Damping Characteristics of Polyurea Nanocomposites Over an Extreme Range of Strain Rates*

Polyurea nanocomposites are used as encapsulants and coatings to increase durability, corrosion, or impact resistance of surfaces. Undoped polyurea has exceptional toughness and impact resistance, derived from a phase-separated hard segment/soft segment microstructure and hydrogen bonding. Here, we measure the impact of nanoparticle filler concentration to enhance polyurea dynamic mechanical properties. Surface modification of the nanoparticles is optimized to promote homogeneous particle dispersion, maximum particle volume fraction, and particle-matrix adhesion. Particle-matrix compatibility significantly affects the ultimate tensile stress for the nanocomposites. The mechanical performance of polyurea nanocomposites is contrasted with that of unfilled polyurea via dynamic mechanical analysis, Hopkinson bar, and shock tube testing, demonstrating the ability of polyurea nanocomposites to damp vibrational energies over an extreme range of strain rates (0.1 – 5,000 s^-1).  In shock tube samples, reduction of the total deflection (75%) and deflection rate (40%) of aluminum plates is observed with nanocomposite coatings.  Finally, highest strain rate insults were imposed by a thermomechanical shock in the Sandia National Laboratories SPHINX electron beam and Z-machine X-ray pulsed power facilities, where the damping of nanocomposites was measured to strain rates of 10⁶ s^-1.  Effects of particle size, concentration, and aggregation level are discussed on the ability of the composites to damp shock, as well as the effect of the radiative environments on the polymer microstructure, composite modulus, and chemical changes in the material.  This work is a comprehensive examination of the role of nanoparticles in shock mitigation across deformation timescales for these polymer nanocomposites.