Molecular dynamics study on anisotropic water cluster formation in polyethylene: A fundamental perspective on water tree initiation

The microscopic mechanisms of water tree inception in polymeric insulating materials remain a critical challenge for high-voltage engineering. In this study, molecular dynamics simulations were employed to investigate the structural and kinetic responses of water nanoclusters in a polyethylene matrix to external electric fields (0 and 2 V/nm). We demonstrate a fundamental disparity between ion-free pure water and ion-containing (Na+ and SO42−) clusters. In pure water systems, external fields induce macroscopic elongation along the field direction, accompanied by a reduction in the activation energy for hydrogen-bond dissociation. This field-induced dynamic softening kinetically facilitates continuous structural reorganization. Conversely, in ion-containing systems, the intense Coulombic attraction of the ions forms an electrostatically locked “hydration core.” This core exhibits an anomalously high activation energy that remains robustly intact even under 2 V/nm fields, causing kinematic freezing of the cluster’s morphology. These findings suggest that while pure water possesses the kinetic flexibility to undergo one-dimensional anisotropic growth under electrical stress, ionic impurities act as stable, non-deformable aggregation centers. Such contrasting dynamic behaviors provide a fundamental perspective on the initial cluster stabilization and directional elongation processes associated with water tree initiation.