Abstract:We find that the splay nematic phase can be chemically induced in binary mixtures of two materials, neither of which exhibits the splay nematic phase in their neat state.
“…We do not observe a splay modulated structure in our simulations (Fig. 5a ); however, this is not unexpected given the modulation period of the N S phase has been measured to be on the order of several microns 3 , 11 , whereas the final dimensions of each simulation box are ~10 × 6 × 6 nm 3 . Fig.…”
Section: Resultssupporting
confidence: 55%
“…the twist deformation 16 . Experimentally, a modulated structure with microns size periodicity has been observed 3 , 11 . It is anticipated that when the material is confined to a layer with a thickness comparable to the periodicity, more metastable structures can occur 17 .…”
Section: Introductionmentioning
confidence: 95%
“…Because molecular shape lacks head-tail symmetry, polar ordering of the molecules causes orientational elastic instability. The transition between the phases is a ferroelectric–ferroelastic transition 3 , in which a divergent susceptibility, typical of a ferroelectric transition, is accompanied by the softening of the splay elastic constant which causes the low-temperature ferroelectric phase to be non-uniform 3 , 11 , i.e. ferroelectric splay-nematic phase (N S ).…”
Nematic liquid crystals have been known for more than a century, but it was not until the 60s–70s that, with the development of room temperature nematics, they became widely used in applications. Polar nematic phases have been long-time predicted, but have only been experimentally realized recently. Synthesis of materials with nematic polar ordering at room temperature is certainly challenging and requires a deep understanding of its formation mechanisms, presently lacking. Here, we compare two materials of similar chemical structure and demonstrate that just a subtle change in the molecular structure enables denser packing of the molecules when they exhibit polar order, which shows that reduction of excluded volume is in the origin of the polar nematic phase. Additionally, we propose that molecular dynamics simulations are potent tools for molecular design in order to predict, identify and design materials showing the polar nematic phase and its precursor nematic phases.
“…We do not observe a splay modulated structure in our simulations (Fig. 5a ); however, this is not unexpected given the modulation period of the N S phase has been measured to be on the order of several microns 3 , 11 , whereas the final dimensions of each simulation box are ~10 × 6 × 6 nm 3 . Fig.…”
Section: Resultssupporting
confidence: 55%
“…the twist deformation 16 . Experimentally, a modulated structure with microns size periodicity has been observed 3 , 11 . It is anticipated that when the material is confined to a layer with a thickness comparable to the periodicity, more metastable structures can occur 17 .…”
Section: Introductionmentioning
confidence: 95%
“…Because molecular shape lacks head-tail symmetry, polar ordering of the molecules causes orientational elastic instability. The transition between the phases is a ferroelectric–ferroelastic transition 3 , in which a divergent susceptibility, typical of a ferroelectric transition, is accompanied by the softening of the splay elastic constant which causes the low-temperature ferroelectric phase to be non-uniform 3 , 11 , i.e. ferroelectric splay-nematic phase (N S ).…”
Nematic liquid crystals have been known for more than a century, but it was not until the 60s–70s that, with the development of room temperature nematics, they became widely used in applications. Polar nematic phases have been long-time predicted, but have only been experimentally realized recently. Synthesis of materials with nematic polar ordering at room temperature is certainly challenging and requires a deep understanding of its formation mechanisms, presently lacking. Here, we compare two materials of similar chemical structure and demonstrate that just a subtle change in the molecular structure enables denser packing of the molecules when they exhibit polar order, which shows that reduction of excluded volume is in the origin of the polar nematic phase. Additionally, we propose that molecular dynamics simulations are potent tools for molecular design in order to predict, identify and design materials showing the polar nematic phase and its precursor nematic phases.
“…The polarity of the splay phase was further proven by second harmonic generation (SHG) imaging, showing that the splay modulation period at the transition is of the order of 5-10 µm. Modulation in the µm range was also reported in a chemically induced N S phase, in the mixture of two materials which neither of them exhibits the N S phase: a non-mesogenic bent-core material, which retains some of the features of RM734, and a nematic phase made of molecules analogous in structure to RM734 but with longer terminal chains [12]. Such mixture exhibits the N S phase at room temperature (8 degrees below the N-N S transition), for which periodicities ~9 µm have been observed optically.…”
The recent discovery of the ferroelectric nematic phase has opened the door to experimental investigation of one of the most searched liquid crystal phases in decades, with high expectations for future applications. However, at this moment, there are more questions than answers. In this work, we examine the formation and structure of large polar nematic domains of the ferroelectric nematic material RM734 in planar liquid crystals cells with different aligning agents and specifications. We observe that confining surfaces have a strong influence over the formation of different types of domains, resulting in various twisted structures of the nematic director. For those cells predominantly showing mm-scale domains, we investigate the optical and second harmonic generation switching behaviour under applications of electric fields with a special focus on inplane fields perpendicular to the confinement media rubbing direction. In order to characterise the underlying structure, the polar optical switching behaviour is reproduced using a simplified model together with Berreman calculations.
“…In the last two years, experiments have actually reported a modulated phase induced by spontaneous splay. [22][23][24][25] In these experiments, the splay elastic constant is reduced, because splay deformation is compatible with molecular shape. As the temperature decreases, the splay elastic constant decreases further, and then the system has a transition from the uniform nematic phase into a modulated phase, which has been called the splay nematic (N S ) phase.…”
Recent experiments have reported a novel splay nematic phase, which has alternating domains of positive and negative splay. To model this phase, previous studies have considered a 1D splay modulation of the director field, accompanied by a 1D modulation of polar order. When the flexoelectric coupling between splay and polar order becomes sufficiently strong, the uniform nematic state becomes unstable to the formation of a modulated phase. Here, we re-examine this theory in terms of a new approach to liquid crystal elasticity, which shows that pure splay deformation is double splay rather than planar single splay. Following that reasoning, we propose a structure with a 2D splay modulation of the director field, accompanied by a 2D modulation of polar order, and show that the 2D structure generally has a lower free energy than the 1D structure.Another type of asymmetry is a bent molecular shape, as occurs in dimers, trimers, and bent-core liquid crystals. 1 In the nematic phase of bent molecules, the bend flexoelectric effect is enhanced 2-4 and the bend elastic constant is reduced, because bend deformation is compatible with the molecular shape. As the temperature decreases, the bend elastic constant decreases further, and then the system has a transition from the uniform nematic phase into a modulated phase with spontaneous bend. Because it is impossible to fill space with pure uniform bend, the phase must have a more complex structure that includes another deformation mode. Normally, the system forms a twist-bend nematic (N T B ) phase, which has a heliconical structure with twist as well as bend. This phase has been investigated through many theoretical 5-13 and experimental [14][15][16][17][18][19][20] studies. An alternative theoretical possibility is a splay-bend nematic (N SB ) phase, which has a planar structure with splay as well as bend. 5,6,8 Considering that a bent molecular shape leads to spontaneous bend, one might ask whether a splayed or pear-like shape leads to spontaneous splay. This issue was considered in a theoretical
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