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

Results are presented from atomistic computer simulations of single molecul es of the liquid crystals 4-n-octyl-4'-cyanobiphenyl and 4-n-heptyl-2-fluor ophenyl 4-octyloxybiphenyl-4'-carboxylate in contact with crystalline polym eric surfaces. The simulations were performed as part of a study of the nat ure of the alignment interactions in liquid crystal displays and other devi ces. In contrast to previous atomistic simulations of this type, the crysta lline polymer surface was represented by a pseudopotential, effectively rep lacing the parallel array of polymer chains with a periodic corrugation. Th e use of a pseudopotential has two main advantages. Firstly, it allows an e xploration of the general principles behind liquid crystal alignment on cry stalline surfaces, free from the obscuring effect of specific chemical inte ractions. Secondly, it permits a significant saving in computer time compar ed with using a surface constructed from explicit atom-pair potentials. In the present work, the aligning capabilities of two simple sinusoidal pseudo potential functions were tested. In each case the wavelength and amplitude of the surface corrugations were varied. It was found that the degree of or ientational order of liquid crystal molecules in contact with the surfaces increased with increasing amplitude and decreasing wavelength of the corrug ations. Aspects of the two potentials were then combined to produce a pseud opotential designed to represent specific polymeric crystal surfaces. In th is case, the (1 0 0) and (1 1 0) faces of polyethylene were modelled. Compa risons with earlier simulations employing atomistic surfaces indicate a goo d 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 time scale.