Juanjuan Su

Toughening of Poly(L-Lactic Acid) by Annealing: The Effect of Crystal Morphologies and Modifications

Poly(L-lactic acid) (PLA) samples prepared by hot pressing were treated by two different processes: process a—quenching of the molten specimens in water, then annealing at high temperatures; process b—direct crystallization at high temperatures from the molten specimens. The crystal modification and morphology of PLA were investigated by differential scanning calorimetry, dynamic mechanical analysis, polarized light microscopy, and wide-angle X-ray diffraction. In the case of process a, the α′ (disordered α) crystal modification was formed at relatively low annealing temperature (T a < 100°C), while the ordered α phase was formed in the case of process a at high T a (> 100°C) and process b. Furthermore, in process a, the nucleation density of spherulites was higher and the radius of the spherulites was much smaller compared with that of the spherulites formed by process b. The effects of crystal modification and morphology on the impact behaviors of PLA were investigated by notched Izod impact testing. The macroscopic fracture toughness was discussed in terms of the microscopic structures. Finally, we suggest an alternative approach for the preparation of high-performance PLA.

Enhanced crystallization behaviors of poly(ethylene terephthalate) via adding expanded graphite and poly(ethylene glycol)

A compound additive system consisting of expanded graphite (EG) and poly(ethylene glycol) (PEG) was designed to enhance the crystallization of poly(ethylene terephthalate) (PET). In this additive system, EG acted as a heterogeneous nucleating agent to reduce energy barrier for nucleation, while PEG played as plasticizer to improve mobility of PET chains. Simultaneously adding EG and PEG resulted in faster crystallization kinetics than the cases of solely adding EG or PEG in both of non-isothermal and isothermal crystallization processes, indicating a synergistic effect of EG and PEG on enhancing PET crystallization. However, for non-isothermal crystallization process, in which crystallization occurred from a cooling melt, EG played a dominant role. As to isothermal crystallization process where crystallization took place in a super-cooling state, PEG seemed to be more important. Moreover, the chain conformation change among the semi-crystalline PET specimens was ascertained by Fourier transform infrared spectroscopy.