Thin Films of Poly(N-isopropylacrylamide) End-Capped with n-Butyltrithiocarbonate

Thin thermoresponsive hydrogel films of poly(N-isopropylacrylamide) end-capped with n-butyltrithiocarbonate (nbc-PNIPAM) are prepared on solid supports having silicon oxide surfaces with spin-coating. The film thickness is varied from 5 to 240 nm. As measured with optical microscopy, atomic force microscopy, and X-ray reflectivity, the films are homogeneous and smooth for films thicker than 5 nm. Microbeam grazing-incidence small-angle X-ray scattering (µGISAXS) shows that these nbc-PNIPAM films are physically cross-linked gels, where the end-group domains form the physical cross-links with a defined nearest-neighbor distance of 25 nm. Along the surface normal, with µGISAXS the presence of long-ranged correlations between substrate and film surface is detected. The thinner the nbc-PNIPAM films are, the stronger is the response to swelling in saturated water vapor atmosphere. A swelling up to a factor of 6.5 as compared to the dry film and a factor of 2.9 as compared to the collapsed film is found. The transition temperature in thin films shifts slightly as compared to the bulk, and the width of the transition is film thickness dependent. Measurements of the bulk solution behavior complete the investigation.