Novel Pickering emulsifiers based on pH-responsive poly(tert-butylaminoethyl methacrylate) latexes

Emulsion copolymerization of 2-(tert-butylamino)ethyl methacrylate in the presence of divinylbenzene (DVB) cross-linker and monomethoxy-capped poly(ethylene glycol) methacrylate (PEGMA) macromonomer at 70 °C afforded sterically-stabilized latexes at approximately 10% solids at pH 9. Dynamic light scattering and scanning electron microscopy (SEM) confirmed that relatively narrow size distributions were obtained. SEM confirmed the formation of spherical particles in the absence of any DVB cross-linker using a simple batch protocol, but in the presence of DVB it was necessary to use seeded emulsion polymerization under monomer-starved conditions to prevent the formation of latexes with ill-defined non-spherical morphologies. Lightly cross-linked latexes acquired cationic microgel character upon lowering the solution pH due to protonation of the secondary amine groups. Increasing the degree of cross-linking led to a progressively lower effective pK(a) of the copolymer chains from 8.0 to 7.3, which implies a gradual reduction in their basicity. Poly(tert-butylamino)ethyl methacrylate latex proved to be an effective Pickering emulsifier at pH 10, forming stable oil-in-water emulsions when homogenized with either n-dodecane or sunflower oil at 12,000 rpm for 2 min. These Pickering emulsions exhibited pH-responsive behavior: lowering the solution pH to 3 resulted in immediate demulsification due to the spontaneous desorption of the cationic microgels from the oil/water interface. Following rehomogenization at high pH, four successive demulsification/emulsification pH cycles could be achieved without a discernible loss in performance. However, no demulsification occurred on acidification of the fifth cycle, due to the progressive build-up of background salt.

Probing particle structure in waterborne pressure-sensitive adhesives with atomic force microscopy

There is a need to know the nanostructure of pressure-sensitive adhesive (PSA) films obtained from waterborne polymer colloids so that it can be correlated with properties. Intermittent-contact atomic force microscopy (AFM) of an acrylic waterborne PSA film identifies two components, which can be attributed to the polymer and the solids in the serum (mainly surfactant). It is found that when the average AFM tapping force, F(av), is relatively low, the polymer particles appear to be concave. But when F(av) is higher, the particles appear to have a convex shape. This observation is explained by a height artefact caused by differences in the indentation depths into the two components that vary with the tapping amplitude and F(av). To achieve the maximum contrast between the polymer and serum components, F(av) should be set such that the indentation depths are as different as possible. Unlike what is found for the height images, the phase contrast images of the PSA do not show a reversal in contrast over the range of tapping conditions applied. The phase images are thus reliable in distinguishing the two components of the PSA according to their viscoelastic properties. At the surface of films dried at room temperature, the serum component is found in localized regions within permanent depression into the film.