Prediction of tensile modulus from calorimetric melting curves of polylactic acid with pronounced cold crystallization ability
J Molnár, A. Hertner-Horváth, Alfréd Menyhárd
Abstract
The modulus of semicrystalline polymers mainly depends on two parameters of the crystalline structure, the crystallinity and the lamellae thickness. These parameters can be obtained from a single calorimetric melting curve, which makes it possible to predict the modulus from a calorimetric measurement. Our equation gives the relation between these two parameters and the elastic modulus. However, in polymers like polylactic acid (PLA), the cold crystallization influences the melting curve and hiders the simple evaluation. This work presents a novel method to eliminate the cold crystallization during the calorimetric measurements and determine the iterative constants needed for the modulus prediction. The crystalline structure of PLA was characterized by differential scanning calorimeter (DSC) and wide-angle X-ray scattering (WAXS) measurements. Tensile properties were determined by standardized tensile tests on injection molded specimens, which were annealed at different temperatures and times to achieve significantly different crystalline structures for the model's development.