以原子層級理論計算探討向列式液晶分子的官能基對旋轉黏度係數的影響

Abstract

液晶分子是目前最廣泛應用的顯示器材料，旋轉黏度是該材料重要的物理特性之一，如果能夠有效降低其旋轉黏度，則可以減少面板的反應時間而直接提升產品所能呈現出之效能。 在本研究中，我們使用分子動態模擬搭配AMBER全原子力場模型，並透過Sarman、Nemtsov-Zakharov以及Fialkowski三種統計力學模型分析預測液晶分子之旋轉黏度係數。我們選擇了三種目前廣泛應用於市面上的液晶分子作為研究的主要目標。這三種液晶分子的化學結構相近，但是旋轉黏度卻有很大的差異，而本計劃即藉由分子模擬來探討影響其物性變化的關鍵機制。我們研究的結果顯示利用我們所建立的分子動態模擬程序能夠準確地預測三種液晶分子的相轉變溫度、光電性質與旋轉黏度係數。我們的結果也顯示旋轉黏度係數及相轉變溫度都與液晶分子間的作用力呈現正相關的趨勢，但是卻與過往文獻認為是主因的結構因子沒有明顯直接的關聯。此外，我們也分析液晶分子各區段結構的旋轉黏度、液晶分子的堆疊方式以及偶極矩分布，我們發現替換官能基會改變旋轉黏度係數的主要原因是官能基會透過偶極-偶極交互作用使得液晶分子形成特定的堆疊方式，影響分子中苯環的旋轉能力，因此改變了液晶分子的旋轉黏度係數。 除了純分子系統，我們也分析了包含兩種不同結構液晶分子的混合分子系統之旋轉黏度係數。我們的結果顯示這三種液晶分子的旋轉黏度係數確實會受到鄰近相異結構的影響。我們分析混合分子系統的堆疊方式，並發現這是由於官能基透過偶極-偶極交互作用，使得混合分子系統中的液晶分子表現出與純分子系統不同的堆疊方式所致。

Liquid crystal displays (LCD) are the most common flat panel displays. The switching time of an LCD is proportional to the rotational viscosity of the liquid crystal. Low rotational viscosity is an absolute prerequisite for the TV application. Based on the atomistic AMBER force-field potential model, we have applied molecular dynamic (MD) simulations to investigate the structural, dynamic, and transport properties of three typical liquid-crystal material systems as well as their important correlations in-between. We can obtain the rotational viscosity of LC molecules via the calculations of their order parameters and the time correlation functions of the LC directors using our in-house Fortran code. Our results first showed that the phase transition temperatures, the optical and dielectric properties and the rotational viscosities of these three LC molecules can be accurately predicted using MD simulations in conjunction with the AMBER force field model. Our results further revealed a new idea that the interactions between LC molecules can be more critical in determining the rotational viscosity than their structure order parameter as suggested in the early literature. We also analyzed the rotational viscosity coefficient the aromatic rings and alkyl chain segments, the stacking configurations, and the dipole moment distribution for each LC molecules. We found that the rotational viscosity of LC molecules can be effectively reduced is mainly attributed to the fact that the rotational motions of the aromatic rings can be largely enhanced by the particular stacking configurations, which were due to the dipole-dipole interaction of the functional groups. Besides the pure molecular systems, we also calculated the rotational viscosity coefficients of the liquid crystal mixtures, which were formed with two kinds of liquid crystals. Our results showed that the rotational viscosity coefficients of these liquid crystals were affected by the liquid crystals nearby with the different molecular structures. We also analyzed the stacking configurations of the liquid crystal mixtures. We found that the difference between the rotational viscosity coefficients of the pure molecular systems and the ones of the liquid crystal mixtures results from the preferred stacking configurations, which were due to the dipole-dipole interaction of the functional groups of the two different liquid crystals.