Microgels at solid/fluid interfaces investigated by atomic force microscopy

  • Untersuchung von Mikrogelen an fest/fluiden Grenzflächen mittels Rasterkraftmikroskopie

Schulte, Marie Friederike; Richtering, Walter (Thesis advisor); Hellweg, Thomas (Thesis advisor)

Aachen (2020)
Dissertation / PhD Thesis

Dissertation, RWTH Aachen University, 2020

Abstract

Microgels are an important model system for soft materials and combine the properties of two fundamental classes: flexible polymers and hard colloids. They are intramolecularly cross-linked, macromolecular networks, swollen by a good solvent, and typically of spherical shape. Microgels containing N-isopropylacrylamide (NIPAM) as the main monomer show fast thermo-responsive behavior. Additionally, they are highly interfacial active, although not being amphiphilic. Their special features make microgels appealing for many applications in the field of advanced polymeric coatings, but require the fundamental understanding of their interfacial properties. Atomic force microscopy (AFM) excels at investigating the morphology and nanomechanical properties of microgels at surfaces. Our topographic AFM measurements reveal, that the behavior of ultra-low cross-linked (ULC) microgels at the solid/liquid interface is very complex: ULC microgels exhibit not only one equilibrium morphology. The final balance between adsorption energy and network elasticity can be trapped in two different states, depending on the adsorption process - controlling the flexible polymer-hard colloid duality at will. Besides the knowledge achieved on the microgels' topography, AFM reveals information beyond. An important parameter is the size of the probe compared to the microgels' dimensions. Sharp tips allow for high lateral resolution but penetrate the polymeric network as they are smaller than the mesh size of the microgel. On one hand, this hampers the imaging strongly. On the other hand, it allows monitoring a local contact resistance in the vicinity of the tip during force-spectroscopic measurements. The depth-dependent contact stiffness closely correlates with the density profiles in bulk solution. In contrast to currently used techniques, e.g., colloidal probe or reflectometry, which only provide an average of the z-profile, sharp tip AFM measurements resolve the real three-dimensional internal structure of individual microgels. We show, that hollow microgels still exhibit a cavity, even though adsorbed and deformed at the solid substrate - essential for drug-release applications. Very low forces - at the edge of the resolution - result in a penetration through the whole network of the ULC microgels, letting them “disappear” in classical imaging modes. Here, we resolve their poorly cross-linked, and entirely homogeneous, soft internal structure. In the case of conventional cross-linked microgels, the spherical symmetry breaks due to the adsorption to the interface. A difference in the internal stiffness gradient along the lateral and the vertical axis is disclosed.

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