Thermal dynamics affected formation and dislocation of PDLA morphology

The morphologies and structures of crystalline polymers depend on the crystallization condition, such as the quenching rate, crystallization temperature, pressure, and molecular weight, tacticity, branching, topology and so on. The morphologies and growth of poly (D-lactic acid) (PDLA) spherulites were observed by POM when the samples isothermally crystallized at various temperatures after cooling from 200 °C to 250 °C. The crystallization behavior of PDLA was characterized by DSC measurements at various temperatures and it is not consistent with the kinetics of spherulite growth, which is due to the difference between the characterized partial and total crystallization behavior. It was found that the formation of the banded spherulites of PDLA is not only dependent on the crystallization temperatures, but also on the melting temperatures. Light fluctuation of the POM images was investigated during heating and it was found that the dark bands or black bands disappeared earlier than the bright ones during heating of the banded spherulites. The observed melting phenomenon implies mutiple lamellar stacking model inside the spherulites. The formation condition of the banded spherulites morphology was discussed based on the crystallization rate and diffusion rate. The proposed mechanism would be helpful to understand the structure and formation of PDLA ring-banded spherulites.