Hole Nucleation and Growth in Free-Standing Polystyrene Ultrathin Films

Nucleation and growth of holes in free-standing ultrathin (< 100 nm) polymer films are examined via optical microscopy in order to gain a better understanding of these phenomena. Hole nucleation is quantified with a free energy barrier based on a simple capillary model. Additionally, holes are found to grow exponentially in accordance with previous studies in the literature. Ultrathin films of polystyrene (between 50 and 100 nm) cast via flow coating are suspended atop lithographically patterned arrays of pillars. The films are then annealed above the glass transition temperature to study the nucleation and growth of holes via optical microscopy. Image analysis is performed to measure the density of nucleated holes as well as hole radius as a function of time. Holes are found to grow exponentially with time in a nonlinear viscoelastic, shear thinning regime under high shear strain. The energy barrier model is applied to the nucleation of holes in free-standing thin films and is found to describe the phenomenon well. This analysis of hole nucleation and growth extends the understanding of ongoing research into suspended fiber formation from the melting of free-standing polymer thin films.