Atomistic Modeling and Simulations of the Effect of Functional Groups on the Rotational Viscosity of Nematic Liquid Crystals

• Main Authors: Li-Yang Su, 蘇立揚
• Other Authors: Chin-Lung Kuo
• Format: Others
• Language: zh-TW
• Published: 2018
• Online Access: http://ndltd.ncl.edu.tw/handle/8uru9a

碩士 === 國立臺灣大學 === 材料科學與工程學研究所 === 106 === 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.