Chapter 3 Fabrication and Measurement Instrument
3.3 Measurement System
After the fabrication of the polymer wall structure, the inspection will be performed to make sure that the polymer structures were conformed to the expectancy.
Thus, polarized optical microscope is required for observing the polymer walls.
Optical microscope with the combination of an ocular and an object lens is the most popular instrument of observing a structure. A transmissive or reflective light can be chosen to observe the micro structure. Besides, a computer-controlled display (CCD) and a computer are usually equipped with the microscope and the picture of observation can be taken from the CCD. In display research, a polarized optical microscope is used to study the behavior of liquid crystal through a pair of polarizer,
especially. Although the magnifying power of general optical microscope has a limitation, a simple operating system still popularizes its application.
A desired angle of the polarized direction between two polarizers can be adjusted manually. In our experiment, a polarized optical microscope was utilized to observe the phase variation between FLC and monomers in encapsulation cells.
Chapter 4
Experimental Results and Discussions _________________________________________
4.1 Introduction
The electric field induced fabrication method of polymer wall structures was introduced in previous chapter. As mentioned before, fast process time and perfect separation quality are the main advantages of electric field induced method. This novel method can be utilized to form polymer walls in not only TNLC device but also FLC species. Therefore, electric field induced method was attempted to form polymer walls in TNLC devices. A polarized optical microscope was utilized to observe the phase separation and experimental results. The results will be shown in this chapter.
After fabricating polymer walls in TNLC device, the proposed process will also be evaluated in R2301 and R3206 FLC devices. The experimental results and comparisons between different materials will then be further discussed.
4.2 Experimental Results in TNLC devices
Polymer wall structure in TNLC device was fabricated by electric field induced method. The specification of AC voltage which was applied to induce phase separation of NOA65 and TNLC was shown in Tab. 4.1. According to the electric field induced process which discussed before, the temperature of polymerization was the most important in this method, and the temperature will greatly affect the result of polymer walls. In order to observe the phase separation temperature in TNLC devices,
a hot stage and polarized optical microscope was required. The temperature of the devices can be exactly controlled by the hot stage. As the observation by the microscope shown, the phase separation temperature of TNLC devices is 46oC
Tab. 4.1 Specification of applied AC voltage in TNLC device.
Parameter Condition
Waveform Square wave
Frequency 1k Hz
Amplitude 100 V (pk-pk)
The phase of TNLC material is nematic at 46 oC. After injecting the mixture of TNLC and NOA65 at 90oC, the temperature of the device was decreased to 46oC and AC voltage was applied to induce electric field between upper and lower ITO substrates. The electric filed can separate TNLC and NOA65 about 5 minutes. The polymer walls can be formed after polymerization by UV exposure at 46 oC. The experimental result of fabricating polymer walls in TNLC device is shown in Fig. 4.1.
The width of polymer walls is about 20 μm and the pixel area is about 200 by 200μ m. In pixel area, the nematic phase of liquid crystal can be clearly observed. The wall structure was formed in the gaps between pixels, and there are no monomers residual in pixel areas. Thus, the contrast ratio of the TNLC device can be maintained and will not be decreased by the residual of monomers in pixels. Fig. 4.2 shows another picture of TNLC device which was taken by an optical microscope with 45degree polarization. The dark and bright states of the TNLC device driven by AC voltage can be observed in Fig. 4.3.
Fig. 4.1 The structure of polymer walls in TNLC device.
Fig. 4.2 The structure of polymer walls in TNLC device at 45degree polarization.
(a)
(c)
(b)
(d)
(e)
Fig. 4.3 The TNLC device which contains polymer walls is driven by an AC voltage of (a) 0V, (b) 1V, (c) 2V, (d) 3V, and (e) 4V.
The temperature of polymerization is quite important, and it will greatly affect the quality of polymer walls. Fig. 4.4 shows the experimental result of the device which was polymerized at 50oC. The polymer walls are incomplete in this case, and there is no liquid crystal phase in pixel areas due to the residual of monomers in pixels.
The temperature of phase separation is 46oC, therefore, phase separation of TNLC and NOA65 is still incomplete at 60oC. If the device is exposed by UV light at this temperature, the formulation of polymer walls will be incomplete.
Fig. 4.4 The structure of imperfect polymer walls in TNLC device.
The final results will be affected by different polarization temperature. If the device is photo-polymerized at the temperature higher than phase separation temperature, polymer walls will not be formed in the gaps between pixels. However, monomers will be solidified in pixel areas and the liquid crystal phase of TNLC will also be destroyed, as shown in Fig. 4.5. The dark and bright states of these imperfect devices can not be observed easily. The contrast ratio of these devices is very poor.
(a) 70 oC (b) 65 oC
(c) 60 oC (d) 50 oC
Fig. 4.5 The photographs of TNLC device polymerized at (a) 70oC, (b) 65 oC, (c) 60 oC, and (d) 50 oC.
The process time of fabricating polymer walls in TNLC devices is about 30 minutes, and the quality of polymer walls in TNLC device is quite complete if the device was exposed by UV light at phase separation temperature. As the results, the electric field induced method can achieve a fabrication process of polymer walls with faster process time and better separation quality.
4.3 Experimental Results in FLC devices
The proposed process was firstly examined to form polymer walls in TNCL device by applying electric field, as shown before. After that, instead of TNLC material, FLC material was mixed with NOA65. The electric field induced process was utilized to form polymer walls in FLC devices. The result of polymer walls in FLC device will be shown in this section.
4.3.1 Polymer Structures in R2301 FLC devices
The FLC material used in this experiment was R2301 FLC. The phase transition of R2301 FLC was isotropic 86.8 oC – 84.8 oC chiral nematic 64.7 oC chiral smectic.
R2301 was firstly mixed with NOA65 as 20 % concentrations. In the beginning, a hot stage and a polarized optical microscope were utilized to observe the phase separation temperature of the mixture of R2301 and NOA65. The phase separation temperature was about 84 oC to 88 oC, as shown in Fig. 4.6. When the temperature of the FLC device is higher than 88 oC, R2301 FLC is in isotropic phase. As the temperature of the device decreased to 88 oC, the phase of R2301 FLC started transferring to chiral nematic phase, as shown in Fig. 4.6.
Fig. 4.6 The phase separation process of the mixture of R2301 FLC and NOA65. The temperature of the device is (a) 88 oC, (b) 87 oC, (c) 86 oC, (d) 85 oC, (e) 84 oC, and (f) 83oC.
(a) 88 oC (b) 87 oC
(c) 86 oC (d) 85 oC
(e) 84 oC (f) 83 oC
The observation of phase separation temperature can help us finding the photo-polymerization temperature more easily. After that, an AC voltage was applied to the FLC device in order to separate R2301 FLC and NOA65 as our expectation.
The specification of the AC voltage is shown in Tab. 4.2.
Tab. 4.2 Specification of applied AC voltage in R2301 FLC device.
Parameter Condition
Waveform Square wave
Frequency 2 kHz
Amplitude 120 V (pk-pk)
The viscosity of R2301 FLC is higher than that of TNLC at the same temperature.
Thus, the separation quality of R2301 FLC and NOA65 is not easy to be controlled, and the separation rate is slower than that of the TNLC device. In order to speed up the process of phase separation, the frequency of the AC voltage applied to induce phase separation was raised to 2k Hz. Besides, the amplitude of the AC voltage was also increased to 120 volt. Phase separation of R2301 and NOA65 can be successfully induced by applying AC voltage to the device. The photographs of separation process were shown in Fig. 4.7. The photographs in Fig. 4.7 were taken by a polarized optical microscope with different temperature.
Fig. 4.7 The phase separation process of the mixture of R2301 FLC and NOA65 with an AC voltage. The temperature of the device is (a) 89 oC, (b) 88 oC, (c) 87 oC, and (d) 86 oC.
(a1) Cross-polarized mode (a2) Non-polarized mode
(b1) Cross-polarized mode
(c1) Cross-polarized mode
(d1) Cross-polarized mode
(b2) Non-polarized mode
(c2) Non-polarized mode
(d2) Non-polarized mode
Fig. 4.7 The phase separation process of the mixture of R2301 FLC and NOA65 with an AC voltage. The temperature of the device is (e) 85 oC, (f) 84 oC, (g) 83 oC, and (h) 82 oC.
(e1) Cross-polarized mode
(f1) Cross-polarized mode
(g1) Cross-polarized mode
(h1) Cross-polarized mode (h2) Non-polarized mode (g2) Non-polarized mode (f2) Non-polarized mode (e2) Non-polarized mode
The phase separation of FLC and NOA65 induced by electric field is shown in Fig. 4.7. When the temperature of the FLC device was lower than 90 oC, the phase of FLC is transferring to chiral nematic phase. As decreasing of the temperature, more FLC molecules transfer to chiral nematic phase. This phase separation process is similar to the process shown in Fig. 4.6 which was applied with no AC voltage. The difference between the separation processes in Fig. 4.6 and Fig. 4.7 is AC voltage.
FLC device was applied with no AC voltage in Fig. 4.6 so that the phase separation of FLC and NOA65 was uniform in the device. Nevertheless, the process which NOA65 was separated to the gap between pixels as a network structure was due to the function of AC voltage. According to the principle of electric field induced phase separation method introduced before, FLC molecules were switched up and down by applying AC electric field and NOA65 were squeezed to the gap of pixels as polymer wall structures.
The best quality of the phase separation process was in 84 oC. However, while the temperature of the device was lower than 84 oC, the separation quality became incomplete, as shown in Fig 4.7(h). The reason is that the viscosity of FLC is larger than TNLC. Besides, the viscosity of FLC will be decreased in high temperature, so that phase separation will be induced more completely in high temperature.
Although NOA65 can be induced and separated to the gaps between pixels by applying an electric field, the polymer wall structures still can not be formed in R2301 FLC device. A UV light was utilized in photo-polymerization process in order to solidify NOA65. During the phase separation process, the R2301 FLC device must be avoided from UV exposure, or NOA65 will be polymerized before phase separation.
The structure of polymer wall in R2301 FLC device was shown in Fig. 4.8. This incomplete polymer structures resulted in lower transmittance of light source in the FLC device.
The switching property of R2301 FLC was affected by the polymer structure in pixel areas. Black and Bright states of the FLC device can not be clearly distinguished.
Fig. 4.9(a) is the bright state of the FLC device which contains incomplete polymer structures. Chiral nematic FLC is confined in small sections, while the dark area is the polymer structures in the device, as shown in Fig. 4.9. Although R2301 FLC and NOA65 can be separated in our device by the electric field induced method, nevertheless, the polymer wall structure still can not be formed successfully. After the phase separation process, the FLC device was then exposed by UV light. However, NOA65 was not photo-polymerized in R2301 FLC device after UV exposure.
Fig. 4.8 The incomplete polymer structures in R2301 FLC device.
Fig. 4.9 The state of incomplete polymer structures in R2301 FLC device.
(a) Cross-polarized mode and (b) 30o polarized mode.
(a)
(b)
The neat NOA65 generally can be photo-polymerized by UV exposure in 5 minutes. However, after mixing NOA65 with R2301 FLC material, NOA65 can not be photo-polymerized by UV exposure in R2301 FLC device. Although the device was exposed by UV light in more than 1 hour, NOA65 was still in liquid phase.
Several experiments were performed in order to solve the photo-polymerization issue in R2301 FLC device. The issues of this photo-polymerization are:
(1) Temperature effect: Photo-polymerization of NOA65 can not be induced in the temperature higher than 60 oC due to the side reaction between NOA65 and FLC.
(2) Electric field effect: Photo-polymerization of NOA65 was restrained by the electric field.
(3) Chemical reaction effect: R2301 FLC reacted with NOA65 or photo-initiators while blending.
In order to figure out the photo-polymerization issue, NOA65 was exposed by UV light at 90 oC without applying an electric field. As a result, NOA65 can be solidified by the UV exposure in 5 minutes, as shown in Fig. 4.10. Thus, the temperature effect was not the key issue in photo-polymerization problem.
Concerning the electric field effect, NOA65 was exposed by the UV light with the effect of an electric field, as shown in Fig. 4.11. NOA65 can be solidified even though it was functioned by an electric field.
According to these tests, chemical reaction between R2301 FLC and NOA65 was the most probable reason of the photo-polymerization issue. Base on this study, R2301 FLC was replaced by R3206 FLC to fabricate polymer walls in our device.
Fig. 4.10 Exposing NOA65 by UV light at 90 oC without applying an electric field.
Fig. 4.11 Exposing NOA65 by the UV light with the effect of an electric field.
4.3.2 Polymer Structures in R3206 FLC devices
R2301 FLC was estimated that it may react with NOA series monomers, so that R3206 was utilized to replace R2301 FLC material. R2301 was mixed with monomers as 20 % concentrations and blended at 100 oC. The phase transition of R3206 FLC was isotropic 107.0 oC – 105.4 oC chiral nematic78.4 oC chiral smectic.
The separation process of R3206 and monomers was the same with that in R2301 device. The specification of the AC voltag applied to induce phase separation of NOA65 and R3206 FLC is shown in Tab. 4.3.
NOA65 was firstly utilized to be blended with R3206 FLC. The process of phase separation between R3206 FLC and NOA65 was shown in Fig. 4.12. Black areas are NOA65 material, and the others are R3206 FLC. Unfortunately, NOA65 can not be concentrated in the gaps under applied AC voltage.
Tab. 4.3 Specification of applied AC voltage in R3206 FLC device.
Parameter Condition
Waveform Square wave
Frequency 500 Hz
Amplitude 120 volt (pk-pk)
Fig. 4.12 The phase separation process between R3206 FLC and NOA65 at (a) 80 oC, (b) 70 oC, and (c) 60 oC.
(a) 80 oC
(b) 70 oC
(c) 60 oC
Although NOA65 can not be separated to the gaps between pixels by switching R3206 FLC molecules in high frequency, photo-polymerization of NOA65 can be successfully induced in R3206 FLC device. Compared with R2301 FLC device, NOA65 can be solidified by UV exposure, as shown in Fig. 4.13. Black areas are polymer structures, and the others are R3206 FLC structures. This photograph showed that NOA65 can not be utilized to form polymer walls in R3206 FLC device.
Chemical reaction of R2301 FLC and NOA series monomers was estimated to be the major reason of photo-polymerization issue. However, after altering FLC material, the photo-polymerization insensitivity in R2301 FLC device was eliminated in R3206 FLC device. NOA65 can be solidified by UV exposure in R3206 FLC device.
Unfortunately, the phase separation quality can not be as sharp as that in R2301 FLC device, as shown in Fig. 4.7. In order to improve the separation quality of R3206 FLC and monomers, NOA74 monomers were utilized to replace NOA65. The separation process of R3206 FLC and NOA74 is shown in Fig. 4.14.
Fig. 4.13 The polymer wall structures with NOA65 monomers in R3206 FLC device.
Compared with NOA65, NOA74 can be separated to the gaps between pixels by R3206 FLC material when the temperature of the device is between 50 oC to 60 oC, as shown in Figs. 4.14 (b) and (c). When the temperature of the device decreases to room temperature, the separation quality becomes poor, as shown in Fig. 4.14 (d). Fig. 4.15 shows the polymer wall structures in R3206 FLC device exposed by UV light at room temperature. The polymer wall structures were not sharp enough compared with the polymer walls in TNLC device, as shown in Fig. 4.1. After that, the temperature of UV exposure was chosen as 55 oC, and the result is showed in Fig. 4.16.
(a) 70 oC (b) 60 oC
(c) 50 oC (d) 30 oC
Fig. 4.14 The phase separation process of the mixture of R3206 FLC and NOA74 with an AC voltage. The temperature of the device is (a) 70 oC, (b) 60 oC, (c) 50 oC, and (d) 30 oC.
The polymer wall structures in R3206 FLC device can be formed sharply by using NOA74 monomers when the device was exposed by UV light at 55 oC, as shown in Fig. 4.16. Polymer structures were solidified in the gaps between pixels while R3206 FLC was confined in the pixel areas.
Fig. 4.15 The incomplete polymer walls in R3206 FLC device exposed by the UV light at room temperature.
(a)
(b)
(a) Cross-polarized mode
(b) 30o polarized mode
Fig. 4.16 The polymer structures in R3206 FLC device exposed by the UV light at 55 oC. (a) cross-polarized mode and (b) 30o polarized mode.
4.4 Discussion
The polymer walls were widely utilized in display technique, especially flexible and PSFLC display. The most popular method to fabricate polymer walls is to use a photo-mask to confine the exposing area. Nevertheless, the electric field induced phase separation method has not been applied in fabricating polymer walls in FLC device due to several issues which will be discussed in his section.
Compared with TNLC material, the viscosity of FLC is very high, so that the phase separation between FLC and monomers will not maintain the same quality at different temperature in FLC device. As a result, the photo-polymerization temperature in FLC device becomes important, and the temperature will greatly affect the structures of polymer walls. In our experiment, the photo-polymerization temperature in TNLC device is 46 oC, while that in R2301 FLC device is 84oC, and 55 oC is that in R3206 FLC device. This photo-polymerization temperature must be narrowly figured out in order to fabricate complete polymer walls in different FLC material. Different photo-polymerization temperature will result in different quality of polymer wall structures, as shown in Fig. 4.17.
(a) room temperature (a) 55oC
Fig. 4.17 Polymer walls in R3206 FLC device exposed by the UV light at (a) room temperature and (b) 55 oC.
Besides, some monomers may be reacted with FLC material, and this chemical reaction will result in photo-polymerization issues. For example, R2301 FLC material will react with NOA series monomers, and NOA65 will not be photo-polymerized in R2301 FLC device even after UV exposure. On the other hand, although the photo-polymerization issue in R3206 FLC device is eliminated, the phase separation between R3206 FLC and NOA65 can not be induced due to its high viscosity of NOA65 in R3206 FLC device. As a result, NOA74 is a better choice in R3206 FLC device due to its lower in viscosity. Thus, the polymer walls can be successfully formed in R3206 FLC device by using NOA74 material. In summary, the choice of FLC and monomers is very important in the fabrication process of polymer walls.
Tab. 4.4 is the comparisons of experimental results with different monomers and FLC materials. Besides, Tab. 4.5 shows the comparisons of proposed and conventional processes.
R2301 FLC R3206 FLC
issue can not be solvedNOA65 can be polymerized by UV light in R3206 FLC device but can not be separated as a issue can not also be solved
NOA74 can be successfully separated and polymerized in R3206 FLC.
NOA83H (250 CPS)
R2301 FLC and NOA83H can be separated as a wall structure faster than that of NOA65 and R2301 FLC. However, curing issue is still there.
NOA83H has a characteristic of heating polymerization. As a result, NOA83H is not suit for
NOA83H has a characteristic of heating polymerization. As a result, NOA83H is not suit for