The beneficial influence of ultrasound in the polymerization of ε-caprolactam to polyamide-6 (Nylon 6). Part II: Additional experiment to understand the “pre-sonication effect”

Ultrasound (US) “pre-sonication effect” is the beneficial effect of US in the hydrolytic polymerization of ε-caprolactam (CL) mixtures with very low water concentrations (about 0.1–1 wt%). It appears after a mild initial treatment of the mixtures with US [17.5–20 kHz, short times (5–15 min), low temperatures (70–110 °C)] followed by heating at 220–260 °C. An explanation is proposed on the basis of the formation in mild conditions (100 °C) of low concentrations of cyclic oligomers never detected in the literature at those conditions. These, under US irradiation, produce linear amino acid oligomers, which are strong activators of polymerization when the mixture of CL and water, after US irradiation, is heated at the suitable polymerization temperature indicated above.