A study by Raman, near-infrared and dynamic-mechanical spectroscopies on the curing behaviour, molecular structure and viscoelastic properties of epoxy/anhydride networks

The curing characteristics of a TGDDM/HHPA formulation have been investigated by Raman spectroscopy, which allowed us to monitor the evolution of the different reactive species (i.e. epoxy, anhydride and ester groups) participating in the curing process. The curing mechanism and, in particular, the role of side processes, were elucidated. NIR spectroscopy was employed to investigate the post-curing process, in view of the superior sensitivity of this technique for monitoring polar groups. Quantitative methods were developed to measure residual concentration of epoxy groups in the high conversion regimes (≥98%). Dynamic-mechanical measurements were performed to gather information on the molecular structure and viscoelastic properties of the investigated networks. For formulation rich in epoxy resin, clear evidence of an inhomogeneous phase structure was found. A viscoelastic analysis in terms of the WLF approach demonstrated that both the free volume and the thermal expansion coefficient of the networks decrease by enhancing the anhydride/epoxy molar ratio.