Ordered and amorphous phases of polyacrylonitrile: Effect of tensile deformation of structure on relaxation and glass transition

Calorimetry and mechanical tests on polyacrylonitrile PAN show two glass transitions (Tg). The lower Tg (370 K) has been attributed to its ordered phase with the amorphous regions showing a glass transition at 420 K. An understanding of these processes at the molecular level is lacking as is the counterintuitive lower Tg for the ordered phase as compared to the amorphous one. Molecular dynamics (MD) simulations of the amorphous and ordered PAN phases result in Tg in the range 445–455 K and 450–465 K, respectively. The amorphous value is in good agreement with the experimental result once rate effects are considered. However, MD predicts a slightly higher Tg for the ordered phase; this is as expected but in disagreement with the experimental observation. To explain this discrepancy, we built a large-scale amorphous PAN sample and mechanically strained it above its Tg to achieve a structure consistent with experiments, including the coexistence of amorphous and ordered regions. The resulting ordered phase shows the hexagonal pattern seen experimentally and, interestingly, these domains are surrounded by nanovoids caused by the large local deformations and molecular alignment. We characterized the backbone torsional mobility of the two phases in the deformed structures as a function of temperature and found that the amorphous region exhibits similar relaxation dynamics as compared with homogeneous samples. However, the ordered phase has significantly faster relaxation dynamics which can explain the lower experimental Tg. [Display omitted] •Study the origin of the two glass transition temperatures (Tg's) of PAN using molecular dynamics simulations.•Apply a large-scale deformation on amorphous PAN structure to achieve a realistic structure.•Ordered region in the deformed structure surrounded by nanovoids is the origin of the long-debated lower Tg.