Metastable Structures of poly(lactic acid)

The coexistence of two poly(lactic acid) crystals (alpha' and alpha) and its impact on the double melting peaks in differential scanning calorimetry is investigated. Solubility differences as well as irreversibility of the alpha' to alpha transformation process indicates that the alpha' crystal is the less stable crystal form. The difference in thermal stability is quantified by measurement of crystal melting enthalpies using the Pyda/Wunderlich method. The relative composition of the crystalline fraction can also be determined by the same method. Crystal annealing is found to be accompanied by an increase in both overall crystallinity and the fraction of the alpha crystal relative to the alpha' crystal. Infrared spectroscopy is used to probe the crystal structures. In comparison to the alpha crystal, the alpha' crystal is found to have a weakened carbonyl-carbonyl intermolecular interaction. Results also suggest that the alpha' crystal does not have a 10/3 helix which constitutes the alpha crystal. The coexistence of the two crystal forms has profound implications in interpretation of double melting peaks in its DSC thermogram.