Towards the Enhancement of the Crystallization Kinetics of a Bio-Sourced and Biodegradable Polymer PLA (Poly (Lactic Acid))

Abstract

PLA (Poly Lactic Acid) is a bio-sourced and a biodegradable polymer. It represents an interesting substitute for some petrochemical based polymers, especially because of its wide range of applications in the biomedical, agriculture and packaging fields. Unfortunately, PLA exhibits slow crystallization kinetics, limiting the amount of crystallinity in the final product, which is a handicap in order to extend its use. Many authors have investigated the crystallization of polymers; nevertheless several physical mechanisms remain not yet understood. This work aims a complete characterization of PLA in order to improve the understanding of its crystallization kinetics. The quiescent crystallization was investigated using Differential Scanning Calorimetry (DSC) measurements in isothermal and non-isothermal conditions for PLA and PLA with 5wt % talc. The flow effect on crystallization was studied using a thermocontrolled hot-stage shearing device (Linkam) coupled with an optical microscope. The number of activated nuclei and the growth rate were measured as functions of temperature. In addition, the linear viscoelastic properties were obtained from a rheometer with plate-plate geometry. The enhancement of the crystallization was quantified and analyzed in terms of the half crystallization time t1/2. This characteristic time t1/2 is found to be drastically decreased by both the talc and the flow which promote supplementary nucleation leading to various crystalline microstructures. The flow is known to orient and stretch molecules leading to an extra nucleation. An original description of this phenomenon is proposed using two characteristic Weissenberg numbers; based on the definition of Rousse and reptation times. Finally, we have proposed a semi-empirical model to quantify the thermal and flow contributions on the crystallization.