Effect of free-volume holes on static mechanical properties of epoxy resins studied by positron annihilation and PVT experiments

The tensile, flexural, and fracture toughness properties of seven chemically different amine-cured epoxy resins were studied. Positron annihilation lifetime and pressure-volume-temperature (PVT) experiments were performed on each epoxy resin to characterize the average size and fraction, respectively, of free-volume holes. A negative correlation between hole fraction and hole size was revealed for these chemically different epoxy resins. Better tensile and flexural mechanical properties (higher tensile modulus and lower tensile strain at break; higher flexural modulus, higher flexural strength, and lower flexural strain at break) were clearly observed for epoxy resins with smaller hole size and higher hole fraction. However, no clear relationship between fracture toughness and hole properties was found. The correlations between (tensile and flexural) static mechanical properties and hole properties for chemically different epoxy resins should provide guidance for further improvements in the mechanical properties of carbon-fiber-reinforced polymers.