Abstract
We report the experimental determination of the structure and response to applied electric field of the lower-temperature nematic phase of the previously reported calamitic compound 4-[(4-nitrophenoxy)carbonyl]phenyl2,4-dimethoxybenzoate (RM734). We exploit its electro-optics to visualize the appearance, in the absence of applied field, of a permanent electric polarization density, manifested as a spontaneously broken symmetry in distinct domains of opposite polar orientation. Polarization reversal is mediated by field-induced domain wall movement, making this phase ferroelectric, a 3D uniaxial nematic having a spontaneous, reorientable polarization locally parallel to the director. This polarization density saturates at a low temperature value of ∼6 µC/cm2, the largest ever measured for a fluid or glassy material. This polarization is comparable to that of solid state ferroelectrics and is close to the average value obtained by assuming perfect, polar alignment of molecular dipoles in the nematic. We find a host of spectacular optical and hydrodynamic effects driven by ultralow applied field (E ∼ 1 V/cm), produced by the coupling of the large polarization to nematic birefringence and flow. Electrostatic self-interaction of the polarization charge renders the transition from the nematic phase mean field-like and weakly first order and controls the director field structure of the ferroelectric phase. Atomistic molecular dynamics simulation reveals short-range polar molecular interactions that favor ferroelectric ordering, including a tendency for head-to-tail association into polar, chain-like assemblies having polar lateral correlations. These results indicate a significant potential for transformative, new nematic physics, chemistry, and applications based on the enhanced understanding, development, and exploitation of molecular electrostatic interaction.
Highlights
The first theoretical treatments of collective molecular orientation in liquids, by Debye [1] and Born [2], were electrostatic versions of the Langevin–Weiss model of the paramagnetic/ferromagnetic transition in solids [3]
Following Born’s model, some calamitic molecules without molecular dipoles were found to exhibit nematic phases [14], while ferroelectricity failed to materialize as a molecular nematic phenomenon
Such polar domains and their boundaries are described by the models, by the elasticity and order parameter energetics of the polar phase, making the domains the signature features of spontaneous polar ordering to be probed and understood in characterizing the nature of the phase transition
Summary
The first theoretical treatments of collective molecular orientation in liquids, by Debye [1] and Born [2], were electrostatic versions of the Langevin–Weiss model of the paramagnetic/ferromagnetic transition in solids [3]. We present the direct observation of such spontaneously broken symmetry in the form of domains of opposite polarization, grown without applied electric field, as a firstprinciples demonstration of ferroelectricity in a thermotropic, uniaxial, nematic liquid crystal (LC) of rod-shaped molecules.
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