Control of Morphology\nin Pattern Directed Dewetting\nof a Thin Polymer Bilayer

Abstract

We report the dewetting of a thin polymer bilayer on\na low surface\nenergy topographically patterned substrate with grating geometry.\nThe bilayer, comprising of a polystyrene (PS) top and poly­(methyl\nmethacrylate) (PMMA) bottom layer was prepared by direct sequential\nspin coating on the patterned substrate, using mutually exclusive\nsolvents. Depending on the coating conditions, three distinct initial\nmorphologies of the as coated bilayer is possible: type 1, a discontinuous\nbottom layer under a discontinuous top layer, resulting in polymer\nthreads confined within the substrate grooves; type 2, discontinuous\nthreads of bottom layer polymer (PMMA) confined within the substrate\ngrooves under a continuous top layer; type 3, continuous bottom and\ntop layers. Our experiments reveal that the initial morphology of\nthe film, particularly, that of the bottom layer significantly influences\nthe final dewetted patterns. For example, in a type 1 or type 2 bilayer\nthe morphology depends significantly on the relative widths of the\nPMMA threads (<i>L</i>_T–PMMA) and that\nof the substrate grooves (<i>L</i>_P). In case <i>L</i>_T–PMMA < <i>L</i>_P, the bottom PMMA layer disintegrates into isolated droplets aligned\nalong substrate grooves, irrespective of the thickness or morphology\nof the top PS layer. On the other hand, the overall morphology of\nthe dewetted film is rather strongly influenced by the thickness of\nthe PS layer and the configuration of the bilayer. In case the PMMA\nthreads span the entire width of the substrate grooves (<i>L</i>_T–PMMA = <i>L</i>_P), the droplet\nformation is suppressed in favor of an intact PMMA thread, with periodic\nundulations, submerged under either an undulating thread or an intact\nlayer of PS. In case of a type 3 bilayer, the continuous PMMA bottom\nlayer in most cases ruptures over the substrate stripes, where it\nis thinnest. This result in the top PS layer coming in direct contact\nwith the substrate and subsequently rupture over the same locations,\nresulting in core shell threads localized over the substrate grooves.\nIn case of a type 3 bilayer with an ultrathin top film, the two layers\nrupture simultaneously at different locations and subsequent dewetting\nresults in an exotic structure comprising alternating array of PS\ndroplets and undulating PMMA threads. For a thicker bottom layer,\nthe PMMA film is seen to remain intact, over which the PS film dewets,\nforming undulating threads. We also construct a morphology phase diagram\nthat depicts the influence of the individual layers on the final dewetted\nmorphology.