The Role of Fast Polymer Dynamics as Quantified by Inelastic Neutron Scattering on the Impact Strength in Solid Polymers

It is generally understood that there is a link between molecular relaxations in a glassy a polymer and its mechanical toughness. The notion is that these relaxations are important to dissipate the energy of impact and enhance toughness. Decades of research have focused on correlating the mechanical toughness of a polymer with the relaxation processes quantified by techniques such as dynamic mechanical analysis, dielectric spectroscopy, and solid state nuclear magnetic resonance. By correlating the strength and temperature dependence of the relaxations with mechanical performance, the community has tried to developed a primitive understanding of the molecular origins of toughness. However, there is a disconnect because the time and length scale of the molecular relaxations or processes that are invoked to rationalize the toughness are typically several orders of magnitude faster and more localized than the experimental techniques used to characterize the motions, especially in the case of ballistic impact events. We revisit this topic by using inelastic and quasielastic neutron scattering to quantify the polymer relaxations that occur on the ns to ps time scales and show how these fast motions correlate well with impact resistance in a series of different solid polymers.