Nanogels and Microgels in Fundamental and Applied SciencesCopyright: A. Scotti
Exploring the colloid-to-polymer transition for ultra-low crosslinked microgels from three to two dimensions
In this work we study ultra-low crosslinked poly(N-isopropylacrylamide) microgels (ULC), which can behave like colloids or flexible polymers depending on their environment, e.g. dimensionality, compression or other external stimuli. Small-angle neutron scattering shows that the structure of the ULC microgels in bulk aqueous solution is characterized by a density profile that decays smoothly from the center to a fuzzy surface. Their phase behavior and rheological properties are those of colloids interacting with a soft potential. However, when these microgels are confined at an oil-water interface, their behavior resembles that of flexible macromolecules. Once monolayers of ultra-low crosslinked microgels are compressed, deposited on solid substrate and studied with atomic-force microscopy, a concentration-dependent topography is observed. Depending on the compression, these microgels can behave as flexible polymers, covering the substrate with a uniform film, or as colloidal microgels leading to a monolayer of particles.
A. Scotti, S. Bochenek, M. Brugnoni, M. A. Fernandez-Rodriguez, M. F. Schulte, J. E. Houston, A. P. H. Gelissen, I. I. Potemkin, L. Isa, and W. Richtering
Nature Communications 10, 1418 (2019).
How do microgels collapse?Copyright: Walter Richtering
Time-resolved structural evolution during the collapse of responsive hydrogels: The microgel-to-particle transition
R. Keidel, A. Ghavami, D.M. Lugo, G. Lotze, O. Virtanen, P. Beumers, J.S. Pedersen, A. Bardow, R.G. Winkler and W. Richtering
Science Advances 06 Apr 2018:
Vol. 4, no. 4, eaao7086
The structural adaption of microgels to the environment involves a unique transition from a flexible, swollen finite-size macromolecular network, characterized by a fuzzy surface, to a colloidal particle with homogeneous density and a sharp surface. In this contribution, we determine, for the first time, the structural evolution during the microgel-to-particle transition. Time-resolved small-angle x-ray scattering experiments and computer simulations unambiguously reveal a two-stage process: In a first, very fast process, collapsed clusters form at the periphery, leading to an intermediate, hollowish core-shell structure that slowly transforms to a globule. This structural evolution is independent of the type of stimulus and thus applies to instantaneous transitions as in a temperature jump or to slower stimuli that rely on the uptake of active molecules from and/or exchange with the environment. The fast transitions of size and shape provide unique opportunities for various applications as, for example, in uptake and release, catalysis, or sensing.
Hollow microgels squeezed in overcrowded environmentsCopyright: Walter Richtering
A. Scotti, M. Brugnoni, A. A. Rudov, J. E. Houston, I. I. Potemkin, and W. Richtering
The Journal of Chemical Physics
148, 174903 (2018)
We study how a cavity changes the response of hollow microgels with respect to regular ones in
overcrowded environments. The structural changes of hollowpoly(N-isopropylacrylamide) microgels
embedded within a matrix of regular ones are probed by small-angle neutron scattering with contrast
variation. The form factors of the microgels at increasing compressions are directly measured. The
decrease of the cavity size with increasing concentration shows that the hollow microgels have an
alternative way with respect to regular cross-linked ones to respond to the squeezing due to their
neighbors. The structural changes under compression are supported by the radial density profiles
obtained with computer simulations. The presence of the cavity offers to the polymer network the
possibility to expand toward the center of the microgels in response to the overcrowded environment.
Furthermore, upon increasing compression, a two step transition occurs: First the microgels are
compressed but the internal structure is unchanged; then, further compression causes the fuzzy shell
to collapse completely and reduce the size of the cavity. Computer simulations also allow studying
higher compression degrees than in the experiments leading to the microgel’s faceting.
Anionic shell shields a cationic core allowing for uptake and release of polyelectrolytes within core-shell responsive microgelsCopyright: A. Gelissen
Arjan P. H. Gelissen, Andrea Scotti, Sarah K. Turnhoff, Corinna Janssen, Aurel Radulescu, Andrij Pich, Andrey A. Rudov, Igor I. Potemkin and Walter Richtering , Soft Matter , 2018, Advance Article,
To realize carriers for drug delivery, cationic containers are required for anionic guests. Nevertheless, the toxicity of cationic carriers limits their practical use. In this study, we investigate a model system of polyampholyte N-isopropylacrylamide (NIPAM)-based microgels with a cationic core and an anionic shell to study whether the presence of a negative shell allows shielding the cationic core while still enabling the uptake and release of the anionic guest polyelectrolytes. These microgels are loaded with polystyrene sulfonate of different molecular weights. By means of small-angle neutron scattering, we evaluate the spatial distribution of polystyrene sulfonate within the microgels. The guest molecules are located in different parts of the core-shell microgels depending on their size. By combining these scattering results with UV-Vis and electrophoretic mobility we gain complementary results to investigate the uptake and release process of polyelectrolytes in polyampholyte core-shell microgels. Moreover, Brownian molecular dynamic simulations are performed to compare experimental and theoretical results of this model. Our findings demonstrate that the presence of a shell still enables efficient uptake into the cationic core of guest molecules. These anionic guest molecules can be released through an anionic shell. Furthermore, the presence of a shell enhances the stability of the microgel-polyelectrolyte complexes with respect to the cationic precursor microgel alone.
Probing the Internal Heterogeneity of Responsive Microgels Adsorbed to an Interface by a Sharp SFM Tip: Comparing Core-Shell and Hollow MicrogelsCopyright: M. F. Schulte
M. F. Schulte, A. Scotti, A. P. H. Gelissen, W. Richtering, A. Mourran, Langmuir 2018 (accepted).
Silica core – PNIPAM shell and corresponding hollow microgels were studied by scanning force microscopy (SFM). We show that swollen microgels are penetrated strongly by a sharp SFM tip. The force profile during insertion of the tip into the polymer network enables to determine a depth-dependent contact resistance which closely correlates with the density profiles determined in solution by small-angle neutron scattering (SANS). Remarkably, while currently used techniques only generate an average of the z-profile, SFM provides spatially resolved internal structure information of individual microgels. We found that the cavity of the swollen hollow microgels is still present when adsorbed to the solid substrate.
Swelling of a Responsive Network within Different Constraints in Multi-Thermosensitive MicrogelsCopyright: M. Brugnoni
M. Brugnoni, A. Scotti, A. A. Rudov, A. P. H. Gelissen, T. Caumanns, A. Radulescu, T. Eckert, A. Pich, I. I. Potemkin, W. Richtering, 2018, Macromolecules, DOI: 10.1021/acs.macromol.7b02722.
Silica-core double-shell and hollow double-shell microgels made of an inner poly(N-isopropylmethacrylamide) and an outer poly(N-isopropylacrylamide) shell are studied by exploiting the distinct temperature sensitivities of the polymers. The swelling states of the two shells can be tuned by temperature changes enabling three different swelling states: above, below, and between the distinct volume phase transition temperatures of the two polymers. This enables to investigate the effect of different constraints on the swelling of the inner network. Small-angle neutron scattering with contrast variation discloses how the expansion of the inner shell strongly depends on the material and swelling state of its constraints. In the presence of the stiff core, the microgels show a considerable interpenetration of the polymeric shells: the inner network expands into the outer deswollen shell. This interpenetration vanishes when the outer network is swollen. Furthermore, as predicted by our computer simulations, an appropriate choice of cross-linking density enables the generation of hollow double-shell nanocapsules. Finally, the inner shell undergoes a push−pull effect. At high temperature, the collapsed outer shell pushes the swollen inner network into the cavity. At lower temperature, the swelling of the outer network contrary pulls the inner shell back toward the external periphery.