Microgels are solvent-swollen nano- and microparticles that show prevalent colloidal-like behavior despite their polymeric nature. Here we study ultra-low crosslinked poly( N -isopropylacrylamide) microgels (ULC), which can behave like colloids or flexible polymers depending on 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 soft colloids. 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.

Microgels are nanometer-to-micrometer-sized cross-linked polymer networks that swell when dispersed in a good solvent. These soft colloids have emerged as versatile building blocks of smart materials, which are distinguished by their unique ability to adapt their behavior to changes in external stimuli. Using X-ray and neutron scattering and molecular simulation methods, we systematically measured and modeled the response to crowding of compressible, deformable microgels with varying cross-link densities and internal architectures. Our experiments and simulations demonstrate that incorporating a solvent-filled cavity during chemical synthesis provides an independent means of controlling microgel swelling that complements the influence of changing cross-link density. In other words, knowledge of the content of cross-links alone cannot be used to define microgel softness, but microgel architecture is another key property that affects softness. These results are potentially important for biomedical applications, such as drug delivery and biosensing.