文摘
Effects of the crystallization temperature on 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-rayscattering, optical microscopy, and differential scanning calorimetory (DSC). PLLA was found to crystallize asthe form when the crystallization temperature Tc was higher than 120 C, while significant change in latticeparameters was seen for Tc's below 120 C. The ratio of the a- and b-axis lengths begins to decrease with Tcbelow 120 C and is 31/2 below 90 C, which suggests a new crystalline form with hexagonal packing, namely,the ' form. The possible reason for ' formation is discussed. High-temperature WAXS and SAXS measurementsshowed that ' crystal transforms into ordered a form during heating. The transition takes place at 150 C withouta decrease in scattering intensity and without heating rate dependence. The mechanism for the transition is discussed.