(44c) Influence of Nanosilica on Phase Separation and Toughness in Multi-Phase Polymers

Toughened thermosetting resins are the basis for many commercially important structural adhesive and composite systems that enable lightweight military structures. Many approaches have been employed to improve the fracture behavior of epoxies including the use of elastomers and thermoplastic resins that phase separate during cure. These approaches have yielded order-of-magnitude increases in resin fracture toughness [1]. The phase separation induces energy dissipating mechanisms during fracture such as increased matrix shear yielding and rubber particle cavitation. At the same time, these approaches frequently have deleterious effects on the modulus and glass transition temperature (Tg) of the resins. Thus materials options for designers are limited to materials with either high toughness or elevated use temperature. More recent approaches have attempted to use nanoscale fillers (particulates, layered silicates, aerogels) to achieve increased strength and toughness but have only shown minor or negligible effects on properties. However, very new research has shown that appropriately functionalized silica nanoparticles will produce an unexpected synergistic toughening effect in phase-separated rubber toughened epoxies [2]. These materials have exhibited high fracture toughness while maintaining both Tg and modulus. A similar synergy has been shown for core-shell particle systems [3] and epoxies with decreased crosslink density [1]. Despite the empirical evidence for property improvements, the fundamental mechanisms that produce these unanticipated enhancements are largely unknown.

In this work, we propose to identify the underlying mechanisms that produce the microstructure and property improvements in these systems. A model epoxy system comprising a DGEBF monomer and diethyltouluene diamine curing agent was modified with a carboxyl-terminated acrylonitrile butadiene (CTBN) elastomer and epoxy-functional nanosilica (20 nm). In rubber-modified thermosets, phase separation of elastomer domains is attributed to the change in Gibbs free energy during cure. Prior to cure, the elastomer is soluble in the matrix (0