Reversible Formation of Thermoresponsive Binary Particle Gels with Tunable Structural and Mechanical Properties

Abstract

We investigate the collective behavior of suspended thermoresponsive microgels, that expel solvent and subsequently decrease in size upon heating. Using a binary mixture of differently thermoresponsive microgels, we demonstrate how distinctly different gel structures form, depending on the heating profile used. Confocal laser scanning microscopy (CLSM) imaging shows that slow heating ramps yield a core-shell network through sequential gelation, while fast heating ramps yield a random binary network through homo-gelation. Here, secondary particles are shown to aggregate in a monolayer fashion upon the first gel, which can be qualitatively reproduced through Brownian dynamics simulations using a model based on a temperature-dependent interaction potential incorporating steric repulsion and van der Waals attraction. Through oscillatory rheology it is shown that secondary microgel deposition enhances the structural integrity of the previously formed single species gel, and the final structure exhibits higher elastic and loss moduli than its compositionally identical homo-gelled counterpart. Furthermore, we demonstrate that aging processes in the scaffold before secondary microgel deposition govern the final structural properties of the bigel, which allows a detailed control over these properties. Our results thus demonstrate how the temperature profile can be used to finely control the structural and mechanical properties of these highly tunable materials.

Decorated networks of native proteins: nanomaterials with tunable mesoscopic domain size

“…To assemble these structures, we develop a strategy to control the interactions of each species independently, similar to Immink and collaborators. 26 Here, we exploit the effects of salts on protein solutions, 18,[27][28][29] based on specific salt-protein interactions. Trivalent ions have been shown to selectively interact with the surface-exposed carboxylic groups of the acidic residues of the proteins, which in turn, leads to gelation or crystallisation.…”

Controlling the morphology of microgels by ionic stimuli

“…9,11,12 This strong morphological change at (biologically relevant) temperatures opens up avenues for tissue scaffolding, 13,14 drug delivery, 15,16 or metamaterials based on hierarchical assembly. 17 In comparison to their neutral counterparts, ionic microgels that have been functionalized with ionizable side groups allow an even larger array of external stimuli, including changes in ion composition and pH. Thus, this responsiveness can be tailored through the chemical characteristic of the ionic side groups, in combination with triggers such as temperature, salinity, acidity, and number density.…”

Reversible temperature-controlled gelation in mixtures of pNIPAM microgels and non-ionic polymer surfactant

“…19,20 This fuzzy corona is known to affect the interactions between microgel particles, and make them more stable to aggregation. 21 There have been few studies describing the surface properties of the pNIPAM microgels, and for most systems the length and density of the extending polymer layer is unknown. 22 pNIPAM is known to spontaneously form cross-linked particles even without addition of a cross-linking agent during synthesis 23 , however, the addition of cross-linker allows the control of swelling capacity of the particles.…”