This work describes the melting and polymorphic behavior of poly(decamethylene terephthalamide) (PA 10T). Both solution-crystallized (SC) and melt-crystallized (MC) PA 10T show double melting endotherms in DSC. The SC crystal form melts at 260-300 degrees C giving the first melting endotherm, and meanwhile undergoes a polymorphic transition forming the MC crystal form. The subsequent melting of the MC crystal form gives the second melting endotherm at 300-325 degrees C. This irreversible polymorphic transition is confirmed by variable-temperature WAXD and IR. Dynamic mechanical thermal analysis (DMTA) shows a glass transition temperature (T-g) at 127 degrees C and the presence of an alpha ' transition at 203 degrees C (0.1 and 1 Hz). This transition could be confirmed by DSC and variable-temperature WAXD experiments. The alpha ' transition correlates with a reversible thermal process and a sudden change in intersheet spacing. (c) 2019 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2019, 57, 465-472