Synthesis of Soluble Polyimides for the Applications of Liquid Crystal Alignment Layers
Dr. Chain-Shu Hsu
|Keywords:||可溶性;聚醯亞胺;液晶;配向膜;桂皮酸酯;光配向;不需摩擦;Soluble;Polyimide;Liquid Crystal;Alignment Layer;Cinnamate;Photoalignment;Nonrubbing|
本研究第二部份的目的是合成出含桂皮酸酯基側鏈之可溶性聚醯亞胺，並探討其在液晶光誘導配向膜上的應用性。實驗結果顯示，藉由導入桂皮酸酯基側鏈所合成的感光性聚醯亞胺高分子（PICA），經由低能量線性偏極化紫外光（LPUV）照射下，具有排列液晶分子的能力。光誘導液晶分子排列方向垂直於LPUV偏極化方向，且液晶配向的均勻性在85℃ 450小時長時間測試都沒有絲毫變差。此外，利用增加側鏈感光基密度或是摻混光增感劑的方式提高光誘導配向的感光度的目的並沒有實現，增加(2+2) photo-dimerization的反應速率反而導致PICA薄膜的光誘導配向能力變差。利用分子模擬計算，本研究成功地描述PICA薄膜的光誘導配向機制。實驗中發現，透過適當塗佈溶劑的選擇，可以提高PICA配向膜的光誘導配向性質與縮短曝光能量。利用二次曝光法可以在PICA配向膜上產生小預傾角，透過材料改質（PICA-co-PIC18）與照光製程（single NPUV exposure）方法，雖然可以使感光性聚醯亞胺的預傾角有所提升，然而這些方法都會使光誘導配向性質變差。由於PICA具有高溶解性，優異的熱穩定性質，且不必摩擦定向，是良好的IPS型液晶顯示器低溫製程配向材料。|
Synthesis of alkyl-branched and solvent-soluble copolyimides (coPIs) and their applications on liquid crystal (LC) alignment layers were described in the first part of this thesis. Synthesis of cinnamate-based and solvent-soluble polyimides (PIs) and their applications on LC photoalignment layers were described in the second part of this thesis. In the first part of this thesis, three series of coPIs containing long alkyl branches were synthesized using the two-step method via poly(amic acid) precursors and chemical imidization. Their applications as LC alignment layers were evaluated, and correlation between coPI backbone structures and LC pretilt angles were discussed. Three dianhydrides and four diamines were used to modify the main chain structures of coPIs. Most of the coPIs prepared are soluble in polar organic solvents. Good liquid crystal alignment was achieved by buffing the spin-coated coPI films on the indium-tin-oxide glass substrates. The LC pretilt angles vary wildly from 0.16 to 15.54 degrees even though the coPIs are branched with the same amount of alkyl group. It was found that both main chain structures and dipole interactions play key roles in determining the pretilt angle. The main chain coPIs with long alkyl side chain, small dipole, linear, symmetric and rigid core structures are favorable for generating large LC pretilt angles. In the second part of this thesis, a new photo-crosslinkable and solvent-soluble polyimide containing cinnamate side chains (PICA) was developed for aligning nematic liquid crystals (LCs). Good LC alignment was achieved by exposing a long-wave linearly polarized ultraviolet (LPUV) light to the PICA film. The LC alignment direction is found perpendicular to the polarization axis of the incident LPUV light. The uniform alignment of LC molecules induced by PICA films remains intact after being heated at 85°C for 450 hours. Two methods were developed for enhancing the photosensitivity of PICA. One is to increase the content of cinnamate side chain in PICA main chain (synthesis of PICA2) and another is to blend some photosensitizer into PICA films. Our experimental results show that increasing the rate of photo-dimerization decreases the alignment ability of the PICA films. Using the molecular simulation method, we successfully describe the photoalignment mechanism of the PICA films. It is also found that photo-induced LC alignment ability of the PICA film could be enhanced through choosing a proper coating solvent. A small pretilt angle on the PICA film was generated by the double exposure method. The pretilt angle on the PICA film could be enhanced by introducing long alkyl side chains to PI main chain (synthesis of PICA-co-PIC18) or by nonpolarized UV single exposure method. However, these two methods also led poor photoalignment ability. The PICA films exhibit good solubility in several polar solvents, low curing temperature and excellent thermally stability while eliminating charge trapping centers. Useful applications for the in-plane switching and low temperature poly-silicon TFT-LCDs are foreseeable.
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