Wrinkling and strain localizations in polymer thin films

Abstract

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.

Onset of Plasticity in Thin Polystyrene Films

“…1). Under compression, the film buckles forming a wrinkle pattern that evolves to a complex collection of delamination blisters and wrinkles as compression is increased [21,22]. Upon release of stress, the topography of regions that were highly curved are measured and found to be flat on the order of the surface roughness (∼ 1 nm).…”

“…shows a single LSCM optical section of a typical sample while under compression (h = 71 nm). The sample shows wrinkles (center), cracks (edge) and delamination (brightest wrinkle peak) as it has been compressed beyond the critical strain for wrinkling, ǫ w , cracking, ǫ c , and delamination, ǫ D [21]. From such an image the wrinkle wavelength can easily be measured, but more importantly, through optical sectioning we also determine the height of each pixel.…”

“…This simple analysis was developed for the ill defined 'near threshold' regime, although for reasons that are not yet clear the analysis seems to be accurate in many situations that are clearly no longer small perturbations [6]. Larger displacements (the so called far from threshold regime) currently challenges elastic theory for a solution, which has prompted significant experimental and theoretical efforts [7,8,20,21]. The far from threshold work often investigates aspects of stress focusing [5], deformations in which the stress distributions become strongly heterogeneous and notably similar to more general types of material failure (e.g.…”

“…High stress also leads to failure, for example the nucleation of fractures (or shear deformation zones and crazes) in a wrinkled film [16] or the formation of delaminations if the wrinkling energy U w ∼ ǫLhĒ 1/6 fĒ 5/6 s exceeds the adhesion energy between the film and the substrate (U a ∼ G c wℓ, where wℓ is the area of the delamination and G c is the energy release rate) [21][22][23]. More importantly, high stress may push the film beyond the material limits of linear elasticity, where plasticity will greatly affect certain aspects of stress focusing [24].…”

Surface patterns in drying films of silica colloidal dispersions

“…Under a biaxial compressive stress, it has been shown that a repetitive wrinkleto-fold transition produces a hierarchical network of folds 11,12 . These patterns are observed in various systems such as glassy polymers 3,13 , polyethylene sheets 2 or foams 14 . Another group of patterns arises in soft layers supported by rigid substrates 1 .…”