Tailoring Interphase Architecture for Toughened Epoxy Systems Using Multilayer Poly(ether imide) Films

Abstract This study presents a systematic investigation into tailoring interphase architecture in hierarchically toughened epoxy systems using multilayer poly(ether imide) (PEI) films as ductile interlayers. Processing parameters, including PEI film thickness (60 and 120 μm), volume fraction (20–50%), and cure cycle (first dwell temperatures of 120–180 °C), were varied to optimize fracture toughness. Single-edge notched bending (SENB) tests, coupled with microscopic analyses, revealed that fracture toughness is governed by the interplay between interphase morphology and the relative thickness of the PEI layer compared to the plastic zone size. Microscopic analysis showed that low cure temperatures reduce gradient interphase formation, leading to weak interfaces, enabling high energy dissipation, while higher temperatures yield strong interfaces with reduced crack tortuosity and hence brittle fracture. A critical crack tortuosity threshold (∼1.8) was identified, beyond which crack deflection and delamination dominate over microstructural mechanisms. Interphase thickness and particle size distribution were governed by diffusion-driven phase separation. In thinner epoxy cavities, PEI saturation occurred quickly, reducing the diffusion gradient and limiting interphase growth, in agreement with predictions from Fick’s law. These findings demonstrate that fracture toughness in PEI–epoxy systems is controlled by the interplay of intrinsic mechanisms (plastic deformation, particle debonding) and geometric mechanisms (crack deflection, tortuosity). Controlling both through process design enables toughness improvements beyond that of pure PEI, providing a framework for developing next-generation damage-tolerant materials.