Simulation of advanced materials with atomic resolution: atomistic modelling of smectic liquid crystal mesophases

Poll, Kristian (2022) Simulation of advanced materials with atomic resolution: atomistic modelling of smectic liquid crystal mesophases. Doctoral thesis, Northumbria University.

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Abstract

Fully atomistic molecular dynamics simulations have been used to investigate the mesophase behaviour of three liquid crystal compounds, an (S)-hexyl lactate derivative, 9HL, 2−{4′−[1′′,1′′−dihydro−2′′−(2′′−perfluorobutoxyperfluoroethoxy)−perfluoroethoxy]}phenyl−5′−octylpyrimidine, 3M 8422, and 4-[3’ nitro-4’-(R)-1(methylhexyloxy)phenyl]phenyl 4-(6-heptylmethoxyltrisiloxyhexyloxy)benzoate, TSiKN65. All of these materials, despite their very different structural features, are reported to exhibit de Vries character.

Force constant parameterisation was performed on molecular fragments representing the perfluoropolyether and trisiloxane terminal chains of mesogens 3M 8422 and TSiKN65, respectively, before the subsequent simulations were used to examine the de Vries behaviour exhibited by these two materials.

For all compounds, simulations were carried out across a broad temperature range, encompassing both the SmA and SmC phases. Simulated trends in orientational order parameters, translational order parameters, layer spacings, and tilt angles were compared to results from experimental investigations and were found to be largely consistent. Further analysis of the simulations revealed the simulated results were inconsistent with many of the conventional models proposed to account for de Vries behaviour, but examination of individual sub-units comprising the liquid crystal compounds showed that distinct aromatic and lactate/fluorinated/siloxane sub-layers were formed by all three compounds, and these sub-layers exhibited very different orientational behaviour across the simulated smectic A and smectic C temperature ranges. The aromatic sub-layers exhibited fairly conventional tilt behaviour, aligning along the layer normal in the smectic A phase, but becoming tilted relative to the layer normal when cooled to the smectic C phase. However, in contrast, the orientation of the lactate/fluorinated/siloxane sub-layers were found to remain largely unchanged and retained a smectic A-like configuration even in the smectic C phase, providing a potential explanation for de Vries behaviour, in which only a fraction of the layer structure exhibits an overall tilt in the smectic C phase. In addition, analysis of the phases formed in simulations of the partially perfluorinated compound 3M 8422 showed that the contraction of the aromatic and aliphatic sublayers caused by the onset of molecular tilt was partially compensated by a simultaneous expansion of the perfluoropolyether sublayers. These observations highlighted that the exact factors that promote de Vries behaviour still remain somewhat unresolved, but a deep insight into molecular behaviour can be gained from atomistic simulations.

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