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

Successive self-nucleation and annealing (SSA) is an efficient way to thermally fractionate semicrystalline polymers. SSA thermal fractions have distinct melting points corresponding to different lamellar thicknesses. SSA was employed to investigate the in-situ evolution of lamellar thicknesses of polyamide 1012 (PA1012) during high-temperature thermal treatments. A competition between chain growth and crosslinking occurred during the thermal treatments as detected (as a function of time) by SSA, rheology, and dissolution behavior. Based on a master curve of "time–temperature superposition" at a reference temperature, the mechanism for lamellar thickness evolution was divided into three stages: initially, chemical crosslinking predominated. As the number of end groups rapidly increased, linear chain growth and crosslinking occurred simultaneously. Eventually, linear chain growth was increased by chain end-group reactions. The structural changes provoked by the thermal treatments enhanced the mechanical and heat resistance properties of PA1012. This work provides guidance for the design for high-performance polyamide materials.