Control of morphology in pattern directed dewetting of thin polymer films

Abstract

We investigate the creation of large area mesoscale structures by controlling the dewetting pathways of thin polymer films on physically heterogeneous substrates comprising a two-dimensional (2-D) array of square pillars. Depending on the initial configuration and thickness of the film, dewetting produces a variety of both ordered and disordered structures. The substrate pattern strongly influences the dewetting pathways as well as the organization and size of the polymer structures. The key findings are: (i) the lateral confinement imposed by the substrate pattern can reduce the length-scale of the dewetted structure by one to two orders of magnitude as compared to dewetting on the same homogeneous substrate. (ii) When the polymer film is thin (<40 nm) and placed in a conformal contact with the patterned substrate, a perfect array of droplets occupying the interstitial spaces of pillar patterns is formed within a narrow range of film thickness. Nonlinear simulations show similar behavior. (iii) In contrast, for a flat polymer film resting only on the pillars and hanging freely over the channels, the dewetted droplets assemble on top of the pillars. (iv) For thicker films (>40 nm), dewetting progresses by nucleation of large holes, uncorrelated to the substrate pattern. The dewetted pattern in this case forms on multiple length-scales consisting of 10 µm droplets resulting from the coalescence of the holes and small (<1 μm) droplets at the interstitial locations due to stick-slip of the retracting contact line.