Packing density, homogeneity, and regularity: quantitative correlations between topology and thermoresponsive morphology of PNIPAM-co-PAA microgel coatings*

We investigated the formation of monolayers of microgel particles comprising poly[(N-isopropylacrylamide)-co-(acrylic acid)] on solid substrates, their surface morphology, and stimuli-responsiveness. Crosslinked microgels with different composition were produced
showing a range of response in swelling with temperature and pH. Microgels were deposited on silicon wafers primed with a bilayer of poly(octadecene-alt-maleic anhydride) and polyethyleneimine. The characterization of the microgel-coated wafers led to the identification of three metrics describing microgel arrangement: density (ρ); heterogeneity (H), which correlates strongly with ρ and depends on deposition temperature and pH; and packing efficiency (PE), which portrays the regularity of microgel arrangement and exhibits no correlation with ρ nor H. The values of ρ, H, and PE calculated for in silico models of microgel coatings confirmed that these three metrics portray distinct characteristics of surface topology. Finally, laser profilometry showed that microgel coatings respond to thermal stimuli with sensible variations in surface roughness; which correlate strongly with ρ and H, and to a lesser extent with PE.