PH-responsive poly(acrylic acid) core cross-linked star polymers: Morphology transitions in solution and multilayer thin films

Star polymers show great promise for a range of applications, including drug delivery and advanced coatings. Their unique solution properties and high functionality also make them attractive building blocks for the preparation of nanostructured thin films. This article describes the preparation of a poly(acrylic acid) (PAA)-based core cross-linked star (CCS) polymer by the "arm first" approach and its assembly into multilayered films by the layer-by-layer (LbL) method. Synthesis of the PAA star polyelectrolyte was achieved by preparing an acid protected polymer, poly(tert-butyl acrylate), by atom transfer radical polymerization and reacting it with a divinyl monomer (divinylbenzene), followed by deprotection. The PAA star polyelectrolyte displayed pH-responsive behavior in solution, as studied by dynamic light scattering, where a reversible size change was observed in response to pH changes. At pH 8710, the acrylic acid segments are fully charged, adopting a more stretched conformation, and larger diameters (similar to 30 nm) were observed. When the pH decreases, and associated protonation of the acid segments occurs, the PAA chains adopt a more coiled conformation, and the diameters decrease to similar to 23 nm at pH 2. The PAA star polymer was LbL assembled with poly(allylamine hydrochloride) (PAH) at different pH conditions to form pH-responsive multilayer films. The PAA star polymer/PAH multilayer films showed distinct morphology changes in response to post-treatment with different pH solutions. At pH 11, the star polymer adopted a stretched conformation and smoother films were obtained, whereas at pH 2, grain domains were visible and comparatively rougher films were formed.