Crystalline, mobile amorphous and rigid amorphous fractions in poly(L-lactic acid) by TMDSC

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 °C, whereas at Tc = 130 °C it starts to appear approximately in correspondence with secondary crystallization. At Tc = 135 °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 °C min−1 seems to be 130 °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.

Research highlights
► The three-phase structure of PLLA has been determined by modulated calorimetry.
► The kinetics of the RAF evolution depends on the crystallization temperature.
► The limit for the formation and disappearance of RAF in PLLA is approximately 130 °C.