Characteristics and interfacial phenomena pf polyimide alignment layer for liquid crystal display = 폴리이미드 배향막의 특성 및 액정과의 계면 현상에 대한 연구
서명 / 저자 Characteristics and interfacial phenomena pf polyimide alignment layer for liquid crystal display = 폴리이미드 배향막의 특성 및 액정과의 계면 현상에 대한 연구 / Sung-Mog Kim.
저자명 Kim, Sung-Mog ; 김성목
발행사항 [대전 : 한국과학기술원, 1996].
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학술문화관(문화관) 보존서고

DCHE 96006







It was investigated the alignment of liquid crystals on a rubbed polyimide alignment layer, which has not been clearly understood yet. It is aimed for the improvement of layer materials and the conventional rubbing technique. Although the alignment of liquid crystal has known to be dominated by the rubbed layer surface, the structural changes of surface resulting from by rubbing has not been fully characterized. Such changes were examined by using several measurement methods; the conoscopy, X-ray diffraction, IR spectroscopy, the contact angle and so on. From the determined changes, the major factors of influencing the alignment of liquid crystal were extracted and an explanation involving them was made upon the rubbed surface. Also, this result was confirmed by applying to LC cells. In the measurement of changes, a devised multiple-coated alignment layers and LB films of PMDA/ODA polyimide and SE150 were used for the high gain of data. The multiple layer was obtained by over-coating one alignment layer on another, in which each layer was made according to the manufacturing sequence of a single alignment layer such as coating, curing, and rubbing. An optical anisotropy of a rubbed polyimide layer for the alignment of liquid crystal was investigated by conoscopic measurement for studying the orientation of polymer chains. Although the layers were over-coated and rubbed, the orientational changes of polymer chains by rubbing were sustained during the over-coating. Following the conoscopies in unrubbed and rubbed layer, the transitional change in the optical anisotropy was observed from the uniaxial to biaxial orientation. Therefore, it was confirmed that the polymer chains oriented due to rubbing and inclined to the film plane. The dichroic ratios of the stretching vibration of $C=0$ group and the X-ray diffraction patterns of multiply coated and rubbed layers of PI alignment film were studied. The orientational change by rubbing was mainly represented from the carbonyl group. It was perpendicular to the chain axis of polyimide in comparison of the dichroic ratios of the $-C=C-$ vibration in bezene moiety at $1500 cm^{-1}$ with the asymmetric stretching vibration of carbonyl group at $1720 cm^{-1}$. It was suggested that the orientational change of the carbonyl group might have originated from the lateral ordering of polyimide chains by the X-ray measurement. If the heat generated by rubbing raised the temperature of surface to $T_g$ or above, the chain segments might begin to move, and then the segmental motion was proceeded to the lateral ordering between the neighboring segments. The orientational changes of PI in the layer surface due to rubbing were investigated with the measurement of the surface tensions and the contact angles. The dispersion component of surface tension was larger than the polar component in the non-rubbed layer surface. While, rubbing resulted in the larger polar component and induced the ordering of polar groups on the surface. Then, the lateral ordering of polar groups could be conjectured to play a dominant role in the alignment of liquid crystal from the fact that the layer with a lower polar component also aligned the liquid crystal along the rubbing direction. We investigated the effect of the lateral packed state of LB films on the alignment of LC. The packed state of LB film was controlled by various deposition pressures. The transferred ratio of monolayer at the air-water interface was increased with an increase in the deposition pressure. A more packed state of LB film deposited on the substrate was obtained. The alignment of liquid crystal was possible at least above a certain level of packed state. The orientation of polymer chains in LB films was not changed by the perpendicular rubbing, but, the alignment occurred along the rubbed direction. Although rubbing could not induce the orientational change of the polymer chains, it might induce some changes in the ordering along the lateral direction of polymer chains. The normal incident light to the cell in which a liquid crystal had an angle with the surface of the cell, had a phase retardation by the refractive anisotropy of liquid crystal and the cell thickness. In the field off-state, the transmission could be calculated from the various methods, but in the field on-state, the variations in tilt angles between the surface anchoring angle and the maximum angle at z=d/2 across the cell made the existing calculation difficult and complete. Considering the distribution of director in the liquid crystal cell under the field with the folded anisotropic layers, the Jones matrix was expanded sequentially and the transmitted intensity was accurately evaluated in consistent with the experiment in order to calculate the light propagating through these layers. When the field was applied to the cross-rubbed LC cell, the transition of the alignment state of LC molecules was observed. The alignment of liquid crystal was changed from the first rubbing direction to the second rubbing direction. The transition seemed to be induced by local weakening of the anchoring of LC molecules at the surface, because the weakening of anchoring brought about the orientational change of LC molecules at the surface under the influence of the field. On the other hand, LC molecules recovered its initial alignment along the first rubbing direction at the field-off state. This represented that the repeating units shorter than main chains could be mobilize relatively with ease and change their orientation along the rubbing direction. The alignment state of FLC was much dependent on the cooling rate. The cooling rate was to be controlled slowly as enough as Nematic or Smectic A phase, which was a thermodynamically stable and ordered phase with a unique axis, settled down on the surface along the rubbing direction. Meanwhile, when an electric field was applied to the cell during slow cooling, it was observed that the alignment was initiated on the surface and followed by the bulk alignment on the contrary to slow cooling only, in which the bulk alignment was first. Owing to the application of the field, the initiation of the alignment on the layer surface influenced the surface layer of liquid crystal to become thicker with increased rubbing strength and then smaller spontaneous polarization was resulted. Also, the decrease in the optical decay time with an increase in the rubbing strength was due to the strong tendency of constraining FLC molecules at the surface. The alignment of liquid crystal on the rubbed layer surface was induced by the ordering of the lateral polar groups. Such ordering was enhanced by the introduction of high polar groups as lateral groups which could give the accompanying effect such as high pretilt angle, and by the development of segments containing the lateral groups, which easily changed their orientation on rubbing.

액정의 전기 광학적특성을 활용하기위해 소자내에서 균일한 배향이 필요하다. 이러한 액정 배향은 표면 처리된 배향막과의 계면에서 이루어지는 상호작용의 결과로써 유도되는 계면 현상이다. 액정의 유동성으로 인해 표면 처리된 배향막의 변화가 액정의 배향에 주도적인 역활을 한다. 그러므로 표면 처리에 따른 배향막 변화를 측정한 결과를 바탕으로 액정 배향을 보다 정확히 규명할 수가 있다. 배향막의 제조는 가공이 용이한 전구체(폴리아믹산)로 부터 목적에 맞게 가공처리한 뒤 열경화 과정을 거친뒤 배향막을 얻게 된다. 그리고 이 배향막은 표면에 문지름과정에 의해서 최종적으로 액정 배향성을 갖는다. 배향막의 표면은 문지름(Rubbing)에 의해 일정 방향으로의 응력을 받게 되고 구조적인 변화를 동반하게 된다. 이러한 구조적인 변화와 배향막이 액정의 배향에 미치는 요인을 폴리이미드계 고분자 배향막의 특성과 함께 X-ray diffraction, IR spectroscopy, Conoscopy, Surface tension 등의 여러 측정 방법등을 통해서 결정하였다. 일정 방향으로 응력을 받은 막 표면은 응력방향으로 고분자 사슬의 배향 변화가 일어나게된다. 사슬의 배향 변화는 conoscopy로 부터 문지름 강도가 증가할 수록 증가하고 있었으며, IR 측정으로 부터 주사슬 보다 곁가지의 배향 변화가 주도적으로 일어나고 있음을 보여준다. 이러한 배향 변화는 막 표면에 존재하는 고분자 사슬들간에 측방향 정렬에 의해 발생함을 X-ray diffraction측정 결과 확인하였다. 문지름에 의한 막 표면만의 물리적인 변화를 AFM, 표면 장력으로써 측정했을 때 비극성 분자들보다 극성인 분자들이 표면에 많이 노출이 되었다. 이로 부터 카르보닐기와같은 극성기가 측방향 정렬에 의해 막 표면에 일정하게 정렬되었을때 액정 분자의 쌍극자와의 상호 작용으로 인해 액정 분자들은 표면과 일정한 각을 이루면서 일정한 배향이 이루어지게됨을 알수 있다. 이러한 결과는 기존의 액정 배향이 주사슬의 방향과 평행한 방향으로 이루어지고 있다는 결과와 상치되고 있다. 위의 결과를 검증하기 위해서, LB(Langmuir-Blodgette)법으로 문지름 이전의 고분자 사슬의 배향을 조절한 다음 문지름 과정에 의해 고분자 배향을 비교했을때 고분자 사슬의 배향 변화는 이루어지지 않고 측방향의 조밀도가 증가하였다. 그리고 액정은 주사슬의 방향이 아니 문지름 방향을 따라 배향이 되었고, 조밀도의 증가는 막 표면에서 측방향 극성분자들의 정렬을 촉진시키게 되고, 액정 배향 상태가 양호해졌다. 이것은 문지름만에 의한 결과를 뒷바침하고 있다. 실제 직교하는 두방향으로 문지름한 액정소자에 적용하였을때 인가한 전계에 대응하는 액정 분자들의 반응은 전계를 증가시켰을 때와는 달리 전계를 감소시킬 때는 다른 반응을 보여 주었다. 문지름에 의한 막표면에서의 고분자 사슬의 배향은 문지름에 의해 쉽게 그 배향을 바꾸지 않지만 상대적으로 짧은 극성기의 배향 변화는 쉽게 이루어짐을 간접적으로 나타내고 있다. 위 결과들을 종합했을때, 배향막내의 곁가지 극성 분자들이 문지름에 의해서 배향막 표면에 노출이 되고 일정한 방향으로 정렬이 될때 액정 분자들의 균일한 배향을 얻을 수 있다.


청구기호 {DCHE 96006
형태사항 xiii, 147 p. : 삽도 ; 26 cm
언어 영어
일반주기 저자명의 한글표기 : 김성목
지도교수의 영문표기 : Jong-Duk Kim
지도교수의 한글표기 : 김종득
학위논문 학위논문(박사) - 한국과학기술원 : 화학공학과,
서지주기 Reference : p. 136-140
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