Crystallization and melting behavior of poly (L-lactic acid)

Effects of the crystallization temperature oil the crystal structure and its melting behavior of poly (L-lactic acid) (PLLA) have been investigated by means of wide-angle (WAXS) and small-angle (SAXS) X-ray scattering, optical microscopy, and differential scanning calorimetory (DSC). PLLA was found to crystallize as the alpha form when the crystallization temperature T, was higher than 120 degrees C, while significant change in lattice parameters was seen for T-c's below 120 degrees C. The ratio of the a- and b-axis lengths begins to decrease with T-c below 120 degrees C and is 3(1/2) below 90 degrees C, which suggests a new crystalline form with hexagonal packing, namely, the alpha' form. The possible reason for alpha' formation is discussed. High-temperature WAXS and SAXS measurements showed that alpha' crystal transforms into ordered a form during heating. The transition takes place at 150 degrees C without a decrease in scattering intensity and without heating rate dependence. The mechanism for the transition is discussed.