Microgels at liquid interfaces

Bochenek, Steffen; Richtering, Walter (Thesis advisor); Isa, Lucio (Thesis advisor)

Aachen : RWTH Aachen University (2020, 2021)
Dissertation / PhD Thesis

Dissertation, RWTH Aachen University, 2020

Abstract

Microgels are three-dimensional, cross-linked polymer networks of colloidal size which are dispersed in and swollen by a good solvent. They combine properties of polymers and colloidal particles, such as their swelling or deswelling in response to external stimuli or their crystallization at higher volume fractions. Furthermore, microgels have been shown to readily adsorb to interfaces and form responsive or ``smart'' emulsions, foams and coatings. Though having an isotropic, spherical shape in bulk solution, the microgels become anisotropic upon adsorption. The structure of regular cross-linked microgels at interfaces is described by a core-corona morphology. In this thesis, the temperature-sensitivity and polymer-to-particle transition of microgels at fluid interfaces are investigated. The two-dimensional phase diagram of poly(N-isopropylacrylamide) (pNIPAM) microgels below and above their volume phase transition temperature is investigated with a combination of compression isotherms, atomic force microscopy, image analysis, and reflectometry methods. At low compression, the interaction between adsorbed microgels is dominated by their highly stretched corona and the phase behavior of the microgel monolayers is the same. The polymer segments within the interface lose their temperature-sensitivity due to the influence of surface free energy. At high compression, however, the portions of the microgels that are located in the aqueous side of the interface become relevant and prevail in the microgel interactions. These portions are able to deswell and, consequently, the isostructural phase transition is altered. It is shown that this effect is presented at both decane- and air-water interface. Thus, the temperature-dependent swelling perpendicular to the interface (``3D'') affects the compressibility parallel to the interface (``2D''). Different pNIPAM-based systems, starting from linear polymers, through regular cross-linked, to hollow microgels were investigated. Similar to microgels in bulk solution, it is shown that the deformability and softness of microgels, due to the constraints of the polymer network, is a dominant property for the interfacial behavior. Their soft interaction potential cannot only be tuned by the variation of cross-linker but also by the architecture of the polymeric network, e.g., the incorporation of a solvent filled cavity. A closer look was taken at the polymer-to-particle transition. Therefore, ultra-low cross-linked (ULC) microgels were investigated, which are the pNIPAM microgels with the lowest number of cross-linking points within their network that can be synthesized by precipitation polymerization. Their phase behavior and rheological properties in bulk are those of soft colloids. However, when these microgels are confined at an oil-water interface, their compression isotherms resemble that of flexible polymers and a concentration-dependent topography is observed. Depending on the compression, these microgels can behave as flexible polymers, covering the substrate with a uniform film, or as colloidal microgels leading to a monolayer of particles.

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