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 alpha-gamma 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 gamma phase and subsequently transform into the alpha phase during cooling, while the 56-unit-rich ones merely crystallize in the gamma phase. For the copolyamide with pseudoeutectic composition, high and low T-c promote the generation of single gamma phase of PA6,6 and double gamma phases of PA5,6 and PA6,6, respectively. Upon cooling, the PA6,6-type gamma phase always transforms into the alpha phase, while the PA5,6-type gamma 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.