Preparation of stable foams using sterically stabilized pH-responsive latexes synthesized by emulsion polymerization

Abstract

Three near-monodisperse poly(2-vinylpyridine) (P2VP) latexes of 380 nm, 640 nm and 820 nm diameter were prepared in turn by emulsion copolymerization of 2-vinylpyridine and divinylbenzene using a monomethoxy-capped poly(ethylene glycol) monomethacrylate (PEGMA) macromonomer as a reactive steric stabilizer. Each of these latexes proved to be effective particulate stabilizers for the production of long-lived foams by either hand-shaking or using foam columns. Scanning electron microscopy studies confirmed that the dried foams contained well-defined latex bilayers in each case, suggesting that the original air bubbles were stabilized with latex monolayers. Dried foams prepared using the smallest latex exhibited interesting optical effects when viewed in reflectance mode. This is most likely due to light diffraction by the latex bilayers, since the mean latex diameter of 380 nm is approximately half that of visible light. These PEGMA–P2VP particles undergo a latex-to-microgel transition at low pH. Such swelling causes catastrophic instability within the wet foams, presumably due to microgel desorption from the air–water interface. Thus these latex foams exhibit pH-responsive behavior.