Photo-oxidation of High Performance Epoxy Networks: Correlation between Molecular Mechanisms of Degradation and Viscoelastic and Mechanical Response.

The photo-oxidative degradation of a highly cross-linked epxide/diamine network has been investigated by FTIR spectroscopy, dynamic-mechanical thermal analysis (DMTA) and compressive mechanical tests. The FTIR measurements allowed us to monitor the degradation process and to obtain reliable kinetic data. Two dimensional infrared (2D IR) correlation spectroscopy, was employed to analyze the spectra collected as a function of time. This approach effectively enhanced the spectral resolution and revealed details about the reaction mechanism hardly detectable in the one-dimensional, frequency domain. In particular, the site of initiation of the auto-oxidative sequence were identified, and the main pathways through which the degradation reaction proceeds were proposed. Dynamic-mechanical measurements performed in a multifrequency mode were used to reveal the effect of the photo-oxidative degradation on the relaxation processes of the epoxy network. It was found that the Tg decreases with increasing the exposure time. An analogous trend was found for the storage modulus, E’, at the temperature above the glass transition. The overall cross-linking density was found to decrease as a consequence of the chain scission reactions. A marked reduction of the mechanichal performances of the material was evidenced at high exposure times, especially in terms of yield-strength and strain.