Electrochemically produced responsive hydrogel films: influence of added salt on thickness and morphology

We report on electrochemically prepared hydrogel layers of poly-N-isopropylacrylamide (pNIPAm) and on the influence that the supporting electrolyte has on their thickness and morphology. Ions that are destabilizing in the Hofmeister sense increase the thickness. The effect correlates well with the ion's tendency to lower the lower critical solution temperature (LCST) of pNIPAm films. AFM micrographs show small-scale globules. When the films were produced in the presence of a destabilizing salt (such as ammonium sulfate) one also observes larger features, resembling wrinkles. We attribute the globules to nucleated growth of surface-attached microgels, whereas the wrinkles presumably are produced by the collapse of hydrogen bubbles underneath a well-crosslinked film. Adding a chain transfer agent to the reactant solution reduces the lateral heterogeneities.