Polymorphic transition in disordered poly(L-lactide) crystals induced by annealing at elevated temperatures

Effects of annealing conditions and molecular weight (MW) on the crystalline phase transition in Poly(L-lactide) (PLLA) were studied by wide-angle X-ray diffraction (WAXD), Fourier transform infrared (FTIR) spectroscopy, and differential scanning calorimetry (DSC). The disordered crystal (alpha'-form) of PLLA was found to transform into the alpha one during annealing process at elevated temperatures. The alpha'-to-alpha transition is quite dependent on the annealing period (t(a): 0-1440 min) and annealing temperature (T-a: 120-160 degrees C). With increasing T-a, the polymorphic transition progresses much more rapidly. The alpha'-to-alpha transition is mainly involved by the slight rearrangement of the chain conformation (especially related to the side groups) and packing manner in the unit cell to the more energy-favorable state, corresponding to the reduction of unit cell dimension. Besides, it was proposed that the alpha'-to-alpha transformation mainly proceeds by the direct solid-solid transition mechanism. Moreover, it was found that MW affects the crystalline phase transition significantly. In the low-MW PLLA sample, the alpha'-to-alpha transition becomes much faster, and it can proceed prominently even when annealed at relatively lower temperature.