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

This study investigates 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 chemical composition were produced to show a broad range of responses in hydrodynamic radius and degree of self-assembly with temperature and pH. Microgels were deposited on silicon wafers primed with a bilayer of poly(octadecene-alt-maleic anhydride) and polyethyleneimine by either incubation or spin coating of aqueous suspension of microgels at different temperature and pH. 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 with statistical significance, but not on microgel composition; 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, profilometry analysis showed that microgel coatings respond to thermal stimuli with sensible variations in surface roughness; notably, the thermal variation of roughness correlates strongly with ρ and H, and to a lesser extent with PE.