Polylactide microspheres prepared by premix membrane emulsification—Effects of solvent removal rate

Polylactide microspheres were prepared by pre-mix membrane emulsification and subsequent extraction of solvent in a coagulation bath, and ultimately to the gas phase. The polymer was dissolved in dichloromethane and emulsified with water or water–methanol mixtures by repeated passage through a glass membrane. During and after emulsification, the droplets are exposed to a bath consisting of a mixture of water and methanol. Transfer of dichloromethane takes place into the bath and (subsequently) to the gas phase. Compared to water, the solubility of dichloromethane is increased when using water–methanol mixtures; the continuous phase can quickly dissolve a significant amount of the solvent, while transfer to the gas phase is strongly enhanced as well. This was observed experimentally and by computer simulation, using a combined model based on the Maxwell–Stefan theory for non-ideal, multi-component mass transfer.With increasing methanol concentration, the size and span of the microspheres became smaller, and was approximately 1 μm at 30% methanol. The surface morphology of these particles was solid and smooth, whereas holes were observed in those prepared in pure water. At methanol concentrations higher than 30%, the size of the microspheres increased again. This is probably due to the swelling of the particles because of the high in-diffusion of methanol which increases the porosity of the particles. Our main conclusion is that particles of defined size and size distribution can be produced by simply adjusting the non-solvent composition of the pre-mix.