Dip-coating deposition of nanocomposite thin films based on water-soluble polymer and silica nanoparticles

Dip-coating deposition of nanocomposite thin films composed of polyvinylpyrrolidone and silica nanoparticles was studied using atomic force microscopy and scanning electron microscopy. The presence of particles (up to 5 wt%) did not modify the film thickness, which followed a classical additive law of an evaporative regime and a draining regime. Within film volume, no preferential retrieval of particles nor polymer occurred during deposition. This was consistent with the size of particles small enough so that particles were not trapped in the meniscus. For the thicker films prepared in pure capillary and draining regimes, the number of particles observed on the surface was independent of the thickness and was in full agreement with a homogenous distribution of the particles in the film. In contrast, for the thinner films (near the critical withdrawal speed) the number of particles differed in the two regimes. In the draining branch, the number followed the theoretical prediction with an increased number of particles as the thickness decreased, due to the presence of an exclusion zone at the interfaces. Unexpectedly, in the capillary branch, a depletion of particles was observed and could result from competition and interplay between time-dependent phenomena that took place during the dip-coating. This behavior will need to be carefully considered when preparing nanocomposite thin films where surface properties such as roughness or wettability need to be considered.