Correction: Structure formation of PNIPAM microgels in foams and foam films.

Correction for 'Structure formation of PNIPAM microgels in foams and foam films' by Matthias Kühnhammer et al., Soft Matter, 2022, 18, 9249-9262, https://doi.org/10.1039/D2SM01021F.

Structure formation of PNIPAM microgels in foams and foam films.

Responsive aqueous foams are very interesting from a fundamental point of view and for various applications like foam flooding or foam flotation. In this study thermoresponsive microgels (MGs) made from poly(N-isopropyl-acrylamide) (PNIPAM) with varying cross-linker content, are used as foam stabilisers. The foams obtained are thermoresponsive and can be destabilised by increasing the temperature. The structuring of MGs inside the foam films is investigated with small-angle neutron scattering and in a thin film pressure balance. The foam films are inhomogeneous and form a network-like structure, in which thin and MG depleted zones with a thickness of ca. 30 nm are interspersed in a continuous network of thick MG containing areas with a thickness of several 100 nm. The thickness of this continuous network is related to the elastic modulus of the individual MGs, which was determined by atomic force microscopy indentation experiments. Both, the elastic moduli and foam film thicknesses, indicate a correlation to the network elasticity of the MGs predicted by the affine network model.

A new model to describe small-angle neutron scattering from foams.

The modelling of scattering data from foams is very challenging due to the complex structure of foams and is therefore often reduced to the fitting of single peak positions or feature mimicking. This article presents a more elaborate model to describe the small-angle neutron scattering (SANS) data from foams. The model takes into account the geometry of the foam bubbles and is based on an incoherent superposition of the reflectivity curves arising from the foam films and the small-angle scattering (SAS) contribution from the plateau borders. The model is capable of describing the complete scattering curve of a foam stabilized by the standard cationic surfactant tetradecyltrimethylammonium bromide (C14TAB) with different water contents, i.e. different drainage states, and provides information on the thickness distribution of liquid films inside the foam. The mean film thickness decreases with decreasing water content because of drainage, from 28 to 22 nm, while the polydispersity increases. These results are in good agreement with the film thicknesses of individual horizontal foam films studied with a thin-film pressure balance.

Exploring water in oil emulsions simultaneously stabilized by solid hydrophobic silica nanospheres and hydrophilic soft PNIPAM microgel.

A general drawback of microgels is that they do not stabilize water-in-oil (w/o) emulsions of non-polar oils. Simultaneous stabilization with solid hydrophobic nanoparticles and soft hydrophilic microgels overcomes this problem. For a fundamental understanding of this synergistic effect the use of well defined particle systems is crucial. Therefore, the present study investigates the stabilization of water droplets in a highly non-polar oil phase using temperature responsive, soft and hydrophilic PNIPAM microgel particles (MGs) and solid and hydrophobic silica nanospheres (SNs) simultaneously. The SNs are about 20 times smaller than the MGs. In a multiscale approach the resulting emulsions are studied from the nanoscale particle properties over microscale droplet sizes to macroscopic observations. The synergy of the particles allows the stabilization of water-in-oil (w/o) emulsions, which was not possible with MGs alone, and offers a larger internal interface than the stabilization with SNs alone. Furthermore, the incorporation of hydrophilic MGs into a hydrophobic particle layer accelerates the emulsions sedimentation speed. Nevertheless, the droplets are still sufficiently protected against coalescence even in the sediment and can be redispersed by gentle shaking. Based on droplet size measurements and cryo-SEM studies we elaborate a model, which explains the found phenomena.

Hydration and Solvent Exchange Induced Swelling and Deswelling of Homogeneous Poly(N-isopropylacrylamide) Microgel Thin Films.

The investigation of the response kinetics of smart colloidal microgel films is crucial for their optimization to enable advanced applications. We study the classical thermoresponsive microgel model system N-isopropylacrylamide cross-linked with N,N'-methylenebisacrylamide. Without the typically used polyelectrolyte coating of the substrate, thin microgel films are prepared in a single spin-coating step. Atomic force microscopy measurements reveal an extremely dense packing, resulting in a homogeneous compact thin film of microgel particles. The hydration kinetics of these films in H2O and D2O atmospheres as well as the kinetics of the solvent exchange between both water species are investigated with in situ time-of-flight neutron reflectometry (TOF-NR) and in situ Fourier-transform infrared (FTIR) spectroscopy. With accounting for a nonconstant humid atmosphere, the intrinsic diffusion dynamics of water molecules into the thin microgel film are modeled and the specific time constant τ and the effective Flory-Huggins interaction parameter χeff are determined. Comparing the results in H2O and D2O atmospheres, TOF-NR and FTIR spectroscopy results show an increased affinity of the microgel films toward H2O as compared to D2O. From the FTIR spectroscopy data, we further identify different kinetics of intermolecular processes and order them according to their temporal evolution.

A comparison of the network structure and inner dynamics of homogeneously and heterogeneously crosslinked PNIPAM microgels with high crosslinker content.

Poly(N-isopropylacrylamide) microgel particles were prepared via a "classical" surfactant-free precipitation polymerization and a continuous monomer feeding approach. It is anticipated that this yields microgel particles with different internal structures, namely a dense core with a fluffy shell for the classical approach and a more even crosslink distribution in the case of the continuous monomer feeding approach. A thorough structural investigation of the resulting microgels with dynamic light scattering, atomic force microscopy and small angle neutron scattering was conducted and related to neutron spin echo spectroscopy data. In this way a link between structural and dynamic features of the internal polymer network was made.