* Corresponding author. Tel.: #49-351-4658-0;-224;-225; fax: #49-351-4658-284. E-mail address: stamm@ipfdd.de, gutmann@mpip- mainz.mpg.de (J.S. Gutmann)  Present address: Institut fu K r Polymerforschung Dresden e.V., Hohe Stra{e 6, D-01069 Dresden, Germany. Physica B 283 (2000) 40}44 Roughness correlations in ultra-thin polymer blend "lms J.S. Gutmann, P. Mu K ller-Buschbaum, D.W. Schubert, N. Stribeck, D. Smilgies, M. Stamm* Max-Planck-Institut fu ( r Polymerforschung, Ackermannweg 10, D-55128 Mainz, Germany Physik Department E13, TU-Mu ( nchen, James-Franck-Stra}e 1, D-85747 Garching, Germany GKSS Forschungszentrum, Max-Planck-Stra}e, D-20502 Geesthacht, Germany Institut TMC, Universita ( t Hamburg, Bundesstra}e 45, D-20146 Hamburg, Germany ESRF, BP 220, F-38043 Grenoble Cedex, France Abstract Ultra-thin "lms of weakly incompatible polymer blends form smooth "lms with correlated interfaces upon suitable preparation. With the poly-(styrene-co-para-bromo-styrene) PBr  S/PBr  S blend system, of slightly di!erent degrees of bromination, a series of samples with varying composition on top of roughened substrates has been investigated. The surface morphology of the thin "lms was characterized by microscopy measurements, while with di!use X-ray scattering the roughness correlation between the interfaces was examined. A lower cut-o! length of the replicated roughness spectrum at small dimensions was obtained. Our results show, that the blend composition has a distinct in#uence on the replicated in-plane lengths.  2000 Elsevier Science B.V. All rights reserved. PACS: 83.30.ES; 61.10.KW; 61.10.Eq; 68.55.-a Keywords: Polymers; Ultra-thin "lms; Roughness correlations; Scattering 1. Introduction Polymeric coatings "nd widespread use in many con- temporary applications such as dielectric coatings and photoresists in the semiconductor industry or biocom- patibilisation of surfaces. Since the individual functional requirements are very di!erent it is a common practice to prepare the "lms from a mixture of two or more compo- nents in order to achieve the required properties. How- ever, due to the low entropy of mixing, polymeric blends are mostly incompatible and phase separate under ap- propriate conditions. For ultra-thin "lms the additional presence of a surface leads to a behaviour which is very di!erent from bulk systems [1]. In incompatible polymer blends the thin "lm may undergo phase separation even during preparation. This behaviour has been observed in a variety of incompatible blend systems where phase separation perpendicular or parallel to the substrate was observed [2}10]. If the blended polymers are only weakly incompatible, the phase separation process may not be fast enough to allow a phase separation during the prep- aration process. In this case essentially smooth blend "lms are obtained over a wide compositional range. While the case of interfacial correlation has previously been observed in single polymer "lms [11,12] and bilayer polymer "lms [13,14], we examine the interface correla- tion in ultra-thin binary-blend "lms of the statisti- cal poly-(styrene-co-para-bromo-styrene) copolymers PBr  S (i.e. at 91% bromination) and PBr  S. Using this very weakly incompatible blend system it is possible to prepare ultra-thin "lms with correlated interfaces by a suitable choice of preparation conditions such as the 0921-4526/00/$ - see front matter  2000 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 4 5 2 6 ( 9 9 ) 0 1 8 8 8 - 8