Morphology of Porous and Wrinkled Fibers of Polystyrene Electrospun from Dimethylformamide

Submicron diameter fibers of polystyrene are electrospun from solutions in dimethylformamide (DMF). When electrospun in a high-humidity environment, the interior of these fibers was found to be highly porous rather than consolidated, despite the smooth and nonporous appearance of the fiber surfaces. The formation of interior porosity is attributed to the miscibility of water, a nonsolvent for the polymers in solution, with DMF. The resulting morphology is a consequence of the relatively rapid diffusion of water into the jet, leading to a liquid−liquid phase separation that precedes solidification due to evaporation of DMF from the jet. When electrospun in a low-humidity environment, the fibers exhibit a wrinkled morphology that can be explained by a buckling instability. Understanding which morphology forms under a given set of conditions is achieved through the comparison of three characteristic times: the drying time, the buckling time, and the phase separation time. The morphology has important consequences for the properties of the fibers such as their mechanical strength and stiffness.