Nanostructure formation in polymer thin films influenced by humidity

The influence of relative humidity (RH) during the film preparation on the surface morphology and on the material distribution of the resulting technical polymer blend films consisting of poly (methyl methacrylate) (PMMA) and poly (vinyl butyral) (PVB) is investigated by atomic force microscopy. Both pure polymers and polymer blends with different compositions of PVB/PMMA dissolved in tetrahydrofuran (THF) were used. Polymer films prepared under dry conditions (RH < 20%) are compared with those that have the same polymer composition but were prepared under increased humidity conditions (RH > 80%). The films consisting of the pure polymers showed a nonporous surface morphology for low-humidity preparation conditions, whereas high-humidity preparation conditions lead to porous PVB and PMMA films, respectively. These pores are explained as the result of a breath figure formation. In the case of the polymer blend films containing both polymers, porous or phase-separated surface structures were observed even at low-humidity conditions. A superposition of the effects of phase separation and breath figure formation is observed in the case of polymer blend films prepared under high-humidity conditions. Atomic force microscopy (AFM) images taken before and after the treatment with ethanol as a selective solvent for PVB indicate that PMMA is deposited on top of a PVB layer in the case of the low-humidity preparation process whereas for high-humidity conditions the silicon substrate is covered with a PMMA film. Copyright (C) 2006 John Wiley & Sons, Ltd.