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Flow-induced crystallization of polylactides in capillary flow Zhang, Ziyue


Polylactide (PLA) is a biodegradable thermoplastic polyester produced from renewable biomass sources. The low crystallinity and the low associated crystallization kinetics have a negative impact on its mechanical and barrier properties. In this Master thesis, the effect of flow on the PLA crystallization is studied. The flow-induced crystallization (FIC) of two PLA with different microstructures (different L-lactic acid content) is studied 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. Lastly, the PLA sample with higher content of PLLA showed slightly higher zero-shear viscosity and a smaller thermodynamic barrier for the onset of crystallization.

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