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

**6.3 Light scattering properties of PDLC films**

*Liquid Crystals and Display Technology*

have developed a single droplet model [116, 133, 134].

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

**Figure 34.**

**48**

*6.4.1 OFF-state transmittance (scattering) (*T*OFF)*

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

indices *Δn* ¼ *ne* � *no* must be as large as possible.

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

In the absence of electric field, the different droplets will have different orienta-

In the absence of field, i.e. OFF state, the directors of LC droplet are randomly aligned. In this situation, most of the light incident normal to the film surface gets scattered giving opaque appearance to the film. The reason behind this is the mismatch of RI (a) between two adjacent droplets, (b) between a LC droplet and polymer and (c) within a LC droplet [135]. The OFF-state transmittance of a PDLC film depends on the birefringence of the LC. To minimize the value of OFF-state transmittance, the difference between LC extraordinary and ordinary refractive

tions. 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.

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 the film.

#### *6.4.3 Contrast ratio (CR)*

Contrast ratio is the term used to evaluate display properties of PDLC films. It is the ratio of the ON- to OFF-state transmittance:

$$CR = \frac{T\_{\text{ON}}(\text{@})}{T\_{\text{OFF}}(\text{@})} \tag{24}$$

#### *6.4.4 Transmittance difference (ΔT)*

Another term used to evaluate the efficiency of PDLC films is transmittance difference (ΔT); it is difference of ON- and OFF-state transmittance:

$$
\Delta T(\text{@}) = T\_{\text{ON}} - T\_{\text{OFF}} \tag{25}
$$

#### *6.4.5 Switching voltages*

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 derived as [136, 137]:

$$V\_{\rm TH} = \frac{d}{3a} \left[ \frac{\rho\_p}{\rho\_{LC}} + 2 \right] \left[ \frac{k \left( l^2 - 1 \right)}{\Delta e} \right]^{\frac{1}{2}} \tag{26}$$

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 voltage or ON-state voltage (VON).

#### *6.4.6 Response time*

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

PDLC film is critical in many applications. Response time is the time required by the LC molecules to align along the electric field upon application of field and to relax to their initial orientation when the electric field is removed. It generally depends on the relative strength of the applied field and the elastic reorientation forces. It can be affected by the degree of phase separation between nematic and polymer phase. Polymer-LC material properties and film morphology which include droplet size, shape, multiple scattering processes, etc. have high impact on response time of composite film [138, 139]. Erdmann observed reduced values of response time for elongated LC droplets [128]. It is calculated by balancing an elastic torque (*Г*d), electric torque (*Г*e) and viscous torque (*Г*v). It is the sum of rise time (*τ*r) and decay time (*τ*d). Rise and decay times are usually defined in terms of an optical response. Rise time is the time in which transmittance of film reaches from 10 to 90% on application of electric field. Similarly decay time is the time in which transmittance reaches from 90 to 10% on removal of electric field [115]. Mathematically

$$\frac{\mathbf{1}}{\tau\_r} = \frac{\mathbf{1}}{\gamma\_1} \left[ \Delta \boldsymbol{\varepsilon} \times \boldsymbol{V}^2 + \frac{k \left( \boldsymbol{l}^2 - \mathbf{1} \right)}{\boldsymbol{a}^2} \right] \tag{27}$$

and

$$\tau\_d = \frac{\gamma\_1 \times a^2}{k \left(l^2 - 1\right)}\tag{28}$$

2.Persistence: The phenomenon when the film does not return to its complete scattering state immediate after removal of field is termed as persistence. The reason behind persistence is high interconnectivity between LC droplets which creates defect structure in any of the droplet. Then the other connected droplets may get trapped in the high field and remain in the same state even

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

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

after the removal of field, until the defect escapes from the trap.

3.Memory effect: It is a semi-permanent persistence because film remains permanently in the transparent state even after removal of field. The reason for memory effect is weak anchoring force which develops between the boundaries of polymer network and LC domain upon application of field.

PDLC films. Persistence and memory effects are due to predominance of some

The sources which give rise to hysteresis phenomenon have been studied by various researchers [117, 135, 138, 140, 141]. Hysteresis depends on the rate at which fields are applied and removed. For quickly (period of microseconds) varying voltages, hysteresis are more as compared to slowly varying (period of seconds) voltages in a same film [19]. It has been proposed that the orientation mechanism of the LC droplet director is a crucial factor responsible for hysteresis. Upon application and removal of field, there is substantial difference in the director direction of LC molecules, which are at the polymer-LC interface and which are inside the LC droplet [138]. The orientation-reorientation of LC directors depends on the polymer-LC compatibility and induced interfacial polarization, influencing the distribution of the relaxation time [136]. Hysteresis might be due to the presence of defect structure within a LC droplet [138, 141]. Depolarization effects might contribute to hysteresis through the generation of electric fields inside the PDLC films [142]. It has been suggested that hysteresis might be due to the residual electric charge (on field removal) which serves as a capacitor during the scan down cycle [84, 135]. On analysing **Figure 35**, it is clear that hysteresis effect is not observed at high fields because at higher voltages of scan down cycle, LC droplets

specific factor.

**Figure 35.**

**51**

*Schematic of hysteresis, persistence and memory effect in a PDLC film.*

There are some common factors in hysteresis, persistence and memory effect in

where *γ*1is the rotational viscosity of LC and other symbols manifest same meaning defined earlier.

For higher electric fields

$$
\pi\_r = \frac{\mathcal{V}\_1}{\Delta \varepsilon \times \mathcal{V}^2} \tag{29}
$$

An analysis of the above equation indicates that *τ<sup>r</sup>* is predominantly a function of the applied voltage, where *τ<sup>d</sup>* typically depends on the configuration of LC domain and their anchoring energy with polymer wall. A good estimation of rise and decay times can be attained by understanding the director direction inside LC droplet.

### *6.4.7 Hysteresis effect*

During the study of electro-optic properties, a well-known hysteresis phenomenon was observed in PDLC films [138]. The hysteresis is a known problem, which must be addressed for practical application of the display material. It has been found that the transmittance values obtained at various voltages during scan down cycle do not follow the same path as that of the scan up cycle. A measure of hysteresis is given by the voltage width at half of maximum transmittance (Δ*V50)*. The lag of transmittance during scan down cycle may trace on any of the three paths as shown in **Figure 35**:

1.Hysteresis: Hysteresis occurs in intermediate voltage range, and the transmission at a given voltage depends on the previous voltage state. The situation when a higher transmission at a given voltage is observed during the scan down cycle as compared to the transmission at the same voltage during the scan up cycle is termed as hysteresis effect.

*An Overview of Polymer-Dispersed Liquid Crystal Composite Films and Their Applications DOI: http://dx.doi.org/10.5772/intechopen.91889*


There are some common factors in hysteresis, persistence and memory effect in PDLC films. Persistence and memory effects are due to predominance of some specific factor.

The sources which give rise to hysteresis phenomenon have been studied by various researchers [117, 135, 138, 140, 141]. Hysteresis depends on the rate at which fields are applied and removed. For quickly (period of microseconds) varying voltages, hysteresis are more as compared to slowly varying (period of seconds) voltages in a same film [19]. It has been proposed that the orientation mechanism of the LC droplet director is a crucial factor responsible for hysteresis. Upon application and removal of field, there is substantial difference in the director direction of LC molecules, which are at the polymer-LC interface and which are inside the LC droplet [138]. The orientation-reorientation of LC directors depends on the polymer-LC compatibility and induced interfacial polarization, influencing the distribution of the relaxation time [136]. Hysteresis might be due to the presence of defect structure within a LC droplet [138, 141]. Depolarization effects might contribute to hysteresis through the generation of electric fields inside the PDLC films [142]. It has been suggested that hysteresis might be due to the residual electric charge (on field removal) which serves as a capacitor during the scan down cycle [84, 135]. On analysing **Figure 35**, it is clear that hysteresis effect is not observed at high fields because at higher voltages of scan down cycle, LC droplets

**Figure 35.** *Schematic of hysteresis, persistence and memory effect in a PDLC film.*

PDLC film is critical in many applications. Response time is the time required by the LC molecules to align along the electric field upon application of field and to relax to their initial orientation when the electric field is removed. It generally depends on the relative strength of the applied field and the elastic reorientation forces. It can be affected by the degree of phase separation between nematic and polymer phase. Polymer-LC material properties and film morphology which include droplet size, shape, multiple scattering processes, etc. have high impact on response time of composite film [138, 139]. Erdmann observed reduced values of response time for elongated LC droplets [128]. It is calculated by balancing an elastic torque (*Г*d), electric torque (*Г*e) and viscous torque (*Г*v). It is the sum of rise time (*τ*r) and decay time (*τ*d). Rise and decay times are usually defined in terms of an optical response. Rise time is the time in which transmittance of film reaches from 10 to 90% on application of electric field. Similarly decay time is the time in which transmittance

reaches from 90 to 10% on removal of electric field [115]. Mathematically

*Δε* � *<sup>V</sup>*<sup>2</sup> <sup>þ</sup>

*<sup>τ</sup><sup>d</sup>* <sup>¼</sup> *<sup>γ</sup>*<sup>1</sup> � *<sup>a</sup>*<sup>2</sup>

where *γ*1is the rotational viscosity of LC and other symbols manifest same

*<sup>τ</sup><sup>r</sup>* <sup>¼</sup> *<sup>γ</sup>*<sup>1</sup>

of the applied voltage, where *τ<sup>d</sup>* typically depends on the configuration of LC domain and their anchoring energy with polymer wall. A good estimation of rise and decay times can be attained by understanding the director direction inside LC

1.Hysteresis: Hysteresis occurs in intermediate voltage range, and the

the scan up cycle is termed as hysteresis effect.

transmission at a given voltage depends on the previous voltage state. The situation when a higher transmission at a given voltage is observed during the scan down cycle as compared to the transmission at the same voltage during

An analysis of the above equation indicates that *τ<sup>r</sup>* is predominantly a function

During the study of electro-optic properties, a well-known hysteresis phenomenon was observed in PDLC films [138]. The hysteresis is a known problem, which must be addressed for practical application of the display material. It has been found that the transmittance values obtained at various voltages during scan down cycle do not follow the same path as that of the scan up cycle. A measure of hysteresis is given by the voltage width at half of maximum transmittance (Δ*V50)*. The lag of transmittance during scan down cycle may trace on any of the three paths

" #

*k l*<sup>2</sup> � <sup>1</sup> � � *a*2

*k l*<sup>2</sup> � <sup>1</sup> � � (28)

*Δε* � *<sup>V</sup>*<sup>2</sup> (29)

(27)

1 *τr* ¼ 1 *γ*1

and

droplet.

**50**

*6.4.7 Hysteresis effect*

as shown in **Figure 35**:

meaning defined earlier. For higher electric fields

*Liquid Crystals and Display Technology*

remain in the same state of orientation. However, when the applied field is reduced further, reorientation of LC droplets director begins, giving rise to hysteresis effect [143].

There is a much scope of improvement in the optical and dielectric properties of PDLC devices, by reducing operating voltages, quickening switching and relaxation times, enhancing image contrast, etc. Before photopolymerization, PDLC films are a homogeneous mixture composed of monomer and LC which transforms into heterogonous system consisting LC droplets embedded in polymer matrix after photopolymerization.

Several groups have suggested that to augment the PDLC film properties, we can alter:


scattering as well as controllable absorbance, modulated by the liquid crystal molecule. The large transition moment of the rod-shaped dye molecules is modulated by the symmetry axis of the LC molecule in the OFF and ON states of electric field [150–153]. In the field "OFF state", dye molecules are randomly oriented along with the LC droplets. The unpolarised incident light is scattered as well as absorbed due to LC droplets and dissolved dye molecules. Due to the scattering of light, the optical path length of light increases, and hence enhancement in absorption has been noticed. In the field "ON state," LC droplets and dissolved dye molecules get aligned along the field direction, making the film feebly absorbing; hence, most of the light is transmitted through the film. **Figure 37** shows the schematic of DPDLC

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

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

LC droplets in all the composite films exhibit predominantly bipolar (LC droplet with cylindrical symmetry axis defined by two-point defects at opposite ends)-type morphology with some radial (LC droplet with spherical symmetry and the only point defect in the volume centre) structures. No preferred orientation of LC droplets was found, which is responsible for the scattering of light. Upon addition of dichroic dye molecule, significant morphological changes arise in LC droplet. The dye, polymer and LC are chosen such that the solubility of dye is high in LC but very low in polymer. **Figure 38** shows POM image of DPDLC film (O36N), prepared using LC BL036 and prepolymer NOA65 in wt/wt% with varying disperse Orange 25 dye concentration. As the dye concentration increases from 0–1%, there is noticeable increase in droplet size from submicron to 10 μ. The reason is dye molecules dissolved in LC absorb some of the UV radiations provided for photopolymerization, which reduces rate of polymerization and in turn influences droplet growth. At lower dye content, faster polymerization rate produces smaller and denser LC droplets. At higher dye content, slow polymerization rate preserves polymer in a liquid state for an extended period, thus allowing the growth and coalescence of small LC droplets to form bigger ones.

film in the OFF and ON states of applied electric field [61, 125].

*A schematic representation of direction dependent absorption of dichroic dye molecule.*

**Figure 36.**

**53**

There is more number of bipolar droplets in the high dye-doped film

To enhance the properties of PDLC films, LC properties are modified suitably by addition of nanoparticles, dyes, polymers and carbon nanotubes. The inclusion of new additives can add new functionalities to the obtained devices. To obtain the desired result from the host LC material, proper selection of size, shape and structure of guest/additive particle is an imperative factor. In has been proved that due to the inherent dipole moment, LC materials respond radically when doped and anchored with elongated species.

The incorporation of small quantity of dye molecules to the LC is one suitable solution, which has been studied by several researchers.

#### **6.5 Operating principle of DPDLC composite films**

DPDLC films are also named as guest-host polymer-dispersed liquid crystal (GHPDLC) composite films. The LC phase acts as the host and dye molecules doped in LC act as the guest. Once the geometrically anisotropic dye molecules are added/ dissolved in LC, the long molecular axis of dye molecule tends to align along the LC director; this is called as guest-host interaction. Dyes used for LC media must have high dichroic ratio, high-order parameter, high stability and good compatibility and solubility with LC but not with monomer unit or polymer matrix. It must be a positive type, i.e. the absorption transition dipole is along the long molecular axis. When the polarization of the incident light is perpendicular to the long axis of the dye molecules, the light is weakly absorbed. When the polarization of the incident light is parallel to the long axis of the dye molecules, the light is strongly absorbed, as shown in **Figure 36**. Considering all the above-mentioned points, dichroic azo dyes, which are N]N substance and absorb light of certain wavelength more along one direction than the other, have been used.

Similar to the PDLC films, the dispersion of the droplets can also be achieved using polymerization-induced phase separation method, in which the dye and LC mixture separate from the polymer binder. These DPDLC films possess controllable *An Overview of Polymer-Dispersed Liquid Crystal Composite Films and Their Applications DOI: http://dx.doi.org/10.5772/intechopen.91889*

#### **Figure 36.**

remain in the same state of orientation. However, when the applied field is reduced further, reorientation of LC droplets director begins, giving rise to

There is a much scope of improvement in the optical and dielectric properties of PDLC devices, by reducing operating voltages, quickening switching and relaxation times, enhancing image contrast, etc. Before photopolymerization, PDLC films are a homogeneous mixture composed of monomer and LC which transforms into heterogonous system consisting LC droplets embedded in polymer matrix after

Several groups have suggested that to augment the PDLC film properties, we

1.Film preparation conditions, such as, reaction temperature, UV irradiation

3.Physical properties of LC such as optical and dielectric parameters, rotational viscosity, temporal characteristics, molecular dynamics, etc. [76, 119, 120, 123,

To enhance the properties of PDLC films, LC properties are modified suitably by addition of nanoparticles, dyes, polymers and carbon nanotubes. The inclusion of new additives can add new functionalities to the obtained devices. To obtain the desired result from the host LC material, proper selection of size, shape and structure of guest/additive particle is an imperative factor. In has been proved that due to the inherent dipole moment, LC materials respond radically when doped and

The incorporation of small quantity of dye molecules to the LC is one suitable

DPDLC films are also named as guest-host polymer-dispersed liquid crystal (GHPDLC) composite films. The LC phase acts as the host and dye molecules doped in LC act as the guest. Once the geometrically anisotropic dye molecules are added/ dissolved in LC, the long molecular axis of dye molecule tends to align along the LC director; this is called as guest-host interaction. Dyes used for LC media must have high dichroic ratio, high-order parameter, high stability and good compatibility and solubility with LC but not with monomer unit or polymer matrix. It must be a positive type, i.e. the absorption transition dipole is along the long molecular axis. When the polarization of the incident light is perpendicular to the long axis of the dye molecules, the light is weakly absorbed. When the polarization of the incident light is parallel to the long axis of the dye molecules, the light is strongly absorbed, as shown in **Figure 36**. Considering all the above-mentioned points, dichroic azo dyes, which are N]N substance and absorb light of certain wavelength more along

Similar to the PDLC films, the dispersion of the droplets can also be achieved using polymerization-induced phase separation method, in which the dye and LC mixture separate from the polymer binder. These DPDLC films possess controllable

2.Properties of monomer unit such as their structure, functionality,

intensity, curing time and film thickness

solution, which has been studied by several researchers.

**6.5 Operating principle of DPDLC composite films**

one direction than the other, have been used.

**52**

concentration in LC, etc.

anchored with elongated species.

hysteresis effect [143].

*Liquid Crystals and Display Technology*

photopolymerization.

144–149].

can alter:

*A schematic representation of direction dependent absorption of dichroic dye molecule.*

scattering as well as controllable absorbance, modulated by the liquid crystal molecule. The large transition moment of the rod-shaped dye molecules is modulated by the symmetry axis of the LC molecule in the OFF and ON states of electric field [150–153]. In the field "OFF state", dye molecules are randomly oriented along with the LC droplets. The unpolarised incident light is scattered as well as absorbed due to LC droplets and dissolved dye molecules. Due to the scattering of light, the optical path length of light increases, and hence enhancement in absorption has been noticed. In the field "ON state," LC droplets and dissolved dye molecules get aligned along the field direction, making the film feebly absorbing; hence, most of the light is transmitted through the film. **Figure 37** shows the schematic of DPDLC film in the OFF and ON states of applied electric field [61, 125].

LC droplets in all the composite films exhibit predominantly bipolar (LC droplet with cylindrical symmetry axis defined by two-point defects at opposite ends)-type morphology with some radial (LC droplet with spherical symmetry and the only point defect in the volume centre) structures. No preferred orientation of LC droplets was found, which is responsible for the scattering of light. Upon addition of dichroic dye molecule, significant morphological changes arise in LC droplet. The dye, polymer and LC are chosen such that the solubility of dye is high in LC but very low in polymer. **Figure 38** shows POM image of DPDLC film (O36N), prepared using LC BL036 and prepolymer NOA65 in wt/wt% with varying disperse Orange 25 dye concentration. As the dye concentration increases from 0–1%, there is noticeable increase in droplet size from submicron to 10 μ. The reason is dye molecules dissolved in LC absorb some of the UV radiations provided for photopolymerization, which reduces rate of polymerization and in turn influences droplet growth. At lower dye content, faster polymerization rate produces smaller and denser LC droplets. At higher dye content, slow polymerization rate preserves polymer in a liquid state for an extended period, thus allowing the growth and coalescence of small LC droplets to form bigger ones. There is more number of bipolar droplets in the high dye-doped film

show relatively higher affinity for the polymer than the low dye-doped LC molecules and hence facilitating strong anchoring conditions at the interface of polymer

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

**Figure 39** shows effect of electric field on the morphology of DPDLC film O36S prepared using disperse Orange 25 dye, LC BL036 and Prepolymer SAM-114. The LC and prepolymer are taken here in 45/55 wt/wt%. In the ON state, when the electric field is low (10–20 V) (**Figure 39(b)**), there is substantial difference in the director direction of molecules which are near the polymer-LC interface and which are inside the LC droplet, and point defects are still at their initial position. On increasing the voltage (higher than anchoring energy at polymer-LC interface) (30 V and above) (**Figure 39(c)** and **(d)**), small and big LC droplets acquire maltese-type structure and twisted arrangement with director direction parallel to field, respectively. The appearance of maltese crosses is due to the interference (recombination) of the two refracted waves at higher fields [125, 159, 160].

The voltage dependence of transmittance for DPDLC composite film (O00N), prepared using dye disperse Orange 25+ LC HPC850100–100 + prepolymer NOA-65, at a constant frequency of 200 Hz is shown in **Figure 40**. LC and prepolymer are taken in 40/60 wt/wt% ratio, respectively, with dye content varying from 0 to 1%. The parameters such as *T*OFF,*T*ON, *CR*, *ΔT*, *V*TH and *V*ON calculated from graph

From the above graph (**Figure 40**) and **Table 4**, it is clear that the DPDLC film with lowest dye concentration (0.007%) is the optimum one. This film has low value of *T*OFF and high value of *T*ON, which results in high *CR* and high *ΔT*. The value of *V*TH and *V*ON is remarkably low, desired for good EO devices.

*Droplet structure of 0.015% O36S DPDLC film at (a) 0 V, (b) 20 V, (c) 40 V and (d) 60 V.*

and LC molecules [155].

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

(**Figure 40**) are summarized in **Table 4**.

**Figure 39.**

**55**

**Figure 37.** *Schematic of DPDLC in (a) OFF state and (b) ON state.*

**Figure 38.** *POM images of O36N DPDLC film with dye concentrations (a) 0%, (b) 0.015%, (c) 0.25% and (d) 1%.*

(**Figure 38(d)**) showing relatively higher affinity for the polymer and thus aiding strong anchoring at the polymer-LC interface than the low dye-doped composite films. Altogether, the addition of dye increases the viscosity of polymer-LC mixture, especially at low temperatures, which increases droplet size and hence optical path length [143, 154–158].

The high dye-doped PDLC film emerges with more number of bipolar droplets, as shown in **Figure 38(d)**. This can be interpreted as high dye-doped LC molecules

## *An Overview of Polymer-Dispersed Liquid Crystal Composite Films and Their Applications DOI: http://dx.doi.org/10.5772/intechopen.91889*

show relatively higher affinity for the polymer than the low dye-doped LC molecules and hence facilitating strong anchoring conditions at the interface of polymer and LC molecules [155].

**Figure 39** shows effect of electric field on the morphology of DPDLC film O36S prepared using disperse Orange 25 dye, LC BL036 and Prepolymer SAM-114. The LC and prepolymer are taken here in 45/55 wt/wt%. In the ON state, when the electric field is low (10–20 V) (**Figure 39(b)**), there is substantial difference in the director direction of molecules which are near the polymer-LC interface and which are inside the LC droplet, and point defects are still at their initial position. On increasing the voltage (higher than anchoring energy at polymer-LC interface) (30 V and above) (**Figure 39(c)** and **(d)**), small and big LC droplets acquire maltese-type structure and twisted arrangement with director direction parallel to field, respectively. The appearance of maltese crosses is due to the interference (recombination) of the two refracted waves at higher fields [125, 159, 160].

The voltage dependence of transmittance for DPDLC composite film (O00N), prepared using dye disperse Orange 25+ LC HPC850100–100 + prepolymer NOA-65, at a constant frequency of 200 Hz is shown in **Figure 40**. LC and prepolymer are taken in 40/60 wt/wt% ratio, respectively, with dye content varying from 0 to 1%. The parameters such as *T*OFF,*T*ON, *CR*, *ΔT*, *V*TH and *V*ON calculated from graph (**Figure 40**) are summarized in **Table 4**.

From the above graph (**Figure 40**) and **Table 4**, it is clear that the DPDLC film with lowest dye concentration (0.007%) is the optimum one. This film has low value of *T*OFF and high value of *T*ON, which results in high *CR* and high *ΔT*. The value of *V*TH and *V*ON is remarkably low, desired for good EO devices.

**Figure 39.** *Droplet structure of 0.015% O36S DPDLC film at (a) 0 V, (b) 20 V, (c) 40 V and (d) 60 V.*

(**Figure 38(d)**) showing relatively higher affinity for the polymer and thus aiding strong anchoring at the polymer-LC interface than the low dye-doped composite films. Altogether, the addition of dye increases the viscosity of polymer-LC mixture, especially at low temperatures, which increases droplet size and hence opti-

*POM images of O36N DPDLC film with dye concentrations (a) 0%, (b) 0.015%, (c) 0.25% and (d) 1%.*

The high dye-doped PDLC film emerges with more number of bipolar droplets, as shown in **Figure 38(d)**. This can be interpreted as high dye-doped LC molecules

cal path length [143, 154–158].

**Figure 38.**

**54**

**Figure 37.**

*Schematic of DPDLC in (a) OFF state and (b) ON state.*

*Liquid Crystals and Display Technology*

moment of LC droplet due to addition of dye molecules, contributing in LC droplet quick orientation, hence low rise time. With the further increase in dye concentration, rise time increases because of increase in viscosity of LC+ dye mixture. Altogether, at higher dye concentration, LC molecules expel dye towards polymer surface creating additional surface anchoring. Upon removal of field, LC droplets of low dye concentration DPDLC film quickly reorients as compared to the un-doped film. In high dye concentration DPDLC film, LC droplets took more time to reorient because of the additional surface anchoring between dye-doped LC droplet and polymer walls. During application of field, charge gets stored in dye-doped LC droplet, which serves as capacitor even after removal of external field affecting

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

For the analysis of experimental data and qualitative evaluation of distribution of relaxation time, Cole-Cole plot is drawn for imaginary part <sup>ε</sup>″ vs. real part <sup>ε</sup><sup>0</sup> of

100 + prepolymer NOA65) DPDLC film, Cole-Cole plot is shown in **Figure 41**. Dielectric parameters calculated using Cole-Cole plot are summarized in **Table 6**. The zero value of *α* listed in **Table 6** clearly shows that DPDLC films show Debye type behaviour. The 0.007% DPDLC film has very high value of dielectric strength.

the dielectric constant. For O00N (dye Orange 25+ LC HPC850100–

*Cole-Cole plots of various dye concentrations O00N DPDLC films at 25°C temperature.*

*Fitting parameters of various dye concentrations O00N DPDLC films.*

**Dye concentration (wt%)** *f* **(MHz)** *ε<sup>s</sup> ε***<sup>∞</sup>** *δε*<sup>0</sup> *τ* **(s)** *α* **0.0** 15.1 11.66 2.77 8.89 1.05 E�<sup>8</sup> 0 **0.007 15.1 17.17 2.15 15.02 1.05 E**�**<sup>8</sup> 0 0.015** 17.4 15.36 1.53 13.83 9.15 E�<sup>9</sup> 0 **0.0625** 15.1 14.03 2.34 11.69 1.05 E�<sup>8</sup> 0 **0.25** 15.1 17.91 1.89 16.02 1.05 E�<sup>8</sup> 0 **1** 13.2 18.96 1.38 17.58 1.21 E�<sup>8</sup> 0

relaxation time of LC droplet.

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

**Figure 41.**

**Table 6.**

**57**

#### **Figure 40.**

*Transmittance vs. voltage curve of various dye concentrations O00N DPDLC films at 200 Hz frequency and 25°C temperature.*


#### **Table 4.**

*Voltage-transmittance data of various dye concentrations O00N DPDLC films.*

The variation in rise time and decay time of DPDLC composite films (O36S) prepared using LC BL036+ Prepolymer SAM-114 in 45/55 wt/wt% ratio and doped with disperse orange dye, at 40 V, is given in **Table 5**.

Generally, the rise time of all composite films decreases with the increase in voltage, whereas decay time is independent of voltage. This observed phenomenon is in good agreement with Eqs. (27) and (28). However, sometimes, there is small increase in decay time with voltage indicating longer transparent state even after field removal. Also, at higher voltages, rise time is of the order of microseconds indicating quick switching of the films. **Table 5** indicates lower value of rise time for low dye concentration DPDLC film. The proposed reason is increased dipole


**Table 5.**

*Rise time and decay time of DPDLC film O36S at 40 V.*

### *An Overview of Polymer-Dispersed Liquid Crystal Composite Films and Their Applications DOI: http://dx.doi.org/10.5772/intechopen.91889*

moment of LC droplet due to addition of dye molecules, contributing in LC droplet quick orientation, hence low rise time. With the further increase in dye concentration, rise time increases because of increase in viscosity of LC+ dye mixture. Altogether, at higher dye concentration, LC molecules expel dye towards polymer surface creating additional surface anchoring. Upon removal of field, LC droplets of low dye concentration DPDLC film quickly reorients as compared to the un-doped film. In high dye concentration DPDLC film, LC droplets took more time to reorient because of the additional surface anchoring between dye-doped LC droplet and polymer walls. During application of field, charge gets stored in dye-doped LC droplet, which serves as capacitor even after removal of external field affecting relaxation time of LC droplet.

For the analysis of experimental data and qualitative evaluation of distribution of relaxation time, Cole-Cole plot is drawn for imaginary part <sup>ε</sup>″ vs. real part <sup>ε</sup><sup>0</sup> of the dielectric constant. For O00N (dye Orange 25+ LC HPC850100– 100 + prepolymer NOA65) DPDLC film, Cole-Cole plot is shown in **Figure 41**. Dielectric parameters calculated using Cole-Cole plot are summarized in **Table 6**. The zero value of *α* listed in **Table 6** clearly shows that DPDLC films show Debye type behaviour. The 0.007% DPDLC film has very high value of dielectric strength.

**Figure 41.** *Cole-Cole plots of various dye concentrations O00N DPDLC films at 25°C temperature.*


#### **Table 6.**

*Fitting parameters of various dye concentrations O00N DPDLC films.*

The variation in rise time and decay time of DPDLC composite films (O36S) prepared using LC BL036+ Prepolymer SAM-114 in 45/55 wt/wt% ratio and doped

*Transmittance vs. voltage curve of various dye concentrations O00N DPDLC films at 200 Hz frequency and*

**Dye concentration (wt%)** *T***OFF (%)** *T***ON (%) Δ***T* **(%) CR** *V***TH** *V***ON 0.0** 1.02 91.43 90.41 90 3.5 40 **0.007 0.41 98.98 98.57 241 1.1 40 0.015** 0.51 98.37 97.86 193 1 40 **0.0625** 7.86 97.96 90.1 12 1.5 29 **0.25** 15.52 97.96 82.44 6 1.3 33 **1** 33.16 97.40 64.24 3 1.5 29

Generally, the rise time of all composite films decreases with the increase in voltage, whereas decay time is independent of voltage. This observed phenomenon is in good agreement with Eqs. (27) and (28). However, sometimes, there is small increase in decay time with voltage indicating longer transparent state even after field removal. Also, at higher voltages, rise time is of the order of microseconds indicating quick switching of the films. **Table 5** indicates lower value of rise time for

low dye concentration DPDLC film. The proposed reason is increased dipole

**Dye concentration (wt%) Rise time (ms) Decay time (ms)** 0 0.4 6.8 0.007 0.2 6.6 0.015 0.3 6.7 1 0.8 8.3

with disperse orange dye, at 40 V, is given in **Table 5**.

*Rise time and decay time of DPDLC film O36S at 40 V.*

*Voltage-transmittance data of various dye concentrations O00N DPDLC films.*

*The significance of bold values are the optimum parameters for the given film.*

**Figure 40.**

**Table 4.**

**Table 5.**

**56**

*25°C temperature.*

*Liquid Crystals and Display Technology*

In order to improve the performance of PDLC films, similar to DPDLC films, LC can be doped with nano-particles or carbon nanotubes. One such example is multiwalled carbon nanotubes (MWCNT)-doped PDLC (CPDLC) film, which is discussed here.

## **6.6 Operating principle of CPDLC composite films**

Similar to the DPDLC composite films, in CPDLC films, LC phase acts as the host and MWCNT doped in LC act as the guest entity. MWCNTs are physically and environmentally stable, mechanically strong, chemically inert and thermally and electrically conducting material with high aspect ratio. MWCNT is also selforganizing, like LC material, but strong attractive Van der Waals forces between adjacent MWCNT lead them to cluster and form unorganized bundles [161]. It is very difficult to disperse MWCNT in a medium, but small percentage of MWCNT can be dispersed well in LC fluid. Till date any experimental confirmations regarding the interaction between MWCNT and LC are unavailable, but on analyzing their structure, π-π interaction (aromatic interaction) between MWCNT walls and phenyl rings of LC molecules is evident. As MWCNT is insoluble in LC, this type of interaction is quite weak to cause any LC director deformation. The well-dispersed MWCNT are generally orientated with their cylindrical axis parallel to the director direction of nematic LC. After the sufficiently stable dispersion of MWCNT in LC, both host LC and guest MWCNT share their intrinsic properties with each other, which are listed below:

**Figure 42.**

**Figure 43.**

*films.* **59**

*Cavities formed after removal of LC from (a) 0%, (b) 0.005%, (c) 0.05% and (d) 0.5%, C00N CPDLC*

*Schematic of CPDLC in (a) OFF state and (b) ON state.*

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

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


Because of this sharing of inherent properties with each other, properties of CPDLC films get affected significantly. Similar to the DPDLC films, in CPDLC films also the dispersion of the LC droplets has been achieved using PIPS method, but in CPDLC films even after completion of polymerization process, MWCNT are found to be well separated entity from LC droplets embedded in polymer matrix. Since MWCNT do not absorb any light, hence operating principle of CPDLC films is based only on controllable scattering of light from randomly dispersed LC droplets. In the absence of field, i.e. "OFF state", MWCNT and LC droplets are separately and randomly oriented inside polymer matrix as shown in **Figure 42(a)**. The unpolarised incident light is scattered because of LC droplets. Upon application of field, i.e. "ON state", LC droplets get aligned along the direction of field, and MWCNT also get partially oriented as shown in **Figure 42(b)** and may form conducting channel at higher MWCNT concentration.

**Figure 43(a)**–**(d)** shows SEM images of some representative CPDLC films (C00N) prepared using LC HPC850100–100 and prepolymer NOA-65 doped with MWCNT concentration 0, 0.005, 0.05 and 0.5%. Here, LC and prepolymer are taken in 60/40 wt/wt% ratio. As MWCNT are insoluble in LC and do not participate in photopolymerization kinetics, therefore, the size of the LC droplets remains invariant upon addition of MWCNT. On analysing SEM images of 0% and 0.005% CPDLC films (**Figure 43(a)** and **(b)**), it is clear that the size of cavities is of few microns. Increase in MWCNT concentration (0.05 and 0.5% CPDLC film) does not affect the size of LC droplets but because of clustering of MWCNT, few bigger size cavities are formed as shown in **Figure 43(c)** and **(d)**.

*An Overview of Polymer-Dispersed Liquid Crystal Composite Films and Their Applications DOI: http://dx.doi.org/10.5772/intechopen.91889*

**Figure 42.** *Schematic of CPDLC in (a) OFF state and (b) ON state.*

In order to improve the performance of PDLC films, similar to DPDLC films, LC can be doped with nano-particles or carbon nanotubes. One such example is multiwalled carbon nanotubes (MWCNT)-doped PDLC (CPDLC) film, which is

Similar to the DPDLC composite films, in CPDLC films, LC phase acts as the host and MWCNT doped in LC act as the guest entity. MWCNTs are physically and environmentally stable, mechanically strong, chemically inert and thermally and electrically conducting material with high aspect ratio. MWCNT is also selforganizing, like LC material, but strong attractive Van der Waals forces between adjacent MWCNT lead them to cluster and form unorganized bundles [161]. It is very difficult to disperse MWCNT in a medium, but small percentage of MWCNT can be dispersed well in LC fluid. Till date any experimental confirmations regarding the interaction between MWCNT and LC are unavailable, but on analyzing their structure, π-π interaction (aromatic interaction) between MWCNT walls and phenyl rings of LC molecules is evident. As MWCNT is insoluble in LC, this type of interaction is quite weak to cause any LC director deformation. The well-dispersed MWCNT are generally orientated with their cylindrical axis parallel to the director direction of nematic LC. After the sufficiently stable dispersion of MWCNT in LC, both host LC and guest MWCNT share their intrinsic properties with each other,

1.Due to the dipolar nature of LC material, asymmetric charges are induced on

Because of this sharing of inherent properties with each other, properties of CPDLC films get affected significantly. Similar to the DPDLC films, in CPDLC films also the dispersion of the LC droplets has been achieved using PIPS method, but in CPDLC films even after completion of polymerization process, MWCNT are found to be well separated entity from LC droplets embedded in polymer matrix. Since MWCNT do not absorb any light, hence operating principle of CPDLC films is based only on controllable scattering of light from randomly dispersed LC droplets. In the absence of field, i.e. "OFF state", MWCNT and LC droplets are separately and randomly oriented inside polymer matrix as shown in **Figure 42(a)**. The unpolarised incident light is scattered because of LC droplets. Upon application of field, i.e. "ON state", LC droplets get aligned along the direction of field, and MWCNT also get partially oriented as shown in **Figure 42(b)** and may form

**Figure 43(a)**–**(d)** shows SEM images of some representative CPDLC films (C00N) prepared using LC HPC850100–100 and prepolymer NOA-65 doped with MWCNT concentration 0, 0.005, 0.05 and 0.5%. Here, LC and prepolymer are taken in 60/40 wt/wt% ratio. As MWCNT are insoluble in LC and do not participate in photopolymerization kinetics, therefore, the size of the LC droplets remains invariant upon addition of MWCNT. On analysing SEM images of 0% and 0.005% CPDLC films (**Figure 43(a)** and **(b)**), it is clear that the size of cavities is of few microns. Increase in MWCNT concentration (0.05 and 0.5% CPDLC film) does not affect the size of LC droplets but because of clustering of MWCNT, few bigger size

2.Nematic LC solvent provides partial orientational order to MWCNT.

3.MWCNT impart their electrical conductivity to the LC molecules.

MWCNT producing permanent dipole moment on MWCNT.

conducting channel at higher MWCNT concentration.

cavities are formed as shown in **Figure 43(c)** and **(d)**.

**58**

**6.6 Operating principle of CPDLC composite films**

discussed here.

*Liquid Crystals and Display Technology*

which are listed below:

#### **Figure 43.**

*Cavities formed after removal of LC from (a) 0%, (b) 0.005%, (c) 0.05% and (d) 0.5%, C00N CPDLC films.*

The voltage dependence of output transmittance for CPDLC films with MWCNT content varying from 0 to 0.5% is shown in **Figure 44**. The parameters such as *T*OFF,*T*ON, CR, Δ*T*, *V*TH and *V*ON calculated from this graph are summarized in **Table 7**.

From the above graph (**Figure 44**) and **Table 7**, it is clear that the CPDLC film with lowest MWCNT concentration (0.005%) is optimum one. This film has low value of *T*OFF and high value of *T*ON, which results in high *CR* and high *ΔT*. The value of *V*TH and *V*ON is remarkably low, desired for good EO devices.

In order to understand the effect of temperature on other dielectric parameters such as dielectric strength and relaxation process, Cole-Cole plot can be drawn between real and imaginary parts of complex dielectric constant. **Figure 45** shows Cole-Cole plots of 0.005% MWCNT-doped C36N CPDLC film (LC BL036 and prepolymer NOA-65 in 50/50 wt/wt % ratio) at different temperatures. It is clear from graphs that the value of dielectric strength {difference of relative dielectric constant at static (at 20 Hz) and optical (at relaxation) frequency *f*} increases up to 40°C and then starts decreasing because small increase in temperature weakens intermolecular interaction and hence relaxes orientation-reorientation process of dipoles, whereas at high temperatures, thermal agitation becomes more predominant than intermolecular interaction which produces randomization of dipoles.

The value of distribution parameter *α* (not shown here) calculated for all Cole-Cole plots drawn for CPDLC films at different temperatures was zero, revealing Debye

*Cole-Cole plots of 0.005% MWCNT concentration C36N, CPDLC film at different temperatures.*

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

The phenomenal optical and dielectric anisotropy of LC has been exploited in various display devices/LC technologies, and one such example is polymer-dispersed liquid crystal films. To improve the optical efficiency of PDLC device, with reduced operating voltages, faster switching time and high image contrast, properties of LC have been modified by doping it with some foreign entity. To obtain the desired result from the host LC material, proper selection of size, shape and structure of guest is an imperative factor. In all cases it has been proven that LC responds radically when doped and anchored to elongated species due to their inherent dipole moment. Therefore, host LC can be doped with dichroic dye guest molecules. Dye molecules tend to line up with the LC director, and dye absorbance is modulated by the alignment of nematic director with an external electric field. The controlled absorption and scattering of light through these materials make them promising candidates for various potential devices. Also the self-organizing properties of nematic LCs can be used to align carbon nanotubes (CNT) dispersed in them. CNT not only well integrate in the matrix but also, even at very low concentration, have a detectable effect on the LC properties that can be very attractive for display applications.

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

type relaxation [162].

**Figure 45.**

**61**

**6.7 Conclusions of PDLC study**

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

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

#### **Figure 44.**

*Transmittance vs. voltage curve of various MWCNT concentrations C00N, CPDLC films at 200 Hz frequency and 25°C temperature.*


**Table 7.**

*Voltage-transmittance data of various MWCNT concentrations C00N CPDLC films.*

*An Overview of Polymer-Dispersed Liquid Crystal Composite Films and Their Applications DOI: http://dx.doi.org/10.5772/intechopen.91889*

**Figure 45.** *Cole-Cole plots of 0.005% MWCNT concentration C36N, CPDLC film at different temperatures.*

The value of distribution parameter *α* (not shown here) calculated for all Cole-Cole plots drawn for CPDLC films at different temperatures was zero, revealing Debye type relaxation [162].

#### **6.7 Conclusions of PDLC study**

The voltage dependence of output transmittance for CPDLC films with MWCNT content varying from 0 to 0.5% is shown in **Figure 44**. The parameters such as *T*OFF,*T*ON, CR, Δ*T*, *V*TH and *V*ON calculated from this graph are summarized

value of *V*TH and *V*ON is remarkably low, desired for good EO devices.

From the above graph (**Figure 44**) and **Table 7**, it is clear that the CPDLC film with lowest MWCNT concentration (0.005%) is optimum one. This film has low value of *T*OFF and high value of *T*ON, which results in high *CR* and high *ΔT*. The

In order to understand the effect of temperature on other dielectric parameters such as dielectric strength and relaxation process, Cole-Cole plot can be drawn between real and imaginary parts of complex dielectric constant. **Figure 45** shows Cole-Cole plots of 0.005% MWCNT-doped C36N CPDLC film (LC BL036 and prepolymer NOA-65 in 50/50 wt/wt % ratio) at different temperatures. It is clear from graphs that the value of dielectric strength {difference of relative dielectric constant at static (at 20 Hz) and optical (at relaxation) frequency *f*} increases up to 40°C and then starts decreasing because small increase in temperature weakens intermolecular interaction and hence relaxes orientation-reorientation process of dipoles, whereas at high temperatures, thermal agitation becomes more predominant than intermolecular interaction which produces randomization of dipoles.

*Transmittance vs. voltage curve of various MWCNT concentrations C00N, CPDLC films at 200 Hz frequency*

**MWCNT concentration (wt %)** *T***OFF (%)** *T***ON (%) Δ***T* **(%) CR** *V***TH** *V***ON 0.0** 1.02 91.42 90.4 90 3.5 40 **0.005 0.71 97.02 96.31 137 0.1 30 0.01** 3.06 93.46 90.4 31 0.4 24 **0.05** 7.14 41.83 34.69 6 1.0 32 **0.1** 10.71 33.36 22.65 3 1.3 20 **0.5** 15.3 21.42 6.12 1 7.4 30

*Voltage-transmittance data of various MWCNT concentrations C00N CPDLC films.*

in **Table 7**.

*Liquid Crystals and Display Technology*

**Figure 44.**

**Table 7.**

**60**

*and 25°C temperature.*

The phenomenal optical and dielectric anisotropy of LC has been exploited in various display devices/LC technologies, and one such example is polymer-dispersed liquid crystal films. To improve the optical efficiency of PDLC device, with reduced operating voltages, faster switching time and high image contrast, properties of LC have been modified by doping it with some foreign entity. To obtain the desired result from the host LC material, proper selection of size, shape and structure of guest is an imperative factor. In all cases it has been proven that LC responds radically when doped and anchored to elongated species due to their inherent dipole moment. Therefore, host LC can be doped with dichroic dye guest molecules. Dye molecules tend to line up with the LC director, and dye absorbance is modulated by the alignment of nematic director with an external electric field. The controlled absorption and scattering of light through these materials make them promising candidates for various potential devices. Also the self-organizing properties of nematic LCs can be used to align carbon nanotubes (CNT) dispersed in them. CNT not only well integrate in the matrix but also, even at very low concentration, have a detectable effect on the LC properties that can be very attractive for display applications.
