A model for translational and orientational relaxation in glass forming materials

Spectroscopic techniques and the MD simulations allow for studying molecular motions in glass forming materials, which are believed to hold the key to understanding the glass transition phenomena. The mean squared displacement and the orientational autocorrelators exhibit several generic features, including: the sub-diffusive regime, dynamic heterogeneity, distinct α- and β-processes, and the fact that in some cases the cos²θ orientational autocorrelator decays slower than the cosθ autocorrelator. A mechanism responsible for these features is poorly understood. The current models are based on the picture of effective energy barriers, which carries a number of well-known conceptual difficulties. In this communication, a random walk model is proposed that does not involve energy barriers. The model relies on the tortuosity of the path a molecule travels to account for the apparent slowing down of mobility. The model captures characteristic features of the translational and orientational relaxation in glass forming materials.