Buckling and Interfacial Deformation of Fluorescent Poly(N-isopropylacrylamide) Microgel Capsules

Charged hollow microgel capsules

Responsive hollow microgels are a fascinating class of soft model systems at the crossover between polymer capsules and microgels. The presence of the cavity makes them promising materials for encapsulation and controlled release applications but also confers them an additional softness that is reflected by their peculiar behaviour in bulk and at interfaces. Their responsivity to external stimuli, such as temperature, pH, and ionic strength, can be designed from their synthesis conditions and the choice of functional moieties. So far most studies have focused on "small" hollow microgels that were mostly studied with scattering or atomic force microscopy techniques. In our previous study, we have shown that large fluorescent hollow poly(N-isopropylacrylamide) (PNIPAM) microgels could be synthesized using micrometer-sized silica particles as sacrificial templates allowing their investigation in situ via confocal microscopy. In this work, we extend this approach to charged large hollow microgels based on poly(N-isopropylacrylamide-co-itaconic acid) (P(NIPAM-co-IA)). Hereby, we compare the structure and responsivity of "neutral" (PNIPAM) and "charged" (P(NIPAM-co-IA)) hollow microgel systems synthesized under similar conditions with the same sacrificial template using confocal and atomic force microscopy and light scattering techniques. In particular, we could demonstrate the extremely soft character of the swollen charged hollow microgels and their responsivity to pH, ionic strength, and temperature. To conclude this study, the buckling behavior of the different capsules was investigated illustrating the potential of such systems to change its conformation by varying the osmotic pressure and pH conditions.

Nonionic Microgels Adapt to Ionic Guest Molecules: Superchaotropic Nanoions

Microgels are commonly applied as solute carriers, where the size, density, and functionality of the microgels depend on solute binding. As representatives for ionic solutes with high affinity for the microgel, we study here the effect of superchaotropic Keggin polyoxometalates (POMs) PW12O403- (PW) and SiW12O404- (SiW) on the aqueous swelling and internal structure of nonionic poly(N-isopropylacrylamide) (pNiPAM) microgels by light scattering techniques and small-angle X-ray scattering. Due to their weak hydration, these POMs bind spontaneously to the microgels at millimolar concentrations. The microgels thus become charged and swell at low POM concentration, surprisingly without strongly increasing the volume phase transition temperature, and deswell at higher POM concentration. The swelling arises because of the osmotic pressure of dissociated counterions of the POMs, while the deswelling is due to POMs acting as physical cross-links in the microgels under screened electrostatics in NaCl or excess POM solution. This swelling/deswelling transition is sharper for PW than for SiW related to the lower charge density, weaker hydration, and stronger binding of PW. The POMs elicit qualitatively and quantitatively different swelling effects from ionic surfactants and classical salts. Moreover, the network softness and topology govern the swelling response upon POM binding. The softer the microgel, the stronger is the swelling response, while, inside the microgel, regions of high polymer density swell/contract more upon electric charging/cross-linking than regions with low polymer density. POM binding thus enables fine-tuning of microgel properties and highlights the role of network topology in microgel swelling. Because POMs decompose at an alkaline pH, these POM/microgel systems also exhibit pH-responsive swelling in addition to the typical temperature responsiveness of pNiPAM microgels.

Influence of Architecture on the Interfacial Properties of Polymers: Linear Chains, Stars, and Microgels

Surface-active polymers have important applications as effective and responsive emulsifiers, foaming agents, and coatings. In this contribution, we explore the impact of the polymer architecture on the behavior at oil-water interfaces by comparing different poly(N-isopropylacrylamide) (pNIPAM)-based systems, namely, monolayers of linear and star-shaped macromolecules, ultralow cross-linked, regular cross-linked, and hollow microgels. Compression isotherms were determined experimentally as well as by computer simulations. The latter provides information about the conformational changes of the individual macromolecules as well as the interfacial properties of the monolayer, including the surface structure and the density distribution of an ensemble of interacting macromolecules near an interface. Surprisingly, the isotherms of the linear polymer, of the star polymer, and of the ultralow cross-linked microgel have an identical shape that differs from the isotherms of regular and hollow microgels. We introduced the mass fraction of adsorbed polymer, which gives a measure of the polymer segments contributing to the isotherm in relation to the most flexible architecture, i.e., the linear polymer, and allows a comparison of polymers with different architectures. The data demonstrate that increasing the number of cross-links leads to a significantly lower amount of polymer in the proximity of the interface as the increase in cross-linker reduces the deformability or softness of the polymers at the interface. The volume fraction profiles along the normal to the interface are essentially different in the microgel monolayers as compared to those in the linear and star polymer. The profiles through the microgel contact line and their growth upon initial compression are similar to those of the linear chains. Herewith, the profiles through the center of mass practically do not change upon compression. Therefore, the initial growth in the microgel surface pressure reveals the polymer-like behavior and is related to the deformation of the peripheral part of the microgel. Further compression of the microgel monolayer leads to 3D interactions of the microgels within the aqueous side of the interface and soft colloid-like behavior.

Exploring the 3D Conformation of Hard-Core Soft-Shell Particles Adsorbed at a Fluid Interface

The encapsulation of a rigid core within a soft polymeric shell allows obtaining composite colloidal particles that retain functional properties, e.g., optical or mechanical. At the same time, it favors their adsorption at fluid interfaces with a tunable interaction potential to realize tailored two-dimensional (2D) materials. Although they have already been employed for 2D assembly, the conformation of single particles, which is essential to define the monolayer properties, has been largely inferred via indirect or ex situ techniques. Here, by means of in situ atomic force microscopy experiments, the authors uncover the interfacial morphology of hard-core soft-shell microgels, integrating the data with numerical simulations to elucidate the role of the core properties, of the shell thicknesses, and that of the grafting density. They identify that the hard core can influence the conformation of the polymer shells. In particular, for the case of small shell thickness, low grafting density, or poor core affinity for water, the core protrudes more into the organic phase, and the authors observe a decrease in-plane stretching of the network at the interface. By rationalizing their general wetting behavior, such composite particles can be designed to exhibit specific inter-particle interactions of importance both for the stabilization of interfaces and for the fabrication of 2D materials with tailored functional properties.