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.  It is generally understood there is a link between the relaxations in a glassy polymer and its corresponding toughness. Our understanding is that relaxations dissipate the energy imparted 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 techniques such as dynamic mechanical analysis, dielectric spectroscopy, or solid-state nuclear magnetic resonance.  However, there is a disconnect when it comes to understanding impact resistance at the strain rates of 10⁶ sec^-1 or higher that are relevant for ballistic impact events.  These time scales are typically several orders of magnitude faster than the time scales of the characterization techniques used by this community to quanitfy the relaxation processes. 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.