The second autocorrelation function examines the rotational diffusion of water molecules relative to the polymer-water interface. In this calculation, waters are first assigned to one of four layers (L 1 , L 2 , L 3 , or L 4 ) defined above the polymer-water interface. Each layer is nominally 2 Å in thickness, such that whether a water molecule is present in a given layer at a specific time is given by