Patterning Thin Polymer Films By Photodirecting the Marangoni Effect Using Photosensitizers

The Marangoni effect causes liquid flow in response to gradients in surface energy. In this study, we direct this effect at a photosensitizer doped into poly(isobutyl methacrylate) (PiBMA) thin films by programming spatial patterns with near UV-visible light (NUV-vis) exposure to oxidize the sensitizer while the film in a glassy state. Upon heating the film to the liquid state, thermal decomposition of the sensitizer oxidizes the host polymer chains by grafting reactions and creates covalent bonds between the sensitizer and the PiBMA chain. As a result, a local surface energy gradient of the film is generated so that the polymer flows from low surface energy regions (unexposed to light) to high surface energy regions (exposed to light) due to the Marangoni effect. This flow creates smooth, three dimensional topographical features reflective of the light exposure pattern. Proper selection of the photosensitizing compound in an otherwise transparent polymer extends the use of this method to more readily available and safer visible light sources. In this work, such a photosensitizer-polymer system was successfully demonstrated by activating the sensitizer with sunlight.