Electrospinning of highly crystalline polymers for strongly oriented fibers

Electrospun nanofibers (NFs) often demonstrate an exponential increase in mechanical and other properties at reduced diameters. Molecular orientation emerges as a key parameter for the performance of amorphous and low crystallinity polymers, but single-fiber structural investigations are still lacking for highly crystalline polymers. Herein, polarized confocal Raman spectroscopy reveals that fibers of highly crystalline poly(ethylene oxide) (PEO) maintain a high orientation over a broad range of diameters, in strong contrast with the usual exponential trend. This observation stands for five electrospinning solvents of widely different properties. By comparison, poly(oxymethylene) (POM) NFs also show a high orientation at low diameters, but it decreases substantially for diameters larger than ∼1400 nm, a result attributed to the lower crystallinity of POM compared to that of PEO. The results show that the exponential orientation dependence on fiber diameter is not universal and stress the importance of polymer crystallinity on the structure and properties of electrospun nanofibers. This work guides the preparation of fibers with optimal orientation-dependent properties and shows that high crystallinity can afford more robust materials whose performance is less affected by variations in experimental conditions, a valuable feature for most applications