  For a wide range of modern soft functional materials the selective transport
of sub-nanometer-sized molecules (`penetrants') through a stimuli-responsive
polymeric membrane is key to the desired function. In this study, we
investigate the diffusion properties of penetrants ranging from non-polar to
polar molecules and ions in a matrix of collapsed Poly(N-isopropylacrylamide)
(PNIPAM) polymers in water by means of extensive molecular dynamics
simulations. We find that the water distributes heterogeneously in fractal-like
cluster structures embedded in the nanometer-sized voids of the polymer matrix.
The nano-clustered water acts as an important player in the penetrant
diffusion, which proceeds via a hopping mechanism through `wet' transition
states: the penetrants hop from one void to another via transient water
channels opened by rare but decisive polymer fluctuations. The diffusivities of
the studied penetrants extend over almost five orders of magnitude and thus
enable a formulation of an analytical scaling relation with a clear
non-Stokesian, exponential dependence of the diffusion coefficient on the
penetrant's radius for the uncharged penetrants. Charged penetrants (ions)
behave differently as they get captured in large isolated water clusters.
Finally, we find large energetic activation barriers for hopping, which
significantly depend on the hydration state and thereby challenge available
transport theories.
