Crystallization of poly(L-lactic acid) probed with dielectric relaxation spectroscopy

The properties of the amorphous component in poly(L-lactic acid), PLLA, depend on its crystalline structure, where the nanoconfinement effects may be monitored using dielectric relaxation spectroscopy. In this work this technique is used to monitor isothermal crystallization in PLLA by probing the evolution of the loss peak with crystallization time. It was found, for two different molecular weight materials and by crystallization from either the glassy or melt states, that the data could be given by a linear combination of three loss processes, here just their intensities vary: the alpha-process of the amorphous material, the constrained alpha-process present in the fully crystallized material, corresponding to the segmental motions of the amorphous phase confined by the crystalline lamellae, and the sub-glass beta-relaxation. The appearance of the confined process was detected in the earlier stages of the crystallization, suggesting that the confinement effects are effective during primary crystallization. In this case, the cooperative segmental motions are restricted by the primary lamellae, leading to a dynamics characterized by a broader relaxation time distribution shifted to higher values. It was observed that the crystallization, as investigated by monitoring the evolution of the amorphous phase, is quicker for lower molecular weight PLLA and is slower for the material crystallized from the melt with respect to the cold crystallization. It was found that the confined dynamics is not very dependent on the thermal history prior to the crystallization step. The features of the beta-relaxation were found to be similar for amorphous and semicrystalline systems, indicating that the sub-glass process in PLLA is not influenced by the crystalline confinement.