Probing Three Distinct Crystal Polymorphs of Melt-Crystallized Polyamide 6 by an Integrated Fast Scanning Calorimetry Chip System

Isothermal crystallization of polyamide 6 (PA6) was studied using a state-of-the-art modified fast scanning calorimetry (FSC) chip over a wide temperature range. Microfocus FTIR spectroscopy was carried out to collect temperature-dependent spectra of PA6 directly on the FSC chip. The changes observed in amide- and methylene-related bands provide direct evidence of three crystalline modifications in PA6 from melt crystallization. The melt-crystallized PA6 was found to develop the mesomorphic β-phase below 105 °C and the α-phase above 150 °C. It has been confirmed that the γ-phase coexists with the β- or α-phase in the temperature region between 105 and 150 °C. Isothermal crystallization kinetics of PA6 in a wide temperature range were extracted directly from exothermic peaks and indirectly using the interruption method. A peculiar discontinuity in crystallization kinetics has been found in the transition region between the β- and γ-phases. In the same temperature region, a sudden transition from crystal aggregates to spherulitic morphology was captured by AFM. PA6 was cooled at rates from 10 to 2000 K/s before isothermal crystallization to mimic a full range of processing relevant conditions. A time-temperature-transformation diagram was built based on kinetic data to illustrate that the isothermal crystallization of PA6 is sensitive to the crystallization pathway. The novel platform built on a modified FSC chip is a powerful tool for the comprehensive study of polymer crystallization at processing-relevant conditions.