3276 Macromolecules 1994,27, zyxwv 3276-3281 Determination of the Micelle Architecture of Polystyrene/ Poly(4-vinylpyridine) Block Copolymers in Dilute Solution Markus Antonietti* and Steffi Heinz Max-Planck-Institut fur Kolloid und Grenzflachenforschung, Kantstrasse 55, 0-14513 Teltow-Seehof, Germany Manfred Schmidt and Christine Rosenauer Makromolekulare Chemie 11 der Uniuersitat Bayreuth, Universitatsstrasse 30, 0-95440 Bayreuth, Germany Received October 18, 1993; Revised Manuscript Received March 18, 1994' ABSTRACT Block copolymersconsistingof polystyrene (PS) and poly(4-vinylpyridine) (4-PVP) with varying chain lengths are synthesized via anionic polymerization. When these block copolymers are dissolved in selective solvents (solvents for PS), well-defined micelles with respect to their size and size distribution are formed. These micelles are characterized by static and dynamic light scattering. The characterization reveals that the micelles exhibit spherical or prolate morphology with a size in the range 20 zyxw I rH I 100 nm. Size and shape depend only on the block molecular weights and the solvent used but exhibit no significant concentration dependence. In certain solvents, the observation of scattering peaks indicates the formation of intermicellar ordering on the scale of the wavelength of light. Viscometric experiments reveal that the block copolymer micelles are unstable under shear applied in common viscometer tubes and decompose to the primary block copolymers. The associated structure, however, is quickly recovered. (I) Introduction For several years it has been known that polymer micelles are formed when block copolymers are dissolved in solvents for one of the components only. These micelles are well-defined with respect to their size and size distribution (see, for instance, refs 1-8). In this paper, we describe the micellar morphology in dilute solution of block copolymers consisting of poly- styrene (PS) and poly(4-vinylpyridine) (4-PVP) with varying chain lengths. The system styrene/4-vinylpyridine was chosen for several reasons. On the one hand, both polymers possess nearly the same index of refractiong with simplifies the evaluation of light scattering data. On the other hand, the solution properties of PS and 4-PVP are sufficiently different to allow micelle formation in nearly all solvents. In addition, these polymers can be made in high quality and in large amounts by anionic polymerization. The micelles of these block copolymers are characterized by static and dynamic light scattering in various solvents. This combination allows for determination of the ag- gregation number, the micelle size, architecture, and polydispersity. The comparison of all these data with theory yields a more detailed picture of block copolymer aggregation and morphology in dilute solution. In addition, the influence of the aggregation number on the stretching of the chains in a micellar morphology can be estimated. (11) Experimental Section (11.1) Polymer Synthesis. The PS/4-PVP block copolymers are synthesized by anionic polymerization following standard procedures.lOJ1 All reactions take place within our all-glass vacuum line which has already been described in the literature.12J3 sec-Butyllithium (12% solution in hexane; Aldrich) is used to initiate the polymerization which is performed at -55 zyxwvuts OC in tetrahydropyran (THP). Styrene is freshly distilled over lithium aluminium hydride, degassed, and added to the initiator solution via the gas phase.lZJ3 The reaction is kept at this temperature for 15 min. A small amount of this solution is taken for @ Abstract published in Advance ACS Abstracts, May zyxwvuts 1, 1994. 0024-929719412227-3276$04.5010 characterization of the polystyrene block. Afterward, the calculated amount of 4-vinylpyridine(twice distilled over calcium hydride) is also added via the gas phase. A marked change of the color from yellow to red occurs, indicating the blocking from styrene to 4-vinylpyridine. In addition, the solution becomes increasingly turbid, an indication for micelle formation in THP where 4-PVP is insoluble. The reaction mixture is allowed to heat up to -20 "C, and the reaction is terminated with methanol. The resulting polymers are precipitated in low-boiling petro- leum ether and dried in vacuo at 50 "C for 2 days. As seen by a complete yield, the added 4-PVP can be regarded as fully polymerized. Therefore, micelle formation seems not to affect further polymerization. (11.2)Polymer Characterization. GPC measurements on the block copolymers are performed in THF (60.0 "C) and DMF (70.0 OC) with a column combination 106-104-103 A from PSS Co. The signals are detected with a two-detector combination (UV detector, Spectra Physics 100; RI detector, Shodex RI 61) which allows for the direct determination of molecular weight and the estimation of the relative composition. In addition, the relative composition of the block copolymers is controlled by 'H-NMR and elementary analysis. (11.3) Static and Dynamic Light Scattering. Light scat- tering experiments were carried out in toluene, tetrahydrofuran, butyl acetate, and butanone at 20.0 OC. The spectrometer and procedure for simultaneous static and dynamic light scattering are extensively described in previous publi~ations.~~J~ The measurements are performed at the 647.1-nm line of a krypton ion laser (Spectra Physics 2025). We adopt for the refractive index increment the values for polystyrene, since PS and 4-PVP are nearly isorefractive. The solutionsare purified by 10-fold filtering through Millipore 0.45-pm PTFE filters. Usually, no significant loss of concentra- tion due to filtering can be detected. The final concentrations range between 100 and g/L. (111) Results and Discussion (111.1) Synthesis and GPC Results. The character- ization of the polystyrene precursors and the corresponding blocks by GPC is summarized in Table 1. We observe that the polystyrene parts of the block copolymers are quite narrowly distributed. The complete blocks become broader which is possibly due to a more heterogeneous chain growth within the micelles which 0 1994 American Chemical Society