On the structure of glassy polymers. VIII. Use of Z contrast to elucidate the microstructure of epoxy and polyimid resins

The technique of ultramicrotomy of polymers, followed by staining of the resulting thin sections with heavy metal ions and viewing with Z enhancement in the scanning transmission electron microscope is described. When applied to anhydride‐cured epoxy resins, the structure is found to be heterogeneously crosslinked on a scale of a few hundred angströms. When the technique is applied to amine‐cured epoxy resins, the microstructure is found to change from homogeneous to inhomogeneously crosslinked, depending on stoichiometries and cure cycles. For amine‐cured resins whose cure conditions are within the range of microstructural change, the bright field of the stained specimens alone does not detect heterogeneities, and the Z contrast becomes crucial to discern the kind of microstructure. A commercial polyimide film examined in the same way is found to exhibit systematic variations in structure through the thickness of the film.