Teaching Softness of Polymer Microgels Employing Atomic Force Microscopy

Studying the properties of soft nanoparticles exposes students to emerging trends in materials science, fosters interdisciplinary knowledge, and prepares them for their own contributions in both academic and industrial settings. We developed a laboratory atomic force microscopy (AFM) experiment using poly-N-isopropylacrylamide (PNIPAM) microgels and investigated single nanogels as well as monolayers by AFM and quantitative image analysis. The experiments show how soft nanogels are deformed at interfaces, and the students learn to quantify the deformation by quantitative analysis of height and phase images. The deformation is related to a core–corona type of cross-linker distribution inside the microgel. Further experiments address the structure of microgel monolayers and demonstrate structural transitions from a hexagonal phase of microgels in corona–corona contact toward a different regime at higher interfacial concentrations, in which microgels form a second hexagonal phase in core–core contact. A quantitative analysis of height images provides the distribution of nearest-neighbor distances. The students use dip-coating to prepare the samples and learn how to correlate AFM experiments in the dry state, i.e., at the solid/air interface after evaporation of the solvent, with properties of the microgel in bulk solution and at the water/air interface.