To: 10/11/2021 14:00
In this seminar the flow-induced crystallization (FIC) of two PLA with different microstructures (different L-lactic acid content) will be presented. The seminar will start with a general presentation of fundamental aspects of FIC and review previous work in the subject for polymeric systems such as PE, PP and PLA. The second part will present our recent work on FIC of PLA using simple shear, uniaxial extension and capillary flow experiments. In simple shear and capillary flow, increase in shear rate and decrease in temperature was found to enhance the crystallization kinetics particularly for Weissenberg numbers (based on the reptation relaxation time, Wi) greater than 1. On the other hand, in uniaxial extensional flow, once a critical Hencky strain is achieved, crystallization starts independently of strain rate and temperature. The amount of mechanical work per unit volume imposed/dissipated onto the polymers during flow to initialize crystallization was also calculated in capillary flow. The critical mechanical work for the onset of flow-induced crystallization was found to be independent of temperature and degree of molecular chain stretch (Wi) as Wi becomes greater than 1. The PLA sample with higher content of PLLA showed slightly higher zero-shear viscosity and a smaller thermodynamic barrier for the onset of crystallization. Lastly, the degree of crystallinity increases linearly from 0% at the start of the flow-induced crystallization region and reaches a plateau at Wi equals to around 1.