SURFACE-INDUCED ORDERING IN ASYMMETRIC BLOCK-COPOLYMERS

The surface-induced ordering in thin films of asymmetric deuterated polystyrene (dPS)-poly(vinylpyridine) (PVP) diblock and triblock copolymers of comparable polymerization index and PVP volume fraction (f approximately 0.25) was studied using transmission electron microscopy, atomic force microscopy, secondary ion mass spectrometry, and neutron reflectivity. The morphology of both di- and triblock copolymer films was found to be cylindrical except for the layer adjacent to the silicon oxide surface, which due to the strong interaction of silica with PVP, was lamellar. The spacing between adjacent cylindrical layers was found to be consistent with mean field theory predictions. In the triblock copolymer films the cylindrical layers were oriented parallel to the silicon oxide surface, and no decay of the ordered structure was observed for at least 12 periods. If the total film thickness t' deviated from t = [(n + 0.71)210 + 182] angstrom, where n is an integer, islands or holes formed at the vacuum interface. The height of the holes or islands reached its equilibrium value, 210 angstrom, after annealing 24 h at 180-degrees-C. In contrast, it was far more difficult to orient parallel to the silicon oxide surface the microphase-separated cylindrical domains in the diblock copolymer films. As a result no islands or holes were observed even after annealing for 5 days at 180-degrees-C. We concluded that the difference in ordering behavior was due to the ability of the triblock copolymer to form an interconnected micelle network while the diblock copolymer formed domains that were free to move with respect to each other. This conclusion was further confirmed by diffusion measurements which showed that the PS homopolymer penetrated easily into the ordered diblock copolymer films and was excluded from the ordered triblock copolymer films.