Surface Structure of Polymer Thin Films Characterized by X-ray Scattering and Atomic Force Microscopy. Jin Wang¹, Metin Tolan², Ajay K. Sood^1,3 , Xiaozhong Wu¹, Zhixin Li⁴, Olaf Bahr², Mariam H. Rafailovich⁴, Jonathan Sokolov⁴, Sunil K. Sinha¹, ¹Argonne National Laboratory, Argonne, IL 60439, ²Christian-Albrechts-Universität Kiel, Kiel, Germany, ³Indian Institute of Science, Bangalore 560012, India, ⁴SUNY at Stony Brook, Stony Brook, NY

X-ray techniques, including diffuse scattering and specular reflectivity, have been employed to characterize the surface structure of polymer thin films. X-ray diffuse scattering (as a function of the wave-vector transfer q_x along the surface) is sensitive to the height fluctuations caused by the capillary waves at polymer thin film surfaces. More specifically, we studied the thickness-dependence of the capillary wave scattering associated with polystyrene (PS) and poly(2-vinylpyridine) (P VP) thin films spun-coated onto silicon substrates. At low value of q_x, the behavior of the capillary wave scattering from a polymer film is dramatically modified from that of a free simple liquid surface. The effective low-q_x cutoff of the capillary waves shifts to higher values as the film thickness decreases. The results can be understood quantitatively by taking into account the strong van der Waals interaction between the polymer thin films and their substrates. By taking advantage of the large coherence length of x-rays at grazing incidence across the sample surface, the x-ray diffuse scattering is utilized to measure the surface morphology of wetted and dewetted polymer films at length scales up to micrometers. In most cases, a dewetted film can be modeled in terms of island-like structures, with domain sizes ranging from 0.1 to a few micrometers. Furthermore, the x-ray diffuse scattering data can be compared with the height-height correlation functions extracted from atomic force microscope (AFM) images of the film surfaces. In conclusion, we have demonstrated that x-ray scattering, complemented by AFM imaging, is a powerful tool for characterizing polymer thin film surfaces. X-ray measurements reveal global, statistical information in reciprocal space, while AFM images provide the complementary local and detailed characteristics of the surface in real space.

supported in part by the U.S. Department of Energy, BES-Materials Science, under contract number W-31-109-ENG-38