Stability and Mechanical Properties of Polyelectrolyte Capsules Obtained by Stepwise Assembly of Poly(styrenesulfonate sodium salt) and Poly(diallyldimethyl ammonium) Chloride onto Melamine Resin Particles Changyou Gao, †,‡ Stefano Leporatti, ‡,§ Sergio Moya, ‡ Edwin Donath,* ,‡ and Helmuth Mo ¨hwald ‡ Department of Polymer Science and Engineering, Zhejiang University, Hangzhou 310027, China, and Max-Planck Institute of Colloids and Interfaces, D-14424 Potsdam, Germany Received November 7, 2000. In Final Form: March 6, 2001 Capsules composed of poly(styrene sulfonate, sodium salt) (PSS)/poly(diallyldimethyl ammonium) chloride (PDADMAC) were prepared by layer-by-layer deposition of the polyelectrolytes on melamine formaldehyde colloidal templates followed by the decomposition of the cores by hydrochloride. A yield of more than 90% of intact capsules was achieved if (i) the core diameter was equal to or less than 3.8 μm and (ii) not more than five pairs of layers were adsorbed. When the cores were larger or the layers were thicker, the osmotic pressure difference caused by core dissolution led to an increased frequency of wall rupture. The elasticity modulus of a multilayer consisting of five pairs of PSS/PDADMAC was about 140 MPa as measured by osmotic pressure induced capsule deformation. Introduction Nano- and micron-sized polyelectrolyte capsules are of both scientific and technological interest because of their potential applications as new colloidal structures in areas such as medicine, drug delivery, and catalysis. 1-3 Biopoly- mers such as proteins, enzymes, or nucleic acids may be encapsulated, transported, and released afterward. Recently, novel nano- and micrometer-sized capsules have been prepared by stepwise adsorption of polyelec- trolytes (PE) 4-6 onto charged colloidal templates, 7-9 fol- lowed by decomposition of the core. 8-9 This fabrication technique allows control of the capsule size and shape, the capsule wall thickness, and the capsule wall composi- tion. The structure and properties of the capsules can be designed to match various applications. Capsules with a given size and a given number of polyelectrolyte layers including surfactants and nanoparticles and derived from different templates can be fabricated. 9-11 So far, these capsules have been characterized by scanning force microscopy (SFM), confocal laser scanning microscopy (CLSM), fluorescence and UV-vis spectroscopy, electro- phoresis, electrorotation, and single particle light scat- tering. Various properties of the capsules, such as the morphology both in the wet state and in the dry state, their stability upon annealing, the elasticity, the surface charge, the capacitance and wall conductance, as well as the permeability have been explored. 12-20 It was shown that organic and inorganic materials can be precipitated inside capsules. 20 Polymerization reactions were conducted to fabricate polymer-containing capsules. 21 A solvent exchange protocol yielded capsules filled with nonpolar liquids. 18 The typical hollow polyelectrolyte capsules produced so far were composed of alternating poly(styrenesulfonate * To whom correspondence should be addressed. Email: edwin.donath@mpikg-golm.mpg.de. † Zhejiang University. ‡ Max-Planck Institute of Colloids and Interfaces. § Present address: Digital Instruments/VEECO Metrology Group, Janderstrasse 9, D-68199 Mannheim, Germany. Email: leporatti@ digmbh.de. (1) Lewis, D. D. 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