含鹵素之強誘電性液晶單體及其側鏈液晶高分子之合成與性質之探討

Abstract

液晶聚合物因有多元化之光電用途，深具研究之潛力;而液晶中的SC*相因具有強誘電性性質，而顯得格外重要。強誘電性液晶(Ferroelectric liquid crystal, FLC)具雙穩定狀想(bistable states)，在電場下可控制其在穩定狀態間切換，由於切換的時間相當短(μs)，比向列相液晶的切換時間快了好幾次方，因此強誘電性液晶可用於高畫質電視之顯示上；而強誘電性液晶高分子則可應用於可擦拭光碟的記錄媒體材料，可以有效的提高其應答速度。 本研究歸納實驗室數年來研究之成果，設計出以MDn12Cl為藍本，改變末端基旋光中心取代基，將氟原子及溴原子取代氯原子。因MDn12Cl液晶分子結構之液晶基堅硬度適當，末端基之部份為雙旋光中心。最重要的是，在MDn12Cl液晶單體系列中，MD112Cl擁有高自發極化值389nC/cm2，以及MD312Cl在SC*相中有數十μs 至1ms的應答時間。以鹵素原子之陰電性趨勢(F>Cl>Br)將氟原子及溴原子取代氯原子，可觀察出偶極矩的變化對液晶分子之熱運動的影響，以及對液晶分子光電特性之影響。 研究分成兩部份： 第一部份為新型液晶單體之合成、液晶相行為之探討。合成二系列之新型液晶單體MDn12F、MDn12Br，以及合成MD312Cl作為比較之基準。液晶相熱行為經由熱微差掃瞄卡計、偏光顯微鏡以及粉末x-ray繞射加以鑑定液晶相的種類與排列結構，探討不同末端旋光基對液晶相行為之影響。 結果顯示，含氟原子及溴原子之液晶單體之液晶相種類與含氯原子之液晶單體類似，其差異點在於各個液晶相溫度之分佈。 第二部份為強誘電性側鏈液晶均聚合物之合成、鑑定與介電測試。選擇第一部份所合成之液晶單體進行與聚矽氧烷之矽氫化接枝反應，。其液晶相的鑑定與分析亦以熱微差掃瞄卡計、偏光顯微鏡以及粉末X-ray繞射來分析液晶相的種類與液晶分子在聚合物中的排列結構。並介電分析討論含不同鹵素偶極之強誘電性側鏈均聚合物對液晶相行為的影響，將此結果作為液晶光電測試之參考。 結果顯示，高分子聚矽氧烷主鏈的導入，不僅擴展了整體液晶相的溫度範圍，並也擴展了Sc*相可達約100℃的範圍。並且由介電分析得知，具較強偶極之含氟強誘電性液晶側鏈均聚合物PS312F在電場之作用下，其Goldstone mode緩和強度在SC*相中隨溫度之升高而加強，且其介電緩和與損失常數均大於含氯強誘電性液晶側鏈均聚合物PS312Cl。此結果表示，若於相同機制之光電測試，也必將有相同之結果。

Liquid-crystalline polymers possess versatile optical-electric applications and therefore are potential research subjects. The Sc* phase of liquid crystals is extremely important because of its ferroelectric property. The ferroelectric liquid crystals (FLC) have bistable states and can be switched between them by controlling the electric field. The switch time is so short (μs) that it is several times order faster than the nematic liquid crystal phase, hence the FLC is applicable in high-fidelity televisions. The FLC polymers can also be adopted as a recording material in the erasable compact discs to elevate the response rate significantly. This research concluded the results over recent years. It was to design molecules based upon MDn12Cl from which the terminal chiral center had been changed by substituting fluorine or bromine atoms for chlorine. The terminal group of MDn12Cl molecule is a two-chiral center due to the appropriate hardness of its liquid crystalline segment. Among the series of the MDn12Cl liquid crystalline monomers, MD112C has high spontaneous polarization (389 nC/cm2) and MD312Cl has fast response time between tens ofμs to 1 ms. According to the electronegativity of halogens (F>Cl>Br), effects of substitution of Cl by F and Br atoms were observed. Both the thermal movement and the electro-optical properties of liquid crystalline molecules are influenced by the change of the dipole moment.