가시광 감응 아조계 폴리이미드를 이용한 네마틱 액정의 이중파장 광배향 및 안정화

다국어 초록 (Multilingual Abstract)

Application of liquid crystals has been enabled by controlling its alignment properties on the different substrates surfaces. Among several other alignment methods proposed and available today, photoalignment technique has the key to the solution of all the rubbing limitations. As photoalignment is a noncontact technique, enables the alignment of LCs on small pixel dimensions and can be performed easily with cost-effective manner.
Azobenzene based photoalignment possess some superiority among the other photoresponsive materials as it results in quick alignment with strong anchoring, fast response time and high contrast ratio. The major problem associated with azobenzene based photoalignment system is its stability against heat and unpolarized light treatment. Since azobenzene goes under continuous isomerization process can easily deteriorate the LC device alignment property in a long span of time at the ambient condition or faster at elevated temperature and high-intensity unpolarized light treatment. Several reports were published addressing the issue of stability, where co-system of reactive mesogens (RMs) and photoresponsive materials were employed resulting in stable and strong alignment conditions. In most of the reports, RM and chromophores were treated with the same wavelength range of UV light for inducing LC alignment and RM polymerization. Both processes of alignment and polymerization can interfere with each other and result in poor or inefficient alignment quality.
To address this issue I have proposed, dual wavelength photoalignment approach, where selective wavelength range of light can be used for inducing LC alignment and different wavelength range for RM polymerization to stabilize the alignment state. I have designed and synthesized visible light sensitive polyimides and employed them for attaining different LC alignment mode and further stabilizing the alignment. In chapter 3, visible light sensitive polyimides PI-MR and PI-DR were used to perform in situ photoalignmnet of LCs. The polyimides with azo-chromophores covalently attached to its side chain result in anisotropic surface upon LPVL irradiation in the isotropic phase of LCs. Obtained aligned state was reversible in nature at this stage, so RM polymerization was done at room temperature with unpolarized UV light. RM layer formed on top of polyimide coating layer results in pacifying the photochromic effect of azo-dyes beneath that layer and now acts as new active alignment layer with irreversible nature. The success of the dual wavelength approach was confirmed by performing stability tests and detailed analysis of surface morphology after polymerization under FESEM and AFM. A similar approach was used for the stabilization of pretilt state in a VA cell discussed in chapter 4. Polyimides with a different molar ratio of modified disperse yellow 7 azo-dye attached in their side chain were shown resulting in planar and vertical alignment (VA) of LCs after coating and backing on ITO substrates. In situ photoalignment process was performed to obtain uniform planar alignment by treating the electro-optic cell in the isotropic phase with linearly polarized visible light. Polymer-stabilized vertical alignment states were reported recently and possess great significance for enhancing viewing angle of LCDs. So far mainly slit-patterned electrodes have been used to stabilize the PS-VA, whereas photoalignment can be an ideal candidate. Photoalignment can produce multidomain patterned alignments and using RM stabilization, PS-VA could be achieved. In an effort to stabilize the pretilt homeotropic alignment, the dual wavelength approach was employed. Vertically aligned cells were treated with oblique linearly polarized visible light (LPVL) in the isotropic phase, results in 0.5 degree pretilt. Further stabilization with UV light in second step yields in higher pretilt of 1 degree with irreversible alignment state. Stability test and surface analysis using FESEM and AFM were performed.
Apart from PI based LC alignment using visible light sensitive chromophore using dual wavelength approach, we tried to employ it in PI-less alignment procedure. We have used a host-guest system containing modified azo-dye with hydrophilic end group and alkyl tail along with polymerizable RMs. Hydrophilic end interaction with ITO interface spontaneously results in vertical alignment of LCs and in situ photoalignment using oblique LPVL results in pretilt state. Further polymerization of RM with UV at room temperature results in the stabilized planar state. The details of surface analysis and stability of the cell were discussed in chapter 5.
The dual wavelength approach possesses significant potential for further exploration and can enhance the quality of LC-based devices.

목차 (Table of Contents)

• Chapter 1: Introduction and Background. 1
• 1.1 Introduction to liquid crystals. 1
• 1.2 Nematic liquid crystals (NLCs) 5
• 1.3 Director and order parameter. 6
• 1.4 Dielectric anisotropy 7
• 1.5 Optical anisotropy 10
• 1.6 Polarization of light. 11
• 1.7 Introduction to alignment of liquid crystals. 14
• 1.8 Alignment by rubbing 16
• 1.9 Oblique evaporation technique 17
• 1.10 Physico-chemical technique 18
• 1.11 Micro-patterned polymers. 19
• 1.12 Photoalignment technique. 19
• 1.12.1 Introduction to photoalignment of liquid crystals 19
• 1.12.2 Photoalignment by photoisomerization process. 21
• 1.12.3 Photoalignment by photodimerization,photocrosslinking or combination of both the process 23
• 1.12.4 Photoalignment of liquid crystals by photodecomposition or photodegradation method 26
• 1.12.5 Photoalignment on azobenzene containing polyimides (Azo-PI) and azo-dye doped systems 27
• 1.13 Motivation 34
• Chapter 2: Materials and Instrumentations. 36
• 2.1 Modification of Methyl Red 36
• 2.2 Polymer synthesis with a hydroxyl aromatic polyimide backbone: (PI) 38
• 2.3 Synthesis of aromatic polyimide containing azo-dye molecules in their side chain: (PI-DR) 40
• 2.4 Synthesis of PI-MR 42
• 2.5 Modification of DY7 azo-dye 44
• 2.6 Synthesis of PI-DY7 46
• 2.7 Modification of Disperse Yellow 7 with hexanoic acid space group: (DY7-C5H10COOH) 49
• 2.8 Instrumentations 51
• 2.8.1 Atomic Force Microscope (AFM) 51
• 2.8.2 Pretilt Angle Measurement System (PAMS) 54
• 2.8.3 Field Emission Scanning Electron Microscope (FESEM). 57
• Chapter 3: Dual Wavelength In Situ Photoalignment for Stable Planar Alignment of Nematic Liquid crystals 59
• 3.1 Introduction. 59
• 3.2 Experimental section 61
• 3.2.1 Materials. 61
• 3.2.2 LC cell fabrication 62
• 3.2.3 In situ alignment control. 62
• 3.2.4 Stabilization of LC alignment. 63
• 3.2.5 Characterization of the stabilization layer 63
• 3.3 Results and discussion. 63
• 3.4 Conclusions 78
• Chapter 4: Generation and Stabilization of Uniform Pretilt and Planar Alignment of Nematic Liquid Crystals: A Dual Wavelength In Situ Photoalignment Approach. 79
• 4.1 Introduction. 79
• 4.2 Experimental section 81
• 4.2.1 Materials. 81
• 4.2.2 Preparation of LC and RM 257 mixture. 82
• 4.2.3 Fabrication of LC cell. 82
• 4.2.4 In situ alignment control for uniform planar and tilted homeotropic state 82
• 4.2.5 Stabilization of LC alignment. 83
• 4.2.6 Characterization of stabilization layer 83
• 4.3 Results and discussion. 84
• 4.4 Conclusions 104
• Chapter 5: Achievement of Homogeneous Planar Alignment of Nematic Liquid Crystals in a Vertically Aligned Cell using Azo-dye Additive and Linearly Polarized Light 105
• 5.1 Introduction 105
• 5.2 Experimental section 107
• 5.2.1 Materials. 107
• 5.2.2 Cell fabrication. 107
• 5.2.3 In situ alignment control. 107
• 5.2.4 Stabilization of LC alignment 108
• 5.2.5 Characterization of the stabilization layer 108
• 5.3 Results and discussion. 109
• 5.4 Conclusions 121
• Chapter 6: Summary and Future Prospects. 122
• References. 125
• List of publications 144
• 국 문 요 약 . 145