MOLECULAR ORIENTATIONAL MOTION IN PCH5 - COMPUTER-SIMULATIONS AND MODELS

The tumbling molecular motion in mesomorphic systems in discussed, bas ed on a molecular dynamics computer simulation of a typical mesogen PC H5 within the united atom approach. It is shown that conformational mo lecular flexibility plays an important role not only in the orientatio nal distribution of the molecules in the mesophase, but also in their dynamics. Being subjected to strongly fluctuating intermolecular force s, the relative position of the atoms in the molecule exhibit relaxati on in the same time-scale as the intermolecular torque, which is much shorter than expected from the corresponding molecular moments of iner tia. This results in a very fast short-time decay of the re-orientatio nal autocorrelation functions of the molecule fragments. In spite of t his rather complicated picture, cage models can be used to fit the re- orientational autocorrelation functions. For a more precise fit to the experimental functions one has to account for the real distribution f unction of the cage potential curvature.