(235d) Swelling-Induced Folding of Crosslinked Polymer Films

When a crosslinked polymer film attached to a rigid substrate is swollen with a solvent, the swelling induces an in-plane compressive stress in the film. If this stress is sufficiently large, a creasing pattern is known to be developed on the free surface. Here we show that the same experiment can also result in a completely different kind of instability: if the film is weakly attached to the substrate, the 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 towards the buckles causing growth of sharp folds of high aspect ratio. The folds persist even after the solvent evaporates. We characterize the dependence of such fold formation on the film thickness and swellability of the film.

We also show that the geometry of folding depends on the size of the region of the film exposed to solvent. A very small region of exposure (realized by placing a small drop of solvent on the film) does not induce delamination. Remarkably, with moderate sized drops, the delamination and folding occurs around the perimeter of the drop, thus culminating in a corral with tall walls. We quantify the parameters (drop volume, film thickness) which demarcate the transitions between no fold formation, corral formation, and multiple fold formation.

Finally this talk will discuss the implications of this research to fabrication of high-aspect ratio features on a surface.