Poly-N-isopropylacrylamide Based Microgels :A Study on Magnetic Microgels and Their Structure

Abstract

The thesis presents studies of the combination of magnetic nanoparticles (MNPs) with N-isopropylacrylamide (NIPAM) based microgels (MGs). Poly-NIPAM (PNIPAM) microgels are used for model systems, e.g. for the development of surface coatings, actuators, sensors or drug containers. Combining MNPs with MGs opens up new applications. The magnetic microgels (MMGs) can be influenced in external magnetic fields. The first part of the thesis studies first obstacles of loading the MGs with MNPs. The part starts by measuring the influence of the electric charge on the loading with MNPs. It is shown that the negatively charged MNPs are preferable embedded in positively charge MGs. The study also shows that there are additional factors influencing the MG loading. The prepared MMGs can be influenced by external magnetic fields. For example MMGs which are adsorbed on a surface, can be deformed in a magnetic field. Additional experiments also show that the MMGs can be separated from the dispersion with magnets. The second study focuses on the internal structure of the MGs by different synthesis methods, in particular on the distribution of cross-linker. Three different methods are studied: the batch method (everything in one reaction vessel), the feeding method (the reactants are fed continuously into the reaction vessel) and the semi-batch method (a combination of the two mentioned beforehand). The reaction rate constants are measured for the different monomers. Additionally, the reaction time could be drastically reduced, without compromising the gel properties. The third study further improves the understanding of the cross-linker distribution. The cross-linker concentration is correlated with the elastic modulus. The batch microgels show a heterogeneous distribution of cross-linker with the highest concentration in the particle center. The feeding microgels show a homogeneous distribution of cross-linker. The cross-linker distribution could also be observed with transmission electron microscopy (TEM). The microgels are loaded with MNPs and the magnetic microgels are imaged with the TEM. The TEM images show that the distribution of the MNPs is correlated to the cross-linker distribution. The fourth study focuses on an additional co-polymer and its influence on the MNP distribution inside the MGs. The added co-polymer is hydrophobic and polymerized into one of two gels. The resulting two MMGs are analyzed by the AG Wende (in particular by Joachim Landers) with a superconducting quantum interference device (SQUID) to measure magnetic properties. The measurements show the relaxation behavior of the MNPs inside the gel network. The relaxation behavior is influenced by the environment and can be used as a measure for the interaction strength between MNP and microgel matrix.