The persistence of interphase regions in epoxy-amine coatings

It has recently been established that interfacial segregation of polymer precursors occurs during the step-growth polymerization of epoxy thermosets. In composite coating formulations, this leads to the establishment of nanoscale, structurally distinct interphase regions around filler or pigment particles. Whilst these under-developed regions are widely considered to be a critical factor in determining corrosion resistant barrier coating performance, the longevity and fate of the interphase, rich in reactive chemical functionality, remains largely unexplored. Here, we investigate the evolution of nanoscale chemical gradients in exemplary systems comprised of diglycidyl ether of bisphenol-A (DGEBA) and triethylenetetraamine (TETA) binder filled with powdered synthetic hematite, (Fe₂O₃). In these systems, it is known that interphase functionality is primarily dependent on entropic segregation processes, since only weak electrostatic binding occurs between the amine and particle surfaces. Surprisingly, here we demonstrate that rather than exclusively segregating upon mixing, unreacted material continues to accumulate in the interphase throughout the ambient cure (including post-vitrification). Moreover, when a post-cure heating regime known to accelerate molecular diffusion and etherification is applied, it is found that the interphase persists and is remarkably unreactive, yielding relatively soft, partially cured regions 50–100 nm in depth around embedded particles.