Abstract
Abstract This paper investigates the crystallization kinetics and morphology development of PLA. The transitory stages in the evolving flow-induced crystallization of PLA are identified and classified in terms of the overall crystallization kinetics and the crystalline morphologies. Under quiescent conditions, temperature governs the crystallization process and the slow crystallization kinetics of PLA is highlighted under these conditions, whereas under shearing conditions, the crystallization is highly enhanced due to the promotion of the nucleation mechanism. The enhancement of the crystallization implies also morphological modifications. Depending on the shear rate and the shearing time the microstructure changes dramatically: spherulitic microstructure, fine grained microstructure and oriented microstructure. For a specific shear rate, depending on the magnitude of the shearing time the microstructure assumes the following states: for low shearing time only an increase of the number of nuclei is observed (leading to fine grained microstructure), followed by a saturation of point-like nuclei, and for a relatively long shearing time (i. e. beyond a critical shearing time) the development of oriented structures looking like “shish-kebabs” is observed. The critical shearing time for the formation of oriented structures in PLA is determined as a function of the shear rate.
Highlights
Biobased polymers have gained more importance during the last decade with applications in various fields including medical sciences, packaging and agriculture
The choice of the crystallization temperature is a crucial step in order to obtain different types of crystalline entities; this temperature should be high enough to elucidate the impact of flow on the crystallization; the temperature was kept higher than 130 8C
We investigated the impact of different values of shear rate on overall crystallization kinetics and morphology development over a constant shearing time of ts = 2 s
Summary
Biobased polymers have gained more importance during the last decade with applications in various fields including medical sciences, packaging and agriculture. The application of a shear flow of short duration to PLA enhances the crystallization kinetics through supplementary nuclei (Zhong et al, 2013) The density of these nuclei increases with the shear rate and the shearing time; for long shearing times one can observe the saturation of point-like nuclei for a given shear rate. This work is focused on the crystallization kinetics of PLA under quiescent as well as flow conditions given the importance of flow conditions during the processing of polymers concerning the kinetics of crystallization and the final microstructure Both crystallization temperature and shear flow are considered with a particular attention to their impact on the overall crystallization kinetics and the morphology development of PLA. A macro-kinetics model has been used for the prediction of nucleation density and the overall crystallization kinetics
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