It is generally understood that there is a link between molecular relaxations in glassy polymers 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 because these studies infer a molecular picture for the relaxation mechanism responsible for toughness.  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 these correlations 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. 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.  Furthermore, 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, a regime where most techniques capable of quantifying polymer relaxation spectra are too slow to be relevant.

Role of fast polymer dynamics as quantified by inelastic neutron scattering on the mechanical toughness of polymeric materials

2019 Spring National Meeting

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