Why Polymer Glasses with Enhanced Sub-nanosecond Relaxation are Tougher

Published

February 12, 2021

Author(s)

Christopher Soles, Adam B. Burns, Kanae N. Ito, Jack F. Douglas, Robert Dimeo, Madhusudan (Madhu) Tyagi, Jingsheng Wu, Albert F. Yee, Yueh-Ting Shih, Liping Huang

Abstract

This manuscript explores the connection between the fast relaxation processes in a polymer glass and essential trends in the mechanical toughness, a non-linear mechanical property that is of practical interest for engineering polymers with high impact strength. In particular, we quantify the timescale of the molecular relaxations in the sub-nanosecond regime for a polycarbonate glass using inelastic and quasielastic neutron scattering and then correlate these processes with the macroscopic brittle-to-ductile transition (BDT), which demarcates a change in the dominant mechanism of failure and a marked increase in material toughness. We show that the macroscopic phenomenon of the BDT corresponds to a change in the dominant dynamical process at the nanoscale. The brittle regime is characterized by vibrational modes (the so-called Boson peak) with a characteristic time scale of τ ≈ 5 ps, while slower relaxational processes with τ ≈ 20 ps become dominant above the BDT. We further establish that the onset of ductility coincides with the appearance of anharmonicity in the mean square atomic displacement , emphasizing that fast anharmonic molecular motions are important for energy dissipation. Our work builds on a previous report correlating toughness with the amplitude of these anharmonic fluctuations across a wide range of polycarbonate glasses.1 Brillioun light scattering measurements were performed to characterize the bulk and shear moduli, revealing a concomitant upturn in Poisson's ratio in the region of the DBT, a phenomenon which has been reported in metallic and oxide glasses. The ratio of transverse acoustic mode velocity and the Boson peak frequency is used to estimate the length scale for these fast-dynamic processes, indicating the dynamic heterogeneities are collective across 100s to 1000s of atoms. These length scales are found to be comparable to the activation volume of yield derived from mechanical measurements, providing evidence that fast col

Citation

Macromolecules

Pub Type

Journals

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Keywords

polymers, dynamics, relaxations, toughness, neutrons, quasielastic neutron scattering, impact strength

Citation

Soles, C. , Burns, A. , Ito, K. , Douglas, J. , Dimeo, R. , Tyagi, M. , Wu, J. , Yee, A. , Shih, Y. and Huang, L. (2021), Why Polymer Glasses with Enhanced Sub-nanosecond Relaxation are Tougher, Macromolecules, [online], https://doi.org/10.1021/acs.macromol.0c02574 (Accessed April 25, 2024)