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

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.