Tailoring the thermal and mechanical properties of injection‐molded poly (lactic acid) parts through annealing

The effect of cold‐crystallization on poly (lactic acid) (PLA) injection‐molded parts was systemically investigated at different annealing temperatures (80/100/120°C) and annealing times (0.5/1/1.5/2 hr). The relative crystallinity (Xc) and crystal form (α' and α) of samples was investigated by differential scanning calorimetry (DSC) and wide X‐ray angle diffraction (WAXD). The dependence of the thermal and mechanical performance on relative crystallinity and crystal form/morphology was discussed in detail. A linear relationship between the increment of heat distortion temperature (HDT) and that of Xc was found. The tensile strength, tensile modulus and storage modulus all increased with annealing time and annealing temperature, while the tensile toughness presented a different behavior. The elongation at break for specimens reached a maximum value of 16.9% after annealing at 80°C for 2 hr, which is a threefold improvement compared to PLA samples prepared without annealing. This work suggests that annealing is an effective method for tailoring the physical properties of PLA products.