The crystal-form transition behaviours and morphology changes in a polyamide 6 cyclic dimer

Abstract

The characteristics of pure α- and β-form of the cyclic dimer (1,8-diazacyclotetradecane-2,9-dione) were systematically and integrally investigated during this study. The results showed that the α-form could dissolve and rapidly transform into the β-form in methanol, and in caprolactam solution at a lower temperature, an interesting transition occurred and formed co-precipitates, which refract colourful light under PLM. However, these dimers can aggregate in water, and they are then transformed into multi-slice layers and compact structures. The detailed transition behaviours between the two forms were further measured by FT-IR, XRD and DSC by varying the temperature from 25°C to 360°C, respectively, which showed that there are two endothermic transitions over the course of the heating programme. At a temperature of approximately 242°C, the β crystals were initially converted into α crystals, and then they melted when the temperature reached over 345°C. A video recorded under a light microscope also showed that the sublimation of the β cyclic dimer occurred after the transition. However, the α-form might sublimate at temperatures lower than 150°C when mixed with volatile matter.

A Novel Synthetic Strategy for Preparing Polyamide 6 (PA6)-Based Polymer with Transesterification

“…Some oligomers can be observed in Figure 9a, which are similar in morphology to the rod-shaped cyclic dimers previously reported in the literature [32]. Most of their directions are the same as the forming direction, and a large amount of oligomers are observed in the cross-section, indicating that too many oligomers will be concentrated in a part of the matrix [32].…”

“…Approximately 2 wt% of the oligomers contained in PEA are mainly composed of cyclic dimers [12]. In PEA, the cyclic dimer is present in the matrix in the form of a rod [32]. In the uniaxial stretching process, the rod crystal is parallel to the stretching direction.…”

One-Amide Thin Films Prepared by Physical Vapor Deposition of Nylon-6 Granules

“…The interface interaction would suppress the rearrangement of the molecules, limiting intramolecular hydrogen bonding in the intrasheet, while the stacking of the molecules would be favored in the deposition process, which can contribute the intermolecular hydrogen bonding in the γ form. On the other hand, we may also consider that some oligomers, such as the cyclic oligoamides, can form the crystal structure in the deposited thin film. , But, the relevant crystal structures were mainly reported with monodisperse/pure oligomers, and their XRD patterns contain a series of clear peaks from 11° to 23° . Thus, it is proposed that the crystal structure of the deposited thin film may contribute to the polymer components or large number of oligomers of different sizes.…”

“…42,43 But, the relevant crystal structures were mainly reported with monodisperse/pure oligomers, and their XRD patterns contain a series of clear peaks from 11°to 23°. 44 Thus, it is proposed that the crystal structure of the deposited thin film may contribute to the polymer components or large number of oligomers of different sizes. Surface/Interface Conditions of the Deposited Thin Film.…”

Unveiling the Mechanisms of Hydrolytic Ring-Opening Polymerization of Caprolactam and Amino-Assisted Ring Opening of Cyclic Dimers: A DFT Study

“…If the meta-amide bond is attacked, a cyclic dimer is produced. 9 In addition to water, the production of cyclic oligomers is also affected by the reaction temperature and time. 10,11 Therefore, a few studies have reported that the formation of cyclic oligomers can be reduced by adjusting the polymerization process.…”