Looking inside Poly(N-isopropylacrylamide) Microgels: Nanomechanics and Dynamics at Solid–Liquid Interfaces

The continuous improvement of synthesis leads to a great variety in the internal architecture and functionality of colloidal hydrogels. A majority of envisioned applications use microgels as colloidal building blocks for layer formation at solid substrates. In this context, a fundamental understanding of the influence of these substrates on the internal structure and physical properties is essential. Especially, the nanomechanical properties of adsorbed poly(N-isopropylacrylamide) microgel particles are important for their application in cell cultivation, functional surface coatings, and others. Furthermore, these properties are closely related to the swelling behavior and the internal structure and dynamics of these microgels. However, the number of methods that are able to probe the viscoelasticity of adsorbed microgels over the entire vertical particle profile is limited. Grazing incidence neutron scattering techniques are suited to probe soft-matter samples with limited sample volumes at planar solid substrates and allow a resolution in the z-direction. We used neutron spin echo spectroscopy under grazing incidence to access fast thermal fluctuations (10–9 s) over the entire vertical particle profile. Atomic force microscopy nanoindentation was used to characterize the nanomechanical properties of adsorbed microgel particles prepared by batch and continuous monomer feeding methods. The resulting force maps revealed that batch microgels were “hard” and heterogeneous in their Young’s moduli, while the microgels from the continuous method were “soft” and homogeneous. Finally, neutron spin echo spectroscopy under grazing incidence revealed major differences in the vertical dynamic profile of both types of microgels.