Multiple melting behaviour of annealed crystalline polymers

The multiple (double) melting behaviour of annealed crystalline polymers including polyamide 6 (PA6), low density polyethylene (LDPE) and high density polyethylene (HDPE) was investigated by means of differential scanning calorimetry (DSC) and wide-angle X-ray diffraction (WAXD). It was found that the multiple melting behaviour can be observed frequently in DSC heating scans for annealed LDPE and PA6 samples, while it cannot be seen for annealed HDPE samples. The WAXD analysis was further performed to characterize the polymorphic structure of annealed samples. It was revealed that the presence of a low-temperature melting peak in annealed PA6 samples was not due to the melting of γ crystals. There was no certain connection between the multiple melting peaks and the melting of polymorphic structure of polymers. Considering the crystallinity difference between LDPE and HDPE samples, we proposed a viewpoint that the multiple melting behaviour was a prevalent phenomenon for annealed samples, providing that the original crystallinity was low. The multiple melting behaviour of annealed samples originated from the different distribution of the lamellar thickness. The high-temperature melting peak was attributed to the melting of thick lamellae generated during the primary crystallization of the injection molding process, while the low-temperature melting peak was attributed to the melting of thin lamellae formed during the secondary long-time annealing process.