Edge Group Chemistry and Position Modulate Rotational Diffusion of Graphene Nanosheet in Polymer Melts

The rotational dynamics of functionalized graphene nanosheets within polymer matrices are crucial for determining the final microstructure and anisotropic properties of nanocomposites. However, the influence of functional group type and substitution site on such dynamics remains poorly understood. This study employed united-atom molecular dynamics (UAMD) simulations to investigate the anisotropic rotational diffusion of graphene nanosheet with various edge functional groups in polyethylene glycol (PEG) melts. The results reveal that the rotational diffusion coefficients are strongly influenced by the interfacial interaction energy, with stronger interaction generally hindering rotation. However, specific effects such as hydrogen bonding and steric hindrance can lead to deviations from this general trend, resulting in complex rotational behavior. While the substitution site of an identical functional group yields similar interaction energies, it induces notable anisotropy in rotational diffusion due to variations in the frictional torque dependent on the distance from the rotation axis. Analysis of the rotational angle distribution demonstrates a transition from non-Gaussian behavior at short times to Gaussian diffusion at long times, which is quantitatively rationalized by a continuous-time random walk (CTRW) model. These findings highlight the critical role of surface chemistry in governing the orientational dynamics of nanofillers and provide fundamental insights for the interfacial design of high-performance graphene/polymer nanocomposites.