Effect of constrained annealing on the microstructures of extrusion cast polylactic acid films

Cast poly(lactic acid) (PLA) films were prepared by extrusion followed by stretching using a vertical three-pinch roll at various take-up speeds from 100 to 1000 cm/min. These cast films were found to be amorphous and unoriented regardless of the take-up speeds applied, using two-dimensional wide angle X-ray diffraction (2D-WAXD), polarized Fourier transform infrared (FTIR), and differential scanning calorimetry (DSC) techniques. However, upon constrained annealing, oriented crystalline structures developed for the films cast at 400 cm/min and over, and the orientation level and crystallization rate of the films increased with the take-up speed at which the film was cast. The findings could be explained by the presence of global orientation during film casting and stress-induced crystallization upon constrained annealing.

► As-cast PLA films were amorphous and unoriented.
► Upon constrained annealing, oriented crystalline structures developed.
► Global orientation occurred during film casting.
► Stress-induced crystallization took place upon constrained annealing.