Influence of self-assembled compliant domains on the polymer network and mechanical properties of POSS-epoxy nanocomposites under cryogenic conditions

Abstract

Although epoxy resins have been widely used for engineering applications because of their excellent mechanical properties, they can be brittle and vulnerable to fracture. Such brittle behavior becomes more severe when such resins are subjected to extremely low-temperature environments. In this study, we employ polyhedral oligomeric silsesquioxane (POSS) reinforcement to improve the mechanical properties under cryogenic conditions (77 K). Nanocomposites are prepared by adding POSS at 0.5–8 wt.% to an amine-cured DGEBF based epoxy resin. The results show that at 77 K the fracture toughness of the neat resin increases compared to room temperature due to the increase in intermolecular forces. The fracture toughness results also showed that the addition of POSS leads to an additional 40% improvement in fracture toughness at 77 K at 5 wt.% of POSS loading, as compared to neat resin. Positron annihilation lifetime spectroscopy indicate no changes in the free volume inside the molecular network with the inclusion of POSS. Addition of POSS leads to the formation of compliant domains, as shown by transmission electron microscopy. Scanning electron microscopy of fractured surfaces shows evidence of extrinsic toughening mechanisms due to POSS inclusion. Tg variation corroborates the existence of soft POSS-POSS domains.