Wrinkling and strain localizations in polymer thin films

Wrinkles and strain localized features are observed in many natural systems and are useful surface patterns for a wide range of applications, including optical gratings and microfluidic devices. However, the transition from sinusoidal wrinkles to more complex strain localized features, such as delaminations or folds, is not well understood. In this paper, we investigate the onset of wrinkling and strain localizations in a model system of a glassy polymer film attached to a surface of an elastomeric substrate. We show that careful measurement of feature amplitude as a function of applied strain allows not only the determination of wrinkle, fold, or delamination onset but also allows clear distinction between each type of feature. We observe that amplitude increases discontinuously as delamination occurs; whereas, the amplitude for a fold deviates gradually compared to the amplitude for a nearby wrinkle as a function of applied strain. The folds observed in these experiments have an outward morphology from the surface, in contrast to folds that form into the plane for a film floating on a liquid substrate. A deformation mode map is presented, where the measured critical strain for localization is compared for films with thickness ranging from 5 nm to 180 nm.