Melting of α'- and α-Crystals of Poly(lactic acid)

The influence of chain structure on thermal stability of alpha'-crystals of poly(lactic acid) (PLA) with high L-lactic acid content (96-100 %) is detailed in this contribution. alpha'-crystals of PLA grow at temperatures below 120 degrees C, and spontaneously transform into stable alpha-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 10(2) K s(-1) could be tested. It was found that reorganization of disordered alpha'-crystals into stable alpha'-crystals can be suppressed by fast heating. The critical heating rate needed to completely melt alpha'-crystals and to avoid formation of alpha-crystals on continuation of heating varies with the chain composition, and decreases upon increase of the D-lactic acid content in the PLA chain.