Electrospinning of polymer solutions is a multifaceted process that depends on the careful balancing of many parameters to achieve a desired outcome, in many cases including mixtures of multiple solvents. A systematic study of how the solution viscosity 𝜼-a good probe of solvent-polymer interactions-and the electrospinnability change when poly(acrylic acid) (PAA) is dissolved in ethanol-water mixtures at varying mixing ratio is carried out. A pronounced maximum is found in 𝜼 at a water-to-ethanol molar ratio of about 2:1, where the solvent mixture deviates maximally from ideal mixing behavior and partial deprotonation of carboxyl groups by water coincides synergistically with dissolution of the uncharged protonated PAA fraction by ethanol. The PAA concentration is tuned as a function of water-ethanol ratio to obtain a common value of 𝜼 for all solvent mixtures that is suitable for electrospinning. For high PAA content, the Taylor cone grows in volume over time despite minimum solution flow rate, even experiencing surface gelation for ethanol-rich solutions. This is attributed to the hygroscopic nature of PAA, drawing excess water into the Taylor cone from the air during spinning.
Core–sheath
electrospinning is a powerful tool for producing
composite fibers with one or multiple encapsulated functional materials,
but many material combinations are difficult or even impossible to
spin together. We show that the key to success is to ensure a well-defined
core–sheath interface while also maintaining a constant and
minimal interfacial energy across this interface. Using a thermotropic
liquid crystal as a model functional core and polyacrylic acid or
styrene-butadiene-styrene block copolymer as a sheath polymer, we
study the effects of using water, ethanol, or tetrahydrofuran as polymer
solvent. We find that the ideal core and sheath materials are partially
miscible, with their phase diagram exhibiting an inner miscibility
gap. Complete immiscibility yields a relatively high interfacial tension
that causes core breakup, even preventing the core from entering the
fiber-producing jet, whereas the lack of a well-defined interface
in the case of complete miscibility eliminates the core–sheath
morphology, and it turns the core into a coagulation bath for the
sheath solution, causing premature gelation in the Taylor cone. Moreover,
to minimize Marangoni flows in the Taylor cone due to local interfacial
tension variations, a small amount of the sheath solvent should be
added to the core prior to spinning. Our findings resolve a long-standing
confusion regarding guidelines for selecting core and sheath fluids
in core–sheath electrospinning. These discoveries can be applied
to many other material combinations than those studied here, enabling
new functional composites of large interest and application potential.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.