Surface Texture of Poly(styrenesulfonate sodium salt) and Poly(diallyldimethylammonium chloride) Micron-Sized Multilayer Capsules: A Scanning Force and Confocal Microscopy Study Changyou Gao, ²,‡,§ Stefano Leporatti, ‡,§ Edwin Donath,* ,‡ and Helmuth Mo 1 hwald ‡ Department of Polymer, Zhejiang UniVersity, Hangzhou 310027, China, and Max-Planck Institute of Colloids and Interfaces, D-14424 Golm/Potsdam, Germany ReceiVed: February 17, 2000; In Final Form: May 3, 2000 Ultrathin polyelectrolyte capsules fabricated by stepwise assembly of poly(styrenesulfonate sodium salt) and poly(diallyldimethylammonium chloride) are examined by confocal laser scanning microscopy and scanning force microscopy. The capsule surface is composed of grains of 124 ( 10 nm in diameter with a roughness of ∼13 nm. These grains are formed by laterally segregated polyelectrolyte complexes. Osmotically and annealing-induced capsule swelling facilitated further grain segregation together with a capsule surface area increase. Incubation in salt solution induced capsule shrinking and grain aggregation. Introduction A novel pathway for fabricating polyelectrolyte capsules with ultrathin multilayer walls based on the layer-by-layer adsorption technique 1-4 was developed recently. 5 Oppositely charged polyelectrolytes were consecutively adsorbed onto charged colloidal templates, followed by decomposition of the templates. The typical polyelectrolyte capsules produced so far were composed of alternating poly(stryrenesulfonate sodium salt) (PSS) and poly(allylamine hydrochloride) (PAH). These cap- sules were characterized with respect to their morphology, surface charge, their wall texture, layer capacitance, and conductance, and their stability upon temperature increase. 5-12 Recently, instead of PAH, another polycation, poly(di- allyldimethylammonium chloride) (PDADMAC), was employed for the preparation of PSS/PDADMAC capsules. Capsules that are impermeable for macromolecules were obtained with a yield higher than 90%. Preliminary studies revealed that PSS/ PDADMAC capsules are different from PSS/PAH capsules concerning their elasticity, and their response to higher tem- peratures and salt addition. It was recently shown that the PSS/PAH capsules exhibit a typical lateral texture in the submicron scale. Grains of a size of about 70 nm in diameter were observed. These grains were interpreted as segregated polyelectrolyte complexes. 9,10 The mechanism of grain formation and their significance for the capsule wall properties is at present not understood. Therefore, it is interesting to study also the texture of the new PSS/ PDADMAC capsules and to compare it with PSS/PAH capsules. One may expect that changing PAH for PDADMAC may influence the layer properties and the appearance of the grains. This would promote a better understanding of the molecular mechanism of capsule formation and of the relation between molecular composition and capsule properties. This study reports data on the nanometer scale lateral structure of PSS/PDADMAC capsules. A grainy texture of the PSS/ PDADMAC capsule wall was indeed observed by SFM at high resolution. The grains were significantly larger than those reported for PSS/PAH capsules. PSS/PDADMAC multilayers deposited on mica had a similar grain size but revealed a more inhomogeneous distribution of the grains. Capsule swelling as a result of an osmotic pressure difference or heating induced a larger separation of the grains. The two cationic polymers that we can now compare differ in their stiffness, dimension of the ionic group, and charge density along the backbone and molecular weight. These features in relation to PSS are discussed in view of consequences for structure and permeability. Materials and Methods Materials. The sources of chemicals were as follows: PSS, M w 70 000 and PDADMAC, medium M w ∼ 200-350 kDa, 20 wt % in water, Aldrich; weakly cross-linked melamine form- aldehyde particles (MF-particles), microparticles GmbH, Berlin, Germany. All chemicals were used as received. The water used in all experiments was prepared in a three-stage Millipore Milli-Q Plus 185 purification system and had a resistivity higher than 18.2 MΩcm. Methods. Capsule and Multilayer Preparation. A membrane filtration technique was employed to consecutively adsorb PSS and PDADMAC onto MF particles. 13 The adsorption of poly- electrolytes (1 mg/mL) was conducted in 0.5 M NaCl solution for 5 min. Three washings in H 2 O followed. Then the respective oppositely charged polyelectrolyte species was adsorbed. After the desired number of layers was adsorbed, the coated particles were treated with HCl (pH 1.1) to decompose the MF cores. The produced MF oligomers and excess HCl were removed by filtration with gentle agitation until a neutral pH was established. Multilayers of PSS/PDADMAC were assembled on mica by consecutively dip-coating the mica in polyelectrolyte solutions alternated with intermediate washings. The films were not dried before the next layer was adsorbed. Capsule Annealing and Incubation in Salt Solution. Aqueous suspensions of capsules (∼4 × 10 8 cm -3 ) were incubated for 2 h at 40 °C. Salt-treated capsules were prepared by exposing them to 0.5 M NaCl at room temperature (23 °C) for 2 h. The * Corresponding author. E-mail: edwin.donath@mpikg-golm.mpg.de. ² Zhejiang University. ‡ Max-Planck Institute of Colloids and Interfaces. § These authors contributed equally to this paper. 7144 J. Phys. Chem. B 2000, 104, 7144-7149 10.1021/jp000615i CCC: $19.00 © 2000 American Chemical Society Published on Web 07/12/2000