Stretching and Bending Moduli of Bilayer Films Inferred from Wrinkle Patterns

Wrinkling patterns were used to investigate the mechanical properties of thin poly(styrene) (PS)/poly(methyl methacrylate) (PMMA) and PS/gold (Au) bilayer films. Films were floated on water with a water drop on the surface to induce wrinkling. The thicknesses and thickness ratios of the films were varied over a broad range. The PS/PMMA bilayer was chosen to provide a contrast in wetting properties, with equilibrium contact angles of θ_PMMA = 68° and θ_PS = 88° with water. The PS/Au bilayer was chosen to provide a large contrast in Young’s moduli, E_Au = 72 GPa and E_PS = 3.4 GPa. The stretching (Y) and bending (B) moduli of the bilayer films were obtained from measurements of the length and number of wrinkles in the wrinkle patterns. The experimentally derived values of Y and B were in reasonable agreement with the values computed from the bulk Young’s moduli and the thicknesses of the two components in the bilayer. The values of Y and B did not depend on which face of the film was exposed to the water droplet or bath when the capillary stresses were considered. Thus, finite size effects from the film thicknesses were unimportant over the range of thicknesses studied, and no relative displacement of the films was found, with the films remaining well-bonded even with deformation associated with wrinkling.