Microstructural study of epoxy-based thermosets prepared by "classical" and cationic frontal polymerization.

A microstructural study of bisphenol-A diglycidyl ether (BADGE), prepared via both "classical" and novel photo- and thermally-induced cationic frontal polymerization, can help to understand the relationships between the microstructure of epoxides and their material properties, as well as the propagation of frontal polymerization waves. Microstructural PALS characteristics, such as the ortho-positronium lifetime (τ o-Ps), lifetime distribution, and void fraction, were investigated in relation to the extension of H bonds obtained from ATR/FTIR and the bulk density. The thermal profiles of differently-induced RICFP revealed that photo-triggered propagation is twice as fast as thermally-induced RICFP, with a comparable maximal reaction temperature (∼283 °C) and heat conductivity. Both RICFP-based samples, induced by UV light and heat, showed a lower τ o-Ps, narrower lifetime distributions, and a reduced void fraction, in comparison to the "classical" cured anhydride-based epoxy sample. These may be the main factors which result in better material properties. In addition, both their radial and angular profiles of free volume fraction confirmed experimentally the rotational movement of the propagating frontal waves and their influence on the microstructural inhomogeneities, and the final material properties.