Cononsolvency of microgels : equilibrium and dynamics
- Cononsolvency von Mikrogelen : Gleichgewicht und Dynamik
Nothdurft, Katja; Richtering, Walter (Thesis advisor); Bardow, André (Thesis advisor)
Aachen : RWTH Aachen University (2021)
Dissertation / PhD Thesis
Dissertation, RWTH Aachen University, 2021
Microgels are three-dimensional, cross-linked polymeric networks swollen by a good solvent. Poly-N-isopropylacrylamide (PNIPAM)-based materials are of ongoing scientific interest due to their unique responsive behavior to external parameters such as temperature and solvent composition. Although PNIPAM microgels are the subject of a variety of application-oriented studies, fundamental aspects about the underlying processes of the responsive behavior are still under discussion. The present work focuses on the sensitivity of PNIPAM to the composition of water-methanol mixtures, the so-called cononsolvency effect. Here, the gels are swollen in either of the pure solvents, whereas water-rich mixtures around 20 mol% methanol cause deswelling of the gels. In the first part of this thesis, the internal properties of the microgels in different swelling states in comparison to their surroundings were studied under equilibrium conditions. Mass balance experiments combined with Raman microspectroscopy revealed an enrichment of methanol inside the macroscopic PNIPAM gels for the cononsolvency-inducing water-methanol mixtures. This preferential adsorption of methanol was confirmed by measurements of the fluorescence lifetime of solvatochromic microgels beads. In addition, the collapsed PNIPAM microgel beads exhibit a lower polarity in unfavorable mixtures than the respective binary water-methanol mixtures. The possibility to control the properties of the microenvironment provided by microgels using external stimuli was exploited to modulate the catalytic activity of PNIPAM-based microgel-catalysts. Furthermore, the mechanical properties of microgel beads were investigated by variation of the external osmotic pressure. Upon compression, a transition from a soft, deformable polymer network to a stiffer, partially collapsed object was observed. In the second part, the dynamics of the volume phase transition were studied. Many applications of responsive microgel systems rely on the fast and reversible adaptability of the polymer network to changes in the environment. To further elucidate the kinetics of the polymer response, fluorescently labeled PNIPAM microgel beads in the micrometer range were studied. A custom-made microfluidic setup allows a fast solvent exchange from pure water to the unfavorable mixture of 20 mol% methanol in water. The deswelling behavior is described by a two-step process. The major volume change occurs in the initial, rapid process where the microgels are still porous. In the second, slower process, only minor changes in size are observed. The dependence of the relaxation times on the microgel’s diameter is discussed taking into account the adhesion-induced deformation of the gels and the physical processes underlying the collapse.