Stem Observations of the Microstructures Present in Polymer Blend Thin Films.
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STEM OBSERVATIONS OF THE MICROSTRUCTURES PRESENT IN POLYMER BLEND THIN FILMS. K. E. SICKAFUS, S. D. BERGER, and A. M. DONALD University of Cambridge, Cavendish Laboratory, Cambridge CB3 01E, U. K.
ABSTRACT This paper examines the effectiveness of scanning transmission electron microscopy (STEM) as an analytical tool for determining composition in multi-phase polymer blend microstructures. The polymer blend polystyrene (PS) - polyether sulphone (PES) thin film used in this study exhibited a two-phase microstructure consisting of PES-rich inclusions, ranging from 0.2gtm to 1.21im in diameter, in a PS-rich matrix. Emphasis in this presentation is placed on the use of annular dark-field (ADF) STEM image contrast to infer information concerning the local composition of adjacent microstructural features.
INTRODUCTION In a scanning transmission electron microscope (STEM), composition is usually determined from spectroscopic analysis, e.g. energy-dispersive x-ray spectroscopy (EDS) or electron energy-loss spectroscopy (EELS). However, recent work has demonstrated that image analysis
may be used to provide chemical information in certain circumstances [1,2]. This has advantages for polymers and other beam-sensitive materials since image analysis is a high-signal technique and image acquisition is rapid. Spectroscopy, on the other hand, is low-signal and spectrum acquisition is slow; hence, spectroscopies are not as useful for specimens susceptible to beam damage. In this study, annular dark-field (ADF) STEM images from a polymer blend thin film consisting of polystyrene (PS) and polyether sulphone (PES) were analyzed in an effort to determine local compositional differences in the microstructure. The principle of ADF imaging of amorphous materials for compositional analysis may be summarized as follows: high atomic number (high-Z) elements have a larger scattering cross-section for elastically scattered electrons than low atomic number (low-Z) elements, and more electrons are scattered to higher angles from high-Z elements. These differences can be used, in principle, to distinguish high-Z and low-Z elements from each other in dark-field STEM [2]. A problem arises, however, in the interpretation of ADF image contrast, when the specimen is thicker than the elastic mean free path for scattering, Ael (as was the case for this PS-PES film), because multiple-scattering effects have to be considered in order to explain the contrast observed in the ADF images.
Mat. Res. Soc. Symp. Proc. Vol. 79. 1987 Materials Research Society
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EXPERIMENTAL PROCEDURE The PS-PES polymer blend thin film used in this study was cast from a solution containing one part (by weight) PES to three parts PS, diluted to -3 wt.% polymer in solvent. The solvent was cyclohexanone, a co-solvent for PS and PES. Details of the thin-film preparation procedure are given elsewhere [3]. The PS-PES film was examined using either a VG HB-5 dedicated STEM operating at 80kV, a VG HB-501 dedicated STEM operating at 100kV, or a Philips 400T TEMISTEM operating at 1
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