*4.3.2 Polymer-liquid crystal composites*

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,

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

*Twisted nematic LCD in (a) OFF state and (b) ON state. [1, unpolarised light; 2, polarizer; 3, PI coated ITO*

overcome these problems, new technologies such as in-plane switching and vertical

In R-LCDs, the necessity of backlight source (as in transmissive-type LCD) has

been seized. They reflect ambient light for displaying images. Therefore, they

alignment mode have been introduced [61].

*glass plates; 4, LC droplets; 5, polarized light].*

*Liquid Crystals and Display Technology*

**4.2 Reflective LCD**

**30**

**Figure 19.**

HPDLC films are free of light scattering. These films have faster response time and require higher switching voltages. In PDLC and HPDLC films, no surface alignment layer is needed [72].

**5.1 Morphological analysis**

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

PSLC film behaviour.

**Figure 21.**

**33**

and in the absence and presence of electric field [80].

**Figure 21** shows POM images of PSLC films which are prepared using LC BL036 and prepolymer NOA-65 in 95/5 wt/wt% ratio under different rubbing directions

**Figure 21** shows the POM image of four types of PSLC films named as "A" (antiparallel rubbing; electric field absent), "B" (antiparallel rubbing; electric field present), "C" (twisted rubbing; electric field absent) and "D" (twisted rubbing; electric field present). On the acute observation of these images in all the four samples, complex geometrical structures of LC and polymer network were found. Samples A and C which were prepared without applying any electric field during polymerization showed rectilinear alignment. Since polymer network has a strong aligning effect on the LC, therefore it tends to keep LC in the aligned state [81]. In these films, polymer chains move throughout the sample parallel to the rubbing direction; therefore shorter chains with smaller domains entrapped between them were observed. In the case of samples B and D, which were prepared by applying electric field during polymerization, bigger LC domains were formed. Upon application of electric field during sample preparation, LC material orients and often indefinitely retains the alignment imposed by an electric field [82]. Because of the adsorption of LC droplets at the polymer wall, polymer network grows in the direction of field. Due to which cross linked, thicker and topologically defective polymeric walls were formed. Also, diffraction of light was observed from polymer nodules. The effect of electric field on the orientation and confirmation of the LC material between the boundaries of polymer network gives deep insight in understanding

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

*POM images of homogenously aligned PSLC films: (A) antiparallel rubbing cured without any voltage, (B) antiparallel rubbing cured by applying 10 V, (C) 90° twist rubbing cured without any voltage and (D) 90°*

*twist rubbing cured by applying 10 V, observed using 5 objective.*
