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Deformation-induced glass transition in segmented elastomers has been proposed to allow highly desirable enhanced energy dissipation. In this study, we investigate the temperature-dependent microscale impact response of polyurea at a fixed impact velocity. We observe a local elevated impact energy absorption around 115 °C, which is attributed to the glass-to-rubber transition temperature under the present high-rate dynamic loading. Dielectric spectroscopy was performed, and the soft-segmental α2-relaxation was extracted and fit with a Havriliak-Negami function. The α2-relaxation frequency at 115 °C correlates well with an order-of-magnitude estimate of the equivalent frequency of deformation. This work further supports the importance of the dynamical Tg as an important consideration in the design of impact resistant materials., Army Research Office and Army Research Laboratory (Contract W911NF-18-2-0048)

This study investigates the molecular dependencies of dynamic stiffening and strengthening through comparison of high strain rate impact responses of various polyurethanes and polyureas. We use an in-house designed tabletop microimpact experimental platform—the laser-induced particle impact test—to perform high strain rate impacts and measure the corresponding material response. Dynamic mechanical analysis and differential scanning calorimetry are used to show that glass transition temperature is a useful predictor of the impact response at ambient temperatures. Meanwhile, solid-state nuclear magnetic resonance spectroscopy identifies segmental dynamics as an important determinant of the variation in both dynamic stiffening and strengthening. The impact responses of polyurethanes and polyureas both show clear dependencies on the molecular weight of the soft segment. This comparison suggests the state of intermolecular hydrogen bonding plays a key role in dynamic stiffening and strengthening. This study aims to identify the molecular dependencies of the impact response and establish a foundation for further design and testing of optimal high strain rate characteristics in synthetic elastomers. Keywords: materials analysis; impact testing; lasers; microscopy; velocity measurement; nuclear magnetic resonance; spectroscopy; polymers; mechanical testing

The laser photoacoustic technique is used to generate and detect picosecond coherent acoustic vibrations in gold film media deposited on Si substrates. As a consequence of the gold crystallites’ canted orientation, the pump-probe picosecond transient reflectivity shows oscillations at the fundamental shear mode frequency. The shear character of the mode is suggested by its dispersion, by the dependence of the signal on the probe laser wavelength, and by x-ray texture analysis.