Melting of alpha'- and alpha-crystals of poly(lactic acid)

The influence of chain structure on thermal stability of ?'-crystals of poly(lactic acid) (PLA) with high L-lactic acid content (96-100 %) is detailed in this contribution. ?'-crystals of PLA grow at temperatures below 120 °C, and spontaneously transform into stable ?-modification during heating. Using conventional differential scanning calorimetry (DSC) and fast scanning chip calorimetry (FSC), a wide range of scanning rates, between about 10-1 and 102 K s-1 could be tested. It was found that reorganization of disordered ?'-crystals into stable ?-crystals can be suppressed by fast heating. The critical heating rate needed to completely melt ?'-crystals and to avoid formation of ?-crystals on continuation of heating varies with the chain composition, and decreases upon increase of the D-lactic acid content in the PLA chain.