title: Solvent-vapor-assisted dewetting of prepatterned thin polymer films: control of morphology, order, and pattern miniaturization.
abstract: Ultrathin (<100 nm) unstable polymer films exposed to a solvent vapor dewet by the growth of surface instability, the wavelength (λ) of which depends on the film thickness (h(f)). While the dewetting of a flat polymer thin film results in random structures, we show that the dewetting of a prepatterned film results in myriad ordered mesoscale morphologies under specific conditions. Such a film undergoes rupture over the thinnest parts when the initial local thickness of these zones (h(rm)) is lower than a limiting thickness h(lim) ≈ 10 nm. Additionally, the width of the pattern grooves (l(s)) must be wider than λ(s) corresponding to a flat film having a thickness of h(rm) for pattern-directed dewetting to take place over surface-tension-induced flattening. We first present an experimentally obtained morphology phase diagram that captures the conditions where a transition from surface-tension-induced flattening to pattern-directed-rupture takes place. Subsequently, we show the versatility of this technique in achieving a variety of aligned mesopatterns starting from a prepatterned film with simple grating geometry. The morphology of the evolving patterns depends on several parameters such as the initial film thickness (h(f)), prepattern amplitude (h(st)), duration of solvent vapor exposure (SVE), and wettability of the stamp used for patterning. Periodic rupture of the film at regular intervals imposes directionality on the evolving patterns, resulting in isolated long threads/cylindrical ridges of polymers, which subsequently disintegrate into an aligned array of droplets due to Rayleigh-Plateau instability under specific conditions. Other patterns such as a double periodic array of droplets and an array of holes are also possible to obtain. The evolution can be interrupted at any intermediate stage by terminating the solvent vapor annealing, allowing the creation of pattern morphology on demand. The created patterns are significantly miniaturized in size as compared to features obtained from dewetting a flat film with the same hf.
authors.affiliation: †Instability and Soft Patterning Laboratory, Department of Chemical Engineering, Indian Institute of Technology Kharagpur, Pin - 721302, Kharagpur, West Bengal, India.
authors.forename: Nandini
authors.initials: N
authors.lastname: Bhandaru
authors.affiliation: ‡Department of Chemical Engineering and Nanoscience Center, Indian Institute of Technology Kanpur, 208016, Uttar Pradesh, Kanpur, India.
authors.forename: Partho Sarathi
authors.initials: PS
authors.lastname: Goohpattader
authors.affiliation: ‡Department of Chemical Engineering and Nanoscience Center, Indian Institute of Technology Kanpur, 208016, Uttar Pradesh, Kanpur, India.
authors.forename: Danish
authors.initials: D
authors.lastname: Faruqui
authors.affiliation: †Instability and Soft Patterning Laboratory, Department of Chemical Engineering, Indian Institute of Technology Kharagpur, Pin - 721302, Kharagpur, West Bengal, India.|‡Department of Chemical Engineering and Nanoscience Center, Indian Institute of Technology Kanpur, 208016, Uttar Pradesh, Kanpur, India.
authors.forename: Rabibrata
authors.initials: R
authors.lastname: Mukherjee
authors.affiliation: ‡Department of Chemical Engineering and Nanoscience Center, Indian Institute of Technology Kanpur, 208016, Uttar Pradesh, Kanpur, India.
authors.forename: Ashutosh
authors.initials: A
authors.lastname: Sharma