Multitechnique characterization of thin films of immiscible polymer systems: PS-b-PMMA diblock copolymers and PS-PMMA symmetric blends

Immiscible polymer systems are known to form various kinds of phase-separated structures capable of producing self-assembled patterns at the surface. In this study, different surface characterization methods were utilized to study the surface morphology and composition produced after annealing thin polymer films. Two different SIMS techniques - static time-of-flight secondary ion mass spectrometry (ToF-SIMS) and dynamic nano-SIMS - were used, complemented by x-ray photoelectron spectrometry (XPS) and atomic force microscopy (AFM). Thin films (spin-coated onto silicon wafers) of polystyrene (PS)-poly(methyl methacrylate) (PMMA) symmetric blends and diblock copolymers of similar molecular weight were investigated. Surface enrichment by PS was found on all as-cast samples. The samples were annealed at 160 degrees C for different time periods, after which the blend and the copolymer films exhibited opposite behaviour as seen by ToF-SIMS and XPS. The annealed blend surface presented an increase in the PMMA concentration whereas that of copolymers showed a decrease in PMMA concentration compared with the as-cast sample. For blends, the nano-SIMS as well as AFM images revealed the formation of phase-separated domains at the surface. The composition information obtained from ToF-SIMS and XPS, as well as the surface mapping by nano-SIMS and AFM, allowed us to conclude that PS formed phase separated droplet-like domains on a thin PMMA matrix on annealing. The three-dimensional nano-SIMS images showed that the PS droplets were supported inside a rim of PMMA and that these droplets continued from the surface like columnar rods into the film until the substrate interface. In the case of annealed copolymer samples, the AFM images revealed topographical features resembling droplet-like domains on the surface but there was no phase difference between the domains and the matrix. In the case of copolymers, owing to the covalent bonding between the blocks, complete phase separation was not possible. The three-dimensional nano-SIMS images showed domain structures in the form of striations inside the film, which were not continuous until the substrate interface. Information from the different techniques was required to gain an accurate view of the surface composition and topographical changes that have occurred under the annealing conditions. Copyright (c) 2005 John Wiley & Sons, Ltd.