Isodimorphic Crystallization and Tunable γ-α Phase Transition in Aliphatic Copolyamides: Critical Roles of Comonomer Defects and Conformational Evolution

Minor comonomer units can be included as defects in crystalline phase during the crystallization of isodimorphic copolyamides, further changing their interchain packing. However, a clear conclusion on how their crystalline structure and phase transition behavior are affected by the inclusion of comonomer defects is lacking. Herein, we systematically studied the polymorphic crystallization and Brill transition of aliphatic even- even random copolyamides, synthesized from nylon-6,6 salt and nylon-8,8 salt. All copolyamides exhibit isodimorphism and undergo an alpha-gamma crystal transition (Brill transition) upon heating. Their Brill transition temperatures are composition-dependent and show a pseudoeutectic characteristic in the intermediate composition range. The copolyamides with pseudoeutectic compositions exhibit a unique irreversible Brill transition, as manifested by the remaining of the gamma phase without transition upon cooling from melt. Intriguingly, the gamma-alpha transition of pseudoeutectic copolyamides is crystallization temperature (Tc) dependent. The gamma phase that crystallized at low Tc includes more minor comonomer units as defects, which raises the energy barrier for gauche-trans conformational transition, thus preventing the gamma-alpha transition. The gamma phase formed at high Tc has less minor comonomer units, facilitating the transformation into alpha phase due to easier activation of the gauche-trans transition. This work has elucidated the crucial role of comonomer defects on conformational evolution and Brill transition of polyamides.

Automated summary

This study systematically investigates the effect of comonomer defects on the crystalline structure and phase transition behavior of copolyamides. The results show that the copolyamides exhibit composition-dependent Brill transition temperatures, with a pseudoeutectic characteristic in the intermediate composition range. The copolyamides with pseudoeutectic compositions undergo an irreversible Brill transition, with the γ phase remaining unchanged upon cooling from melt. The γ-alpha transition of pseudoeutectic copolyamides is found to be temperature-dependent.