(79a) Thermomechanical Properties of Polyurea Nanocomposites over Extreme Strain Rates

Polyurea is commonly used as an encapsulant, a coating, and/or a binder to increase durability, corrosion, or impact resistance of surfaces. Unlike materials with similar chemistries, such as polyurethane, polyurea has a phase-separated microstructure consisting of hard-segments that are dispersed throughout a soft-segment matrix. This phase separation and hydrogen bonding in the material results in exceptional impact resistance and toughness, even at high strain rates and high strain levels. Here, we demonstrate enhanced thermal and mechanical properties of polyurea through the incorporation of nanoparticles. Surface modification of the nanoparticles was 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 dispersibility of particles is measured via rheology while the phase segregated microstructure and particle segregation is observed via atomic force microscopy. The mechanical performance of polyurea nanocomposites is compared to that of unfilled polyurea over an extreme range of strain rates (0.1-10,000 Hz) via dynamic mechanical analysis, Hopkinson bar, and shock tube testing, demonstrating the ability of polyurea nanocomposites to damp vibrational energies over a wide range of rates. Finally, highest strain rate insults were imposed in the Sandia National Laboratories SPHINX electron beam and Z-machine X-ray radiation pulsed power facilities, where the damping of mechanical energies of polyurea was measured to strain rates of 10⁶ s^-1 by imposing a thermomechanical shock in the material. Assessment of the damage to the polyurea as a result of electron beam radiation shows an exceptional resistance of the nanocomposites to radiation and thermal damage.

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