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 C<sub>60</sub> 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<sub>S</sub>) 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 (<i>C</i><sub>NP</sub>) and γ<sub>S</sub>, we identify three distinct stability regimes. In regime 1 (<i>C</i><sub>NP</sub> < 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<sub>S</sub> < γ<sub>C60</sub> (NP surface energy) and larger as γ<sub>S</sub> exceeds γ<sub>C60</sub>. In regime 2 (0.3% < <i>C</i><sub>NP</sub> < 0.75%) the films dewet partially, and the extent of dewetting is seen to strongly dependent on the relative magnitudes of γ<sub>C60</sub> and γ<sub>S</sub>. While dewetting proceeds up to the stage of partial hole growth and coalescence when γ<sub>S</sub> < γ<sub>C60</sub>, some random isolated holes are seen to form when γ<sub>S</sub> > γ<sub>C60</sub>. 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<sub>S</sub> > γ<sub>C60</sub>. We show complete suppression of dewetting in regime 3 (<i>C</i><sub>NP</sub> > 1.0%), where the particles are seen to migrate to the substrate for all values of γ<sub>S</sub>. The work highlights that entropy driven migration of particles takes place on substrates with any γ<sub>S</sub> only above a critical NP concentration (<i>C</i><sub>NPC</sub>) and only on substrates with γ<sub>S</sub> > γ<sub>C60</sub> when <i>C</i><sub>NP</sub> < <i>C</i><sub>NPC</sub>. The findings, apart from dewetting suppressing, can guide potential design criteria for applications such as electron extracting layer in organic photovoltaic.