulation method*

*6.1.1 Phase separation*

*Liquid Crystals and Display Technology*

properties and characteristics.

applied in mask patterning.

**44**

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 poly-

a. Polymerization-induced phase separation: It is usually an irreversible process, in which initially miscible, single-phased mixture of prepolymer (monomer or oligomer) and LC is filled into a prefabricated cell. On application of suitable energy in the form of heat, light or radiation, the polymerization is induced. The growing polymer chains phase separate LC droplets by forming an enclosing polymer matrix because LC is less soluble in polymer than in prepolymer. The phase-separated system has lower entropy than the mixed one. Polymerization may be initiated by heat, photo (UV irradiation) or free radicals. Generally, photopolymerization is preferred over the other two because it has low activation energy, has a good control over final properties of PDLC film and can occur in broader temperature range [120, 123, 124]. Phase separation process continues even if the UV exposure is interrupted during the cure process. Nowaday's photopolymerization technique is also

b. Solution-induced phase separation: In this method, a polymer and LC are mixed and dissolved in a common organic solvent to form a single-phase homogeneous mixture. Such solution is then poured (using suitable technique) on an optically flat, transparent and conducting substrate for solvent evaporation. Evaporation of solvent causes phase separation between polymer and LC domains. Droplets start growing as the polymer and LC come out of solution and grow up till the polymer solidification. Another substrate (generally ITO) is laminated and annealed on the film using pressure and heat to form a PDLC cell. The electro-optic properties, which depend on LC droplet size and morphology, can be tailored by controlling evaporation rate. Droplet size and shape can be manipulated after the PDLC cell formation, by heating the sample cell till the clearing temperature of LC and then cooling. During heating LC get dissolved in polymer and again phase separates on cooling. By

c. Temperature-induced phase separation: It is useful for thermoplastics which melt below their decomposition temperature. Thermoplastic and LC are heated until the clear solution is formed, such solution is filled into a

prefabricated cell and then cooled. When the system is cooled, the LC phase separates from the solidifying polymer. Droplet size can be tailored by

varying cooling rates, LC droplet size can be varied.

mer 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

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