Atomistic simulations of nematic phases formed by cyano-biphenyl dimers

TL;DR

This study uses atomistic molecular dynamics simulations to explore the structure of nematic phases in cyano-biphenyl dimers, revealing a polar-twisted nematic phase with helical molecular ordering, aligning well with experimental data and challenging traditional models.

Contribution

It provides detailed atomistic insights into the nematic phases of cyano-biphenyl dimers, especially the polar-twisted nematic phase, and compares simulation results with experimental and theoretical models.

Findings

Identification of a polar-twisted nematic phase with helical molecular order.

Simulation results agree with experimental NMR measurements.

Traditional twist-bend models fail to explain the observed phase structure.

Abstract

Molecular dynamics simulations of selected members of the cyano-biphenyl series of dimers (CBnCB) have been set up using atomistic detail interactions among intermolecular pairs of united atoms and allowing fully for the flexibility of the spacer chain. 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 8nm. 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.