Velocity criterion for rate-dependent deformation and failure mechanisms in glassy polymers under quasi-isentropic uniaxial strain tension

Glassy polymers, owing to their sophisticated molecular structures and chain aggregation states, can undergo multiple micromechanisms in response to increasing strain-rate loading. Employing the quasi-isentropic (QI) technique, we systematically investigate the dynamic tensile behavior and the underlying molecular mechanisms of pyromellitic dianhydride (PMDA)/4,4′-oxidianiline (ODA) polyimide (PI) across seven orders of magnitude in strain rate (10⁸ to 10¹⁵ s⁻¹) through large-scale all-atom molecular dynamics simulations. Our results reveal three distinct strain-rate-dependent regimes in the dynamic tensile response of PI: low (10⁸–10¹¹ s⁻¹), moderate (10¹¹–10¹⁴ s⁻¹), and high (>10¹⁴ s⁻¹) strain rates. The predicted maximum sustained tensile stress, being consistent with experimental data, follows a three-segment power-law relationship with strain rate, exhibiting a slow-rapid-saturation increase trend. Concurrently, the mechanisms of deformation and failure transition from cavitation-crazing at low strain rates to uniform deformation-interchain decohesion at moderate rates, and ultimately to chain scission at high strain rates. Our detailed analysis indicates that the mismatch between deformation velocity (V_D) and thermal velocity (V_T) of polymer atoms is critical in governing the dynamic tensile behavior. Based on these results, we propose an analytical velocity criterion—specifically, the logarithmic ratio of log(V_D/V_T)—that quantitatively predicts the transition among the three distinct regimes. This study provides significant theoretical insights and molecular-level understanding of the rate-dependent tensile behavior of glassy polymers, offering a foundation for the design and development of advanced polymers capable of withstanding extreme loading conditions.