Tuning the gel structure with temperature controlled sequential gelation; Relating mechanical properties and structure in microgel self-assemblies

Abstract

Control over the properties of amorphous materials requires thorough understanding of the relation between structure and elasticity. Colloidal gels have been used as model system to study morphology and mechanical properties, but correlation between these features has not been addressed. No previous work has exploited the potential of sequential gels to control the gel structure, and an extensive study into its morphological and mechanical properties has not been performed yet. In this thesis, the synthesis and characterisation of a binary thermoresponsive microgel system was performed, which was used to form novel gel structures via sequential gelation. To increase resolvability, a binary core-shell system was synthesised with a fluorescently stained polystyrene core and multiple undyed poly(N-isopropylacrylamide) or poly(N-isopropylmethacrylamide) microgel shells. Aggregation and the formation of secondary nucleates limited the synthesis conditions to a low monomer to core surface ratio and low monomer concentration. The particles could be synthesised in multiple sequential batches with centrifugation rounds in between to remove small newly formed secondary nucleates, which extended the working synthesis conditions and resulted in secondary nucleate-free samples. Excellent control over size was achieved via scaling of the reactants with the core concentration determined with fluorescence spectroscopy. The core-shell particles exhibited very similar swelling behaviour and elasticity in the pair potential as pure microgel colloids, i.e. microgel particles without a core, and its results could be directly put in the context of pure microgel systems. Morphological features were extracted from three-dimensional reconstructions of the gel volume obtained from confocal microscopy data and simulations, which were compared with experimentally obtained rheological moduli. A clear increase in the rheological moduli was found at the gel-gel transition, indicating an increased rigidity, which was explained from the change of the gel structure and increased interparticle attraction. The second particle type deposited homogeneously on the inner network formed in the first gelation step, and created new links between nearby strands, which could be quantified with respectively the volume over surface and the indirectness. The homogeneous deposition appeared to be caused by a weaker attraction between the decorating particles, which could be a result of the gradual collapse or different density of the two particle types at the experimental temperatures. The sequential gel exhibited ageing in the form of large collective rearrangements. However, before temperature equilibration, the gel rearranged quickly and compacted severely, probably as a result of thermally and mechanically activated rearrangements driven by the increase in temperature. Hence, the exact execution of the temperature quench appeared to be a determining factor in the resulting gel structure, and control over this could be essential to obtain reproducible results.