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Molecular Dynamics Simulations of an Antiferroelectric Liquid Crystalline Molecule


Molecular dynamics simulations using Materials Explorer were carried out for an antiferroelectric liquid crystalline molecule, (S)-4-[1-methylheptyloxycarbonyl] phenyl 4'-octyloxybiphenyl-4-carboxylate (MHPOBC), to analyze its conformational property in smectic liquid crystalline phases. The simulation reproduces a phase transition through the solid-SmCx-SmA-isotropic states. The MHPOBC molecules in the solid-SmCx phase are packed in a tilted layered structure, while in the SmA-isotropic phase the molecular tilt disappears producing a fluid like conformation.



Figure 1. Structure of MHPOBC liquid crystal system from MD simulation at 298K.

The simulations were carried out on a system of 64 MHPOBC molecules under periodic boundary conditions using NTP ensemble at various temperatures and DREIDING and OPLS force-field parameters.



Figure 2. Structure of system at 400K representing the SmCx phase.

The molecular dynamics simulation reproduces the X-ray structure at 298K (Fig. 1) and this is identified as the solid phase. At 400K (Fig. 2), the biphenyl group is packed in an alternately tilted structure of the SmCx phase. At 450K (Fig. 3) the tilted later structure disappears as the molecules uniaxially align along the layer direction. The molecules take on more elongated conformations and this phase is identified as the SmA phase. The average molecular bend angle in this phase agrees with 13C-NMR and 2H-NMR experimental data. At 750K (Fig. 4), the MHPOBC molecules undergo free rotational and translational motion as well as internal bond rotation. The orientation correlation function has a computed value of zero. This phase is thus identified as the fluid like isotropic state.



Figure 3. Structure of system at 450K. The molecules take on a more elongated shape representing the SmA phase.

Materials Explorer is therefore capable of generating the four distinct phases (solid, SmCx, SmA and isotropic) of the MHPOBC liquid crystal.



Figure 4. Structure of system at 750K. The MHPOBC molecules undergo free motion and internal bond rotation representing the fluid like isotropic state.


References: Torumi et al., Molecular Crystals and Liquid Crystals (2003), 402, Number 1, 1.