Emergence of a Persistent Surface-Normal Dynamical Mode in Planar Polymer Brushes

Polymer grafting affects chain relaxation dynamics in ways that can strongly influence the physical properties of polymer nanocomposites and surface layers. Here, we performed energy-conserving dissipative particle dynamics (eDPD) simulations with proper orthogonal decomposition (POD) to investigate the dominant relaxation modes of planar polymer brushes. While the dynamics parallel to the substrate remain largely consistent with classical Rouse behavior, the perpendicular dynamics collapse into a single persistent, surface-localized mode that remains remarkably robust across polymer chain lengths and grafting densities. We show that this mode can be represented by a localized exponential contribution together with a small number of low-order Rouse modes. While the spatial extent of the localized contribution increases with grafting density, the local strength of the graft-induced perturbation becomes approximately independent of grafting density once a brush is formed. Further analysis of the contour-resolved dynamical memory confirms that grafting produces increasingly anisotropic dynamics that extend farther along the polymer contour with increasing grafting density. These results provide a mechanistic framework for understanding how grafting reorganizes polymer dynamics and identify a surface-localized dynamical mode that may underlie the suppression of long-wavelength Rouse modes observed in neutron scattering experiments.