Molecular origins of Epoxy-Amine/Iron oxide interphase formation

Hypothesis: Interphase properties in composites, adhesives and protective coatings can be predicted on the basis of interfacial interactions between polymeric precursor molecules and the inorganic surface during network formation. The strength of molecular interactions is expected to determine local segmen-tal mobility (polymer glass transition temperature, Tg) and cure degree. Experiments: Conventional analysis techniques and atomic force microscopy coupled with infrared (AFM-IR) are applied to nanocomposite specimens to precisely characterise the epoxy-amine/iron oxide inter-phase, whilst molecular dynamics simulations are applied to identify the molecular interactions under-pinning its formation. Findings: Attenuated total reflectance Fourier transform infrared (ATR-FTIR) spectroscopy and high-resolution AFM-IR mapping confirm the presence of nanoscale under-cured interphase regions. Interfacial segregation of the molecular triethylenetetraamine (TETA) cross-linker results in an excess of epoxy functionality near synthetic hematite, (Fe2O3) magnetite (Fe3O4) and goethite (Fe(O)OH) particle surfaces. This occurs independently of the variable surface binding energies, as a result of entropic seg-regation during the cure. Thermal analysis and molecular dynamics simulations demonstrate that restricted segmental motion is imparted by strong interfacial binding between surface Fe sites in goethite, where the position of surface hydroxyl protons enables synergistic hydrogen bonding and elec-trostatic binding to Fe atoms at specific sites. This provides a strong driving force for molecular orienta-tion resulting in significantly raised Tg values for the goethite composite samples. (C) 2022 Elsevier Inc. All rights reserved.