Why Enhanced Sub-Nanosecond Relaxations are Important for Toughness in Polymer Glasses

It is understood that there is a link between molecular relaxations in glassy polymers and mechanical toughness. The notion is that these relaxations dissipate the energy of 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 because the time and length scale of the molecular mechanisms are typically several orders of magnitude faster and more localized than the experimental techniques used to characterize them. We revisit this by using quasielastic neutron scattering (QENS) to quantify both the collective excitation and molecular relaxations that occur on the time scale of ps to ns, and see how these motions correlate with mechanical toughness. We demonstrate a strong correlation between these fast polymer relaxations and toughness. We show that these fast polymer relaxations play a critical role in predicting the ability of a material mitigate impact under ballistic conditions where the strain rates can approach an inverse microsecond.