Polymer-Mediated Interactions and Phase Behaviour of Polymer-Particle Dispersions

Abstract: Interactions between colloidal particles can be modelled by particles grafted with polymers. In this work, structural and physical properties of colloids are investigated under variation of parameters such as pH, ionic strength, and temperature, where aggregation and cluster formation can be monitored in aqueous solution. Being the subject of our work, in particular, we show that linear or polymer-like clusters can be formed if long-ranged repulsive barriers are combined with very short-ranged attractive minimums stimulating particles to form highly anisotropic structures. This is adjusted by changing the properties of particles and the dispersing medium. Besides, we utilize Metropolis Monte Carlo (MC) simulation to investigate the behavioural change of these particles with a focus on the types of clusters formed. A simplistic potential of mean force is adopted for the simulations, but we also invoke a more elaborate model, to demonstrate that similar interactions can be obtained when the grafted chains are treated explicitly. An important criterion in these studies is that the particle size is large enough to allow structural analyses via microscopy. The range of electrostatic interactions is adjusted by the ionic strength, and the strength of the short-ranged attraction is changed via hydrophobicity regulation of the grafted layer through temperature variation. The results revealed that highly anisotropic structures which resemble linear or branched polymers were the clusters at equilibrium. We could also investigate the effect of polymer addition to the particle dispersions. We could detect a non-monotonic temperature dependent aggregation of particles from attraction to repulsion to attraction, where the polymer-mediated interactions were repulsive. The results were validated against experiments.The next phase of this work is devoted to the study on capillary induce phase transitions with an experimental focus on polymer solutions containing PNIPAM at the presence of hydrophobic surfaces (mesoporous silica) as a function of pH, temperature and chain length. The capillaries/confined geometries are known to influence the phase diagram of polymer solutions where condensation of bulk solutions may occur close to the surfaces. This work is performed using a combination of experiments and theories where a shift to the LCST (lower critical separation temperature) is presumed to occur, resulting in a capillary-induced decrease in the LCST.