MOLECULAR-DYNAMICS SIMULATIONS OF THE GLASS FORMER ORTHO-TERPHENYL

An 18-site, three-ring model has been developed for the van der Waals system ortho-terphenyl (OTP) which has been studied extensively experi mentally because of its glass forming ability. The method of constrain ts 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 provi des a reasonable representation of the properties of OTP in the liquid and supercooled liquid states, including the volume-temperature behav ior and diffusion coefficients. The glass transition temperature has b een obtained from the break in the slope of the volume-temperature cur ve 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 disp lacements, 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 t he transient cages increase significantly with decreasing temperature until, in the glass, cage breakup can no longer occur on the time scal e of the simulations. Two distinct regimes are seen in the single part icle motion in the liquid; these correspond to localized motion:within the cage and the diffusive motion that sets in after cage breakup. Ar ound the supercooled liquid region, a subdiffusive behavior occurs bet ween the two regimes-indicative of the increasing difficulty encounter ed by the cage breakup mechanisms. (C) 1995 American Institute of Phys ics.