Chapter 2 Principle
2.3 Polymer-dispersed Liquid Crystals (PDLC)
2.3.4 Two-dimensional Radially Stretched PDLC Film
Since the transmission axis of the stretched PDLC films is normal to the stretching direction, special polarizers can be obtained by applying strain in different directions on the PDLC films. The azimuthal polarizer which can be used as a polarization axis finder is fabricated by radially stretching of the film. Thus the transmission axis is along the azimuthal direction, and the unpolarized light can be converted into the state of azimuthal polarization, as shown in Fig. 23.
Fig. 23 Mechanism of an azimuthal polarizer by radially stretching of PDLC film
Chapter 3
Fabrication and Measurement Instruments
3.1 Preparation of Stretched PDLC Films
3.1.1 Fabrication of PDLC Films
PDLC films were prepared by encapsulation method in this research[16]. The system was heterogeneous during the whole fabrication process. Liquid crystal was dispersed in a polymer solution , the solvent of which did not dissolve liquid crystal.
Polyvinyl-alcohol (PVA) was chosen as the PDLC binder. The solvent evaporation stabilized the obtained composite structure due to polymer solidification.
The detailed steps of fabrication are listed below, and the preparation of the PDLC films is shown schematically in Fig. 24.
(a) PDLC solution preparation:
(1) PVA, a water-soluble polymer, was used as the solvent. The nematic liquid crystal E7 (Merck Display Technology, Ltd.) was mixed with a 20 wt% aqueous solution of PVA (PVA 81381, molecular weight 31000, Fluka Analytical) , and the liquid crystal concentration in the PDLC films ( PVA with E7) was set 20-45 wt %.
(2) The solution was then emulsified by agitators, and the bubbles in the solution were driven out by soaking the beaker containing the solution into a ultrasonic
tank.
(b) Film process:
(1) The emulsion was coated on a polyethylene terephthalate (PET) substrate using a Meyer Bar (coating rod) driven by hands.
(2) The thin film was dried in the sweatbox to have the water evaporated from the film surface.
(3) The dried PDLC films were peeled from the PET substrate. The film thickness was 8 to 24 µm depending on the wire size of the Meyer Bar.
(4) The samples were cut into the H-shape according to the ASTM Standard D 1708 (American Standards for Testing and Materials) and one-dimensionally stretched by a micro-tensile tester with the drawing rate of 0.5 mm/min.
The refraction indices of E7 and PVA is shown in Table 2.
Tab. 2 Refraction indices of E7 and PVA
(a)
(b)
(c)
(d)
Fig. 24 Flow of preparation of PDLC films (a) the emulsion applied on the PET substrate (b) Meyer Bar coating (c) the film dried in sweatbox (d) the film one-dimensionally stretched by micro-tensile tester
3.1.2 Real-time Measurement Set-up of Optical Properties
The measurement of optical transmittance under different strain as defined in Eq.
(12) can be achieved by a real-time measurement set-up as shown in Fig. 25 and 26. The unstretched PDLC film were cut in the shape defined by the American Society for Testing and Materials (ASTM) Standard D 1708, as shown in Fig. 27. The sample was connected to a stress sensor, with a Helium-Neon (He-Ne) laser operating at 633 nm as the light source directed at normal incidence on it. A rotating polarizer was set between the laser and the sample, and polarizations parallel and perpendicular to the stretching direction were investigated. The sample was then one-dimensionally stretched by the micro-tensile tester with the draw rate of 0.5 mm/min. The intensity of the transmitted light was detected by a photodetector while the sample is simultaneously stretched.
length film
Unstretced
n deformatio
Strain= Film (12)
Fig. 25 Schematic representation of real-time measurement set-up
Fig. 26 The pictures of real-time measurement set-up
Fig. 27 Samples cut in the shape defined by ASTM standard D 1708
3.1.3 Radially Stretching Process
In the former section, the PDLC films were one-dimensional linearly stretched to obtain linear polarizer. Polarizers with specific polarization function can be achieved by varying the stretching direction. Here the azimuthal polarizer was obtained by radially stretching of the PDLC film.
The steps of fabrication are listed below, and the set-up of radially stretching is shown in Fig. 28.
Power-meter Load Cell
Movable Clips Polarizer
He-Ne Laser
(1) The PDLC films whose concentration of E7 is 25 wt% was cut into ‘donut’
shape with radius of 16 mm and attached to the film holder. An o-ring was used at the center of the sample to prevent the sample from breaking.
(2) A screw across the central hole of the sample was attached to the micro-tensile tester.
(3) The samples were radially stretched by applying strain at the center of the samples, and the drawing rate of the micro-tensile tester was 0.5 mm/min.
Fig. 28 The set-up of radially stretching process
3.2 FT-IR Spectrometer
A Fourier transform infrared spectrometer (FT-IR) (Nicolet 380, Thermo Electron) with a rotating polarizer, as shown in Fig. 29, is used to investigate the macroscopic orientation of the liquid crystal directors.
Fig. 29 Inside layout of the FT-IR Spectrometer
The C≡N band is representative functional group in E7. When the light is polarized in the direction parallel to the optical axes of E7, a specific wave band at wavenumber 2230 cm-1 will be absorbed by the C≡N band .The orientation of E7 can be represented by the ordering parameter S defined by Eq. (13)
2
where θ is the angle between the optical axes of E7 and the stretching direction. S is converted into Eq. (14) by infrared dichroism technique [17].
2
where A∥ and A⊥ are the absorbances of the C≡N band of E7 at 2230 cm-1, with the infrared beam polarized parallel and perpendicular to the stretching direction of the film.
A sample of the IR dichroism in Fig. 30 is indicative of a macroscopic orientation of E7 aligned in the stretching direction where A∥ is greater than A⊥.
Fig. 30 Polarized infrared spectra of a stretched PDLC film (PVA/E7) with the polarizations of the incident beam parallel and perpendicular to the stretching direction
Chapter 4
Experimental Results and Discussion
4.1 Introduction
The scattering polarizers with polarization recycling can be employed in liquid crystal displays instead of the conventional absorbing polarizers to improve the optical efficiency as mentioned before. Because the scattering polarizers are relatively simple in fabrication process, which can be a potential candidate in portable LCDs where power saving is one of the key issues. Stretching of the PDLC films is one of the most effective way to fabricate scattering polarizers. Besides, the azimuthal polarizer which converts the unpolarized light into azimuthal polarization can be achieved by two-dimensional radially stretching of the PDLC films instead of one-dimensional linearly stretching. A study of the optical properties under different strain will be investigated in this chapter, and the experimental results will discussed.
4.2 Optical Properties of Stretched PDLC Films
4.2.1 Elongated LC Droplets in Stretched PDLC Films
The PDLC films were stretched under different strain where the liquid crystal droplets were elongated and attained different range of deformation as shown in Fig. 31 and Table 3. The liquid crystal droplet were on the order of several microns as indicated in the optical microscope image. As the films were stretched longer, the aspect ratios of the liquid crystal droplets increased, and the droplets were aligned along the stretching direction.
(a) (b)
(c) (d)
(e) (f)
Fig. 31 The PDLC films stretched under the strains of (a) 0% (unstretched) (b) 20% (c) 30% (d) 40% (e) 50% (f) 70%
Tab. 3 Strain and range of deformation of the stretched PDLC films
Strain (%) Range of deformation
0 1 : 1
20 1.6-2 : 1
30 1.6-2 : 1
40 2 : 1
50 2.5-2.8 : 1
70 3-4 : 1
4.2.2 Dependence of LC Concentration on Optical Properties
The dependence of LC concentration on transmittance under different strain was discussed. The film thickness was fixed at 10 µm in this experiment. The concentration of the liquid crystal (E7) in the PDLC films were adjusted, and the optical transmittances under different strain were measured while the PDLC films were stretched. The measurement results are shown in Fig. 32 where T∥ and T⊥ are the transmittances of the PDLC films, with the parallel and perpendicular polarization state to the stretching direction of the film. When the concentration of E7 was 20 wt%, most of the light transmitted at the normal direction without being scattered by the liquid crystal droplets, and the transmittance was highest among all the samples. The highest T∥ (50%) was achieved under the strain of 30%. The defects which occurred on the surfaces of the films occasionally due to air bubbles in the films as the stain was higher than 50% would result in intensive and predominantly forward scattering for both ∥ and ⊥ polarizations and strongly altered T∥ and T⊥ accordingly.
Fig. 32 The strain-transmittance curve of the PDLC films with different concentration of E7
In order to further evaluate the film quality, the extinction ratio was defined as
1 2
T ratio T
Extinction = (15)
where T1 (T⊥) was the transmittance with polarization parallel to the transmission axis, and T2 (T∥) was the transmittance with polarization perpendicular to the transmission axis. An ideal polarizer has extinction ration = 0. For real polarizers, extinction ratio is always larger than 0. The extinction ratio properties of the PDLC films with different
concentration of E7 are shown in Fig. 33. The extinction ratio dropped more rapidly as the concentration of E7 was over 25 wt% than it was 20 wt%. When the concentration of E7 was 25-45 wt%, the extinction ratio was lower than 0.1. Thus a sample with the concentration of E7 over 25 wt% had better extinguishing efficiency. Considering both transmittance and extinction, the sample had better performance with 25 wt% of E7.
Fig. 33 The extinction ratio properties of the PDLC films with different concentration of E7
4.2.3 Effect of Relative Humidity during Drying Process
The optical performance of the films was unstable when the films are dried in ambient condition. The behavior of the relative humidity which was a parameter of evaporation of water from the film surface was investigated. The concentration of E7 was fixed at 25 wt%. The definition of the relative humidity is the ratio of the partial
pressure of water vapor in air-water mixture to the saturated vapor pressure of water at a prescribed temperature. The relative humidity was controlled while the thin film was dried in the sweatbox. After the film was dried and peeled off from the substrate, the optical transmittance was measured during the stretching process, and the measurement result is shown in Fig. 34 and 35. The transmittance and extinction ratio of the films dried under different relative humidity were almost equal. It can be concluded that only the evaporation rate was steadily controlled by keeping the same relative humidity during the drying process, the optical properties of the film can be kept unison. In the following experiment, 50% of relative humidity was chosen as the experimental parameter since it was close to the environmental condition of the laboratory and easy to be controlled.
Fig. 34 The strain-transmittance curve of the PDLC films dried under the condition of different relative humidity
Fig. 35 The extinction ratio properties of the PDLC films dried under the condition of different relative humidity
4.2.4 Dependence of Film Thickness on Optical Properties
The film thickness is an important parameter of transmittance. In this experiment, the film thickness was controlled by the diameter of the stainless wires on the Meyer Bar and had a range of 8-24 µm. The concentration of E7 was fixed at 25 wt%, and the relative humidity was controlled at 50% during drying process. The dependence of film thickness on transmittance and extinction ratio under different strain is shown in Fig. 36 and 37.
Fig. 36 The strain-transmittance curve of the PDLC films with different film thickness
Fig. 37 The extinction ratio properties of the PDLC films with different film thickness
The thinner film had higher transmittance while the extinction ratio was comparable.
However, the film will break easily during the stretching process when the film thickness is under 8 µm for our experience. In order to keep robust mechanical property of the stretched film, the thickness of the thin film is supposed to be higher than 8 µm.
4.3 Ordering Parameter of Liquid Crystal
The alignment of liquid crystal molecules in the droplets can be represented by the ordering parameter of liquid crystal which is derived from anisotropic absorption of the representative C≡N band of E7 as Eq. (14). The incident beam with polarization parallel or perpendicular to the stretching direction of the film was detected by the FT-IR spectrometer, and the absorbance of the C≡N band of E7 at 2230 cm-1 was used to calculate the ordering parameter as shown in Table 4. Since the ordering parameter stopped increasing when the strain arrived at 30%, the alignment of liquid crystal molecules in the droplet had also completed at 30% strain.
Tab. 4 Ordering parameter of liquid crystal under different strain
Strain (%) A|| A⊥ Ordering Parameter
20% 0.889 0.835 0.0209
30% 0.943 0.853 0.0337
40% 0.937 0.849 0.0335
50% 0.939 0.854 0.0321
4.4 Function of Azimuthal Polarizer
In order to obtain azimuthal polarizers, the fabrication process was extended from one-dimensional linearly stretching to two-dimensional radially stretching. The
concentration of E7 was fixed at 25 wt%, and the film was dried under the relative humidity of 50%. The strain in the radially stretching process is defined as Eq. (16), and the parameters of Eq. (16) are shown in Fig. 38.
2
Fig. 38 Definition of the strain in the radially stretching process
The sample was stretched until the strain reach 100% and measured by the polarization testing set-up, as shown in Fig. 39, right after the stretching process. The Helium-Neon (He-Ne) laser operating at 633 nm was used as the light source. The polarization of the incident beam can be controlled to be radial or azimuthal polarization by the radial polarization converter. The incident beam was detected by the CCD camera after passing through the sample. The CCD image appeared in bright state when the incident beam polarized in the azimuthal direction, as shown in Fig. 40. When the incident beam was radially polarized, the CCD image changed from bright state to dark state which proved the function of the azimuthal polarizer achieved by two-dimensional radially stretching.
Fig. 39 The polarization testing set-up
(a) (b)
Fig. 40 CCD images with (a) azimuthally polarized (b) radially polarized incident beam
4.5 Discussion
As the measurement results mentioned before, the transmittance at the transmission axis starts to decrease when the strain ratio is over 30% which agrees with the measurement results of the ordering parameter of the liquid crystal. However, the range of deformation of the liquid crystal droplets keeps increasing with the strain ratio. This implies the alignment of the liquid crystal molecules in the liquid crystal droplets has completed at 30% strain ratio ,and the effect of the strain arising from the stretching process has saturated.
Besides, the detected highest transmittance at transmission axis is 50% since part of light is scattered to large angle and not detected by the detector located in the normal direction. The light scattering results from the surface roughness of the film, the misalignment of liquid crystal droplets and the existence of the ‘anomalous’ droplets as shown in Fig. 41. Even the film is stretched under high strain, part of liquid crystal droplets are not aligned in the stretching direction which causes the light polarized in the direction of transmission axis to be scattered, as shown in Fig. 41(a). At the same time, the portions of the bipolar configuration in a portion of the droplets do not coincide with the major axis of ellipsoidal cavities. The possible portions of the poles of the bipolar configuration in this case are schematically shown in Fig. 41(b). This kind of liquid crystal droplets also lead to the light scattering to large angle.
(a) (b)
Fig. 41 Schematic representation of (a) the typical orientation of the bipolar director configuration, and (b) the orientation of the bipolar configuration in an ‘anomalous ’ droplet
Chapter 5
Conclusions
5.1 Conclusion
The function of the one-dimensional linearly stretched PDLC films as scattering polarizers has been demonstrated in this thesis. We found that in addition to distortion of droplet shape the polymer orientation during stretching process also contributes to the liquid crystal molecule alignment within the droplets. By simultaneously measuring the strain characteristics and transmittance at transmission or forbidden axis of PDLC films, the relationship between the polarization properties and strain properties of PDLC scattering polarizers has been determined. The film performed 50% transmittance at transmission axis and 0.06 extinction ratio has been demonstrated with determined fabrication parameters as shown in Table 5. Moreover, the azimuthal polarizer can achieved by 2D radial stretching process. The function of azimuthal polarizers has been demonstrated by the polarization testing set-up.
Tab. 5 Fabrication parameters of stretched PDLC films
Concentration of LC (E7) 25 wt%
Strain ratio 30%
Relative humidity 50%
Thickness of PDLC film 8 um
5.2 Future Works
The stretched PDLC films which have polarization selectivity can be successfully fabricated by mentioned fabrication process. In order to meet the size of LCDs, the working area of the film will be expanded, and the uniformity should be kept to ensure the optical performance. In order to further control the droplet size which is a key parameter of scattering behavior, the preparation process will be changed to phase separation method instead of encapsulation. The optical properties with oblique incident beam will also be investigated.
The azimuthal polarizers can be achieved by 2D radial stretching. However, the surface roughness is still s key issue to suppress surface scattering. This can be fixed by further improving the fabrication process.
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