Effects of curing temperature on the structure and properties of epoxy resin-poly(ε-caprolactam) blends

Abstract

Improving the toughness of epoxy resins (EP) with diverse thermoplastic polymers through curing reaction induced phase separation (RIPS) has been widely studied, with the prerequisite of designing and tailoring the phase morphology evolution of the blend. Herein, two typical amine-type curing agents, 4,4′-diamino diphenylmethane (DDM) with precuring temperature at around 150 °C and diethylenetriamine (DETA) at 80 °C were utilized to regulate and control the phase structures of EP-poly(ε-caprolactone) (PCL) blends. The curing temperature above or below the melting temperature (Tm) of PCL resulted in different states of PCL. The effect of curing temperature on the miscibility between EP matrix and PCL phase, and the correlation between phase morphology evolution and thermomechanical behavior, mechanical properties were comprehensively investigated for the first time. Results revealed that when the curing temperature of EPDDM-PCL blends was higher than the Tm of PCL, curing took place without phase separation; while distinct phase separation took place when EPDETA-PCL was cured at a temperature below the Tm of PCL. When the curing temperature was high, the PCL was able to dissolve completely in the resin before crosslinking began, so that semi-interpenetrating networks were formed; and that when the curing temperature was low, the PCL was unable to dissolve, and therefore formed separate particles. Consequently, the EPDDM-PCL blends exhibited all-leading mechanical properties due to the entanglement and slippage between PCL chains and EP networks. The strength of EPDETA-PCL was reduced but the toughness was improved for island-shaped PCL particles bridging or pinning the cracks during failure. These results elucidated the effect of the phase morphologies formed at different curing temperature on mechanical properties, and provided significant insights regarding the toughening of thermosets.