Bridging Strength and Ductility in Crosslinked Epoxy Networks

By introducing a monofunctional epoxy compound in epoxy-amine structural adhesives, a series of polymer glasses are studied which systematically varies the apparent molecular weight between crosslinks ( M c,a ) from 500 to 1800 g/mol while maintaining a glass transition temperature ( T g ) above 110 °C. Young’s modulus and yield strength, measured in uniaxial tension, increased with increasing M c,a breaking from common trends in highly crosslinked polymers. Positron annihilation spectroscopy elucidated a local densification of the relative free volume with increasing monofunctional epoxy content, providing a plausible origin to the increase in tensile properties. While fracture toughness ( K IC ) remained relatively constant across the series, the high-rate impact performance increased as a function of tensile strength achieving a 10% improvement in impact resistance. Overall, this study highlights the sensitivity of macroscale, mechanical performance on polymer topology, and local density within the polymer glass.