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The hybrid assembling of thermoplastics with thermosets through controlled interdiffusion is an attractive approach towards integrated structural composite bonding. The fracture toughness of the interphase between the high Tg thermoplastic polyetherimide and the RTM6 epoxy is found to be four times higher than the corresponding RTM6 value under specific curing cycle. The origin of this remarkable cracking resistance is elucidated by analyzing the crack path in relationship with the morphology and local elastoplastic and fracture properties of the interphase. The crack is trapped inside a morphological gradient involving a relatively low PEI concentration. Uniaxial compression and nanoindentation tests on homogenous blends exhibit an unexpected softening in this PEI content window. This efficient crack trapping mechanism originates thus from a synergistic coupling between a lower local strength and higher local fracture toughness. This finding opens to many new options for interface toughening beyond the system addressed in this research.

A new block copolymer-thermoplastic dual toughening concept for high performance RTM6 epoxy matrix composites is elaborated through a comprehensive microstructural study of well-defined model systems. A blend of phenoxy and MAM (copolymer of poly(methyl methacrylate) and poly(butyl acrylate)) increases the toughness of the corresponding carbon fiber reinforced composite by 125%, a five-fold synergistic improvement over the situation where the tougheners are used alone at the same total concentration. Selective intercalation of blended toughener thin films in the preform gives rise to a diffusion-induced composition gradient during the RTM process, which generates a morphology gradient upon curing. The observed microstructures results from MAM self-assembly in the miscible mixture of phenoxy and the epoxy resin precursor, followed by reaction induced phase separation, forming wormlike micelles, interconnected micelles and co-continuous morphologies. The resulting microstructures activate different toughening mechanisms responsible for the observed synergy.

The mechanical properties of an unmodified reference composite panel based on a highly crosslinked epoxy resin are compared with several corresponding toughened versions, all containing the same amount of representative hard or soft tougheners. The panels are prepared and tested under identical conditions. In terms of tougheners, soft MAM block copolymers and CTBN rubber are compared with two high glass transition temperature (Tg) amorphous thermoplastics, PES and phenoxy. The high Tg thermoplastic tougheners yield considerable improvements of the composites mechanical properties, especially the interlaminar fracture toughness, as compared to the reference panel. On the contrary, MAM and CTBN modification are much less effective. The analysis of the morphology strongly suggests that effective toughening of composite panels based on densely crosslinked epoxy resins requires the combination of at least two key factors: (i) a fine dispersion of the toughener and (ii) strong matrix-toughener and matrix-carbon fibers interfaces. The study provides valuable insights for the choice of suitable tougheners for high performance epoxy systems, based on the link with the physical toughening mechanisms.

The use of phenoxy nanocomposite films as carriers of nanofillers involving multiwalled carbon nanotubes and nanoclays is successfully demonstrated for application in epoxy carbon fibers reinforced composites (CFRC) processed by RTM. Model studies on individual nanocomposite filaments embedded in epoxy precursors show that the nanofillers are passively transported by the interdiffusion gradient during heating over distance around 800 lm. A morphology gradient is generated after reaction induced phase separation and the nanofillers end up in the epoxy, despite their initial dispersion in the phenoxy. The proof of concept is extended to CFRC panels where nanocomposite phenoxy films are prepositioned between every odd carbon layer of the preform. Carbon nanotubes are filtered by the carbon fabrics, which limits their full diffusion and that of phenoxy through the preform. This has negative consequences on fracture toughness (GIc). For nanoclay, GIc is rather slightly improved although the origin is not fully clear.

We explore the influence of the interdiffusion of two thermoplastic tougheners and RTM6 epoxy resin precursors on the resulting morphologies after curing and the consequences on delamination toughness of the corresponding carbon fiber reinforced composites. Two thermoplastics with contrasting Tg and compatibility, i.e. poly(ether sulfone) (PES) and phenoxy are compared. The dramatic improvement of the interlaminar fracture toughness (GIC) found for the phenoxy-RTM6 system as compared to the pure thermoset reference can be ascribed to the broad morphology gradient only observed for that system. By contrast, the PES-RTM6 system is characterized by a steep morphology gradient and a corresponding decrease of as compared to the reference.