Molecular orientational motion in PCH5: computer simulations and models

The tumbling molecular motion in mesomorphic systems in discussed, based on a molecular dynamics computer simulation of a typical mesogen PCH5 within the united atom approach. It is shown that conformational molecular flexibility plays an important role not only in the orientational distribution of the molecules in the mesophase, but also in their dynamics. Being subjected to strongly fluctuating intermolecular forces, the relative position of the atoms in the molecule exhibit relaxation in the same time-scale as the intermolecular torque, which is much shorter than expected from the corresponding molecular moments of inertia. This results in a very fast short-time decay of the reorientational autocorrelation functions of the molecule fragments. In spite of this rather complicated picture, cage models can be used to fit the reorientational autocorrelation functions. For a more precise fit to the experimental functions one has to account for the real distribution function of the cage potential curvature.