Bulk Enthalpy of Melting of Poly (l-lactic acid) (PLLA) Determined by Fast Scanning Chip Calorimetry

The bulk enthalpy of melting of a-crystals of poly (L-lactic acid) (PLLA) is evaluated by fast scanning calorimetry (FSC), by correlating the melting enthalpy of samples of different crystallinity with the corresponding heat capacity at 90 degrees 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 degrees C towards the heat capacity of fully solid PLLA yields a value of 104.5 +/- 6 J g(-1) when melting occurs at 180-200 degrees C. The analysis relies on a two-phase structure, that is, absence of a vitrified rigid amorphous fraction (RAF) at the temperature of analysis the solid fraction (90 degrees C). Formation and vitrification of an RAF was suppressed by avoiding continuation of primary crystallization and secondary crystallization during cooling the system from the crystallization temperature of 140 degrees C to 90 degrees C, making use of the high cooling capacity of FSC. Small-angle X-ray scattering (SAXS) confirmed 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.