Nanoparticle-Mediated Stabilization of a Thin Polymer Bilayer

We show that an unstable thin polymer bilayer comprising a polystyrene (PS) top and polymethyl methacrylate (PMMA) bottom layer remains completely stable on a non-wettable, hydrophobized silicon wafer substrate, even after 600 h of thermal annealing at a temperature that is higher than the glass transition temperature (T-G) of both the polymers, when thiol-capped gold nanoparticles (AuNPs) are added only to the PS top layer at a concentration (C-NP) approximate to 10.0% (wt. of NP/wt. of polymer). We also show that in films with C-NP as low as 2.5%, the bottom PMMA layer becomes completely stabilized while the top PS layer undergoes partial dewetting, the extent of which progressively reduces with an increase in C-NP. X-ray reflectivity (XRR) studies reveal that the AuNPs added to the top PS layer migrates toward the film-substrate interface within a few minutes of thermal annealing, imparting stability to the bottom PMMA layer. On the other hand, in films with a higher C-NP approximate to 10.0%, particles are also seen to accumulate in the vicinity of the free surface, in addition to the film-substrate interface, resulting in complete stabilization of the bilayer. The extracted electron density profile from XRR shows that the AuNPs accumulate both at the free surface as well as at the film-substrate interfaces. Grazing incidence small-angle X-ray scattering (GISAXS) suggests the formation of an in-plane ordering of Au-polymer-Au clusters at the polymer-air and polymer-substrate interfaces with thermal annealing, which plays a pivotal role in arresting the dewetting of the bilayer by strengthening the interfaces. The novelty of the technique reported here lies in complete stabilization of an unstable bilayer by adding particles only to the top layer. This technique is likely to be helpful in obtaining stable multicomponent ultrathin films even on non-wettable surfaces.