Influence of solvent and molecular weight in wrinkle formation in spin-cast polystyrene thin films

Abstract

The surface morphology of polystyrene thin films formed from various molecular weight polystyrene and solvent conditions is studied. When spin-cast from tetrahydrofuran (THF) wrinkles are formed at the extremities that have periodicity with wavelengths in the μm range and amplitudes in the nm range but varies with molecular weight. A mixed solvent system consisting of THF and dimethylformamide (DMF) leads to periodic structures only with THF-rich compositions. THF and DMF have similar properties relevant to spin-casting: density, surface tension, molecular weight, and viscosity but different boiling points and room temperature vapor pressures, demonstrating that formation periodicity requires a volatile solvent. The formation of the surface structures is attributed to the Marangoni effect and the film thicknesses and wave parameters are shown to be consistent with literature models.

Keywords

• morphology
• polymer
• solvent casting
• wrinkle formation
• spin-coating