Computer simulations of the interactions between liquid crystal molecules and polymer surfaces III. Use of pseudopotentials to represent the surface

Abstract

Results are presented from atomistic computer simulations of single molecules of the liquid crystals 4-n-octyl-4'-cyanobiphenyl and 4-n-heptyl-2-fluorophenyl 4-octyloxybiphenyl-4'-carboxylate in contact with crystalline polymeric surfaces. The simulations were performed as part of a study of the nature of the alignment interactions in liquid crystal displays and other devices. In contrast to previous atomistic simulations of this type, the crystalline polymer surface was represented by a pseudopotential, effectively replacing the parallel array of polymer chains with a periodic corrugation. The use of a pseudopotential has two main advantages. Firstly, it allows an exploration of the general principles behind liquid crystal alignment on crystalline surfaces, free from the obscuring effect of specific chemical interactions. Secondly, it permits a significant saving in computer time compared with using a surface constructed from explicit atom-pair potentials. In the present work, the aligning capabilities of two simple sinusoidal pseudopotential functions were tested. In each case the wavelength and amplitude of the surface corrugations were varied. It was found that the degree of orientational order of liquid crystal molecules in contact with the surfaces increased with increasing amplitude and decreasing wavelength of the corrugations. Aspects of the two potentials were then combined to produce a pseudopotential designed to represent specific polymeric crystal surfaces. In this case, the (1 0 0) and (1 1 0) faces of polyethylene were modelled. Comparisons with earlier simulations employing atomistic surfaces indicate a good agreement between the orientation functions produced by the two methods. However, the pseudopotential approach uses significantly less computer time, allowing a more reliable determination of orientation within a given timescale.