Langmuir 2003, 19, 10449-10453
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Notes In Situ Atomic Force Microscopy Imaging of Block Copolymer Micelles Adsorbed on a Solid Substrate Simon D. Connell,† Stephen Collins,‡ Johan Fundin,‡ Zhuo Yang,‡ and Ian W. Hamley*,‡ Department of Chemistry, Department of Physics and Astronomy, and the Astbury Centre for Structural Molecular Biology, University of Leeds, Leeds LS2 9JT, U.K. Received July 21, 2003. In Final Form: September 16, 2003
Micelles of block copolymers are useful for applications in bulk and when adsorbed at a solid surface.1 The latter provides a route, for example, for the controlled deposition of inorganic nanoparticles, prepared from precursors (for example, metal salts) sequestered within the micellar core.2,3 The structure of block copolymer micelles on solid substrates has been probed on dried films by atomic force microscopy.4-7 In addition, transmission electron microscopy can be applied for specimens that have been suitably cryocooled (sometimes the specimens are also stained to enhance contrast).4,8-13 However, there have been surprisingly few studies of the structure of block copolymer micelles in situ in solution. Webber et al.14 investigated the adsorption of poly(2-(dimethylamino)ethyl methacrylate)-b-poly(methyl methacrylate) (PDMA-PMMA) onto mica as a function of pH. They observed surface micelles at low pH and regions of close-packed structure at higher pH for this polyelectrolyte diblock. Subsequently, they have observed hexagonal close-packed structures in an adsorbed layer of a related diblock, poly(2-(dimethylamino)ethyl methacrylate) [10% quaternized]-b-poly(2(diethylamino)ethyl methacrylate).15 The structure of the polymer layer was influenced by the electrostatic interac* To whom correspondence may be addressed. † Department of Physics and Astronomy. ‡ Department of Chemistry. (1) Fo¨rster, S.; Antonietti, M. Adv. Mater. 1998, 10, 195-217. (2) Spatz, J. P.; Roescher, A.; Mo¨ller, M. Adv. Mater. 1996, 8, 337340. (3) Spatz, J. P.; Mo¨ssmer, S.; Hartmann, C.; Mo¨ller, M. Langmuir 2000, 16, 407-415. (4) Spatz, J. P.; Sheiko, S.; Mo¨ller, M. Macromolecules 1996, 29, 32203226. (5) Fo¨rster, S.; Hermsdorf, N.; Leube, W.; Schnablegger, H.; Lindner, P.; Bo¨ttcher, C. J. Phys. Chem. B 1999, 103, 6657-6668. (6) Spatz, J. P.; Mo¨ssmer, S.; Hartmann, C.; Mo¨ller, M.; Herzog, T.; Krieger, M.; Boyen, H.-G.; Ziemann, P.; Kabius, B. Langmuir 2000, 16, 407-415. (7) Wu, C.; Liu, T.; White, H.; Chu, B. Langmuir 2000, 16, 656-661. (8) Price, C.; Woods, D. Eur. Polym. J. 1973, 9, 827-833. (9) Booth, C.; Naylor, T. d.; Price, C.; Rajab, N. S.; Stubbersfield, R. B. J. Chem. Soc., Faraday Trans. 1 1978, 74, 2352-2362. (10) Zhang, L.; Eisenberg, A. Science 1995, 268, 1728-1731. (11) Mortensen, K.; Talmon, Y. Macromolecules 1995, 28, 88298834. (12) Breulmann, M.; Fo¨rster, S.; Antonietti, M. Macromol. Chem. Phys. 2000, 201, 204-211. (13) Gohy, J.-F.; Antoun, S.; Je´roˆme, R. Macromolecules 2001, 34, 7435-7440. (14) Webber, G. B.; Wanless, E. J.; Armes, S. P.; Baines, F. L.; Biggs, S. Langmuir 2001, 17, 5551-5561. (15) Webber, G. B.; Wanless, E. J.; Bu¨tu¨n, V.; Armes, S. P.; Biggs, S. Nano Lett. 2002, 2, 1307-1313.
tion between the PDMA block and the negatively charged mica. Prior to this work, Regenbrecht et al.16 probed the structure of micelles of the neutral-polyelectrolyte diblock copolymer poly(ethylethylene)-b-poly(styrene sulfonic acid) adsorbed on solid substrates in aqueous solution by in situ atomic force microscopy (AFM). A comparison was made between adsorption onto a neutral and hydrophobic substrate and onto mica modified to be positively charged by a coating of poly(ethyleneimine) or by ion exchange. They observed a range of structures from spherical micelles to network-like structures, by varying ionic strength. They also reported that strongly charged substrates led to the formation of structures not found in the bulk. In the present study, we describe AFM studies on the structure of neutral diblock copolymer micelles adsorbed onto a neutral substrate in situ in aqueous solution. There can thus be no influence of charge on the surface micellar structure. We find that adsorption of micelles is correlated to the bulk critical micelle concentration (cmc). At higher concentrations, images of close-packed structures are obtained, where the spacing between micelles is similar to the micellar diameter obtained via light scattering experiments. The copolymer P94E316, where E denotes an oyxethylene unit OCH2CH2 and P an oxypropylene unit OCH2CH(CH3), was prepared by sequential anionic polymerization of propylene oxide followed by ethylene oxide.17 The polymer is methoxy terminated on the P end and has a narrow molecular weight distribution, Mw/Mn ) 1.07. Full micellar properties have been reported previously.17 The spontaneous adsorption of P94E316 diblock copolymer micelles from solutions of varying concentration (0.1-2.0 wt % in Milli-Q water) was studied. The silicon had been prepared by thermal oxidation at 1100 °C for 1 h in air, which has the effect of increasing the oxide thickness from 2.3 nm to around 90 nm.18 It was found that this treatment produced more hydrophobic surfaces, due to the removal of silanol -OH groups to form siloxanes, with a water contact angle of 60°, compared with an angle of
