可溶性聚醯亞胺之合成及其在液晶配向膜上的應用

Abstract

本論文主要分為二個部份，第一部份為含長碳鏈側鏈之可溶性聚醯亞胺的合成及其在液晶配向膜上的應用，第二部份為含桂皮酸酯基側鏈之可溶性聚醯亞胺的合成及其在光誘導液晶配向膜上的應用。 本研究第一部份的目的是利用二步合成法合成出含18個烷基長碳鏈側鏈的可溶性聚醯亞胺，並探討其在液晶配向膜上的應用性。為了探討主鏈結構的改變對含長碳鏈側鏈之聚醯亞胺的溶解性、玻璃轉移溫度、熱安定性質以及液晶配向性質的影響，本研究選擇三個雙酸酐單體以及四個雙胺單體以共聚合的方法改變主鏈結構合成出三個系列的共聚醯亞胺。大部分的共聚醯亞胺可溶解在極性溶劑中，所合成9種可溶性共聚醯亞胺薄膜在定向摩擦後，都可以均勻地排列液晶分子。儘管各共聚醯亞胺都含相同比例的長碳鏈側鏈，所測得的預傾角值卻分布很廣，最低至0.16°，最高到15.54°。實驗結果發現，主鏈立體結構和主鏈與液晶分子間的偶極作用力是影響液晶預傾角最主要的關鍵。本研究所得的結論是，線性的、對稱的、剛硬的以及偶極值較小的聚醯亞胺主鏈，在導入長碳鏈側鏈之後會有較高的機會得到高預傾角。 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.