On the crystal stabilization during two-step isothermal crystallization of poly(butylene terephthalate) examined by fast scanning calorimetry

The Isothermal Melting Kinetics of Ultra-High Molecular Weight Polyethylene Crystals

The isothermal melting behaviors of ultra-high molecular weight polyethylene (UHMWPE) with different entangled states (i.e., nascent and melt-crystallized samples) are studied. For two kinds of UHMWPE samples, the result shows that the relative content of survived crystals (Xs) exponentially decreases with time and reaches a constant value. It is suggested that such a melting behavior is related to the observed nonlinear growth of crystals induced by the kinetically rejected entanglements accumulated at the growth front. Additionally, the exponential decay of Xs with time provides a characteristic melting time (τ) for the melting process. Compared to the melt-crystallized UHMWPE, the τ value of nascent UHMWPE is generally longer even in a higher temperature range, which is mainly because the former has a larger entanglement density difference. Furthermore, these observations demonstrate that UHMWPEs with different entangled states have an analogous melting mechanism since they exhibit a similar melting activation energy (≈1300 kJ mol-1).

Bulk enthalpy of melting of poly (l-lactic acid) (PLLA) determined by fast scanning chip calorimetry

The bulk enthalpy of melting of α-crystals of poly (l-lactic acid) (PLLA) has been evaluated by fast scanning chip calorimetry (FSC), by analysis of the correlation between the measured enthalpy of melting of sets of samples of different crystallinity and the corresponding heat capacity at 90°C, that is at a temperature higher than the glass transition temperature of the bulk amorphous phase and lower than the melting temperature. Extrapolation of this relationship for crystals formed at 140°C towards the heat capacity of fully solid PLLA yields a value of about 104.5±6 J/g when melting occurs at 180-200°C. The analysis is based on the presence of a two-phase structure, that is, absence of a vitrified rigid amorphous fraction (RAF) at the temperature of analysis of the solid fraction of the material (90°C). Formation and vitrification of an RAF was suppressed by avoiding both continuation of primary crystallization and secondary crystallization during cooling the system from the crystallization temperature of 140°C to 90°C, making use of the high cooling capacity of FSC. Small-angle X-ray scattering (SAXS) confirmed distinct thickening of initially grown lamellae which only is possible if these lamellae are not surrounded by a glassy RAF. Linear crystallinity values obtained by SAXS and calorimetrically determined enthalpy-based crystallinities agree close to each other. This article is protected by copyright. All rights reserved.