A Solution Selection Model for Coaxial Electrospinning and Its Application to Nanostructured Hydrogen Storage Materials

Coaxial electrospinning was used to encapsulate the complex hydride ammonia borane in polystyrene to improve its properties as a hydrogen storage material. A solvent selection system was developed by using the Hansen solubility parameters to facilitate the choice of compatible solvents for core and shell. This enabled systematic optimization of the parameters needed for successful coelectrospinning. This approach has general application for any multiphase electrospinning system, including ones where the core is highly conducting or nonpolymeric. The resulting fiber morphologies depend strongly on the degree of miscibility of core and shell solutions. Fibers spun from immiscible core−shell solutions had a classic coaxial structure. Fibers produced from semimiscible core−shell solutions were highly porous, with inclusions extending through the fiber and an ordered radial and longitudinal distribution of nanoscale pores on the fiber surface. We suggest that this type of porosity may be due to an instability created in the nonaxisymmetric modes at the core−shell interface, resulting in intrusion of the core into the shell polymer. These controllably porous structures have numerous potential applications including materials templating or drug delivery. In the porous fibers, the temperature of the first hydrogen release of ammonia borane is reduced to 85 °C. This result suggests a nanostructured hydride, but a large mass loss indicates that much of the ammonia borane is expelled on heating. The coaxial fibers, in contrast, appear to encapsulate the hydride successfully. The coaxial and porous fibers alike showed no significant release of borazine, suggesting two different suppression mechanisms for this impurity.