Effect of annealing and orientation on microstructures and mechanical properties of polylactic acid

Two types of polylactic acid (PLA) films (one amorphous and one semi-crystalline) were produced by sheet extrusion. Talc was used as a nucleation agent for the semi-crystalline PLA. The films were annealed above their T-g or were uniaxially orientated in two ways: (1) via a drawing system in front of the extruder and die or (2) via a three-roller stretching system. The slower crystallization rate and lower melting stress of the PLA resulted in amorphous film using the drawing system. Annealing above T-g increased crystallinity and polymer chain relaxation, which resulted in increases in both strength and toughness. Stretching above T-g also produced simultaneous crystallization and chain relaxation, which resulted in increases in both modulus and toughness. Both modulus and tensile strength in the stretching direction were higher than in the crosswise direction. Talc acted not only as a rigid filler to reinforce the PLA, but also as a nucleation agent for the PLA, especially during annealing.