Effects of Hyperbranched Polyethylenimine on the Properties of Commercial Epoxy Resins for Composite Applications

Hyperbranched polymers are commonly used as surface and cure modifiers, along with nanoparticles, of epoxy resins used in medical and infrastructure applications; however, their impact on structure and performance of the epoxy resins is not fully understood. Here, a commercial epoxy-aromatic amine resin (EP-AA) system is blended with off-stoichiometric amounts of hyperbranched amine (HA), and the cure kinetics, structure, polymer dynamics, and mechanical properties of the different compositions are evaluated. Cure kinetic studies indicate that the curing of the hyperbranched amine is nearly complete prior to that of the aromatic amines. At its highest concentration (1% by mass), the hyperbranched amine is found to autocatalyze the epoxy and the aromatic amine cure, but the bulky HA groups slow the diffusion kinetics during the final stages of the cure process. This results in a heterogeneous, cocontinuous network structure consisting of unreacted secondary amines that increase the side-group/pendant contribution and increase the free volume within the polymer network. The restricted cooperative segmental mobility of the polymer network in the 1% by mass HA blend results in poor energy dissipative properties, as evidenced by reduced strength and strain to failure.