Dottorato in FISICA

Titolo della tesi: Numerical simulations of monomer-resolved and coarse-grained microgels: electrostatic effects, solvent interplay and internal elasticity

Microgels are one of the most investigated types of soft colloids, a category of systems consisting of mesoscopic particles that have the ability to deform and compress, generating a rich macroscopic phenomenology. Specifically, thermoresponsive microgels have the ability to change size by temperature tuning, undergoing a so-called Volume Phase Transition (VPT), close to ambient temperature. Ionic microgel particles are intriguing systems in which the properties of thermo-responsive polymer are enriched by the presence of weak acidic or alkaline co-monomers, developing a bare charge making them responsive also to pH. In order to rationalize their properties and predict the behaviour of microgel suspensions, it is necessary to develop a coarse-graining strategy that starts from the accurate modelling of single particles. Recent progress has been made in the numerical modelling of neutral microgel particles with a realistic, disordered structure. With this thesis we extend this approach to address the effect of electrostatics on their structural properties, that is of fundamental importance both for ionic microgels and for the only thermo-responsive ones obtained through free-radical polymerisation techniques, involving the presence of charged molecules in the synthesis process. Firstly we address the behaviour of ionic microgels by comparing the cases where counterions directly interact with microgel charges or are modelled implicitly through an effective screened potential. We do so by performing extensive numerical simulations of single microgels across the volume phase transition (VPT) varying the temperature and the fraction of charged monomers. We find that the presence of charges considerably alters the microgel structure, and we observe significant deviations between the implicit and explicit models, with the latter comparing more favourably to available experiments. Then, we provide a numerical advancement of the model for co-polymerized microgels by improving the treatment of ionic groups in the polymer network. We find that, when charged groups are considered to be hydrophilic at all temperatures, highly charged microgels do not achieve a collapsed state within the experimentally available window, in favorable comparison to experiments. In addition, we explicitly include the solvent in the description and put forward a mapping between the solvophobic potential in the absence of the solvent and the monomer-solvent interactions in its presence, which is found to work very accurately for any charge fraction of the microgel. Finally, we use the developed model to give a detailed microscopic look at the VPT of only thermo-responsive microgels, assessing the role of charges in their deswelling process. We study the behaviour of Poly(N-isopropylacrylamide)-based microgels, where the constituent monomers are neutral but charged groups arise due to the initiator molecules used in the synthesis. Here we address this point combining experiments with state-of-the-art simulations to show that the microgel collapse does not happen in a homogeneous fashion, but through a two-step mechanism, entirely attributable to electrostatic effects. Our work paves the way for comparing single-particle properties and swelling behaviour of both ionic and thermo-responsive microgels to experiments and to tackle the study of these charged soft particles at a liquid-liquid interface and in many microgels systems. Our results also have direct relevance on fundamental soft condensed matter science and on microgel applications ranging from materials to biomedical technologies.