where 1 d 0 (t) is an indicator function that specifies whether a water molecule is within a distance d 0 of the water-polymer interface at time t (i.e., 1 d 0 (t) = 1 if d(t) < d 0 and is zero otherwise). We specify d 0 = 2 Å and use a multi-exponential fit with parameters a i and τ res,i to estimate contributions from different relaxation processes. We select n τ = 3 or 4 based on model quality and parsimony according to a Bayesian information criterion (see Supporting Information, Section S9). A characteristic mean relaxation time τres can be computed by integrating Eq. ( ) from t = 0 to t = ∞.