Thermoresponsive surfaces by spin-coating of PNIPAM-co-PAA microgels: A combined AFM and ellipsometry study

Thermosensitive coatings are fabricated by spin-coating of microgels consisting of the cross-linked copolymer poly(N-isopropyl acrylamide-co-acrylic acid) (P(NIPAM-co-AA)) on silicon wafers. The microgels were synthesized with two different cross-linker molar ratios and the thin films were prepared at pH 2. At this pH the particles are negatively charged only due to the starter used for the polymerization. Scanning force microscopic (AFM) images indicate a dense packing of the particles and a strong flattening in the adsorbed state. This effect is stronger for microgels containing less cross-linker. Coatings consisting of these microgel particles show a reversible thermoresponsive swelling/shrinking in the region of the lower critical solution temperature (LCST) of NIPAM. For the ellipsometric study of this process a standard setup was modified in order to allow temperature dependent measurements of the optical thickness in a liquid cell. The temperature induced transition is sharper in the case of microgels with lower amount of cross-linker and smears out with increasing amount of cross-linker. No significant desorption of the particles occurs at pH 2, which was shown by AFM of the dried films before and after the ellipsometric measurements. In the dry state the average thickness of the prepared films is approximately 30 nm and a thickness of about 400 nm is reached in the swollen state.