Atomistic Simulations of a Thermotropic Biaxial Liquid Crystal

Abstract

The full-text may be used and/or reproduced, and given to third parties in any format or medium, without prior permission or charge, for personal research or study, educational, or not-for-pro t purposes provided that:• a full bibliographic reference is made to the original source • a link is made to the metadata record in DRO • the full-text is not changed in any way The full-text must not be sold in any format or medium without the formal permission of the copyright holders.Please consult the full DRO policy for further details. We have performed molecular dynamics simulations of a 2,5-bis-(p-hydroxyphenyl)-1,3,4-oxadiazole mesogen ODBP-Ph-C 7 at a fully atomistic level for a range of temperatures within the region that has experimentally been assigned to a biaxial nematic phase. Analysis of the data shows that the simulated nematic phase is biaxial but that the degree of biaxiality is small. The simulations show also the formation of ferroelectric domains in the nematic where the molecular short axis is aligned with the oxadiazole dipoles parallel to each other. Removal of electrostatic interactions leads to destabilization of ferroelectric domains and destabilization of the biaxiality. An additional simulation shows the slow growth of a mesophase directly from the isotropic fluid over a period of approximately 50 ns. This is the first time this has been achieved within the framework of an all-atom model.

Biaxial Nematic Phase in Model Bent-Core Systems

“…The atomistic simulations [21] of the molecular systems studied experimentally [2,3] have confirmed the existence of a weak biaxiality in that system. Similarly, for a system of GB bent-core molecules with long arms the biaxial nematic phase was observed on cooling [22].…”

Molecular simulation of liquid crystals: progress towards a better understanding of bulk structure and the prediction of material properties

“…It is now possible to grow a nematic phase from an isotropic liquid, thereby demonstrating thermodynamic stability. 26 For united atom models without the complication of partial charges, this appears to occur relatively easily within around 10-15 ns, as shown in the work of the work of McBride et al 15 and Hanna et al 27 However, for fully atomistic models, with long range partial-charge interactions, the time scales required appear to be longer. Zannoni has specifically looked to predict transition temperatures for systems of a series of molecules which exhibit an odd-even effect.…”

“…Early simulations of smectic phases have been published; 29 and the first simulation of the recently discovered biaxial nematic phase in a low molecular weight material, showed very unusual ordering of molecules into ferroelectric domains. 26 As yet, what can be achieved is severely limited by system sizes but it would be extremely interesting in the future to get a ''molecular picture'' of the ordering of molecules in banana phases; or spontaneous layer polarization in ferroelectric and antiferrorelectric smectic-C phases; or even to get a better picture of molecular packing, the degree of flexibility and the relative ordering of the molecular core and alkyl chains in tilted smectic phases. It is immensely difficult to get experimental probes of this type of molecular behaviour, and so simulation is likely to provide the major breakthroughs here.…”

Computer Simulations of Biaxial Nematics

“…Experimental values are usually much smaller, in particular for the observed biaxialities R 2 2,2 . Differently from molecular models which offer at most semi-quantitative results, the atomistic simulations with predictive capabilities [58,34,59] can provide reliable estimates of such order parameters as well as other macroscopic properties.…”