(654c) Nanoparticle Migration in Fiber-Reinforced Polymeric Composites

The incorporation of nanoparticles to the fiber-matrix interphase has been previously shown to improve the mechanical properties of fiber-reinforced polymeric composite systems, as measured by the interfacial sheer strength and tensile strength. Most methods for incorporating nanoparticles to the fiber-matrix interphase rely on a combination of electrostatic attraction, covalent silane coupling chemistries, or nucleation and growth directly on fiber surfaces. All of these methods introduce several additional fiber processing steps, complicating their manufacture. Here we present an alternative method for directing nanoparticles to fiber surfaces by incorporating a thermoplastic additive, referred to as migrating agent, directly into the resin formulation. Depending on the fiber, resin, and nanoparticle chemistry, as well as the identity of the migrating agent, nanoparticles are found to be concentrated in the fiber-matrix interphase, post-cure, upon addition of the migrating agent. The migrating agent phenomenon is investigated using a combination of several microscopy techniques. Scanning electron microscopy (SEM) and atomic force microscopy (AFM) were used to characterize the extent of nanoparticle migration in many systems by examining mode II fracture surfaces and polished composite cross sections. Results from SEM and AFM indicate that the surface chemistry of the nanoparticle and fiber, as well as migrating agent concentration are critical factors for successful nanoparticle migration. Fluorescent microscopy, using fluorescent core-shell silica particles prepared using a modified Stöber process, was also used to monitor the relative concentrations of nanoparticles throughout the composite during the cure process. Result from fluorescence microscopy indicate that at low temperatures the nanoparticles do not migrate to the fiber surface, likely due to viscosity limitations of the resin. Above a threshold temperature, particles can be seen to preferentially accumulate near fiber surfaces. While the mechanism for particle migration is still uncertain at this time, it is believed to be either a particle induced phase separation in which the particles and fibers are preferentially wet by a phase rich in the migrating agent, or an osmotically driven depletion interaction induced by excess migrating agent excluded near the vicinity of the fibers and particles. Some predictive methods have been employed to predict the strength of depletion interactions by considering the osmotic second virial coefficient of the migrating agent dissolved in the epoxy resin. A large positive second virial coefficient would result in a higher affinity for nanoparticles to deposit on fiber surfaces, and therefore would be indicative of a good migrating agent.