Erasable and Reversible Wrinkling of Halogenated Rubber Surfaces Alae El Haitami, †,§ Fre ́ de ́ ric Bretagnol, ‡ Patrick Assuid, ‡ Gilles Petitet, ‡ Sabine Cantournet, † and Laurent Corte ́ * ,† † Centre des Mate ́ riaux, UMR 7633 Mines-ParisTech, BP 87, 91003 Evry, France ‡ Valeo Wiper Systems Product Group, R&D/P2, Rue Marie Curie, 63500 Issoire, France * S Supporting Information ABSTRACT: Few surfaces can exist at rest in either wrinkled or unwrinkled states and switch reversibly between these states. Here, we report a new approach to creating reversibly wrinkling systems using the halogenation of rubber to induce a local increase in the glass-transition temperature within a thin layer at the surface. Such systems are obtained by the bromination of molded rubber fi lms. By means of thermomechanical experiments and in situ observations, we show that microscopic wrinkles are produced by unstretching a stretched film below the glass-transition temperature of the brominated layer. These surface patterns are erased within seconds when the wrinkled layer is heated to above its glass transition and recovers its initial equilibrium dimensions. New wrinkles can be produced and erased repeatedly on the same surface. A model is proposed that takes into account the existence of a gradient in bromine content along the thickness of the modified layer. It describes the viscoelastic behavior of these brominated films and captures the temperature dependencies of the thickness of the glassy layer and of the wrinkle wavelength. 1. INTRODUCTION The wrinkling of soft solids produces fascinating surface morphologies with a rich family of symmetries and multiple- scale ordering. 1,2 These surface patterns usually result from a mechanical instability when a thin, stiff coating buckles to allow its softer substrate to contract. 3-5 Since the pioneering work of Bowden and co-workers, 6 a number of wrinkled rubber or gel films 7-10 have been developed with a promising potential for applications ranging from metrology, 11 optics 12,13 and organic electronics 14,15 to biopatterning. 16 Most of these wrinkling methods rely on a three-step process as follows: (i) the rubber substrate is stretched using, for instance, a tensile device, solvent swelling, or thermal expansion; (ii) a thin stiff layer is created at the surface of the stretched substrate by coating or additional cross-linking; (iii) stretching is released, allowing the substrate to recover its initial size, which compresses the stiff layer and causes it to wrinkle. With most of these approaches, wrinkles are formed permanently. The design of surfaces that wrinkle and unwrinkle reversibly in response to a stimulus has attracted particular attention lately as a way to produce smart dynamical surfaces or sensors. 17,18 In such reversible systems, wrinkling is often obtained by maintaining a compressive strain using an active external drive such as a mechanical 17 or osmotic 18 force. Unwrinkling is then simply obtained by releasing this strain. Very few systems can exist passively in both wrinkled and unwrinkled states and switch reversibly between these states. In a recent study, 19 Li et al. have shown that metal coatings atop hard shape memory polymer substrates can produce such a bistable system. However, cracks in the coating significantly altered the reversibility of the wrinkling- unwrinkling process. Here, we explore a new approach using the halogenation of rubber surfaces to make soft surfaces that can wrinkle and be erased to recover their unwrinkled state. In these erasable systems, the surface can exist in a wrinkled or unwrinkled state without the application of a constant external force, and new wrinkle patterns can be created repeatedly on the same surface. In this approach, depicted in Figure 1, a rubber surface is modified chemically to increase the glass-transition temperature above room temperature within a microscopic layer. It is then possible to take advantage of the very slow glassy dynamics of the thus-modified layer to produce wrinkles. In particular, there exists a temperature range between the glass-transition temperature of the unmodified rubber T g,R and that of the modified layer T g,layer in which the system consists of a glassy layer on top of a rubber substrate. As the system is heated to above T g,layer (step 1), it becomes fully rubbery and can be deformed by stretching, for instance (step 2). The modified layer can then be frozen in an out-of-equilibrium stretched state by maintaining the deformation while cooling to between T g,R and T g,layer (step 3). Stable wrinkles are produced by allowing the unmodified rubber substrate to recover elastically (step 4). Received: August 27, 2013 Revised: November 13, 2013 Published: November 20, 2013 Article pubs.acs.org/Langmuir © 2013 American Chemical Society 15664 dx.doi.org/10.1021/la403295g | Langmuir 2013, 29, 15664-15672