**6.3 Light scattering properties of PDLC films**

Light scattering properties of PDLC films which can be controlled by electric field is a topic of great interest for scientific and technological reasons. The light scattering effects are insensitive to the initial polarization of light, and hence PDLC films can modulate light without the use of alignment layers or polarizers. The light scattering property of a PDLC film depends on many parameters such as LC droplet shape and size, droplet configuration, droplet density, refractive indices of LC and polymer, wavelength, etc. However, the nematic director orientation within the LC droplet dominates the light scattering properties of the films. Upon application of electric field, directors can be oriented along the direction of electric field causing transformation from highly scattering to highly transparent film. For simplicity, to explain the scattering and/or propagation of light at different voltage levels, we have developed a single droplet model [116, 133, 134].

*6.4.2 ON-state transmittance (*T*ON)*

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

the film.

*6.4.3 Contrast ratio (CR)*

*6.4.5 Switching voltages*

derived as [136, 137]:

voltage or ON-state voltage (VON).

*6.4.6 Response time*

**49**

the ratio of the ON- to OFF-state transmittance:

*6.4.4 Transmittance difference (ΔT)*

In the ON state, when sufficient voltage is applied across the PDLC which overcome the anchoring at polymer-LC interface, the LC droplets attain minimum free energy by completely aligning themselves parallel to the field direction by the action of dielectric torque. In such a situation, light incident normal to the film surface experiences RI, *np*= *no*, and gets transmitted through

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

Contrast ratio is the term used to evaluate display properties of PDLC films. It is

*CR* <sup>¼</sup> *<sup>T</sup>*ONð Þ %

Another term used to evaluate the efficiency of PDLC films is transmittance

One of the most important parameters of PDLC films is the voltage required to achieve an electro-optic effect. Initially the LC droplets are randomly oriented in PDLC films. When low electric field is applied, LC droplets starts orienting along the field; the voltage required to increase the transmittance of PDLC film by additional 10% of OFF-state transmittance (*T*OFF) is termed as threshold voltage (*V*TH). The theoretical model for threshold voltage has been developed by balancing the elastic forces, surface interaction and applied electric force and is mathematically

difference (ΔT); it is difference of ON- and OFF-state transmittance:

*<sup>V</sup>*TH <sup>¼</sup> *<sup>d</sup>* 3*a*

*ρp ρLC* þ 2 � � *k l*<sup>2</sup> � <sup>1</sup> � �

where *d* is the film thickness; *ρ<sup>p</sup>* and *ρLC* are the resistivities of polymer and LC, respectively; *k* is the elastic constant; the aspect ratio *l* ¼ *a=b*, where *a* and *b* are the length of the major and minor axes of LC droplet, respectively; and *Δε* is the dielectric anisotropy of the LC. With further increase in voltage, more and more LC align along the field. When sufficient voltage is applied across the PDLC which overcome the anchoring at polymer-LC interface, the LC droplet attains minimum free energy by completely aligning itself parallel to the field direction. At this stage film becomes fully transparent, and corresponding voltage is termed as saturation

Another decisive factor in evaluating the performance of the polymer-LC composite film is its dynamic response to an applied electric field. Quick response of a

*<sup>T</sup>*OFFð Þ % (24)

*ΔT*ð Þ¼ % *T*ON � *T*OFF (25)

*Δε* " #<sup>1</sup>

2

(26)

In the absence of electric field, the different droplets will have different orientations. The droplet under investigation (**Figure 34(a)**) will scatter the light at polymer-LC interface. When a little voltage, below threshold voltage *V*TH (defined later) (**Figure 34(b)**) is applied, because of the strong anchoring at interface, the alignment of LC droplet directors will not change much except the internal portion of the droplet. The internal portion experiences the electric field effect and aligns LC directors along the direction of the field. When the field (**Figure 34(c)**) is further increased up to the intermediate level, the majority of the LC droplets gets oriented along the direction of applied electric field, except the LC directors, which are on the polymer-LC interface, experiencing enough anchoring forces. At adequately high electric field (**Figure 34(d)**), i.e. above saturation voltage VON (defined later), all the directors get aligned along the direction of electric field. In such situation, light encounters only ordinary RI of LC, which is very close with the RI of the polymer. Therefore, a clear transparent film is observed at sufficiently high voltages.

**Figure 34.**

*Single LC droplet model: (a) V=0, (b) V<VTH, (c) VTH* ≤*V*≤ *VON, (d) V>VON.*

#### **6.4 Electro-optic properties of PDLC films**
