Why Enhanced Subnanosecond Relaxations Are Important for Toughness in Polymer Glasses

Abstract

This manuscript explores the connection between the fast relaxation processes in an undeformed polymer glass and essential trends in the mechanical toughness, a nonlinear mechanical property that is of practical interest for engineering polymers with high impact strength. We quantify the time scale of the molecular relaxations in the subnanosecond regime for a quiescent polycarbonate glass using inelastic and quasi-elastic 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 the 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 collective vibrational modes (the so-called Boson peak) with a characteristic time scale τ ≈ 0.5–0.8 ps, while slower collective relaxations with τ ≈ 3 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 ⟨u 2⟩ on a picosecond time scale, emphasizing that fast anharmonic molecular motions are important for energy dissipation. This builds upon our previous report correlating toughness with the amplitude of these anharmonic fluctuations across a wide range of polycarbonate glasses. Brillouin light scattering measurements are used to characterize the bulk and shear moduli of the material, revealing a concomitant upturn in Poisson’s ratio in the region of the BDT, a phenomenon that 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 processes, indicating that the dynamic heterogeneities are collective across 100–1000s of atoms. These length scales are strikingly similar to the activation volume of yield derived from mechanical measurements, suggesting that these fast and collective relaxation processes may be related to the mechanisms of yield.

Supersonic Impact Response of Polymer Thin Films via Large-Scale Atomistic Simulations

“…Since the MFC and ρ e are different between the two polymers, the raw values of MSD are not a good metric to compare the relaxation processes of the two polymers; however, MSD is a good metric to illustrate enhanced chain mobility in response to different impact conditions. Notably, the MSD for both polymers increases with increasing V i (and E p *), indicating a connection between chain mobility and toughness that has been recently observed experimentally. , Due to a high ρ e , PE initially has a low MSD that continuously increases with V i . PS has an initial increase in MSD up to 1.5 km s –1 , after which an elbow in the data is observed and the MSD increases at a slower rate, suggesting that as the temperature increases with increasing V i , the MFC is reduced and chain mobility is enhanced.…”

“…Notably, the MSD for both polymers increases with increasing V i (and E p *), indicating a connection between chain mobility and toughness that has been recently observed experimentally. 27,28 Due to a high ρ e , PE initially has a low MSD that continuously increases with V i . PS has an initial increase in MSD up to 1.5 km s −1 , after which an elbow in the data is observed and the MSD increases at a slower rate, suggesting that as the temperature increases with increasing V i , the MFC is reduced and chain mobility is enhanced.…”

High Strain Rate Failure Behavior of Polycarbonate Plates due to Hypervelocity Impact

“…If τ e were to become greater than the impact event, then relaxations would not engage the entanglements, and the influence of ν e during HVI would not be significant. Even if the entanglements play a role in HVI (although small), the literature reports have shown that polycarbonate displays a fast collective relaxation process, ∼10 –12 s involving a few monomeric segments . HVI testing at different molecular weights, particularly at low molecular weights, would be needed to validate if ν e has any significant contribution.…”

Systematic coarse-graining of epoxy resins with machine learning-informed energy renormalization

“…Flexibility introduced by D400 increases mobility and reduces density as well as mechanical properties of the DGEBA+D400 system 53 . A quantitative comparison of 〈𝑢 〉 between simulations and future experiments should be done with caution, since in experiments 〈𝑢 〉 is extracted from the neutron scattering intensity 60 , can depend on the scattering wavelength Q and the very definition of Debye-Waller Factor includes the whole exponential term 𝐷𝑊𝐹 = 𝑒𝑥𝑝(− 〈 〉 ), while it is customary for molecular simulation studies to use the term 𝐷𝑊𝐹 as a definition of the 〈𝑢 〉 value extracted from MSD functions 61 .…”