Self-Organization of Water-Soluble Complexes of a Poly(2-vinylpyridinium)-block-poly(ethylene oxide) Diblock with Fluorinated Anionic Surfactants Jean-Franc ¸ ois Gohy, †,§ Sandrine Mores, † Sunil K. Varshney, ‡ and Robert Je ´ ro ˆ me* ,† 1 Centre for Education and Research on Macromolecules (CERM) - Institute of Chemistry B6, University of Lie ` ge, Sart-Tilman, B-4000 Lie ` ge, Belgium, and 2 Polymer Source, 771 Lajoie Street, Dorval, PQ H9P 1G7 Canada Received September 11, 2002 Revised Manuscript Received February 24, 2003 Nowadays, much attention is paid to strategies for polymers to self-organize into nanomaterials. The so- called “hairy-rod” or “comblike” polyelectrolyte-surfac- tant complexes are typical examples, which result from electrostatic interactions between a polyelectrolyte and oppositely charged ionic surfactants. 1 These objects are organized at a typical size of 1-6 nm, and their properties can be tuned by, e.g., the structure of the ionic surfactant. For example, partly fluorinated sur- factants can impart low friction properties to the complexes. 2 Block copolymers are another class of polymeric materials able to self-organize, in bulk and in a selective solvent for one of the constitutive blocks. Moreover, they are at the origin of a wide variety of morphologies on a typical length scale of 10-200 nm. 3 Combination of block copolymers with surfactants in the bulk is a valuable tool to prepare hierarchical structures thus to associate within a single material driving forces to self-organization at two different length scales. 4 Moreover, water-soluble hierarchical systems can be prepared by mixing a double hydrophilic diblock copoly- mer 5 with a low molar mass surfactant known for specific interaction with one of the blocks of the copoly- mer. These complexes self-aggregate into micellar ob- jects in water, with a core consisting of the polymer- surfactant complexes and a corona formed by the water- soluble uncomplexed blocks. 6-9 These types of micellar systems have potential as drug carriers 6 and in quite different fields, such as coatings, cosmetics, food, and enhanced oil recovery. 7 This communication deals with the preparation and preliminary characterization of an aqueous hierachical system based on the complexation of a poly(2-vinylpy- ridinium)-block-poly(ethylene oxide) copolymer (P2VP- b-PEO) with partially fluorinated anionic surfactants (FSA and FSE). The P2VP 41 -b-PEO 204 copolymer was prepared by sequential living anionic polymerization of 2-vinylpyridine and ethylene oxide, as reported else- where. 10 The numbers in the subscripts are the average degree of polymerization for each block. The copolymer polydispersity is 1.05. The diblock copolymer was dis- solved in water at pH ) 3(C ) 1 g/L; 0.1 mol/L phosphate buffer), for the P2VP block to be protonated and water-soluble. The two blocks being solvated, no micelles nor aggregates were formed in water. 10 This solution was added with different amounts of an aque- ous solution of a partially fluorinated anionic surfactant (C ) 1 g/L, pH ) 3, 0.1 mol/L phosphate buffer). Two surfactants were actually used, i.e., FSA and FSE (Scheme 1), purchased from E. I. du Pont de Nemours and used as received. They were selected because of solubilility in water, partial negative charges at pH ) 3 and a low surface energy component that could be attached to the polyelectrolyte by complexation. 11 At pH ) 3, the P2VP blocks and the FSA (or FSE) molecules were oppositely charged and formed electrostatic com- plexes that were water soluble. These strong electro- static interactions are believed to screen the weaker dipolar interactions between negatively charged FSA or FSE molecules and PEO blocks. The complexes were first analyzed by dynamic light scattering (DLS), with a Brookhaven Instruments Corp. BI-200 apparatus equipped with a BI-2030 digital correlator and an Ion Laser Technology argon laser at a wavelength of 488 nm. The intensity correlation function was measured, and its analysis by a cumulant expansion led to the average hydrodynamic diameter (D h ). Figure 1 shows the dependence of the normalized scattered intensity (I) and the average D h on the stoichiometry of the P2VP 41 -b-PEO 204 /FSE complexes. From these data, it appears that the characteristic size of the aggregates formed by the P2VP 41 -b-PEO 204 /FSE complexes does not fall in the range known for starlike spherical micelles and that the smaller and more dense aggregates (higher I and lower D h , in Figure 1) are formed when all the 2VP units of the P2VP block are complexed by FSE. The same type of dependence was also observed for the P2VP 41 -b-PEO 204 /FSA complexes and is reminiscent of complexes formed by cationic surfactants and a poly(sodium methacrylate)-block-poly- (ethylene oxide) diblock. 8 Moreover, the position of the minimum in the curve shown in Figure 1 is strongly sensitive to pH, in direct relation with the ionization degree of both the interacting moieties. In this respect, a small increase in pH results in a decrease of the number of positively charged 2VP units, and the mini- mum in the curve (Figure 1) is in turn shifted to a lower x-axis value. 12 Whenever the pH is higher than 5, the P2VP block is almost completely deprotonated and no electrostatic complex with FSE is observed so far. 12 The average D h for the P2VP 41 -b-PEO 204 /FSE com- plexes does not change with dilution, which indicates that the structure of the aggregates is extensively “frozen” in. The electrostatic origin of the complexation * To whom correspondence should be addressed. † University of Lie `ge. ‡ Polymer Source. § J.-F.G. is “Charge ´ de Recherches” by the Belgian National Fund for Scientific Research (FNRS). Scheme 1. Chemical Structures of the FSA and FSE Surfactants 2579 Macromolecules 2003, 36, 2579-2581 10.1021/ma025665v CCC: $25.00 © 2003 American Chemical Society Published on Web 03/27/2003