Interplay of Substrate Surface Energy and Nanoparticle Concentration in Suppressing Polymer Thin Film Dewetting

It is known that dewetting of a polystyrene (PS) thin film on a silicon substrate gets completely suppressed upon addition of small amount of C₆₀ nanoparticles (NP). 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.