Effect of crystallization temperature on crystal modifications and crystallization morphology of poly(L-lactic acid)

In this work, amorphous PLLA films and crystalline PLLA samples were prepared at different crystallization temperatures (80-160°C). The crystal modifications and crystallization morphologies of PLLA samples were studied by Infrared spectroscopy (FT-IR), X-ray diffraction (XRD), and polarized optical micrographs (POM). The results demonstrated that the change of crystallization temperature has a great effect on the crystal structure, phase transition, and crystallization morphology of PLLA. At crystallization temperature <100°C, disorder α′-crystal is mainly produced for the crystal structure of PLLA samples; however, at crystallization temperature ≥ 120°C, the order α-crystals PLLA are mainly proposed; whe the crystallization temperature is between 100 to 120°C, both α′- and α-forms coexist. Besides, the crystal morphologies of PLLA samples were illustrated, and it was found that the crystal size increases and the number of spherulite decreases with increasing the crystallization temperature. It was found that the disorder-to-order (α′-to α-crystalline form) transition occures with increasing crystallization temperature above 120°C.