Thermal and barrier properties of stretched and annealed polylactide films

In this study, both stretched and annealed polylactide (PLA) films were prepared by a twin-screw extruder with different draw ratios, and the effects of the orientation, crystallization, ambient temperature and humidity on the barrier properties were evaluated. The wide-angle X-ray diffraction results revealed that the crystallinity increase with the increase of draw ratio, and the alpha'-crystal was produced by uniaxially stretching. Moreover, the alpha'-crystal was easy to form when the film was annealed at 90A degrees C. Based on the scanning electron microscopy observation, the film surface cracks occurred at a certain draw ratio. Modulated differential scanning calorimetry (MDSC) results showed that the glass-transition temperature and crystallinity increased with the increase of draw ratio. The barrier behavior results showed that, both stretched and annealed PLA films exhibited much better oxygen and water vapor barrier than undrawn PLA at middle draw ratio. Upon further stretching, their barrier ability was reduced. The annealed PLA film had higher barrier ability than that of the stretched PLA film. With regard to the temperature and humidity, the oxygen permeability coefficient increased with the increase of temperature, however, the water vapor permeability coefficient reduced slightly. The oxygen and water vapor permeability coefficient were insensitive to humidity.