Synthesis, analysis and modification of microgels with functional oligoglycidol comonomers

Willems, Christian; Pich, Andrij (Thesis advisor); Möller, Martin (Thesis advisor)

Aachen (2017)
Dissertation / PhD Thesis

Dissertation, Rheinisch-Westfälische Technische Hochschule Aachen, 2017

Abstract

Microgels are colloidal polymer networks, which can take up a lot of solvent (in this case water). Depending on the chemical makeup microgels are sensitive to an external stimulus, like changes in the temperature, pH or ion strength of a solution. Because of their properties there are many different applications for microgels. For some applications, they have to be further modified. The challenge lies in the specific functionalization of the microgel surface. Since microgels are only loose polymer networks, it is difficult to fix functional groups at the periphery, as they can be distributed throughout the whole microgel network. The aim of this work is the synthesis, characterization and functionalization of microgels, based on N-vinylcaprolactam (VCL), which are copolymerized with different oligoglycidol macromonomers in a precipitation polymerization. The macromonomers differ in the number of glycidol repeating units. The first part of this work discusses the characterization of the synthesized microgels. It can be shown, that the addition of macromonomer leads to changes in the microgel size, temperature sensitivity and colloidal stability. By increasing the amount of glycidol groups and the amount of macromonomer during the synthesis, the size of the finished microgels decreases along with the thermal responsivity. The addition of too much macromonomer leads to very small particles, which is why a second synthesis method was developed. Instead of adding the whole amount of crosslinker at once, it was slowly added during the polymerization process. This leads to microgels, whose size is independent of the amount of used macromonomer. In the end, a system can be developed, where the size and temperature responsivity of a microgel can be tuned depending on the requirements. 1H-NMR studies could show, that the macromonomer is exclusively located in the microgel shell, which makes them more accessible for other molecules. The second part concerns the functionalization of the microgels with allyl, vinyl sulfonate- or thiol groups. In all three cases, the functionalization was successful and it could be shown, that the concentration of functional groups on the surface could be varied without changing the temperature responsivity of the microgel. Apart from functional groups, polymers can be fixed on the microgel surface too. The third part concerns the decoration of oligoglycidol-modified microgels with polymer brush-like shells by polymerization of several monomers initiated from the microgel surface (grafting-from). Two types of grafting-from polymerization processes were studied: a) a cerium salt induced redox polymerization and b) the single-electron transfer living radical polymerization. Several monomers were tested and the reaction was realized in water as a solvent. In the first case it was possible to polymerize [2-(methacryloyloxy) ethyl]-dimethyl (3sulfopropyl) ammonium hydroxide (sulfobetain), di (ethylene glycol) ethyl ether acrylate (DEGA), 2-methoxy ethyl acrylate (MEA), N-isopropyl acrylamide (NIPAAM) and acrylonitrile. With the second method is was possible to polymerize sulfobetain and styrene sulfonic acid (SSA). Those monomers could be polymerized on the microgel surface. This shows, that the synthesized microgels can be easily further modified for different applications.