Controlling Dual Scale Morphologies Of Epoxy And Poly(Etherimide) Towards Improved Interlayer Toughening Of Composites

A common approach to toughen epoxy matrix systems is to dissolve a thermoplastic phase, in the uncured thermosetting monomers. These can interdiffuse within the thermoplastic, followed by reactioninduced phase separation, leading to intricate graded morphologies with a high fracture toughness. Here, we first architect the thermoplastic phase, made of poly(etherimide) films as a macroscopic layered scaffold, and infiltrate it by an epoxy system, leading to dual-scale morphologies with distinct spatial control of morphological feature at the microscopic and macroscopic scale. The fracture toughness of the modified epoxy system is investigated as a function of varying cure temperature (120–200 °C) for interphase formation and poly(etherimide) film thickness (50–120 μm). Results show that the fracture toughness of the heterogeneous system is mainly controlled by the macroscopic feature, the final PEI layer thickness. Remarkably, as the PEI layer thickness exceeds the plastic zone around the crack tip, around 60 μm, the fracture toughness of the dual scale morphology surpasses the property of bulk PEI. Additionally, decreasing the gradient microscale interphase morphology triggers higher crack tortuosity, which seems to be the dominating mechanism for the synergistic toughening. Ultimately, this knowledge will lead to novel toughening approaches to increase the damage tolerance of fibre reinforced composites by suppressing delamination damage modes.