Molecular dynamics simulation of a nematic liquid crystal

Abstract

The article describes molecular dynamics simulations of 4-n-pentyl4'-cyanobiphenyl(5CB) in the nematic phase at 300 K using two interaction models. The first model comprises united atoms, while in the second, shorter simulation,-the hydrogen atoms are explicitly included. Liquid crystalline order parameters were calculated using various definitions of molecular frames and were found to be in reasonable agreement with experiments. Distributions of dihedral angles and relative populations of various conformations in the alkyl chain have been determined. Translational and rotational diffusion processes were investigated using time correlation functions, and were compared with experimental results. Local order parameters, relevant for deuterium nuclear magnetic resonance (NMR) spectra, were determined for the segments in the alkyl chain. Proton NMR line shapes were calculated from the trajectory using an approximate method for determination of the dipole-dipole Hamiltonian matrix. These line shapes were found to be very sensitive to conformational distributions and therefore to the force field used in the simulation.

Thermotropic nematic order upon nanocapillary filling

“…If one assumes a perfect cylindrical geometry and an intermolecular distance of 0.4 nm [35], the molecules added in the next filling steps correspond to ∼1.5 and 2 monolayers (f =0.17 and 0.225). They exhibit sizeable increases in δ(∆n)(T ) upon cooling, which are characteristic of a rearrangement between partial collective orientational order (a paranematic state) to full collective orientational order (nematic order).…”

Paranematic-to-nematic ordering of a binary mixture of rodlike liquid crystals confined in cylindrical nanochannels

“…In our evaluation we rely on intermolecular spacings a 0 of about 5Å, reported in Ref. [48] for 5CB, which is expected to be valid also for 6CB and 7CB. We assume that the thickness of the monomolecular layer next to the pore wall roughly corresponds to the intermolecular distance a 0 resulting in a volume fraction of this layer in a total pore volume,…”

Monte Carlo simulations of systems of rodlike molecules with semiflexible terminal groups

“…The model molecules have been found to be totally extended (with end tails in the all-trans conformation) at all temperatures, both in the anisotropic and in the isotropic phase. This is quite surprising, when it is considered that the MD calculations of Cross and Fung 32) and those of Komolkin, Laaksonen and Maliniak 33) on realistic and pseudorealistic models of 5CB indicate that all-trans conformations account for less than 50% of all conformations in the nematic phase (no data are reported for the isotropic liquid).…”

“…For instance, the translational diffusion coefficient is 0.034 r 2 /cycle in the most sluggish 300 K isotropic phase of system F 60 , while the rotational diffusion coefficients R || and R k are 0.018 and 0.0066 per cycle, respectively (diffusion coefficients are defined and calculated as in ref. 33) by substituting time with number of cycles). Both translational and rotational mobilities are obviously much higher for system RC.…”