Selective solute adsorption and partitioning around single PNIPAM chains

TL;DR

This study uses molecular dynamics simulations to investigate how single PNIPAM chains selectively adsorb and partition various solutes, revealing the influence of temperature, polymer elongation, and stereochemistry on binding affinities.

Contribution

It provides detailed molecular insights into solute binding mechanisms in PNIPAM, highlighting how polymer state and solute properties affect adsorption and partitioning.

Findings

Adsorption increases with solute size.

Temperature effects on affinity are highly solute-specific.

Polymer elongation significantly influences binding behavior.

Abstract

Thermoresponsive polymer architectures have become integral building blocks of 'smart' functional materials in modern applications. For a large range of developments, e.g., for drug delivery or nanocatalytic carrier systems, the selective adsorption and partitioning of molecules (ligands or reactants) inside the polymeric matrix are key processes that have to be controlled and tuned for the desired material function. In order to gain insights into the nanoscale structure and binding details in such systems, we here employ molecular dynamics simulations of the popular poly(N-isopropylacrylamide) (PNIPAM) polymer in explicit water in the presence of various representative solute types with focus on aromatic model reactants. We model a PNIPAM polymer chain and explore the influence of its elongation, stereochemistry, and temperature on the solute binding affinities. While we find that the excess adsorption generally raises with the size of the solute, the temperature- dependent affinity to the chains is highly solute specific and has a considerable dependence on the polymer elongation (i.e., polymer swelling state). We elucidate the molecular mechanisms of the selective binding in detail and eventually present how the results can be extrapolated to macroscopic partitioning of the solutes in swollen polymer architectures, such as hydrogels.