Ordered Polyelectrolyte Multilayers. Rules Governing Layering in Organic Binary Multilayers Xavier Arys, † Peter Fischer, ‡,§ Alain M. Jonas,* ,† Marc M. Koetse, ‡,| Andre ´ Laschewsky,* ,‡,⊥ Roger Legras, † and Erik Wischerhoff ‡,# Contribution from the Unite ´ de physique et de chimie des hauts polyme ` res and Unite ´ de chimie des mate ´ riaux, UniVersite ´ catholique de LouVain, Place Croix du Sud, 1, B-1348 LouVain-la-NeuVe, Belgium Received September 3, 2002 ; E-mail: jonas@poly.ucl.ac.be; andre.laschewsky@iap.fhg.de Abstract: We study the growth and internal structure of polyelectrolyte multilayers obtained by combining three polyanions with nine polycations of the ionene family, of systematically varied chemical architecture. We find that, contrary to a generally held belief, ordered organic multilayers are by no way exceptional, provided one of the polyelectrolytes bears groups which induce structure in water, such as long hydrophobic segments or mesogenic groups. However, this condition is not sufficient, as order will or will not emerge in the multilayer depending on the specific pairing of the polyelectrolytes. The results support the notion that layering in the multilayer results from some degree of prestructuring of a water-swollen layer adsorbed during each step of deposition. These findings pave the way to new possible uses of polyelectrolyte multilayers, for example, for applications requiring preferential alignment or strong confinement of specific functional groups. Introduction Electrostatic self-assembly (ESA) of polyelectrolytes (also referred to as layer-by-layer assembly, LBL) is a powerful technique enabling the fabrication of thin organic functional films with nanoscale control over the film structure. 1-7 The versatility and deceptive simplicity of the assembly process, which consists of the sequential adsorption of polyions of opposite charges (with rinsing and drying steps being applied between), resulted in a growing interest for this method over the last 10 years. However, in contrast with naive cartoons sometimes encountered in the literature, the internal structure of most organic binary (A/B) n ESA films was shown to be highly disordered: Successively adsorbed layers of polyelec- trolytes interpenetrate so strongly that the compositional fluctua- tion perpendicular to the surface is completely smoothed out. 4 Although stratified in the sense that the location of a chain is directly related to the step at which it was deposited in the succession of adsorption events, 4,8 most organic (A/B) n films should be considered as molecular-level blends of polyelectro- lytes. 9,10 Consistently, no Bragg reflection due to layering appears in X-ray or even neutron reflectograms of such films, 11-13 even though Bragg reflections are observed for more complex purely organic systems of the {(A/B) m /(C/D) p } n - type, 4,14-17 or for hybrid binary (A/B) n systems made of rigid inorganic platelets combined with an organic polyion. 18-22 The lack of true layering in ESA films may be a limitation of the technique, because well-organized multilayers are required for † Unite ´ de physique et de chimie des hauts polyme `res. ‡ Unite ´ de chimie des mate ´riaux. § Present address: Bayer AG; KU-FE/PCS; Rheinuferstrasse 7-9; D-47829 Krefeld; Germany. | Present address: Macromolecular and Organic Chemistry, Eindhoven University of Technology, The Netherlands. ⊥ Present address: Fraunhofer Institut fu ¨r Angewandte Polymerforschung FhG-IAP; Geiselbergstrasse 69; D-14476 Golm; Germany. # Present address: Glaucus Proteomics B.V.; P.O. 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