Competition between Chain Extension and Crosslinking in Polyamide 1012 during High-Temperature Thermal Treatments as Revealed by Successive Self-Nucleation and Annealing Fractionation

Linearly-Changed Thermal Behavior and Depressed Brill Transition in Long-Chain Polyamides Substituted by Methyl Side Groups

Side substitution is an effective way of functionalizing and modifying the properties of polyamides. Meanwhile, side substitution would significantly influence the crystallization kinetics and polymorphic phase transition of polyamides, which, however, has not been well elucidated. Herein, we synthesized the side-substituted long-chain polyamides with various content of methyl pendent groups and investigated their crystallization and phase transition behaviors. We find that the thermal parameters of side-substituted polyamides vary linearly with the side group content, analogous to the isomorphic crystallization of random copolymers. All the solution-crystallized polyamides experience the α-γ Brill transition during heating, with the Brill transition temperature linearly decreasing as the side group content increases. Intriguingly, the γ-α transition of polyamides during cooling is suppressed with the presence of side methyl groups due to the difficulty in H-bond reorganization and gauche-trans conformational changes. This work has demonstrated the critical role of side substitution in the polymorphic crystallization and phase transition of long-chain polyamides.

Thermal Fractionation of Polyolefins: Brief History, New Developments and Future Perspective

Abstract

For semi-crystalline polymer materials, the difference in chain structure often leads to different physical properties; therefore, in-depth analysis of the chain structure is of great significance. With the continuous development of advanced instruments, many research means have emerged to characterize the structure of molecular chains. Among them, fractionation techniques provide effectively structural information on inter- and intra-molecular comonomer distribution, branching degree, and sequence length, etc. This work briefly presents the history of developments of various classical fractionation means such as temperature-rising elution fractionation, stepwise crystallization and successive self-nucleation and annealing, while focusing on the present and future of their applications.