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Archive for March, 2020

Molecular dynamics simulations of nematic phases formed by cyano-biphenyl dimers

March 15th, 2020 Comments off

AG Vanakaras, DJ Photinos, Liquid Crystals 45 (13-15), 2184-2196, (2018).

Abstract: Molecular dynamics simulations of selected members of the cyano-biphenyl (CB) series of dimers (CBnCB) have been set up using atomistic detail for the description of the alkyl spacer and coarse-grained representation of the CB end-segments. Detailed results are presented for the CB7CB dimers, showing an isotropic fluid phase and two nematic phases. The positional and orientational correlation functions extracted from the simulations are used to elucidate the structure of the low-temperature nematic phase. Polar molecular ordering is clearly identified along a direction undergoing helical twisting at right angles to the helical axis, with a constant pitch of about of 8 nm. The local ordering of the various molecular segments is calculated and found to be in excellent agreement with experimental NMR measurements. Key findings of the simulation are shown to be correctly predicted by the theoretical model of the polar-twisted nematic (NPT) phase [A.G. Vanakaras, D.J. Photinos, Soft Matter. 12 (2016) 2208–2220]. The complete failure of the usual twist bend model (NTB) to account for these findings is demonstrated.

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Polar molecular ordering in the Nx phase of bimesogens and enantiotopic discrimination in the NMR spectra of rigid prochiral solutes.

March 15th, 2020 Comments off

A. Kumar, A. G. Vanakaras, D. J. Photinos, J. Phys. Chem. B, 121(47), 10689-10703 (2017).

Abstract The potential of mean torque governing the orientational ordering of prochiral solutes in the two nematic phases (N and NX) formed by certain classes of symmetric achiral bimesogens is formulated and used for the analysis of existing NMR measurements on solutes of various symmetries dissolved in the two phases. Three distinct attributes of the solvent phase, namely polarity of the orientational ordering, chirality of the constituent molecules, and spatial modulation of the local director, are identified as underlying three possible mechanisms for the generation of chiral asymmetry in the low temperature nematic phase (NX). The role and quantitative contribution of each mechanism to enantiotopic discrimination in the NX phase are presented and compared with the case of the conventional chiral nematic phase (N*). It is found that polar ordering is essential for the appearance of enantiotopic discrimination in small rigid solutes dissolved in the NX phase and that such discrimination is restricted to solutes belonging to the point group symmetries Cs and C2v.

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