Influence of wetting on morphology and core content in electrospun core-sheath fibers.

Coaxial electrospinning allows easy and cost-effective realization of composite fibers at the nano- and microscales. Different multifunctional materials can be incorporated with distinct localization to specific regimes of the fiber cross section and extended internal interfaces. However, the final composite properties are affected by variations in internal structure, morphology, and material separation, and thus, nanoscale control is mandatory for high-performance application in devices. Here, we present an analysis with unprecedented detail of the cross section of liquid core-functionalized fibers, yielding information that is difficult to reveal. This is based on focused ion beam (FIB) lift-out and allowing HR-TEM imaging of the fibers together with nanoscale resolution chemical analysis using energy dispersive X-ray spectroscopy (EDS). Unexpectedly, core material escapes during spinning and ends up coating the fiber exterior and target substrate. For high core injection rate, a dramatic difference in fiber morphology is found, depending on whether the surface on which the fibers are deposited is hydrophobic or hydrophilic. The latter enhances postspinning extraction of core fluid, resulting in the loss of the functional material and collapsed fiber morphology. Finally, in situ produced TiO2 nanoparticles dispersed in the polymer appear strikingly different when the core fluid is present compared to when the polymer solution is spun on its own.