A. Sepe 1 , E. T. Hoppe 1 , S. Jaksch 1 , D. Posselt 2 , D. Magerl 1 , Q. Zhong 1 , J. Perlich 3 , D.-M. Smilgies 4 and C. M. Papadakis 1 !" ! # $% & ’( ’) *’) + , -’. #/0 000 *’) & 1 2%34%- &53 6’ ##/01 278" ! 9’)) 2" 5" ’’ ’8 :925; <)’ 47’’ 9’)) + $ 6 3’ 1 Diblock copolymer thin films are prominent candidates for a range of applications requiring nanostructured surfaces, such as the creation of nanoporous films and of arrays of nanowires as well as their use as photonic crystals and biosensors [1]. Thermal annealing has frequently been applied to remove defects and to improve the long-range order [2]. Annealing temperature is a key parameter because it influences both, the interfacial tension between the two blocks and the polymer mobility. We wish to elucidate the mechanisms of structural changes induced by thermal treatment. The poly(styrene-b-butadiene) (P(S-b-B)) system studied by us is well-suited to address this question because it has initially the perpendicular lamellar orientation [3]. In-situ grazing-incidence small-angle X-ray scattering (GISAXS) at beamline BW4, HASYLAB at DESY and VIS- interferometry as well as ex-situ atomic force spectroscopy (AFM) and X-ray reflectivity (XR) were used. The samples were heated to the target temperature within ~35 min and were then cooled down to room temperature within ~40 min, while GISAXS images were taken. At BW4, the wavelength was λ = 1.38 Å, and the beam was focused by Beryllium compound refractive lenses to a size of ~40 μm × 20 μm (horizontal × vertical) at the sample position, which limits the footprint to ~6 mm at an incident angle of 0.2°. A tantalum rod with a diameter of 1.5 mm served as a beam stop for the intense reflected beam and the strong diffuse scattering in the incident plane. Parasitic scattering around the intense specularly reflected beam was in some cases blocked with an additional, disk-like beamstop. A MarCCD area detector was used for the detection of the scattered intensity. The pixel size was 79.1 μm x 79.1 μm. The sample-detector distance was 1.915 m. The AFM height image taken after spin-coating and drying at RT shows small and randomly oriented lamellar domains (Figure 1a). The lamellae themselves are not continuous but consist of small round grains. The 2D GISAXS image displays straight and elongated Bragg rods (BRs) at q y = ± 2π/D lam which indicate the perpendicular lamellar orientation (Figure 1b). From the q y -positions of the BRs the lamellar thickness is determined to be 861 ± 5 Å. In-situ GISAXS measurements elucidate the structural changes during heat treatment at temperatures between 60 and 130°C. Thermal treatment below 100°C does not destroy the perpendicular lamellar order, but the shape of the BRs changes (Figure 2a,b) with respect to RT (Figure 1b): In addition to the straight part extending to high values of q z , a contribution which is slightly bent inwards is present as well. The higher the temperature, the shorter is the bent part. We attribute the bending to tilting of the lamellae away from the purely perpendicular orientation. In contrast, treatment between 105 and 120°C leads to a broad distribution of lamellar orientations which only partially recovers upon subsequent cooling (Figure 2c,d, left picture), the elongated Figure 1. (a) AFM height image, size 3 m × 3 m with the 2D Fourier transform in the inset. The circle marks a lamellar domain where the lamellae are composed of grains. The height scale is in nm. (b) 2D GISAXS image of the as- prepared film, both measured at RT.