Atomic Insight into Multilevel Microstructure Evolution and Energy Variation Mechanisms of Polyimide under Shock Compression

Polyimide (PI) stands as a foundational material in the aerospace industry and high-energy-density science, and its shock response under extreme conditions constitutes an increasing focus within the academic community. However, given the sophisticated molecular structure and stacking state of chains, the understanding of how PI mobilizes multiple micromechanisms in response to external shock loading remains incomplete. To fill this knowledge gap, aided by the multiscale shock technique (MSST), the dynamic behaviors of pyromellitic dianhydride (PMDA)/4,4′-oxidianiline (ODA) PI over shock pressure range of 0-20 GPa are systematically studied through all-atom molecular dynamics (MD) simulations. The multilevel microstructures evolution (spanning from the covalent bond to molecular chain and to aggregation structures) and the diverse energy variation mechanisms are comprehensively analyzed. The results indicate a favorable agreement between the MD-calculated Hugoniot relations of PI and the experimental data in literature. The shock response of PI reveals two distinct stages: low shock strength (0 < P < 5 GPa, I) and high shock strength (5 < P < 20 GPa, II). In the low shock strength stage, diverse energy mechanisms engage in competitive interactions, with their sensitivity to shock pressure determined by the interaction intensities, wherein the relatively weak interchain energy emerges as the most active mechanism. In the high shock strength stage, energy mechanisms transform to coordinate with each other, maintaining consistent contributions to the total energy at a specified proportion. Microstructure analysis identifies the linkage between PMDA and ODA, as well as the ether linkage in ODA, as the most changeable bonded sites. Correspondingly, the C_p-N, C_p-O-C_p, and C_p-C_p-O_c-C_p are the most changed bond length, bond angle, and dihedral angle, respectively. The rearrangement of rigid segments between the ether linkages is the primary reason for chain conformation, resulting in the topological structure of the entire molecular chain remaining constant, exhibiting self-similarity under varying shock pressure. Concurrently, X-ray diffraction (XRD) examinations on aggregation structure reveal a 42.6% reduction in d-spacing, indicating that the contraction of free volume is responsible for the observed shock compressibility of PI materials. This study offers insights into the molecular-level mechanisms of shocked polymers, which may serve as a blueprint for the development of advanced polymers in extreme conditions.