It is generally understood that there is a link between molecular relaxations in the glassy state of a polymeric material and mechanical toughness. The notion is that these relaxation processes are important to dissipate the energy of impact and thereby enhance toughness. In fact, 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 relaxation processes with mechanical performance, the community has tried to develop a primitive understanding of the molecular origins of toughness. However, there is a disconnect here 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. In this presentation, we revisit this topic by using inelastic and quasielastic neutron scattering to quantify both the collective excitation and molecular relaxations that occur on the time scale of ps to ns, and length scales from angstroms to nanometers, and see how these motions correlate with mechanical toughness. These measurements are done on a series of polycarbonates where the molecular mobility is systematically varied by either blending in a diluent that stiffens the material in the glassy state (antiplasticizer) or inserting cyclohexyl linkages into the main chain that facilitate mobility. This reveals a positive correlation between the amplitude of the molecular mobility in the glassy state and the ability of the material to dissipate energy under quasi-static impact testing.

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

256th National Meeting (2018)

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