Surface-Induced Orientation of Symmetric, Diblock Copolymers: A Secondary Ion Mass Spectrometry Study

Abstract

Secondary ion mass spectrometry, SIMS, has been used to investigate a surface-induced orientation of symmetric, diblock polystyrene/poly(methyl methacrylate) copolymers, PS/PMMA, where either the PS or PMMA block was perdeuterated. SIMS yields a secondary ion intensity for carbon, hydrogen, and deuterium produced by sputtering of the copolymer film as a function of time (depth) from the surface with a measured depth resolution of 125 A. Solution cast films of the copolymer from toluene, with thicknesses of 5 x 103A or less, were found to exhibit no preferential orientation of the microdomain morphology with respect to the surface. Annealing the copolymer films at 170 °C for 24 h produced dramatic orientation of the microdomains parallel to the surface of the film. This preferential alignment resulted in a periodic variation in the composition of either block that persisted through the entire thickness of the film. The periodicity, determined from model calculations, agreed with small-angle X-ray scattering results on the same but thicker films. PS exhibited an affinity for the free surface, while PMMA was preferentially located at the copolymer/substrate interface. At equilibrium and near-equilibrium conditions, the copolymer microdomains were aligned primarily parallel to the film surface with extensive lateral coherence. The thickness of the PS-rich layer at the air/copolymer surface was found to be one-half that of the microdomain in the bulk. Finally, interference microscopy results show that the thickness of an annealed film is quantized in that it is dictated by the alignment of the microdomains with respect to the surface and the periodicity of the microdomain morphology. Annealed films of the copolymer exhibit steps near the perimeter of the specimen, the height of which corresponds to precisely one period of the copolymer morphology. © 1989, American Chemical Society. All rights reserved.