Crystal Polymorphism and Phase Transition of Isodimorphic Copolyamides Based on Even–Even and Odd–Even Comonomer Units: Synergistic Effects of Copolymer Composition and Crystallization Temperature

Copolymerization of diacids and diamines can produce copolyamides with diversified physical properties. Due to the interplay between even–even and odd–even diamine-diacid units, the even–even/odd–even copolyamides can show much different crystal polymorphism and phase transition behavior from the typical (co)polyamides. Herein, we synthesized the novel even–even/odd–even copolyamides from nylon 5,6 and nylon 6,6 salts and investigated their crystallization kinetics, crystalline structure, and phase transition behavior. The copolyamides display the typical isodimorphic behavior, and the solution-crystallized copolyamides experience α–γ Brill transition upon heating. Intriguingly, the crystalline structure and phase transition of copolyamides are strongly influenced by the copolymer composition and initial crystallization temperature (Tc) upon cooling due to the possible change of H-bonding structure regularity induced by the incorporated comonomer units in the crystalline phase. The 66-unit-rich copolyamides initially form the γ phase and subsequently transform into the α phase during cooling, while the 56-unit-rich ones merely crystallize in the γ phase. For the copolyamide with pseudoeutectic composition, high and low Tc promote the generation of single γ phase of PA6,6 and double γ phases of PA5,6 and PA6,6, respectively. Upon cooling, the PA6,6-type γ phase always transforms into the α phase, while the PA5,6-type γ phase is maintained. This study has elucidated the synergetic effects of copolymer composition and Tc on the crystallization and phase transition of isodimorphic copolyamides.