Polymer solution electrospraying: A tool for engineering particles and films with controlled morphology

Polymer solution electrospray is a relatively recent research area which has become increasingly active due to its potential for engineering particle morphologies at the micro- and nanoscales, which can be useful for pharmaceutical applications, energy devices, and other fields. Despite significant efforts in this research area, and the fact that electrospray (ES) theory for pure solvents is well-established, improved understanding is still needed on how polymeric structures form during electrospray, to be able to predict the diversity of particle morphologies encountered. This situation is due partly to the complexity of the mechanisms involved, which combine the rich behavior of polymer solutions with the unique physics of ES (with disparate temporal and spatial scales). In this article, we start by reviewing which application fields have driven research in polymer solution electrospraying, and which morphologies matter. We then develop a theoretical framework on the mechanisms responsible for the transformation of a polymer solution droplet made by ES into a solid morphology. We discuss the applicability of a droplet drying model used in Spray Drying to the case of electrospray droplets. We further highlight the importance of Coulombic instabilities in developing polymeric structures. These structures include particles with pointed ends, sometimes with long nanofilaments, as well as other elongated shapes, and globular shapes with a broad size distribution. We identify the conditions necessary to prepare monodisperse globular particles (e.g. spheres). Finally, we analyze the roles played by the presence of nonsolvent vapors during droplet drying. This includes ambient humidity when electrospraying non water-soluble polymers, whose effects on particle porosity have often been overlooked. We close with recommendations and clarifications of practical importance for practitioners.