DOI: 10.1002/adma.200600315 Controlling Tack with Bicomponent Polymer Brushes** By Haris Retsos , Anton Kiriy , Volodymyr Senkovskyy , Manfred Stamm, Mikhail M. Feldstein, and Costantino Creton* The development of technologically simple responsive coat- ings that could provide selective surface properties is becom- ing increasingly important for numerous applications from the microelectronic to pharmaceutical industries. [1] Scientifically, a substantial effort has been made toward controlling the wet- ting, [2–4] swelling, [5,6] and adsorption [7–10] properties of the coatings with significant results. In contrast, very few results have been reported on selective adhesive properties and, as far as we are aware, only by contact experiments with an atomic force microscopy (AFM) tip. [11] This stems from the fact that adhesion, considered as a macroscopic property, still tends to be a less-well understood and much more complex is- sue despite its high practical relevance. The approach of chemically heterogeneous polymer brushes has been chosen to modify surface chemistry in a well-controlled and reproducible way. Silicon surfaces were grafted with bicomponent polystyrene–poly(2-vinyl pyridine) (PS–P2VP) brushes by the “grafting to” method, [12] and their adhesive properties against a hydrophilic pressure-sensitive adhesive (PSA) have been tested. We have used a well-stud- ied [13,14] blend of poly(vinyl pyrrolidone)–poly(ethylene gly- col) (PVP–PEG) which is a simplified version of what is typi- cally used for applications like transdermal drug delivery and skin adhesives. Tackiness of the PVP–PEG adhesive on the mixed brushes, after the treatments with selective solvents, has been investigated with an instrumented probe test, a well- known and versatile experimental technique to test the tack properties of PSA. [15,16] A key aspect of our investigation is the pretreatment of the polymer brushes with a selective solvent which determines the structure of the brush when it is put in contact with the PVP–PEG adhesive. When the binary PS–P2VP brush is im- mersed in toluene the PS brush swells, and after a thorough drying of the 10 nm thin film, a surface layer enriched in PS is left (Fig. 1a). On the other hand, after ethanol treatment, a se- lective solvent for P2VP, this component occupies the free sur- face of the dried film. Furthermore, the treatment with acidic water (pH 2) protonates the P2VP and keeps it in its salt form for several hours even after drying the sample with nitrogen flow as water contact angle measurements have shown (Fig. 1b). [17] The reversible switching of the wetting properties of the surface and the reversible switching of adhesion with a hydrophobic PSA has been previously demonstrated [17] and is due to the different level of hydrophilicity of the two compo- nents of the layer. However, we noted a much higher degree of switching of the contact angle with water than those of the adhesive properties against a hydrophobic adhesive. It was hence a logical extension to test the switchability of the adhe- sive properties against a hydrophilic adhesive. We present first the results of tack on single-component brushes of PS or of P2VP. If the PVP–PEG adhesive is ap- plied on the PS, the probe tack curve shown in Figure 2, shows a peak followed by a sharp drop in stress, a clear indica- tion of an interfacial fracture mechanism caused by poor in- terfacial adhesion. [15] This result, confirmed by a video of the debonding process, remains identical regardless of the solvent used for the pretreatment. Things are quite different on the other hand for the pure P2VP brush. In this case the debond- ing is initiated by the formation of cavities. The foam formed by the cavities is then stretched in the tensile direction and fi- nally the separation of the two surfaces is achieved by the de- COMMUNICATIONS 2624 © 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim Adv. Mater. 2006, 18, 2624–2628 – [*] Prof. C. Creton, Dr. H. Retsos Laboratoire de Physico-Chimie des Polymères et des Milieux Dispersés UMR 7615, Ecole Supérieure Physique et de Chimie Industrielles 10, rue Vauquelin, 75231 Paris (France) E-mail: Costantino.Creton@espci.fr Dr. A. Kiriy, V. Senkovskyy,Prof. M. Stamm Leibniz-Institut für Polymerforschung Dresden Hohe Straße 6, 01069 Dresden (Germany) M. M. Feldstein A. V. Topchiev Institute for Petrochemical Synthesis Russian Academy of Sciences 29 Leninsky prospekt, 119991 Moscow (Russian Federation) [**] We are grateful to the joint CNRS-DFG program for funding this project and also to Yvette Tran for her comments and fruitful discus- sions of the manuscript. Dry state Wet state Wet state Dry state Toluene Acidic Water (a) (b) Figure 1. Schematic representation of the switching scenario from the hy- drophilic to the hydrophobic state and vice versa, in the wet and dry state by using selective solvents.