pH-Responsive Diblock Copolymer Micelles at the Silica/Aqueous Solution Interface: Adsorption Kinetics and Equilibrium Studies Kenichi Sakai,* ,² Emelyn G. Smith, ‡ Grant B. Webber, ², ⊥ Christophe Schatz, ‡,# Erica J. Wanless, ‡ Vural Bu 1 tu 1 n, § Steven P. Armes, | and Simon Biggs ² School of Process, EnVironmental and Materials Engineering, UniVersity of Leeds, Leeds LS2 9JT, United Kingdom, School of EnVironmental and Life Sciences, The UniVersity of Newcastle, Callaghan, N.S.W., 2308, Australia, Department of Chemistry, Eskisehir Osmangazi UniVersity, Campus of Meselik, Eskisehir 26040, Turkey, and Department of Chemistry, Dainton Building, The UniVersity of Sheffield, Brook Hill, Sheffield S3 7HF, United Kingdom ReceiVed: May 9, 2006; In Final Form: June 1, 2006 The adsorption behavior of two examples of a weakly basic diblock copolymer, poly(2-(dimethylamino)ethyl methacrylate)-block-poly(2-(diethylamino)ethyl methacrylate) (PDMA-PDEA), at the silica/aqueous solution interface has been investigated using a quartz crystal microbalance with dissipation monitoring and an optical reflectometer. Dynamic and static light scattering measurements have also been carried out to assess aqueous solution properties of such pH-responsive copolymers. In alkaline solution, core-shell micelles are formed above the critical micelle concentration (cmc) by both copolymers, whereas the chains are molecularly dissolved (as unimers) at all concentrations in acidic solution. As a result, the adsorption behavior of PDMA-PDEA diblock copolymers on silica is strongly dependent on both the copolymer concentration and the solution pH. Below the cmc at pH 9, the cationic PDMA-PDEA copolymers adsorb as unimers and the conformation of the adsorbed polymer is essentially flat. At concentrations just above the cmc, the initial adsorption of copolymer onto the silica is dominated by the unimers due to their faster diffusion compared to the much larger micelles. Rearrangement of the adsorbed unimers and/or their subsequent displacement by micelles from solution is then observed during an equilibration period, and the final adsorbed mass is greater than that observed below the cmc. At concentrations well above the cmc, the much higher proportion of micelles in solution facilitates more effective competition for the surface at all stages of the adsorption process and no replacement of initially adsorbed unimers by micelles is evident. However, the adsorbed layer undergoes gradual rearrangement after initial adsorption. This relaxation is believed to result from a combination of further copolymer adsorption and swelling of the adsorbed layer. Introduction Understanding the interfacial adsorption of amphiphilic block copolymers is one of the most important topics in the field of materials science and engineering as a result of existing or proposed technological applications for nanostructured polymer films. These include uses as template structures for the second- ary synthesis of microelectronic components, the preparation of surfaces with nanoscale variations in friction and adhesion, and possible uses in novel drug delivery systems. The use of amphiphilic block copolymers as the building blocks for self- assembled surface coatings having a high spatial order offers various advantages over the analogous small molecule surfactant coatings. For example, the rate of desorption of polymer chains adsorbed at a solid/solution interface is much slower than that of surfactants. It has already been reported that copolymer coatings can improve various industrial products and processes, including paints, paper production, wastewater treatment, and mineral flotation. In addition, recent studies focusing on the self-assembly of amphiphilic block copolymers at the solid/ aqueous solution interface have demonstrated their potential application in the world of nanotechnology, including as stimulus-responsive surface coatings, 1 nanostructured templates, 2-5 and nanoreactors for preparing metal nanoparticles. 6-9 The adsorption behavior of amphiphilic diblock copolymer micelles from aqueous solution has been investigated extensively both theoretically 10 and experimentally. 11-15 For instance, Walter and co-workers reported the adsorption of a zwitterionic diblock copolymer poly(methacrylic acid)-block-poly(2-(dimethylami- no)ethyl methacrylate) (PMAA-PDMA) on silica. 11,13 It was shown that the adsorbed amount of this copolymer was strongly influenced by the solution pH and that direct adsorption of the copolymer micelles from solution can occur. An alternative adsorbed layer morphology has been reported by a number of other researchers: 16-19 this consists of a brush-like conformation where the copolymer has a small anchor block adsorbed on the solid surface and a large “buoy” block extending into the solution phase. When studying the adsorption of diblock copolymers, atten- tion should not only be paid to the equilibrium state but also to the adsorption kinetics, which allows the adsorption mechanism * To whom correspondence should be addressed. E-mail: k.sakai@ leeds.ac.uk. ² University of Leeds. ‡ University of Newcastle. § Eskisehir Osmangazi University. | University of Sheffield. ⊥ Current address: Particulate Fluids Processing Centre, Department of Chemical and Biomolecular Engineering, The University of Melbourne, Parkville, Victoria, 3010, Australia. # Current address: Laboratoire de Chimie des Polyme `res Organiques, LCPO (UMR5629) - ENSCPB, 16 Avenue Pey Berland, 33607 PESSAC cedex, France. 14744 J. Phys. Chem. B 2006, 110, 14744-14753 10.1021/jp062830q CCC: $33.50 © 2006 American Chemical Society Published on Web 07/08/2006