Core/Shell Microgels Decouple the pH and Temperature Responsivities of Microgel Films

Abstract

We investigate the impact of pH on the swelling properties of thermoresponsive microgel layer-by-layer films. Microgels were synthesized from N-isopropylacrylamide and/or N-isopropylmethacrylamide. At pH 3, films composed of traditional (core) microgels, where acrylic acid has been copolymerized throughout the microgel network, have sharp temperature responsivities as observed by increases in the intensity of scattered light. This is contrasted by results at pH 7.4, where responsivity is diminished, likely as a result of deprotonated carboxylic acid groups, which changes the osmotic pressure within the component microgels. To probe this phenomenon further, core/shell microgels were synthesized such that the acidic comonomer was isolated to the shell and the core was purely temperature-responsive. Films assembled from such core/shell microgels exhibited pH-independent temperature responsivities. Temperature controlled in-liquid atomic force microscopy was used to directly measure the thickness of microgel films at pH 3 and 7.4 at ∼26 and 50 °C. Film thickness measurements were predominately in agreement with light scattering data, where decreases in film thickness corresponded with increases in scattered light intensity. Thus, we find that spatially separating the responsivities within multiresponsive microgels can result in more predictive power over the responsivity of microgel assemblies.