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 temperaturedependent 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 βor αphase 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 βand 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.

Polyamide 66 (PA66) and polyamide 6 (PA6) share many comparable properties due to their similar chemical structures. However, their crystallization kinetics and morphological differences are not as well understood as other properties. This work establishes the crystallization kinetics and morphology of additive-free PA66 and PA6 at high undercooling conditions using a modified fast scanning calorimetry technique. Two polyamides show similar kinetics profile and morphology, but the transitions associated with polymorphs occur at different temperatures. Regarding kinetics, PA66 always crystallizes faster than PA6 regardless of the polymorphs formed, supported by the temperature-dependent Avrami kinetics coefficients k. Both PA66 and PA6 show a bimodal kinetics profile with a local crystallization rate minimum at 135 and 110 °C, respectively. Apart from the crystallization rate, a sudden broadening of the exothermic crystallization peak is found near the rate minimum. The broadening is described by a drastic change of the Avrami index n from 3 to 2. The morphology at the micro- and nanoscales of polyamides was followed by a polarized optical microscope (POM) and atomic force microscopy (AFM). The POM reveals that both polyamides turn translucent from transparent near the rate minimum. The temperature-dependent AFM micrographs show multistep transitions from amorphous-like morphology, cauliflower-like crystal, crystal aggregates, and lamellar structure after T c changes from near T g to above the kinetics break temperature. Although two polyamides have similar molecular weight and the same content of amide groups, the morphological transition in PA66 is found to always be 20 °C higher than in PA6, suggesting a difference in their thermodynamic drive to nucleate. The conclusions drawn from the Avrami analysis in the final part of this study provide a universal explanation of the drastic peak broadening observed in many previously studied thermoplastics.

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Heterobimetallic title complexes can be prepared in which one ring is bound to tungsten and the other to rhodium, as shown here, or to cobalt.