Phase behavior of polylactides in solvent-nonsolvent mixtures

Isothermal phase diagrams for the semicrystalline poly-L-lactide (PLLA) and the amorphous poly-DL-lactide (PDLLA) in combination with several solvent-nonsolvent combinations (dioxane/water, dioxane/methanol, chloroform/methanol, and NMP/water) have been determined. The locations of the liquid-liquid miscibility gap, the solid-liquid miscibility gap and the vitrification boundary in the isothermal phase diagrams at 25°C were identified. The liquid-liquid miscibility gap for the systems with PLLA was located in the same composition range as the corresponding systems with PDLLA. For the systems containing PLLA solid-liquid demixing was thermodynamically preferred over liquid-liquid demixing. Attempts were made to correlate the experimental findings with predictions on the basis of the Flory-Huggins theory for ternary solutions using interaction parameters derived from independent experiments. Qualitative agreement was found between the theoretical predictions and the experimentally obtained liquid-liquid miscibility gap. No good agreement was found for the solid-liquid miscibility gap.