Nylon 7,10: An odd-even polyamide with a complex melting and a peculiar high temperature polymorphic form

The crystalline structure of nylon 7,10, an odd–even polyamide derived from biobased sebacic acid, has been investigated to elucidate its complex polymorphism and thermal behavior. Solution-crystallized samples display two intense X-ray reflections at 0.439 nm and 0.380 nm, features that challenge a conventional sheet-type arrangement considering the presence of odd diamide units and an extended chain conformation. In contrast, melt-crystallized specimens adopt a well-defined pseudohexagonal structure at room temperature. Upon heating, α-modified crystals evolve toward a pseudohexagonal arrangement through a process involving partial melting, whereas cooling from the melt does not restore the α-modified packing. Close to fusion, an additional monoclinic/triclinic structure emerges, also detectable in the early stages of melt crystallization. Depending on solvent and thermal history, mixed structures are frequently obtained. These findings support the formation of hydrogen-bonded packings in which intermolecular H-bonds can be established in more than one direction. Calorimetric data further reveal a complex behavior, including the appearance of a high-temperature crystalline form that depends strongly on processing conditions. An equilibrium melting temperature of approximately 228 °C was estimated from the evolution of the main melting peak with crystallization temperature. Overall, these results highlight the pronounced structural plasticity of odd–even polyamides and the key role of thermal history in governing their polymorphic behavior. • Nylon 7,10 exhibits a rich polymorphism depending on processing and thermal history • ●A modified α-form stabilized by two hydrogen-bonding directions is identified • ●A high-temperature metastable δ-phase appears repeatedly during thermal cycling • Pseudohexagonal packing develops through rearrangements involving partial melting • ●Processing conditions strongly influence hydrogen-bond topology and lamellar structure