무배향막 네마틱 액정 배향을 위한 인시츄 광배향 및 이의 전기광학 소자 응용

국문 초록 (Abstract)

액정의 배향 제어는액정연구 분야에서, 특히 광학 및 전기 광학 응용 분야에서 매우 중요하다. 액정의우수한 배향을 달성하기 위해 경사 증착, 이온빔 처리, 러빙 및 광배향과 같은 다양한 기술이 제안되었다. 그 중 러빙기술은 액정 디스플레이의 제조 공정에서 성공적으로적용되어 왔다. 그러나 작은 픽셀 크기에 따른 고르지 않은 표면 및 배향 균일도, 먼지 및 정전기의 발생과 같은 심각한 단점에 따라 새로운 배향기술의 개발이 요구되고 있다. 이러한 관점에서 비접촉 방식인 광배향 기술은 기존의 러빙 방식과 비교하여 그 장점으로 인해많은 연구가 진행되고 있다. 또한, 최근에는 “PI-less in situ 광배향”이라고 불리는 전처리 된 배향막을 사용하지 않은 in situ 인시츄 광배향이 보고되었다. 이 경우 감광성 화합물을액정 호스트에 도핑하고 혼합물을 배향막이 없는 빈 셀에 주입하여 광배향으로 액정 배향을 유도하였다. 이 방법은 비용 효율적이기 때문에 대량 생산에 매우 유용하며 초 고해상도 디스플레이 제조에 대한 요구 사항을 충족시킬 수 있다. 본 논문은 “PI-less in situ 광배향”에 초점을 맞추고 있다.
아조벤젠계 물질은액정 분야에서많이 사용되고 있는 화합물 중 하나이다. 아조벤젠계 물질은트랜스-시스 광 이성질화를 거치며, 이 물질의 매력적인 특징은이성질화 과정에서의 가역성이다. 그러나액정 정렬에 관해서는, 충분한 안정성을 갖는 견고한 정렬 층이 요구되며, 중합 가능한 물질에 의해 정렬된 상태를 동결 안정화시키는 것이 중요하다. 두 번째 장에서는 “PI-less in situ 광배향”을 위한 간단하고 신뢰할 수 있는 방법이 제안되었다. 액정 정렬 배향의 목적과 정렬 배향 과정의 메커니즘 연구를 위해 형광 지시약으로 Acidified Chrysophenin (ACP)과 Thiazole yellow G (TYG)라는 두 가지 가시 광선에 민감한 화합물을 각각 선택하였다. Azo 아조 발색단 및 반응성 메소겐(RM)을액정 호스트에 도핑하여 하나의 액정 조성물을 준비하였다. 선 편광된 가시 광선(LPVL)을 조사 시, 초기 무작위 평면 배열로부터 균일한 수평배향이 유도되었다. 정렬된 상태를 유지하기 위해 비편광자외선을 사용하여 RM의 중합을 통해 안정화 공정을 수행하였다. 이러한 순차적 공정을 통하여 정렬 및 안정화 층 형성 간의 간섭을 최소화하여 양질의 액정배향을 달성할 수 있었다. 이 연구에서 사용된 아조 염료의 주된 특징은 화학 구조에서 이온 그룹의 존재로 인한 LC의 제한된 혼합성이다. 그것은 혼합물이 빈 셀에 주입되면 아조 염료 분자가 표면에 국지화되고 흡수되게 하였다. 정렬 메커니즘 및 안정화 공정은 Confocal Fluorescence Microscopy (CFM), polarized fluorescence microscopy (PFM), Field Emission Scanning Electron Microscopy (FE-SEM) 등과 같은 다양한 방법을 사용하여 분석하였다.
3장에서는 이전의 것과는 달리, 전혀 다른 특성을 지닌 독특한 아조 염료 화합물이 종래의 배향막이 없는 in situ 수평배향에 사용되었다. DA6DMA 및 RM을 액정 호스트에 도핑하고 추가 전처리없이 비어 있는 샌드위치 셀에 주입하였다. 1 단계 광 배향 공정은 LPVL 조사를 이용하고, 이어서 편광 되지 않은 UV 빛으로 2 단계 RM-안정화(즉, 이중 파장 접근법)를 수행하였다. 이 작업에서 사용된 아조 염료는 흥미로운 배향유도 특성을 나타냈다. 이 아조 염료의 UV-vis 스펙트럼은 가시 영역과 UV 영역 모두에서 넓은 범위를 가지고 있다. 이중 파장 공정 외에도 PI가 없는 in situ 광 배향 및 RM 안정화는 "단일 파장 접근법"이라고도 불리는 자외선 광 조사만으로 이루어졌다. 또한 가시 광선 영역에서 광 발광을 나타내므로 정렬 메커니즘을 모니터링하기 위해 추가적으로 형광 지시약이 사용되지 않았다. 게다가, 이것은 이전의 아조 염료와 달리, 액정 호스트 에서 우수한 용해도를 가졌다. 따라서 CFM에서 얻은 셀 전반에 걸친 azo 발색단의 공간적 분포는 ACP 아조 염료와는 다른 공간 분포의 특성을 나타내었다.
4 장에서는신나메이트 기반 아크릴 모노머를 사용하여폴리이미드가 없는인시츄 수평배향을 성공적으로달성하였다. C3 모델 화합물은 인시츄 광 배향 공정을 위하여 포괄적인 연구를 통해 설계되고 채택되었다. 이 경우, 액정 배향을 이루기 위해 세 가지 주요 공정이 필요하였다. 초기에는 광중합 반응, 광이량화 반응과 함께트랜스-시스 광이성질화 반응이 일어났다. 그러나, 광이성질화 반응은 [2+2] 첨가반응보다 수평배향을 유도하기 위한 과정에서 주도적인 역할을 하는 것으로 나타났다. 이 다기능성 광 반응 화합물은 또한 형광 특성을 나타내어 CFM, PFM 및 편광 형광 분광법(PFS)을 사용하여 액정배향 메커니즘을 연구하는데 매우 유용하였다. 파장 의존성 연구는 긴 파장의 자외선(λ > 350 nm)이 더 효과적이라는 것을 확인하였다. 아조벤젠 및 신나메이트 기반 재료를 사용하여 제안된 새로운 액정배향 방법은 산업 응용 분야에 있어서 큰 잠재력을 지닌 “PI-less in situ 광배향”을 위한 신뢰성 있고 실용적이며 비용 효율적인 액정 배향법을 제공하였다.

다국어 초록 (Multilingual Abstract)

The control of the alignment of liquid crystals is of great importance in the field of LC research, especially for their optical and electro-optical applications. To achieve an outstanding alignment of liquid crystals, various techniques such as oblique evaporation, beam treatment, rubbing and photoalignment have been proposed. Rubbing process successfully became dominant in the manufacture of liquid crystal displays (LCDs). However, some serious disadvantages such as uneven surfaces for the small-sized pixels, generation of dust and static electricity triggered the researchers to explore a new technique to deal with the rubbing deficiencies. Therefore, photoalignment technique as a non-contact method has been considerably succeeded due to its advantages compare to the classical rubbing method. Polyimide (PI) as a popular polymer has been ubiquitously used either in rubbing or photoalignment method as an alignment layer. Nevertheless, recently the in situ photoalignment without using pretreated alignment layers which is called PI-less in situ photoalignment has been reported. In this case, photosensitive compounds were doped to LC host and one-bottle mixture was injected to an empty cell with no alignment layer. This method is very useful for mass production because it is cost-effective and it can meet the demanding for manufacturing ultra-high resolution displays. The present dissertation focuses on “PI-less in situ photoalignment”.
Azobenzene-based materials are one of the most utilized and prominent compound in the field of LCs. Azobenzene-based materials undergo trans-cis photoisomerization and an attractive feature of these materials is the reversibility of the process from cis to trans form. However, as for a LC alignment a robust alignment layer with sufficient stability is required, it is crucial to freeze the aligned state by a polymerizable material. In the second chapter, a simple and reliable method was proposed for PI-less in situ photoalignment. Two visible-light-sensitive model compounds, Acidified Chrysophenin (ACP) and Thiazole yellow G (TYG) as a florescent indicator, were selected for LC alignment purpose and screening the mechanism of alignment process, respectively. The azo chromophore and a reactive mesogen (RM) were doped to LC host (i.e., one-bottle approach). Upon linearly polarized visible light (LPVL) irradiation, a uniform planar alignment was induced from initially random planar alignment. To maintain its aligned state, the stabilization process was performed by using unpolarized UV light via polymerization of RMs. This process was called “dual-wavelength approach”. Such thrilling stepwise process was capable of avoiding interference between forming alignment and stabilization layer. The predominant characteristic of the azo-dye used in this work was the limited miscibility in the LC due to existence of ionic group in the chemical structure. It caused the azo-dye molecules were localized and adsorbed at the surface once the mixture was injected into the empty cell. The alignment mechanism and stabilization processes were characterized by employing various methods such as Confocal Fluorescence Microscopy (CFM), polarized fluorescence microscopy (PFM), Field Emission Scanning Electron Microscopy (FE-SEM) and so on.
In chapter 3, contrary to the previous one, a unique azo-dye compound with quite different characteristics was employed for PI-less in situ planar alignment. DA6DMA and RM were doped to LC host and injected to empty sandwiched cell without additional pretreatment. First step photoalignment process was performed by using LPVL irradiation, followed by second step RM-stabilization with unpolarized UV light (i.e., dual-wavelength approach). The azo-dye served in this work, exhibited an interesting electronic properties. Its UV-vis spectra possessed a broad range in both visible and UV area. In addition to dual wavelength process, the PI-less in situ photoalignment and RM-stabilization was accomplished by only UV light irradiation also which was so called “single-wavelength approach”. Moreover, it showed photoluminescence in visible range and thus, no additional fluorescence indicator was used to monitor the mechanism of alignment. Besides, it had an excellent solubility in the LC host, unlike the previous azo-dye. Therefore, the spatial distribution of azo chromophores throughout the cell obtained from the CFM, presented different observation from the ACP azo-dye.
In chapter 4, the PI-less in situ planar alignment was successfully presented by using the cinnamate-based acrylic monomer. The C3 model compound was designed and employed for a comprehensive study for in situ photoalignment process. In this case, three major processes were involved to achieve a LC alignment. At early stage photopolymerization and subsequently trans-cis photoisomerization along with photodimerization were taken place. However, it was shown that the trans-cis isomerization had a leading role to obtain the planar alignment rather than [2+2] cycloaddition reaction. This multifunctional photoresponsive compound exhibited fluorescence properties also, which was very beneficial to explore the mechanism of the alignment by employing the CFM, PFM and polarized fluorescence spectroscopy (PFS). A wavelength dependence study confirmed that the longer wavelength of UV light ( > 350 nm) was more effective.
The above proposed methods for azobenzene- and cinnamte-based materials provided a practical, reliable and cost-effective process for PI-less in situ photoalignment which has a great potential for industrial applications.

목차 (Table of Contents)

• Chapter 1. Background and Overview 1
• 1.1 Introduction to Liquid Crystals 1
• 1.2 Nematic Liquid Crystal (NLC) 6
• 1.3 Anisotropic Physical Properties of Liquid Crystals 9
• 1.3.1 Optical Anisotropy 9
• 1.3.2 Dielectric Anisotropy 13
• 1.4 Introduction to Molecular Alignment of Nematic Liquid Crystals 14
• 1.5 Alignment of Liquid Crystals by Rubbing Technique 18
• 1.6 Alignment of Liquid Crystals by Photoalignment Technique 21
• 1.6.1. Trans-Cis Photoisomerization 23
• 1.6.2 Photodimerization 29
• 1.6.3. Photodegradation 33
• 1.7 PI-Less in Situ Photoalignment of Nematic LCs (Guest-Host System) 33
• 1.8 Motivation and Aims 35
• Chapter 2. Polyimide-Free Planar Alignment of Nematic Liquid Crystals: Sequential Interfacial Modifications through Dual-Wavelength in Situ Photoalignment 37
• 2.1 Introduction 37
• 2.2 Experimental Section 40
• 2.2.1 Materials 40
• 2.2.2 Acidification of Chrysophenine 41
• 2.2.3 Cell Fabrications 41
• 2.2.4 In Situ Photoalignment and Stabilization 41
• 2.2.5 Confocal Fluorescence Microscopy (CFM) 42
• 2.2.6 Polarized Fluorescence Microscopy (PFM) and Polarized Fluorescence Spectroscopy (PFS). 42
• 2.2.7 Characterizations 43
• 2.3 Results and Discussion 45
• 2.3.1 Conceptual Approach for PI-Less in Situ Photoalignment 45
• 2.3.2 Interfacial Adsorption and Photochemical Responsiveness 47
• 2.3.3 Interfacial Modification by in Situ Photoalignment 51
• 2.3.4 Stabilities Against Heat and Light Irradiation 57
• 2.3.5 Interfacial Stabilization of LC Alignment 60
• 2.3.6 Electro-Optical Characteristics 68
• 2.4 Conclusions 71
• Chapter 3. PI-Less in Situ photolignment for Stable Planar Alignment of Nematic Liquid Crystals through the Single and Dual-Wavelength Approaches 72
• 3.1 Introduction 72
• 3.2 Experimental Section 74
• 3.2.1 Materials 74
• 3.2.2 Cell Fabrication 75
• 3.2.3 In situ Planar Alignment and Stabilization 77
• 3.2.4 Fluorescence Microscopy and Spectroscopy 77
• 3.2.5 Characterizations 78
• 3.3 Results and Discussion 79
• 3.3.1 Schematic Demonstration for PI-Less in Situ Photoalignment 79
• 3.3.2 Azo-dye Compounds Selection for Sequential Mechanism Study 81
• 3.3.3 PI-Less in Situ Photoalignment for Mono- and Multi-Domain 83
• 3.3.4 Molecular Orientation and Spatial Distribution of Azo-Dye Chromophores 85
• 3.3.5 Stability Test against Heat and Light Illumination 87
• 3.3.6 Stabilization of LC Alignment 89
• 3.3.7. Morphological Observation of Stabilized Alignment Layer 91
• 3.3.8. Electro-Optical Properties 95
• 3.3.9. PI-Less in Situ Photoalignment through the Single-Wavelength Approach 98
• 3.4. Conclusions 104
• Chapter 4. In Situ Planar Photoalignment of Nematic Liquid Crystals: Interfacial Polymerization and Wavelength Dependent Photochemical Responses of Cinnamoyl-Based Acrylic Monomers 105
• 4.1 Introduction 105
• 4.2 Experimental Section 107
• 4.2.1 Materials 107
• 4.2.2 UV-Vis Spectroscopy 108
• 4.2.3 Cell Fabrications 108
• 4.2.4 In Situ Photoalignment 108
• 4.2.5 Electro-Optical Characterizations 109
• 4.2.6 Fluorescence Microscopy and Spectroscopy 109
• 4.2.7 Morphology and Thickness Measurements 110
• 4.3 Results and Discussion 113
• 4.3.1 Material Design for a Sequential Mechanism Study 113
• 4.3.2 PI-less in Situ Photoalignment 118
• 4.3.3 Photochemical Responses of the Cinnamte-Based Acrylic Monomers 122
• 4.3.4 Wavelength Dependence of the in Situ Photoalignment 126
• 4.3.5 Polymerization and Molecular Reorientation Actuated by LPUV 131
• 4.3.6 Reversibility of the in Situ Photoalignment 134
• 4.3.7. Interfacial Alignment Layer formed by in Situ Photoalignment 148
• 4.3.8. Stepwise Understandings on the PI-less in Situ Photoalignment 153
• 4.4. Conclusions 155
• Chapter 5. Conclusions and Outlook 156
• References 160
• List of Publications 179
• List of Patents 180
• Abstract (Korean) 181