From wrinkly elastomers to Janus Particles

We have studied the control and manipulation of tuneable sub-micron scale equilibrium structures in urethane/urea elastomer films [1] and spheres [2]. These are obtained by UV-irradiation of the elastomers followed by mechanical deformation or swelling in a solvent. The chemistry is standard and no complex deposition techniques are required. The resulting textures have been investigated using atomic force microscopy, scanning electron microscopy, small-angle light scattering and polarised optical microscopy. They can be interpreted by assuming that each film or sphere consists of a thin, stiff surface layer ('skin') lying atop a thicker, softer substrate ('bulk'). The skin's higher stiffness is hypothesized to result from the more extensive cross-linking of the polymer chains located near the surface by the UV radiation. Textures then arise from competition between the effects of bending the skin and compressing the bulk, as the applied stress is removed or the solvent evaporates. In the case of spheres, this provides a very convenient route for the fabrication of Janus particles, which we describe in some detail. References: [1] M. H. Godinho, A. C. Trindade, J. L. Figueirinhas, L. V. Melo, P. Brogueira, A. M. Deus, P .I. C. Teixeira, Eur. Phys. J. E 21, 319 (2006). [2] A. C. Trindade, J. P. Canejo, L. F. V. Pinto, P. Patricio, P. Brogueira, P. I. C. Teixeira, M. H. Godinho, Macromolecules 44, 2220 (2011).