Cross-sectional tem analysis of solvent-cast SBS thin films

Abstract

Block copolymers can assume a range of microphase-separated morphologies, and the dependence of morphology on temperature and composition is an area of active research. Our work has been studying the morphology of solvent-cast thin films of polystyrene-polybutadiene-polystyrene (SBS) as a function of solvent evaporation rate and post-specimen annealing. This paper describes the analysis of thin film cross-sections to distinguish between possible morphologies. ∼50 nm-0.5 μm thick films were cast from a 0.1 wt% solution of SBS (30 wt%, Mw=105) in toluene. GPC analysis indicates PS/PB diblock and PS homopolymer is present (<5wt% total). 50 μl of solution was deposited on water-polished salt. The solvent evaporation rate was roughly controlled between 200 nl/sec and 0.2 nl/sec. After evaporation, the film was cut, floated from the salt, collected on grids, and exposed to OsO4 vapor (20 min) to stain the PB. After plan view study, some specimens were cross-sectioned. A carbon layer was evaporated on both sides prior to embedding in epoxy and ultramicrotomy(−130°C). Microscopy used a Philips CM3 0ST TEM.