Growth of Noncircular and Faceted Holes in Liquid–Liquid Dewetting of Thin Polymer Bilayers

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

Different surface structures are fabricated via adjusting the instability mode from a thermodynamically controlled one to a kinetically controlled one.

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

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.

Short chains enhance slip of highly entangled polystyrenes during thin film dewetting

We investigate the effect of short chains on slip of highly entangled polystyrenes (PS) during thin film dewetting from non-wetting fluorinated surfaces.

Instability, self-organization and pattern formation in thin soft films

The free surface of a thin soft polymer film is often found to become unstable and self-organizes into various meso-scale structures. In this article we classify the instability of a thin polymer film into three broad categories, which are: category 1: instability of an ultra-thin (<100 nm) viscous film engendered by amplification of thermally excited surface capillary waves due to interfacial dispersive van der Waals forces; category 2: instability arising from the attractive inter-surface interactions between the free surface of a soft film exhibiting room temperature elasticity and another rigid surface in its contact proximity; and category 3: instability caused by an externally applied field such as an electric field or a thermal gradient, observed in both viscous and elastic films. We review the salient features of each instability class and highlight how characteristic length scales, feature morphologies, evolution pathways, etc. depend on initial properties such as film thickness, visco-elasticity (rheology), residual stress, and film preparation conditions. We emphasize various possible strategies for aligning and ordering of the otherwise isotropic structures by combining the essential concepts of bottom-up and top-down approaches. A perspective, including a possible future direction of research, novelty and limitations of the methods, particularly in comparison to the existing patterning techniques, is also presented for each setting.