Control of Morphology in Pattern Directed Dewetting of a Thin Polymer Bilayer

Abstract

We report the dewetting of a thin polymer bilayer on a low surface energy topographically patterned substrate with grating geometry. The bilayer, comprising of a polystyrene (PS) top and poly(methyl methacrylate) (PMMA) bottom layer was prepared by direct sequential spin coating on the patterned substrate, using mutually exclusive solvents. Depending on the coating conditions, three distinct initial morphologies of the as coated bilayer is possible: type 1, a discontinuous bottom layer under a discontinuous top layer, resulting in polymer threads confined within the substrate grooves; type 2, discontinuous threads of bottom layer polymer (PMMA) confined within the substrate grooves under a continuous top layer; type 3, continuous bottom and top layers. Our experiments reveal that the initial morphology of the film, particularly, that of the bottom layer significantly influences the final dewetted patterns. For example, in a type 1 or type 2 bilayer the morphology depends significantly on the relative widths of the PMMA threads (L T−PMMA ) and that of the substrate grooves (L P ). In case L T−PMMA < L P , the bottom PMMA layer disintegrates into isolated droplets aligned along substrate grooves, irrespective of the thickness or morphology of the top PS layer. On the other hand, the overall morphology of the dewetted film is rather strongly influenced by the thickness of the PS layer and the configuration of the bilayer. In case the PMMA threads span the entire width of the substrate grooves (L T−PMMA = L P ), the droplet formation is suppressed in favor of an intact PMMA thread, with periodic undulations, submerged under either an undulating thread or an intact layer of PS. In case of a type 3 bilayer, the continuous PMMA bottom layer in most cases ruptures over the substrate stripes, where it is thinnest. This result in the top PS layer coming in direct contact with the substrate and subsequently rupture over the same locations, resulting in core shell threads localized over the substrate grooves. In case of a type 3 bilayer with an ultrathin top film, the two layers rupture simultaneously at different locations and subsequent dewetting results in an exotic structure comprising alternating array of PS droplets and undulating PMMA threads. For a thicker bottom layer, the PMMA film is seen to remain intact, over which the PS film dewets, forming undulating threads. We also construct a morphology phase diagram that depicts the influence of the individual layers on the final dewetted morphology.

Ordered Alternating Binary Polymer Nanodroplet Array by Sequential Spin Dewetting

“…It is worth highlighting that similar core−shell PS−PMMA droplet array is obtained by dewetting of a polymer bilayer on a topographically patterned substrate, which takes approximately 6 h of thermal annealing and dewetting. 31 In contrast, the array in Figure 1B is obtained instantaneously, which highlights the strength of spin dewetting in terms of rapidness over conventional thermal or solvent vapor assisted dewetting. Figure 1B highlights that it is necessary to suppress the interfacial energy driven migration of PS over the existing PMMA droplets during the second spin dewetting step, in order to obtain an alternating array.…”

Dewetting of a pre-patterned thin polymer bilayer: influence of the instability mode

“…We can nd that the PMMA lm bends to conform to the buried PS-PMMA interface in the case of PS-2.4K. Mukherjee et al 33 have presented that the continuous PMMA bottom layer ruptures over the substrate stripes due to the geometry of substrate, while the top PS layer comes in direct contact with the substrate and subsequently rupture over the same locations, resulting in core shell threads localized over the substrate grooves. In our case, the high viscous of PMMA lm is much more stable against local variations in its thickness because it requires less energy to make a simple bending.…”

Synergism of Dewetting and Self-Wrinkling To Create Two-Dimensional Ordered Arrays of Functional Microspheres

“…Thus, in the past decades, enormous experimental and theoretical endeavors have been made to improve the ordering degree of the dewetting-induced features. 9−12,14−28 Previously, three preferred strategies have been introduced for directing dewetting: chemically 14,15 or topographically patterned substrates [9][10][11][20][21][22][23][24][25]28 or externally structured templates. 16 −19 In the above directed dewetting, lithography-based techniques such as photolithography or e-beam etching have been directly or indirectly involved in the fabrication of the required patterned substrates and structured templates.…”