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 (T_c) 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 T_c 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 T_c on the crystallization and phase transition of isodimorphic copolyamides.