Achieving Stable Patterns in Multicomponent Polymer Thin Films Using Marangoni and van der Waals Forces*

Topographically patterned polymer films are promising for applications such as fabrication of integrated circuit elements in microelectronics, optical gratings, photonic crystals, and sensors and actuators. Liquid-air interfaces can be deformed by surface-tension gradients to create topography, a phenomenon useful for polymer film patterning. A recently developed method creates these gradients by photochemically patterning a solid polymer film. Heating the film to the liquid state leads to flow driven by the patterned surface-tension gradients, but capillary leveling and diffusion of surface-active species facilitate eventual dissipation of the topography. However,  experiments demonstrate that using blends of high- and low-molar-mass polymers can considerably delay the decay in topography. To gain insight into this observation, we develop a model based on lubrication theory that yields coupled nonlinear partial differential equations describing how the film height and species concentrations evolve with time and space. Incorporation of a non-monotonic disjoining pressure is found to significantly increase the lifetime of topographical features, making the model predictions qualitatively consistent with experiments. A parametric study reveals the key variables controlling the kinetics of film deformation and provides guidelines for photochemically induced Marangoni patterning of polymer films.