Ordering at Diblock Copolymer Surfaces

The temperature dependence of the surface excess of a series of symmetric microphase-separated diblock copolymers of polystyrene (PS) and poly(methyl methacrylate) (PMMA) was determined by using X-ray photoelectron spectroscopy. For diblock copolymer systems in the strong segregation limit, under equilibrium conditions, a pure layer of PS was located at the surface, whereas for systems in the weak segregation limit, a layer composed of a mixture of PS and PMMA was located there. The surface excess of PS of a weakly segregated diblock copolymer system was observed to decrease with increasing temperature. We show that the chemical potential difference that favors PS at the surface decreases with increasing temperature. The segmental interactions in the vicinity of the surface are modified such that the surface should order at a temperature above the bulk microphase-separation transition (MST). The length scale over which the segmental interactions in the vicinity of the surface are perturbed is on the order of a statistical segment length. Finally, an estimate of the proximity of the weakly segregated system to the bulk MST is made based on the experimental data. © 1991, American Chemical Society. All rights reserved.