Swelling-Induced Delamination Causes Folding of Surface-Tethered Polymer Gels

Abstract

When a polymer film that is weakly attached to a rigid substrate is exposed to solvent, swelling-induced compressive stress nucleates buckle delamination of the film from the substrate. Surprisingly, the buckles do not have a sinusoidal profile, instead, the film near the delamination buckles slides toward the buckles causing growth of sharp folds of high aspect ratio. These folds do not result from a wrinkle-to-fold transition; instead, the film goes directly from a flat state to a folded state. The folds persist even after the solvent evaporates. We propose that patterned delamination and folding may be exploited to realize high-aspect ratio topological features on surfaces through control of a set of boundary constraints arising from the interrelation of film-surface adhesion, film thickness and degree of swellabilty. KEYWORDS: folds, delamination, creases, buckles, wrinkles, swelling The swelling of polymers with solvent can lead to one class of such buckling phenomena in which an osmotic stress applied to a homogeneous body leads to nonhomogeneous deformation.10−12 One of the simplest geometries in which this can happen is the swelling of a cross-linked polymer film that is attached to a rigid substrate. If the extent of swelling is sufficiently large, the film swells nonhomogeneously and the free surface develops cusp-like creases and rounded wrinkles.12−15 Recent research 13,16 has clarified the mechanism of crease formation: lateral film swelling (i.e., swelling perpendicular to the film normal) is constrained by the rigid substrate, and hence the free surface of the film is under compressive stress. It is this compression that induces the creasing instability at the free surface. In this paper, we show that a new buckling phenomenon can also occur in the same situation: if the film is not strongly bound to the rigid substrate, a second kind of buckling instability can lead to large scale folding of the film. This Letter describes the basic phenomenon, clarifies the conditions under which folding does or does not occur, and proposes a schematic model for the process.Experiments were conducted using PDMS (Sylgard 184) films swollen with toluene. Sylgard is a two-part silicone rubber formulation in which the base resin and the cross-linker are usually mixed in a 10:1 ratio. For most of our experiments however, the ratio of the base resin to cross-linker was 10:0.4 which resulted in a lower cross-link density. The shear modulus of these films was 18 kPa (in contrast to 54 kPa for 10:1 ratio), and the films swelled 380% by weight with toluene.17 Films were prepared by spincoating the PDMS mixture onto acrylic sheets, letting them cross-link overnight, and then completing the cross-linking reaction for one hour at 75°C. Film thickness was measured with a profilometer at three locations on the spincoated films. Thickness variation was less than 15%. Sections of the films, roughly 25 × 25 mm in size were peeled off the acrylic sheets and carefully placed (without entrapping air bubbles) on glass slide...

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