Tuning of the Exchange Dynamics of Unimers between Block Copolymer Micelles with Temperature, Cosolvents, and Cosurfactants Jan van Stam, ‡,⊥ Serge Creutz, ‡,§ Frans C. De Schryver,* ,² and Robert Je´ roˆ me ‡ Departement Scheikunde, Katholieke Universiteit Leuven, Celestijnenlaan 200F, BE-3001 Heverlee, Belgium, and Center for Education and Research on Macromolecules (CERM), University of Lie` ge, Sart-Tilman, B6, BE-4000 Lie` ge, Belgium Received December 29, 1999; Revised Manuscript Received May 30, 2000 ABSTRACT: The dynamics of unimer exchange between aqueous micelles, formed by two amphiphilic block copolymers, i.e., poly(styrene-b-sodium methacrylate) and poly(tert-butylstyrene-b-sodium meth- acrylate), has been investigated by steady-state fluorescence spectroscopy. The kinetics are so slow at room temperature that no exchange could be detected over several hours, while at 60 °C the exchange rate constants could be estimated. These results corroborate our previous findings that the rate is slowed down by increasing the hydrophobicity of the core. In addition to the temperature, the exchange can also be tuned by the addition of either a cosolvent or a cosurfactant. The efficiency of these additives to speed up the exchange process is related to their water solubility and their compatibility with the hydrophobic core of the micelles. The most pronounced effect on the exchange process is observed when the water solubility is low and the mixing of the additive with the hydrophobic core is favorable. 1. Introduction When dissolved in a solvent selective for one of the constitutive components, amphiphilic block copolymers self-assemble into micelles in a manner similar to classical surfactants. Compared with classical surfac- tants, the exchange rate of amphiphilic block copolymer molecules, unimers, between the aggregates is substan- tially slowed down. On one hand, if the rate is slowed down too much, block copolymers will be of limited use for applications that require a quite fast unimer release. For instance, adsorption from a selective solvent onto surfaces, which is a prerequisite for the stabiliza- tion of solid dispersions, has been shown to be driven by the ease of unimer release. 1 On the other hand, slow exchange rates do open new opportunities, e.g., drug delivery. Both theoretical and experimental studies on the aggregation behavior of block copolymers in aqueous solutions, 2-23 as well as studies of the exchange of unimers between aqueous block copolymer micelles, 24-34 have been very active research domains in the recent years. Special attention has been paid to poloxamers, i.e., di- or triblock copolymers of ethylene oxide and propylene oxide, and block copolymers of polystyrene as the hydrophobic block. The former have an exit rate of approximately 10 3 s -1 , 28 which is slow in comparison with what is found for ordinary micelles, 35-38 while poly- (styrene-b-ethylene oxide) copolymers exchange so slowly, that a significant rate only could be determined at elevated temperatures. 26 In other words, the exchange rate of copolymer molecules between block copolymer micelles can, roughly, be expected to be found in the range between 10 3 s -1 and 0. Recently, the exchange kinetics of poly(dimethylami- noalkyl methacrylate-b-sodium methacrylate) copoly- mers, determined by nonradiative energy transfer, were reported. 33,34 The unimer exchange kinetics were in between those of poloxamers and polystyrene-based copolymers and could be related to the hydrophobic character of the aminated block, the composition, and the architecture of the copolymer. Any increase of the copolymer hydrophobicity, by changing either the co- polymer composition or the aminated monomer, slows down the exchange. In this study, the influence of the hydrophobic char- acter on the unimer exchange rate has been further elaborated on by replacing the hydrophobic methacry- late block by blocks with varying lipophilicity, i.e., styrene and tert-butylstyrene. According to earlier reports on polystyrene copolymers, extremely slow exchange rates are expected. 26,27 The possibility to tune the exchange rate by changing the temperature or addition of a cosolvent or cosurfactant is investigated and discussed in detail. 2. Experimental Section 2.1. Materials. Pyrene (Acros Janssen) was twice recrystal- lized from absolute ethanol. Distilled water of Milli-Q quality was used for all solutions. Triton X-100 (Tx-100) (Acros Janssen), sodium dodecyl sulfate (SDS) (BDH, specially pure), toluene, and 1,4-dioxane (dioxane) (both from Rathburn, PA quality) were used as received. 2.2. Block Copolymer Synthesis. The anionic synthesis of poly(styrene-b-sodium methacrylate) copolymers has been reported by Desjardins et al. 39 and Ramireddy et al. 40 Desjar- dins et al. highlighted the need for end-capping the polystyrene with 1,1-diphenylethylene; otherwise, side reactions were observed. The labeling procedure was described by Ramireddy et al. and was applied in the present study. The method used differs from their only by the type of initiator used, i.e., R-methylstyrene (RMeS) in combination with sec-butyllithi- um. 41 This initiator is preferred, compared to cumylpotas- sium, 40 since it is formed in situ and does not require any preliminary synthesis. In addition, the used initiator can easily be extended to other poly(styrene-b-alkyl methacrylate) co- ² Katholieke Universiteit Leuven. ‡ University of Lie`ge. § Present address: Dow Corning S.A., Parc Industriel C, BE- 7180 Seneffe, Belgium. ⊥ Present address: Department of Chemistry, Karlstad Uni- versity, SE-651 88 Karlstad, Sweden. * To whom correspondence should be addressed. E-mail Frans.DeSchryver@Chem.KULeuven.ac.be. 6388 Macromolecules 2000, 33, 6388-6395 10.1021/ma992174a CCC: $19.00 © 2000 American Chemical Society Published on Web 08/03/2000