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 (α′-form) of PLLA was found to transform into the α one during annealing process at elevated temperatures. The α′-to-α transition is quite dependent on the annealing period (ta: 0−1440 min) and annealing temperature (Ta: 120−160 °C). With increasing Ta, the polymorphic transition progresses much more rapidly. The α′-to-α 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 α′-to-α 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 α′-to-α transition becomes much faster, and it can proceed prominently even when annealed at relatively lower temperature.