Fracture toughness and bonding strengths of epoxy composites containing POSS-PCLn: An impact of phase separation from macro-to nano-scales

Epoxy thermosets have been widely used in engineering applications due to their excellent thermal stability and chemical resistance. However, their inherent brittleness causes low bonding strength. Therefore, improvement of toughness is required. By using a variety of modifiers, epoxy thermosets can be toughened via the generation of fine phase-separated morphologies. In this work, we reported the toughening of epoxy thermosets with polyhedral oligomeric silsesquioxane (POSS)-capped poly(ε-caprolactone)s (PCL) . By taking control of types of R groups ( i.e., isobutyl and/or isooctyl) of POSS cages and the lengths of PCL chains, the epoxy thermosets were regulated with the morphologies from nano-to macro-scales. It was found that the improvement of toughness was quite dependent on types of R groups of POSS cages. Under the identical condition, the thermosets contain isooctyl POSS-PCL n had the fracture toughened larger than those containing isobutyl POSS groups. For the thermoset containing 15 wt% of isooctyl POSS-PCL 1.00 , the critical stress intensity factor ( K IC ) was enhanced from 0.70 to 1.69 MPa m 1/2  at room temperature and from 1.11 to 2.51 MPa m 1/2  at low temperature and the thermal stability remained invariant. Furthermore, the bonding strength on stainless steel and anodized aluminum substrates were significantly improved with the incorporation of 15 wt% of isooctyl POSS-PCL 1.00 . These findings provide a novel approach to regulate phase structure in epoxy thermosets, offering the insights into structure-performance relationships, thus opening up a new way for designing epoxy resins with high toughness and adhesive strength.