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.

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Electrospun polymer fibers are shown to have wrinkled surface topographies that result from buckling instabilities during processing. A glassy shell forms on the surface of the gel-like core during solvent evaporation; continued evaporation leads to a contraction mismatch between the core and shell that triggers buckling of the shell. The wrinkled topographies are quantified in terms of the critical buckling wave number and wavelength. The results explain the observed wrinkled topographies and provide a framework for designing fibers with high specific surface areas and textured/patterned surface topographies to enhance surface dominated properties in fibers and fibrous mats.

Adequate mechanical integrity of nonwoven fabrics is generally a prerequisite for their practical usage. Nonwoven fiber mats of poly(trimethyl hexamethylene terephthalamide) (PA 6(3)T) with average fiber diameters from 0.1 to 3.6 microns were electrospun from solutions in dimethylformamide and formic acid and their in-plane mechanical response characterized. Two quantitative microstructure-based models that relate the Young's moduli of these fabrics to those of the fibers are considered, one assuming straight fibers and the other allowing for curved fibers. It is found that the model allowing for curved fibers provides a quantitative relationship between the Young's moduli of the mats and those of the fibers themselves. The governing factors that affect the mechanical properties of nonwoven mats are the porosity of the mats, the intrinsic fiber modulus, and the average fiber diameter, curvature (or "curl") and distance between fiber-to-fiber junctions. Especially for submicron diameter fibers, both the intrinsic fiber properties and fiber curvature make important contributions to the mechanical behavior of their nonwoven fabrics.2

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