Odd–Even and Even–Odd Copolyamides PA(56-co-65): Synthesis, Crystallization, and Properties

Random odd–even and even–odd copolyamides PA(56-co-65) were synthesized by one-pot polycondensation. The crystallization kinetics, crystalline structure, and morphologies, as well as the phase transition of the copolyamides, were investigated. The decrease in T_c and T_m with the increasing content of PA65 segments in copolymers indicates an isodimorphic crystallization behavior. The results obtained from DSC and SAXS suggest that the incorporation of even–odd segments strongly perturbs the symmetry and regularity of the polyamide, thereby hindering the crystallization of PA56. At a PA65 fraction (f₆₅) of 0.5, the lamellar thickness of the copolyamide decreased to 1.3 nm, accompanied by a concurrent reduction in crystallinity to 15.2% and a decrease in the melting point to 164 °C. It was found that the crystalline structures of the copolyamides are composition-dependent. The crystalline structure and phase transition behavior of PA(56-co-65) with f₆₅ of 0.1–0.2 resembled those of PA56. However, the difference in hydrogen bond strength between melt- and solution-crystallized PA(56-co-65) samples was reduced by adding even–odd PA65 segments. The solution-crystallized PA(56-co-65) with f₆₅ of 0.3–0.5 exhibited an incomplete α→γ Brill transition upon heating, and γ-56, HT-α-56, and α′-65 phases were observed in the melt-crystallized samples. The low crystallinity and polymorphism structure of PA(56-co-65) resulted in high toughness and transparency, with the elongation at break and the transmittance reaching optimal values of 475.2 and 84.0% for coPA-50, respectively.