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Each set of images is of an epoxy-amine film that was spin cast onto a silicon substrate. The sample in the top left images was made with a stoichiometric ratio between epoxy and amine. The sample in the bottom left images was prepared with an excess of epoxy (ratio 1.25/1). Both of these samples were < 15 nm in thickness. The stoichiometric imbalance between epoxy and amine led to changes in the resulting microstructure with remarkably bigger nodules. Also, significant phase image contrast was observed between the nodular phase and the matrix, which indicates significant differences in the properties of the two regions. In epoxy systems, stoichiometric imbalances can occur near interfaces due to preferential segregation of one of the reacting species to the substrate, fiber, or filler surface. Thus, the difference in microstructure observed in these two samples might indicate potential differences in the interphase microstructure and resulting properties compared to the bulk epoxy.
In the two sets of images on the right, the influence of the silicon substrate on the microstructure of an epoxy-amine film that was approximately 150 mm thick is shown. The top right image set is of the surface of the film exposed to air while the bottom right image set is of the surface that was in contact with the silicon substrate during processing. Again, a significant difference in microstructure is observed, somewhat similar to the difference between the microstructures of the on- and off-stoichiometry thin films on the left.
This study was done in collaboration with Marine Giraud (NIST Guest Researcher) of Claude Bernard University (Lyon, France).