Influence of Shell Structure on Stability, Integrity, and Mesh Size of Polyelectrolyte Capsules: Mechanism and Strategy for Improved Preparation

Novel polyelectrolyte microcapsules are developed using alternating layer-by-layer (LBL) adsorption of oppositely charged polyelectrolytes onto charged templates followed by core removal. These capsules have a continuous wall with a hollow interior and exhibit tunable permeability, which can be utilized for both sustained release from the capsule and precipitation reactions from the bulk to the capsule interior. The purpose of this work is to understand core removal mechanism and elucidate the structure−property relationships governing capsule stability and yield, integrity, and mesh size. To this end, melamine formaldehyde (MF) templates with polyelectrolyte multilayers (PEMs) of poly(styrene sulfonate) (PSS) and poly(allylamine hydrochloride) (PAH) were investigated by confocal Raman microscopy, surface force microscopy, and confocal laser scanning microscopy. The capsule structure is dominated by two critical factors: the wall thickness and the core decomposition condition. After optimization of the core removal conditions, capsule permeability is shown to be strongly dependent on the thickness, which is controlled by the variation of solvent quality (i.e. salt concentration) or the number of bilayers. The critical multilayer thickness for capsule preparation is about 10 nm. Above this value, capsules of three different structures are obtained as a function of wall thickness. The mesh size of intact capsules is also shown to decrease from meso- to microporous as thickness increases. To improve the capsule integrity, a new mode of core decomposition is proposed and demonstrated by control of the acid excess during core release.