Surface-induced ordering in graft copolymer thin films

The surface-induced ordering of deuterated poly(ethyl acrylate)-polystyrene (dPEA-PS) graft copolymers with different average number x of graft chains per copolymer chain (dPEA-g-xPS, x = 1, 3, 5) was investigated using neutron reflection, secondary ion mass spectrometry, and atomic force microscopy. The ordering starts only at the vacuum surface and decays through the bulk of the film to the silicon surface. The dPEA-g-3PS thin films (f, fraction of styrene monomers = 0.28) appear to have dPEA-PS lamellar structures and a nearly pure dPEA layer next to the vacuum surface. The dPEA-g-5PS thin films (f = 0.48) are found to be cylindrical except for the top layer, adjacent to the vacuum surface, which is lamellar. On comparison with diblock copolymers, graft copolymers show a phase diagram shift and smaller micelle spacing. We find that long-range order in graft copolymers only occurs within similar to 15 lamellar spacings of the vacuum surface. Linear viscoelasticity of bulk samples reveals that none of the shear-history and low-frequency terminal response complications observed for diblock copolymers are observed for graft copolymers, owing to their lack of long-range order.