Competition between Chain Extension and Crosslinking in Polyamide 1012 during High Temperature Thermal Treatments as Revealed by SSA Fractionation

Abstract

Self-nucleation and annealing (SSA) is an efficient way to thermally fractionate semi-crystalline polymers. The thermal fractions produced by SSA have distinct melting points that correspond to different average lamellar thickness. In this research, SSA was adopted to investigate the in-situ evolution of lamellar thickness of polyamide 1012 (PA1012), which was affected by high temperature thermal treatments. SSA successfully fractionated PA1012 into 4 thermal fractions with different average lamellar thicknesses. The integrated area of the first or second SSA fraction against the total endothermic integrated area was plotted as a function of thermal treatment time to study the kinetics of lamellar thickness changes. Two opposing structural effects, chain growth and crosslinking, occurred during the applied thermal treatment (which consisted in thermally treating the material by holding it isothermally at temperatures in the range of 140-250 ºC) and they were detected as a function of time by SSA, rheology and dissolution behavior. The structural changes increased the viscosity and Tg and decreased the overall crystallization rate. Based on the construction of a master curve of “time-temperature superposition” at a reference temperature (T0) of 190 oC, the mechanism for lamellar thickness evolution was divided into three stages: (a) Stage I: Initially, PA1012 end groups reacted rapidly with active sites to generate chemically crosslinked structures. (b) Stage II: As the number of end groups rapidly increased, amidation reactions between carboxylic end groups and amine end groups resulted in linear chain growth. Linear chain growth and crosslinking occurred simultaneously, and there was no change in lamellar thickness or its distribution. (c) Stage III: Eventually, an increasing number of end groups was formed in the system, most of which led to linear chain growth via chain end-group reactions. These structural changes during the applied thermal treatments enhanced the mechanical properties and the heat resistance of PA1012. This work provides specific guidance for improving the toughness, strength and heat resistance of polyamide materials.