Phase Contrast Imaging of Styrene/Isoprene Block Copolymers

SAXS shows that in polystyrene (PS)/polyisoprene (PI) block copolymers with lamellar morphologies there is a sufficient electron density difference for x-ray scattering [1]. It has been accepted, however, that this difference is insufficient for TEM imaging by amplitude contrast. Therefore, contrast has been produced by staining the PI phase with OsO4 [2]. Staining changes the mechanical and chemical structure of the PI phase perhaps altering the phase separation and the domain interfaces (figure 1).Mean inner potential calculations, however, show potentials of 8.29V for PS and 7.57V for PI. This indicates that for a simple specimen geometry (e.g. lamellar) with sharp phase interfaces oriented parallel to the incident beam, a film 500Å thick will provide a phase change (Δψ) of 0.3 radians at 100 KV. This phase change is sufficient for imaging under defocus conditions and contains information about the interface region between the domains. The phase change may be increased by using lower voltage (50% increase at 40 KV).