Title: Role of relaxations in material’s mechanical toughness

Abstract

The mechanical toughness in materials is defined as the amount of energy that a material can absorb per unit volume without experiencing a catastrophic failure. Despite the societal importance of tough engineering thermoplastics, it is currently not possible to predict why one material might display a tough mechanical response while another material might be brittle and breaks. In this work, we look at the origin behind mechanical toughness in a variety of polycarbonate-based systems in detail by Inelastic and Quasi-elastic Neutron Scattering (QENS) and were able to develop a framework that quickly correlates macroscopic mechanical toughness to the nanosecond dynamics. We find striking correlations between the fast relaxations in the quiescent glass and essential trends in mechanical toughness. The onset of macroscopic ductility (accompanied by a substantial increase in toughness), demarcated by the brittle-to-ductile transition (BDT), correlates with the onset of anharmonic motions in the mean-squared atomic displacement, (u2), on time scales faster than 1 ns. This finding1 emphasizes the role of anharmonic motions in dissipating energy in the glassy state. Poisson’s ratio, characterized by Brillouin light scattering, shows an upturn in the same temperature region. Further investigation2 into the full inelastic neutron scattering spectra reveals two nanoscopic processes: (1) collective vibrational modes (the so-called Boson peak) with a characteristic time scale τ ≈ 0.5–0.8 ps and (2) collective relaxations with τ ≈ 3 ps. We show that the macroscopic phenomenon of the BDT corresponds to a change in the dominant nanoscale process from vibration-dominated to relaxation-dominated dynamics. This work clearly shows relaxation processes are how tough materials dissipate the energy and shows a way forward to tune the mechanical toughness in polymer glasses.

Biography

Madhusudan Tyagi works at the National Institute of Standards and Technology (NIST) and the Department of Materials Science and Engineering at the University of Maryland. He has also been working as an instrument scientist for the High Flux Backscattering spectrometer (HFBS) for the last 15 years. His research interests include the dynamics and structure of liquids, polymers, and biomolecules.