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Thermoresponsive microgels based on strong polycations

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Title: Thermoresponsive microgels based on strong polycations
Author(s): Bandini, Elena
Contributor: University of Helsinki, Faculty of Science, none
Discipline: none
Degree program: Master's Programme in Chemistry and Molecular Sciences
Specialisation: Polymer Materials Chemistry
Language: English
Acceptance year: 2019
The aim of this work is to synthetize a series of thermoresponsive microgels that have never been reported before, based on strong polycations, and study their properties such as the change in volume in response to a temperature stimulus. Polymer microgels are interesting materials for practical applications as drug delivery systems, in separation techniques and catalysis. The interest on these materials arises from their physical properties of colloids combined with gel properties. The microgels presented in this work can undergo phase transitions not only in water but also in DMF/water mixtures. A crosslinked polymer that displays cloud point behaviour when heated forms a temperature-sensitive gel network. Cloud point is the temperature above which an aqueous solution of a water-soluble polymer becomes turbid in the case of polymer with LCST (Lower Critical Solution Temperature) behaviour. Upon heating such a gel, the gel shrinkage is observed by expelling water over a temperature range. The transition is largely driven by the entropy gain associated with the release of water from the network, and the concomitant collapse of the polymer chains. In addition, the size of the microgels is tuneable by adding NaCl at different concentration. The synthesis is carried out as a normal radical polymerization always in the same conditions except for the solvent mixture. The homopolymer, synthetized for comparison, is polymerized with RAFT (Reversible Addition-Fragmentation chain-Transfer) method. Nuclear Magnetic Resonance (NMR) confirmed the structure of the microgels validating the synthetic method. The hydrodynamic radius of the microgels after the addition of salty solutions at different concentration is determined by Dynamic Light Scattering (DLS). The thermo-responsive properties are investigated in terms of polarity using fluorescence and turbidity measurements and in terms of changes in volume calculated from the hydrodynamic radius with DLS at different temperature. The microgels show a thermo-responsive behaviour in the temperature range between 10 °C and 90 °C. In fact, the raise in temperature causes an increase in volume and hydrophobicity. Finally, it is reported a trend that follows the NaCl concentration of salt solutions added to the microgels. These microgels can be used for a wide range of applications, amongst them, they are useful support for metal nanoparticles for catalytic purposes. Here, AuNPs are formed directly on the microgel and the formation is ascertained by DLS and TGA (Thermogravimetric Analysis). Then, they are tested to effectively work during a catalysis experiment.

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