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 (10 8 to 10 15 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 (10 8 –10 11 s −1 ), moderate (10 11 –10 14 s −1 ), and high (>10 14 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. • All-atom MD with QI technique examines strain rate effects on polymer spallation. • Three strain-rate regimes identified in PI tensile response from 10 8 to 10 15 s −1 . • Spall strength follows a three-segment power-law with a slow-rapid-saturation trend. • Micromechanism evolution: cavitation-crazing → interchain decohesion → chain scission • Velocity criterion log( V D / V T ) predicts deformation and failure transitions.