Cure behavior of nanostructured hierarchical composites with functionalized carbon nanotubes

Electrophoretic deposition is a promising technique to hybridize nanomaterials with conventional reinforcing materials for multifunctional applications. It utilizes the principle of electrophoresis, where electric potential drives charged particles dispersed in a liquid towards counter-charged substrates. Polymer matrix can be infused into the hybridized fibers to produce hierarchical polymer composites with reinforcements spanning several orders of magnitude in scale. This research addresses a key challenge associated with nanostructured composites produced by first dispersing the nanoscale reinforcement into the polymer matrix and then infusing into the fiber reinforcement (direct mix/infusion method). The key limitation is on the volume fraction of the nanoscale particles due to the drastic increase of the resin viscosity and the potential filtering effect of the particles during resin infusion. Our model system consists of an aqueous dispersion of carbon nanotubes (CNT) functionalized with a cationic polymer, polyethyleneimine (PEI), non-conductive glass fabric and epoxy resin. Amine functional groups of PEI are protonated under mildly acidic conditions, producing positively charged CNTs. A stable dispersion is formed through repulsive electrostatic forces among charged CNTs, which also facilitates deposition under applied electric fields. CNT-PEI films uniformly deposited via EPD on each filament throughout the fabric form a unique interface between reinforcing fiber and epoxy matrix. Concentrated amines from CNT-PEI coatings possibly alter the curing mechanism of infused epoxy resin, thereby creating the graded mechanical properties at the interface. In this study, curing kinetics and thermomechanical properties of epoxy resin are investigated with added PEI which provides stoichiometrically excessive amines. It is expected that the curing temperature profile can be designed to optimize the interfacial properties of electrophoretically processed CNT-PEI multiscale composites.