Homogeneous Crystal Nucleation in Poly (butylene succinate-ran-butylene adipate): Challenging the Nuclei-Transfer Step in Tammann's Method.

The kinetics of homogeneous crystal nucleation and the stability of nuclei were analyzed for a random butylene succinate/butylene adipate copolymer (PBSA), employing Tammann's two-stage crystal nuclei development method, with a systematic variation of the condition of nuclei transfer from the nucleation to the growth stage. Nuclei formation is fastest at around 0 °C, which is about 50 K higher than the glass transition temperature and begins after only a few seconds. Due to the high nuclei number, spherulitic growth of lamellae is suppressed. In contrast, numerous μm-sized birefringent objects are detected after melt-crystallization at high supercooling, which, at the nanometer-scale, appear composed of short lamellae with a thickness of a few nanometers only. Regarding the stability of nuclei generated at -30 °C for 100 s, it was found that the largest nuclei of the size-distribution survive temperature jumps of close to 80 K above their formation temperature. The critical transfer-heating rate to suppress the reorganization of isothermally formed nuclei as well as the formation of additional nuclei during heating increases with the growth temperature at temperatures lower than the maximum of the crystallization rate. This observation highlights the importance of careful selection of the transfer-heating rate and nuclei development temperature in Tammann's experiment for evaluation of the nucleation kinetics.