Reorganization and Recrystallization in α_c-Mobile and Crystal-Fixed Semicrystalline Polymers Studied by Fast Scanning Calorimetry (Flash-DSC) and SAXS

Semi-crystalline polymers can undergo processes of reorganization/recrystallization during a heating scan in a DSC measurement. As a result, several melting peaks are observed or the melting peak shifts depending on the applied heating rate. We investigated these processes by a Flash-DSC for α_c-mobile and crystal-fixed polymers in comparison. With NMR it is possible to distinguish these classes of sc polymers, depending on the presence of α_c-relaxation - an intra-crystalline chain mobility. As model samples we chose PEO (α_c-mobile) and PCL (crystal-fixed). Typical experiments consist of heating scans after isothermal crystallization with a broad range of different heating rates which in the limit of large heating rates allow a suppression of the reorganization. At such high heating rates additional experimental effects like thermal lag and superheating become important and need to be taken into account. Our results show that crystallization of PCL leads to the formation of marginally stable crystallites that constantly reorganize during heating, while in PEO due to the presence of the α_c-relaxation, much more stable lamellar crystals form, which melt only at much higher T. Crystallization at low T on the other hand seems to suppress the α_c-induced stabilization in PEO.