Reactivity, viscoelastic behaviour and mechanical performances of hybrid systems based on epoxy resins and reactive polyhedral oligosilsesquioxanes

An epoxy formulation was modified by adding various amounts of an epoxy functionalized polyhedral oligomeric silsesquioxane (POSS). The obtained networks were characterized in terms of reactivity, viscoelastic behaviour and mechanical properties. Spectroscopy measurements by FT-NIR demonstrated that the epoxy groups of POSS were almost as reactive as those of the DGEBA in the curing stage, while in the post-curing a lower reactivity of the latter was found, possibly due to steric hindrance effects. The dynamic-mechanical analysis showed an increase in the storage modulus in both glassy and rubbery regions. On the other hand, Tg and Tβ decreased with POSS content. The viscoelastic analysis showed that the addition of POSS enhanced the free volume and the tendency of the network to undergo thermal expansion. In particular, the β transition was made increasingly localized by increasing the POSS content. With respect to mechanical performances, the yield process was facilitated by incorporation of the POSS nanocages within the network, possibly due to an enhancement of the chain mobility. Fracture measurements showed an improvement of the parameters Kc and Gc up to 5 wt% of POSS, after which a decrease of toughness was observed. These results were supported by a fractographic analysis.