Confinement induced ordering in dewetting of ultra-thin polymer bilayers on nanopatterned substrates

Abstract

We report the dewetting of a thin bilayer of polystyrene (PS) and poly(methylmethacrylate) (PMMA) on a topographically patterned nonwettable substrate comprising an array of pillars, arranged in a square lattice. With a gradual increase in the concentration of the PMMA solution (Cn-PMMA), the morphology of the bottom layer changes to: (1) an aligned array of spin dewetted droplets arranged along substrate grooves at very low Cn-PMMA; (2) an interconnected network of threads surrounding each pillar at intermediate Cn-PMMA; and (3) a continuous bottom layer at higher Cn-PMMA. On the other hand the morphology of the PS top layer depends largely on the nature of the pre-existing bottom layer, in addition to Cn-PS. An ordered array of PMMA core-PS shell droplets forms right after spin coating when both Cn-PMMA and Cn-PS are very low. Bilayers with all other initial configurations evolve during thermal annealing, resulting in a variety of ordered structures. Unique morphologies realized include laterally coexisting structures of the two polymers confined within the substrate grooves due to initial rupture of the bottom layer on the substrate followed by a squeezing flow of the top layer; an array of core-shell and single polymer droplets arranged in an alternating order etc., to highlight a few. Such structures cannot be fabricated by any stand-alone lithography technique. On the other hand, in some cases the partially dewetted bottom layer imparts stability to an intact top PS layer against dewetting. Apart from ordering, under certain specific conditions significant miniaturization and downsizing of dewetted feature periodicity and dimension as compared to dewetting of a single layer on a flat substrate is observed. With the help of a morphology phase diagram we show that ordering is achieved over a wide combination of Cn-PMMA and Cn-PS, though the morphology and dewetting pathway differs significantly with variation in the thickness of the individual layers.

Transition from Spin Dewetting to continuous film in spin coating of Liquid Crystal 5CB

“…Apart from thermal annealing 5 – 10 , solvent vapor exposure 63 , or quenching the film in a mix of solvent and non-solvent 64 , which are the commonly adopted approaches for engendering instability in a thin polymer film, a novel approach is to achieve dewetting during spin coating itself 65 – 69 . The method relies on the rupture of an extremely dilute solution layer on the substrate during spin coating itself, forming isolated solute patches.…”

“…While spin dewetting is totally undesired from the stand point of achieving a continuous thin film, it is finding potential utilization as a rapid meso fabrication technique for solution processable materials. Spin dewetting has already been utilized for fabricating array of polymers (PS and PMMA) droplets on topographically patterned substrate 65 – 69 . Very recently by Bandyopadhyay et al .…”

Thermodiffusion as a means to manipulate liquid film dynamics on chemically patterned surfaces

“…Dewetting of thin liquid films (∼1-100 nm) on substrates with tailored heterogeneity has been recognized as a promising strategy for templating and nanopatterning. [1][2][3][4][5] When thin liquid films dewet physically or chemically patterned substrates, their dewetting mechanisms are more complex, as additional length scales pertaining to the pattern geometry need to be accommodated. This will also result in accelerated instability growth, [6][7][8][9][10] complex film morphologies, and/or different rupture locations, if any.…”

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

“…[15][16][17][18][19][20][21][22][23][24][25][26][27][28][29][30] The template-directed and template-assisted dewetting of the thin polymer bilayer have been investigated extensively due to their technological and scientic importance. [31][32][33][34][35] Mukherjee et al 32,33 reported the dewetting process of a thin bilayer of polystyrene and poly(methylmethacrylate) on a topographically patterned nonwettable substrate, and using this method they fabricate many different structures.…”

“…Previous studies [10][11][12][13][14][31][32][33][34][35] on the template-directed or template-assisted dewetting of the thin polymer lms mainly focus on the inuence of the geometry and size of the template pattern on the dewetting structures. In this paper, we concentrate on the inuence of instability mode on the templateassisted dewetting process of the thin polymer bilayer.…”