Selective Molecular Transport in Thermoresponsive Polymer Membranes: Role of Nanoscale Hydration and Fluctuations

Abstract

For a wide range of modern soft functional materials the selective transport of sub-nanometersized 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.

Transfer Free Energies and Partitioning of Small Molecules in Collapsed PNIPAM Polymers

“…Equilibrated molecular structures of the collapsed PNIPAM polymers with water were taken from the previous study [41]. First, 48 polymer chains were solvated in excess water above the transition temperature (≈305 K), which caused the polymers to phase separate from the water phase.…”

“…Our broader research goal lies in a qualitative molecular-level understanding of thermodynamic and transport properties in PNIPAM-based hydrogels. Employing atomistic, explicit-water simulations, we studied solute partitioning in swollen gels [40] and diffusion inside collapsed gels [41]. In this study, we adopt the previous models to investigate the solvation and partitioning in collapsed PNIPAM polymers of several polar and nonpolar molecules, featuring small alkanes, simple alcohols, and aromatic molecules.…”

Repertoire Builder: high-throughput structural modeling of B and T cell receptors

“…We also showed that in a less hydrated gel, the the diffusion of solutes is lower (i.e., higher ∆F a ) and at the same time the diffusion coefficients indeed decay faster with solute size. 113 In conclusion, all-atom MD simulations offer insights into the molecular nature of the transport and solvation properties of molecules in hydrogels. These mechanisms are not only important for PNIPAM hydrogels, but most probably play important roles also in other responsive hydrogels, and their understanding is important for the rational design of novel materials.…”

“…9C. Its connected blob-like structure suggests that the diffusion advances via the hopping mechanism 182,183 : A penetrating solute resides for longer time in a local cavity and suddenly performs a longer jump into a neighboring cavity through a transient water channel 113 that forms between the chains (schematically illustrated in the bubbles in Fig. 9C).…”

“…The amount of sorbed water between the polymeric chains is chosen such that it corresponds to the chemical equilibrium with bulk water. 113 The amount of water in the collapsed state depends on temperature, and amounts to around 20 wt. % (somehow less than experimental estimates of around 30 wt.…”