A MORPHOLOGICAL-STUDY OF MEMBRANES OBTAINED FROM THE SYSTEMS POLYLACTIDE-DIOXANE-METHANOL, POLYLACTIDE-DIOXANE-WATER, AND POLYLACTIDE-N-METHYL PYRROLIDONE-WATER

The influence of liquid-liquid demixing, solid-liquid demixing, and vi trification on the membrane morphologies obtained hom several polylact ide-solvent-nonsolvent systems has been investigated. The polymers inv estigated were the semicrystalline poly-L-lactide (PLLA) and the amorp hous poly-DL-lactide (PDLLA). The solvent-nonsolvent systems used were dioxane-water, N-methyl pyrrolidone-water and dioxane-methanol. For e ach of these systems it was attempted to relate the membrane morpholog y to the ternary phase diagram at 25 degrees C. It was demonstrated th at for the amorphous poly-DL-lactide the intersection of a glass trans ition and a liquid-liquid miscibility gap in the phase diagram was a p rerequisite for the formation of stable membrane structures. For the s emicrystalline PLLA a wide variety of morphologies could be obtained r anging from cellular to spherulitical structures. For membrane-forming combinations that show delayed demixing, trends expected on the basis of phase diagrams were in reasonable agreement with the observed memb rane morphologies. Only for the rapidly precipitating system PLLA-N-me thyl pyrrolidone-water were structures due to liquid-liquid demixing o btained when structures due to solid-liquid demixing were expected. Pr obably, rapid precipitation conditions promote solid-liquid demixing o ver liquid-liquid demixing, because the activation energy necessary fo r liquid-liquid demixing is lower than that for crystallization. (C) 1 996 John Wiley & Sons, Inc.