Pattern Driven Stress Localization in Thin Diblock Copolymer Films

When an elastic plate is fixed to a soft substrate and compressed, it accommodates applied strain by buckling and forming a sinusoidally wrinkled topography. At large strains, the regular wrinkled pattern is replaced by sharp, localized folds. Such folds are ubiquitous in biology; however, strains are not necessarily large in all cases, suggesting that a different mechanism may contribute to the formation of folds. In this work, we use thin films of a symmetric diblock copolymer coupled to thick elastomer substrates to explore the progression from isotropic wrinkles to localized folds in films with secondary structure. The block copolymer molecules organize into lamellae parallel to the elastomer substrate, and the balance of film thickness, lamellar dimensions, and elasticity dictate the development of topographic structures in a systematic manner. This "self-assembled" topography in the bounding film leads to stress localization when the pattern has a lateral spacing of the same order as the wrinkling wavelength. This first systematic exploration of pattern driven localization reveals the importance of a new emergent length scale which also appears in more traditional localization experiments.