with τ rot,k and β rot,k as fitting parameters. In Eq. ( ), α d is a time-dependent unit vector in the direction of the water dipole, such that α d • x = cos α d with x being an (arbitrary) unit vector, here chosen in the x-direction. Therefore, RACF k (t) isolates orientational decorrelation resulting from decoupled motion of the water and the polymer-water interface. A characteristic relaxation time for rotation in each layer is then computed as