Selective molecular transport in thermo-responsive polymer membranes: role of nanoscale hydration and fluctuations

TL;DR

This study uses molecular dynamics simulations to reveal how nanoscale hydration and polymer fluctuations facilitate selective molecular transport in thermo-responsive membranes, highlighting a hopping diffusion mechanism influenced by water cluster structures.

Contribution

It uncovers the role of nanoscale hydration and polymer fluctuations in molecular transport, providing a detailed mechanistic understanding and an analytical scaling relation for diffusion in responsive membranes.

Findings

Water forms fractal-like clusters in the polymer matrix.

Diffusivity varies over five orders of magnitude depending on penetrant size.

Hopping mechanism dominates penetrant diffusion through transient water channels.

Abstract

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.