Controlled evolution of surface patterns for ZnO coated on stretched PMMA upon thermal and solvent treatments

Poly (methyl methacrylate) (PMMA) possesses the excellent optical properties and thermal stabilities, its shape memory effect can be triggered by heating or solvent absorption. Zinc oxide (ZnO) films, the most common semiconductor materials, were usually deposited on polymer substrates as a transparent conductive element for applications to electro-optical devices. In this paper, ZnO films with different thicknesses were deposited on PMMA substrates with different pre-strains. The influences of the shape memory effect (SME), thermal expansion mismatch, and ethanol soaking on the evolutions of surface topographies were systematically investigated. Results revealed that the isotropic wrinkles without any preferential orientation were induced by the thermal expansion coefficient mismatch between ZnO film and PMMA substrate during annealing. The well-aligned wrinkles perpendicular to the direction of pre-strain resulted from the SME of PMMA triggered by heating. In addition, cracks appeared instead of wrinkles since the isotropic swelling of PMMA upon soaking in ethanol.