“Polymers for impact mitigation: New measurements provide insights into an old problem”
Polymers are widely used in protective equipment or structures where the objective is to absorb or dissipate the energy from a mechanical impact. Examples include foams or gels incorporated into helmets, shear thickening particle suspensions for stab resistant body armor, high strength fibers woven into ballistic resistant fabrics, and tough glassy polymers in structural armor for military and law enforcement vehicles. It is generally understood that there is a link between the molecular relaxations in a glassy polymer and its mechanical toughness. The notion is that these relaxations dissipate the energy imparted to the material during impact and thereby enhance toughness. Decades of research have focused on correlating the mechanical toughness of a polymer with the relaxation processes quantified by relatively slow characterization techniques such as dynamic mechanical analysis, dielectric spectroscopy, or solid-state nuclear magnetic resonance. However, there is a disconnect when it comes to understanding relaxations and impact resistance at the strain rates of 106 sec-1 or higher that are relevant for ballistic impact events. The time and length scale of the molecular mechanisms that are relevant for ballistic conditions are typically several orders of magnitude faster and more localized than the typical experimental techniques used by this community. We revisit these correlations between toughness and polymer relaxations by using quasielastic neutron scattering (QENS) to quantify the collective excitations and molecular relaxations that occur on the time scale of pico- to nanoseconds, and then see how these polymer motions correlate with mechanical toughness. What emerges is a strong correlation between the ratio of the population of the fast relaxations (dissipative) to the fast collective (many atom) vibrations deep in the glassy state and the mechanical toughness. We interpret this ratio as the number of successful relaxations per the vibrational attempt frequency and show a strong correlation between the number of successful relaxations and toughness. To explore these connections in greater detail, we are also developing a laser induced projectile impact testing (LIPIT) technique as a novel way to perform ballistic impact tests using micron scale particles at strain rates that exceed an inverse microsecond, a regime where most techniques capable of quantifying polymer relaxation spectra are too slow to be relevant.