Quantitative understanding of two distinct melting kinetics of an isothermally crystallized poly(ether ether ketone)

The multiple melting peaks of isothermally crystallized poly(ether ether ketone) were investigated by fast-scanning chip calorimetry over a wide range of heating rates from 500 to 60,000 K s(-1). The analysis of heating rate dependence of melting temperatures revealed that there are two crystal populations with distinct melting kinetics for the samples prepared at different crystallization temperatures, 170 degrees C <= T-iso <= 270 degrees C. For each Tiso, the two melting temperatures of the metastable crystals without prior reorganization or superheating (i.e. zero-entropy-production melting temperatures) were constantly 20 K and 30 K, respectively, higher than Tiso. We first discuss this behavior quantitatively based on the Lauritzen-Hoffman nucleation approach. Finally we conclude that these increases in temperature are linked to the number of stems which construct the secondary surface nuclei. The existence of two crystal populations can be caused by differences in the stability of the crystal or "local" differences in the free energy of the surrounding melt. (C) 2016 Elsevier Ltd. All rights reserved.