A detailed analysis of the three-phase structure of poly(L-lactic acid) was performed by temperature-modulated differential scanning calorimetry upon quasi-isothermal and non-isothermal crystallizations. The TMDSC analyses revealed that the relative amounts of the three fractions of poly(L-lactic acid) depend on the temperature at which crystallization occurs. It was found that rigid amorphous fraction grows in parallel with crystalline phase during quasi-isothermal crystallization at 90 degrees C, whereas at T(c) = 130 degrees C it starts to appear approximately in correspondence with secondary crystallization. At T(c) = 135 degrees C RAF practically does not develop. The results prove that RAF development during crystallization depends on the mobility of the chains. The temperature of complete devitrification at 10 degrees C min(-1) seems to be 130 degrees C: this temperature could be the limit for the formation and the disappearance of rigid amorphous fraction in the PLLA sample analyzed in the present study. (C) 2011 Elsevier B.V. All rights reserved.