Partial Dewetting and Labyrinthine Pattern Formation on Polystyrene Films Immersed in Ternary Solvent Mixtures

This research examines partial dewetting and labyrinthine morphologies in thin polystyrene (PS) films using a ternary solvent mixture of water, acetone, and methyl ethyl ketone (MEK) (15:3:7 by volume). The films are initially glassy but destabilize due to the solvent into mixed structures containing labyrinth-like patterns and holes, not droplets. Morphological changes result from solvent–nonsolvent interactions. MEK is a good solvent that swells the film, lowers the glass transition temperature, and increases chain mobility, while acetone, as a cosolvent, provides uniform solvent uptake, and water as a nonsolvent reduces film–substrate affinity and promotes spreading-driven dewetting. The opposing influence of these factors leads to viscosity gradients that drive heterogeneous nucleation of holes, their growth to large size, and partial coalescence. The process ends before equilibrium is reached as rapid evaporation of MEK balances driving forces with viscosity, and traps intermediate structures. The physics behind the existence or a mechanical pathway to intermediate structures is provided by studies of the spreading coefficient and hole growth. This work demonstrates engineering solvent mixtures for controlled non-equilibrium surface patterning in polymer thin films.