Atomistic computer simulations on large ensembles of high-molecular-weight polyaromatic hydrocarbons: The influence of molecular shape on carbonaceous mesophase

Molecular modelling is a particularly useful tool for studying the behaviour of high-molecular-weight pitch components, since these very large and irregular compounds are not available in pure form for fundamental experimental studies. To date detailed computational chemistry has been carried out only on small clusters of such large discotics, while simulations of bulk phases have used simple molecular potentials (e.g. Gay-Berne) that cannot be directly related to specific molecular structure. The present paper presents a isobaric-isothermal Monte Carlo technique for simulating large ensembles of discotic molecules based on massive summation of atomic pair potentials. The technique uses explicit description of atom locations with no adjustable potential parameters and thus allows parametric study of molecular structural features. We report computer simulation results on the phase behaviour of a large regular molecule circumcoronene. These molecules showed the transition from order to disorder via a columnar phase. The simulations show detailed features of stacking and mesophase formation as a function of temperature. Rotational movement was the preferred choice for regular molecules at all temperatures, and this may be responsible for the loss of orientational order concurrently with loss of positional order (simple melting). In contrast, irregular PAH molecules show a predominant preference towards translational rather than rotational motion, especially in the lower temperature simulations.