Dynamics of Aromatic Molecules in Nanochannels of Crystalline Syndiotactic Polystyrene Studied by Molecular Dynamics Simulation

This study investigated the characteristic dynamics of translational diffusion and reorientational relaxation of substituted benzenes confined within nanoporous channels in the ε form of crystalline syndiotactic polystyrene, employing atomistic molecular dynamics simulations. Reorientational motions were analyzed using the autocorrelation functions of the vectors embedded on the phenyl rings. The motions were classified into two main modes: in-plane motion of the phenyl ring associated with the inversion of the molecular long axis and out-of-plane motion of the ring around the long axis. Interestingly, the former obeys the Arrhenius law, whereas the latter does not. This non-Arrhenius behavior was attributed to temperature-dependent variations in the free energy landscape along the reaction coordinates. It was found that the enhancement of the fluctuation of the nanochannels, which is promoted by thermal expansion, played an important role in this landscape modification. The activation energy of the inversion motion was lower for o-xylene than for toluene. The mechanism of this fast relaxation of bulkier ortho-substituted benzene was interpreted in terms of the activation free energy for the inversion motion, which is specifically associated with the deformation of the host matrix in a confined environment. The fundamental aspects obtained provide crucial information for controlling the functionality of nanoporous materials.