Molecular dynamics simulations of the glass former ortho-terphenyl

Abstract

An 18-site, three-ring model has been developed for the van der Waals system ortho-terphenyl (OTP) which has been studied extensively experimentally because of its glass forming ability. The method of constraints has been used to freeze out the fast internal modes of the molecule, but the model retains some internal motion in the form of side-ring torsions. When used in molecular dynamics simulations, the model provides a reasonable representation of the properties of OTP in the liquid and supercooled liquid states, including the volume–temperature behavior and diffusion coefficients. The glass transition temperature has been obtained from the break in the slope of the volume–temperature curve and found to agree with experimental values, given the high cooling rates of the simulations. The short time dynamics of the system have been probed using velocity autocorrelation functions, mean-square displacements, van Hove correlation functions, and intermediate scattering functions. The dynamics of the model can be interpreted consistently within a molecular cage framework. It is found that the lifetimes of the transient cages increase significantly with decreasing temperature until, in the glass, cage breakup can no longer occur on the time scale of the simulations. Two distinct regimes are seen in the single particle motion in the liquid; these correspond to localized motion within the cage and the diffusive motion that sets in after cage breakup. Around the supercooled liquid region, a subdiffusive behavior occurs between the two regimes—indicative of the increasing difficulty encountered by the cage breakup mechanisms.