Controlling the nanostructure of electrospun polymeric fibers

The high strain rate extensional flow of a semi-dilute polymer solution can cause substantial stretching and disentanglement of the polymer network. In this study, we conducted a theoretical and experimental investigation of the effects of electrospinning, a flow governed by high strain rate and rapid evaporation, on the polymer matrix of the resulting nanofibers. Modeling of the dynamic evolution of the entangled polymer network in an electrospinning jet predicted substantial longitudinal stretching and radial contraction of the network, a transformation from an equilibrium state to an almost fully-stretched state. This prediction was verified by X-ray phase-contrast imaging of electrospinning jets, which revealed a noticeable increase in polymer concentration at the jet center, within a short distance from the jet start. Additionally, polymer entanglement loss in consequence of stretching was evidenced in jet fragmentation and appearance of short nanofibers, affecting the entanglements density and molecular orientation of as-spun fibers. The stretching model was expanded to semi-flexible conjugated polymer chains, and scanning near field optical microscopy of electrospun nanofibers of such optically active polymers revealed that the network's dense elongated conformation effectively remains after jet solidification. By tuning the electrospinning conditions, the unique size-dependent properties of nanofibers can be controlled and improved, potentially leading to novel applications in engineering and life sciences.