Particle toughening for improving fracture toughness

A curable prepreg ply comprising:

• a layer of resin-impregnated reinforcement fibers having a top surface and a bottom surface;
• two resin films applied to the top surface and the bottom surface of said layer, respectively,
• wherein said layer of resin-impregnated reinforcement fibers comprises reinforcement fibers impregnated with a first curable resin matrix, and said resin films comprises a second curable resin matrix,
• said second curable resin matrix comprises at least one thermoset resin, and a mixture of two different types of particles:
• (i) insoluble toughening particles, and (ii) partially soluble toughening particles, and
• said first curable resin matrix comprises at least one thermoset resin, but is void of said insoluble toughening particles and said partially soluble or swellable toughening particles, and
• wherein said insoluble toughening particles are insoluble in the second resin matrix upon curing of the prepreg ply, and said partially soluble toughening particles are thermoplastic particles that decrease in volume by more than 5% upon curing of the prepreg ply, but remain as discreet particles after curing.