Thermal effects on the network structure of electron-beam polymerized diglycidylether of bisphenol-A

Electron beam (EB) induced polymerization of diglycidylether of bisphenol A (DGEBA) in the presence of a diaryliodonium salt (DAIS) was studied by FTIR spectroscopy and by dynamic mechanical thermal analysis (DMA). The obtained results allow to draw the gradual increase of the temperature for the network thermomechanical transition (Ta, associated wth the glass transition temperature Tg) over a broad range of conversion (it) and reveal a peculiar behavior at high conversion. In this domain (it> 0.90), the material's Tg is shown to decrease when conversion approaches unity. Moreover, the postirradiation thermal treatment of the materials, that generally yields effective "dark curing', appears to induce a decrease of Tg, with an amplitude correlated with the amount of DAIS in the formulation. Owing to the particular nature of the propagating centers in cationic polymerization, the thermal relaxation of ionic clusters trapped in the glassy matrix can be reasonably invoked as a possible cause for this behavior.