It is known that dewetting of a polystyrene (PS) thin film on a silicon substrate gets completely suppressed upon addition of small amount of C 60 nanoparticles (NP). 1 The NPs migrate to the film−substrate interface and forms an enriched surface layer of the particles that eventually stabilizes the film by apparent pinning. In this article we quantitatively highlight the unexplored effect of substrate surface energy (γ S ) on the migration of the NPs to the film−substrate interface and their contribution on dewetting suppression. Depending on the relative magnitudes of NP concentration (C NP ) and γ S , we identify three distinct stability regimes. In regime 1 (C NP < 0.2%) there is no suppression of dewetting and the final polygonal arrangement of droplets closely resemble dewetted structures in particle free films. However, the size of the polygons becomes smaller in NP containing films when γ S < γ C60 (NP surface energy) and larger as γ S exceeds γ C60 . In regime 2 (0.3% < C NP < 0.75%) the films dewet partially, and the extent of dewetting is seen to strongly dependent on the relative magnitudes of γ C60 and γ S . While dewetting proceeds up to the stage of partial hole growth and coalescence when γ S < γ C60 , some random isolated holes are seen to form when γ S > γ C60 . On the basis of direct AFM imaging, we show that in both regimes 1 and 2 the NPs migrate to the substrate−film interface only when γ S > γ C60 . We show complete suppression of dewetting in regime 3 (C NP > 1.0%), where the particles are seen to migrate to the substrate for all values of γ S . The work highlights that entropy driven migration of particles takes place on substrates with any γ S only above a critical NP concentration (C NPC ) and only on substrates with γ S > γ C60 when C NP < C NPC . The findings, apart from dewetting suppressing, can guide potential design criteria for applications such as electron extracting layer in organic photovoltaic.

We investigate the influence of fullerene (C60) nanoparticle (NP) additives on a thermodynamically miscible polymer blend thin film of polystyrene (PS) and polybutadiene (PB). In this system both homopolymer components individually dewet from the commonly used silicon substrate. Three NP concentration regimes having distinct blend nanocomposite film morphologies are observed: (a) In the neat blend and low NP mass (0–1%) range, the blend films rapidly dewet, apparently due to fluctuations in the polymer surface tension arising from the composition fluctuations of a surface enrichment layer at the film air boundary. This behavior is in sharp contrast to the corresponding NP-filled homopolymer films where dewetting is progressively slowed by the segregation of NPs to the solid substrate in this same concentration range. (b) In the intermediate NP concentration range of 1–5 mass %, the C60 additive acts as a “compatibilizing agent”, progressively reducing the size of the dewetted droplets with increasing NP concentration. Dewetting is fully suppressed in the homopolymer films in this NP concentration range. We conclude that C60 segregation to polymeric interfaces within blend film competes with the NP film stabilizing effect. (c) At higher NP concentrations between 5 and 10 mass %, the NPs enrich the substrate sufficiently to fully inhibit the blend film dewetting through a percolating blend–NP structure. At very high NP concentrations (10–15 mass %), the NPs form clusters within the blend film giving rise to a “spinodal clustering” NP morphology.

We show that temporary confinement of polystyrene thin films by an elastomeric capping layer possessing nanoimprinted subcapillary wavelength (λ << λcap (20 μm)) line channels (amplitude A ≈ 120 nm) can suppress film dewetting on thermodynamically unfavorable substrates by arresting the amplitude growth and in-plane propagation of the destabilizing surface capillary waves. Confinement by either a smooth elastomer capping layer (A ≈ 1 nm) or with pattern features above the threshold dimension only retards dewetting but does not prevent it. The nanoimprint pattern is therefore essential to preventing dewetting, illustrating that only the penalty of elastomer deformation and interfacial tension reduction is insufficient.

We investigate the effects of fullerene nanoparticles (f-NP) on the dewetting morphology in the immiscible temperature regime of blend films of polystyrene (PS) and polybutadiene (PB) on silicon substrate. As in our former work in the miscible temperature regime of this blend film, competitive partitioning of the f-NPs to the polymer–polymer and the substrate interfaces in blend films requires a larger concentration of f-NPs (∼10 mass %) to suppress film dewetting than in homopolymer components (∼2 mass %). In contrast, however, phase-separated blend films rapidly dewet into hemispherical droplets due to finite interfacial tension of internal blend components unlike irregular shape droplets obtained in miscible blend films. The effect of the f-NPs (1 mass % to 5 mass % f-NP) is to simultaneously reduce the size and contact angle of the dewet droplets, but the hemispherical shape of droplets is maintained, suggesting the f-NPs act to (a) only reduce the phase-separated blend interfacial tension but not fully compatibilize it into single phase and (b) reduce the blend substrate interfacial tension progressively. Selective solvent etching of the PS blend component reveals a spherical PS core enclosed within a circular PB shell at all f-NP concentrations. Confocal fluorescence microscopy reveals that the f-NPs are distributed in both phases consistent with the hemispherical shape and calculations that predict only a weak reduction of interfacial tension. The dewet blend droplet contact angle (likewise polymer–substrate interfacial tension) measured by atomic force microscopy shows a bimodal behavior, reducing rapidly (by 40%) at low (0.1 mass %) f-NP levels and significantly slowing down at higher f-NP concentrations. Molecular surfactant like behavior of the f-NPs in the blend films then provides an effective means of tuning dewetting blend film morphology dimensions without compromising phase behavior for potential applications in nanotechnology and nanomedicine.

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