Effect of processing conditions on morphology and mechanical properties of injection-molded poly(L-lactic acid)

This work investigates the relationship among the processing, morphology, and the mechanical properties of injection-molded poly(L-lactic acid) (PLLA). Melt processing temperature, mold temperature, injection flow rate, and holding pressure were systematically changed following a design of experiments array. The thermomechanical environment imposed during processing was estimated by computer simulations for the mold-filling phase, which allows the calculation of shear stress, shear rate, and the thickness of frozen skin layer. The morphology was characterized by differential scanning calorimetry and hot recoverable strain measurements. The analysis of variance results of influence of processing factors on the morphology are in good agreement with the analysis of thermomechanical parameters on the morphology. The primary factor for inducing the crystallinity in PLLA product was the stress-induced crystallization, whereas the thermal induced crystallization had a little effect. The morphology-mechanical property relationships were established. The crystallinity developed during processing has little effect on elastic modulus, increases the yield strength, and severely decreases the elongation at break. The level of molecular orientation developed during processing has little effect on elastic modulus, but increases both the yield strength and the elongation at break.