DEWETTING OF THIN POLYMER FILMS IN NON-SOLVENT - SOLVENT ENVIRONMENT
Dewetting of polymers is a central phenomenon in polymer thin films which can affect film stability and morphology. If the dewetting process is controlled and directed, it can also be harnessed for patterning micro and nano features. The process of polymer thin film dewetting typically begins with the rupture of the film and formation of holes. These holes continue to grow until they collapse and create small droplets.
In this research, we investigate the behavior of spontaneous dewetting of polystyrene thin films in a non-solvent environment under exposure to solvent vapors (solvent annealing). This process leads to the formation of spherical polystyrene particles with very high contact angle, compared to the contact angle obtained in thermal dewetting process, as seen with scanning electron microscopy (SEM) cross-sectional imaging. The particles’ size and their spatial arrangement on the surface has a strong correlation to the polymer molecular weight and film thickness, as revealed by SEM and light microscopy. Controlled experiments of arrested annealing shed light on the kinetics of the process and revealed the gradual transformation from holes to particles. Interestingly, the particles also exhibit porosity on their surface. We attribute this surface porosity to non-solvent induced phase separation that occurs at the polystyrene - non-solvent (water) - solvent (chloroform) boundaries.
Figure 1: Top down (a) and cross section (b) SEM images of polystyrene 118k particles after the dewetting process of polystyrene thin film.