Programming Feature Size in the Thermal Wrinkling of Metal Polymer Bilayer by Modulating Substrate Viscoelasticity

We report a novel strategy for creating stress-induced self-organized wrinkles in a metal polymer bilayer with programmable periodicity (λ_S) varying over a wide range, from ∼20 μm down to ∼800 nm by modulating the viscoelasticity of the bottom polymer layer. Substrates with different viscoelasticity are prepared by precuring thin films of a thermo-curable poly dimethylsiloxane (PDMS) elastomer (Sylgard 184) for different durations (t_P) prior to deposition of the top aluminum layer by thermal evaporation. Precuring of the Sylgard 184 film for different durations leads to films with different degrees of viscoelasticity due to variation in the extent of cross-linking of the polymer matrix. The λ_S as well as the amplitude (a_S) of the wrinkles progressively decrease with an increase in the extent of elasticity of the film, manifested as an increase in the storage modulus (G'). Based on the variation in the rate of decay of λ_S with G', we identify three clearly distinguishable regimes over predominantly viscous, viscoelastic, and elastic bottom layers. While λ_S and a_S drop with an increase in G' for both the first and the third regimes, it remains nearly independent of G' for the intermediate regime. This is attributed to the difference in the mechanisms of wrinkle formation in the different regimes. We finally show that simultaneous modulation of λ_S and a_S can be used to engineer surfaces with different wettability as well as anti-reflection properties.