Microgels under internal stress - interactions with nano-ions

RWTH Publications (RWTH Aachen)

Abstract

The diversity of microgels with regard to structure, properties, and functionality makes them fascinating research objects. In particular, the incorporation and targeted release of guest molecules from the polymer network make microgels interesting for catalysis and drug delivery systems. However, different guest molecules can strongly influence the properties of microgels in solution. In order to understand how microgels adapt to solute binding, the influence of interface-active molecules, including anionic amphiphiles and large anions (nano-ions) lacking amphiphilic structure, was investigated by scattering techniques. The anionic amphiphiles and the nano-ions interact with soft matter materials due to different water-mediated driving forces, i.e. the hydrophobic effect for amphiphiles and the superchaotropic nature of nano-ions. In this thesis, it is shown that millimolar nano-ion concentrations strongly swell or deswell non-ionic microgels below the volume phase transition temperature (VPTT), depending on concentration. The underlying mechanism is given by electric charging and counterion osmotic pressure at low nano-ion concentrations, whereas deswelling at high concentrations is mainly caused by physical crosslinking. Additionally, nano-ions preferentially bind to regions of high polymer density inside the microgels, with a much stronger response in weakly crosslinked microgels compared to highly crosslinked ones. Finally, the pH stability of the nano-ions enables the introduction of additional pH sensitivity to the microgels. Comparable to the behavior in dilute microgel suspensions, the addition of nano-ions also strongly affects microgel size in more crowded environments, inducing additional structuring at low nano-ion concentrations that disappears with increasing concentration. Moreover, the macroscopic properties of the microgel solution vary non-monotonically with nano-ion concentration, showing maxima in the cloud point and viscoelastic response. Interactions of pNiPAM microgels with amphiphiles of varying chain lengths show two limiting behaviors: short-chain amphiphiles decrease swelling below the VPTT and induce aggregation above it, similar to classical salts, while long-chain amphiphiles increase both swelling and the VPTT. The swelling suggests formation of smaller microgel-bound aggregates, driven by polymer charging and counterion osmotic pressure. Comparison of chain lengths indicates that a minimum hydrophobic chain length is required for binding to the microgel network and inducing swelling. This study highlights the different mechanisms of nano-ion and amphiphile interactions with microgels.

Authors 1

1. Jasmin Simons corresponding Aachen

   RWTH Aachen University

   Affiliation as printed

   RWTH Aachen