Complexities in STEM analyses of polymer blend thin films

Polystyrene (PS)-polyether sulphone (PES) polymer blend thin films were prepared for examination in a scanning transmission electron microscope. The microstructures observed in 75 wt % PS-25 wt % PES films consisted of spherical inclusions, ranging from 0.2 to 1.2m in diameter. X-ray spectrometric analysis in the microscope revealed that the inclusions were PES-rich, while the matrix contained only PS. Attention in this paper is paid to the contrast in the annular dark-field detector (ADF) images from these thin films. This image contrast has a complicated dependence on both the angular range subtended by the dark-field detector and mass-thickness variations within the films. On microscopes with appropriate lens controls which permit the acceptance angle of the ADF detector to be varied, it becomes possible actually to reverse the contrast between the two phases.