Investigation of phase transitional behavior of poly(L-lactide)/ poly(D-lactide) blend used to prepare the highly-oriented stereocomplex

By using the highly oriented 1:1 stoichiometric blend of high-molecular-weight PLLA and middle-molecular-weight PDLA species and by choosing a suitable annealing condition, we have succeeded for the first time to observe the phase transition phenomenon in the heating and cooling processes by measuring the temperature dependence of the X-ray fiber diagram and polarized IR spectra. The as-drawn sample was found to consist of the two crystalline phases; disordered α form (α′) and stereocomplex (β_c). In the heating process, the α′ form was found to be reorganized into the oriented α phase above 120°C, which melted around 180°C. However, by heating this sample some more, some parts of the molten chains are found to crystallize on the surface of the β_c crystallites with high degree of chain orientation. In the subsequent cooling process from the β_c phase, the unoriented α crystal appeared at around 120°C and coexisted with the highly oriented β_c crystals.