Temperature-modulated differential scanning calorimetry studies on the origin of double melting peaks in isothermally melt-crystallized poly(L-lactic acid)

In this work, the melting behaviors of nonisothermally and isothermally melt-crystallized poly(L-lactic acid) (PLLA) from the melt were investigated with differential scanning calorimetry (DSC) and temperature-modulated differential scanning calorimetry (TMDSC). The isothermal melt crystallizations of PLLA at a temperature in the range of 100-110 degrees C for 120 min or at 110 degrees C for a time in the range of 10-180 min appeared to exhibit double melting peaks in the DSC heating curves of 10 degrees C/min. TMDSC analysis revealed that the melting-recrystallization mechanism dominated the formation of the double melting peaks in PLLA samples following melt crystallizations at 110 degrees C for a shorter time (<= 30 min) or at a lower temperature (100, 103, or 105 degrees C) for 120 min, whereas the double lamellar thickness model dominated the formation of the double melting peaks in those PLLA samples crystallized at a higher temperature (108 or 110 degrees C) for 120 min or at 110 degrees C for a longer time (>= 45 min). (c) 2007 Wiley Periodicals, Inc.