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

Abstract

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.

Crystallization Rate Minima of Poly(ethylene brassylate) at Temperatures Transitioning between Quantized Crystal Thicknesses

“…Most of the high-temperature crystallization rate minima in polymers have been found at the transition between two polymorphic phases. In addition to our work on precision polyethylenes with halogens and long-spaced polyacetals, minima have been found in polyamides, , polyesters, − polyketones, syndiotactic polystyrene, isotactic polypropylenes, , dendron-like giant molecules, and amphiphilic molecular brushes of poly­(ethylene oxide) and n -alkyl side chains . Minima that appear in the low-temperature region approaching the glass transition have been interpreted as a change from heterogeneous to homogeneous nucleation. − …”

“…Prior rate minima or discontinuities found in the temperature gradient of the crystallization rate of precision polyethylenes and in other semicrystalline polymers have been invariably associated with a polymorphic transition. ,− The minima displayed by PEBs in Figure are of interest because PEB does not undergo any apparent polymorphic transition in the whole range of T c analyzed. Such continuous isomorphic behavior and the characteristics of the rate minima, namely a temperature gradient that becomes positive approaching the melting point of a thinner metastable crystal structure, are equivalent to the minimum of the crystallization rate found in long-chain n -alkanes by Ungar and Keller. − …”

Key Insights into the Differences between Bimodal Crystallization Kinetics of Polyamide 66 and Polyamide 6

“…However, controlling crystallization, a major step of solidification during melt processing, strongly relies on the knowledge of kinetics at high undercooling and rapid cooling conditions and can help to design an efficient pathway to make products with desired properties. As previously reported, PA66 and PA6 can develop more than one type of crystal depending on the crystallization conditions. − Amorphous phase due to incomplete crystallization and mesophase formed at low crystallization temperatures undergo an irreversible reorganization to a more stable phase after annealing, , which is believed to be one of the reasons for part deformation in some high-temperature or high-frequency loading applications.…”

“…One of the major challenges of studying polyamide crystallization under processing-relevant conditions is the historical limitations of thermal instrumentation to follow the ultrafast crystallization process, which usually completes within 10 s. This problem can now be solved by the fast scanning calorimetry (FSC) technique. The crystallization kinetics fingerprints of many common thermoplastics have been revealed for the first time. ,− For example, it was found that the solidification of PA66 and PA6 is extremely fast but behaves differently in their isothermal crystallization and nonisothermal crystallization kinetics. − ,− …”

Hydration, Refinement, and Dissolution of the Crystalline Phase in Polyamide 6 Polymorphs for Ultimate Thermomechanical Properties

“…Upon increasing temperature, the two characteristic diffraction signals tend to merge, which leads to a pseudo-hexagonal packing prior to melting and is referred to as the Brill transition . High cooling rates may lead to crystal imperfection and pseudo-hexagonal structures existing at temperatures below the glass transition temperature, as recently demonstrated by fast scanning chip calorimetry experiments and successive FTIR and X-ray diffraction . The origin of the Brill transition in even-even polymers has been enlightened by temperature-dependent wide-angle X-ray diffraction and complemented by detailed FTIR and solid-state NMR spectroscopic methods. − The molecular organization of polyamide 6, the hydrogen bonding directionality of polyamide 6 in the monoclinic and pseudo-hexagonal crystallographic packing, and the defined interchain d - spacings are illustrated by projection along the c -axis of the unit cells in Figure .…”