Epoxy networks modified by a new class of oligomeric silsesquioxanes bearing multiple intramolecular rings formed through Si-O-C bonds

A new class of silsesquioxane (SSO), containing species with two to none Si atoms bearing multiple intramolecular rings formed through Si-O-C bonds, was synthesized as a glassy powder. It was characterized by UV-MALDi-TOF MS, Si-29 NMR and FT IR. Solutions containing different amounts of SSO in the diglycidyl either of bisphenol A (DGEBA), were homopolymerized in the presence of (4-dimethylamino)pyridine (DMAP) as initiator, leading to SSO-modified epoxy networks. SSO species were covalently bonded to the epoxy network without any evidence of phase separation. The SSO addition provoked an increase in the elastic modulus in the glassy state explained by an increase in the cohesive energy density. The SSO addition gave also place to an increase in the intensity of tan delta and a decrease in both the glass transition temperature and the elastic modulus in the rubbery state. This was explained by a decrease in crosslink density associated with the flexibility of SSO structures. DMAP was much more effective than other usual initiators (like benzyldimethylamine, BDMA), in increasing the crosslinked density of the resulting epoxy network. This led to high values of the glass transition temperature and the elastic modulus in the rubbery state.