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

The characteristics of pure alpha- and beta-form of the cyclic dimer (1,8-diazacyclotetradecane-2,9-dione) were systematically and integrally investigated during this study. The results showed that the alpha-form could dissolve and rapidly transform into the beta-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 degrees C to 360 degrees C, respectively, which showed that there are two endothermic transitions over the course of the heating programme. At a temperature of approximately 242 degrees C, the beta crystals were initially converted into alpha crystals, and then they melted when the temperature reached over 345 degrees C. A video recorded under a light microscope also showed that the sublimation of the b cyclic dimer occurred after the transition. However, the alpha-form might sublimate at temperatures lower than 150 degrees C when mixed with volatile matter.