Abstract
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.
Highlights
The idea of making fibers by electrospinning is approaching its centennial anniversary,[1] it has only been in the last two decades that the technique has truly flourished.[2−8] The introduction of core−sheath electrospinning using nested capillary spinnerets, often coaxial, has led to an explosion of creativity, with a diversity of functional nano- and microfibers with a variety of internal morphologies being successfully electrospun.[9−13] Pioneering contributions in demonstrating the potential of dual-phase coaxial electrospinning for making controlled core−sheath fibers were published by Sun et al.,[14] Yu et al.,[15] and Li and Xia.[16]
When attempting to electrospin the liquid crystals (LC) RO-TN 651, which has negligible miscibility with water, as a core inside the PAA− water sheath solution, the relatively high γcs causes significant problems, as seen from a detailed frameby-frame analysis of Supporting Information Movie S1, showing the Taylor cone dynamics during a run with flow rates optimized for maximum fiber filling
As seen at the beginning of Supporting Information Movie S1 and in Figure 3a, the LC pumped from the inner needle forms a nearly spherical droplet inside the external sheath solution, hovering far above the Taylor cone apex from which the jet is ejected
Summary
The idea of making fibers by electrospinning is approaching its centennial anniversary,[1] it has only been in the last two decades that the technique has truly flourished.[2−8] The introduction of core−sheath electrospinning using nested capillary spinnerets, often coaxial, has led to an explosion of creativity, with a diversity of functional nano- and microfibers with a variety of internal morphologies being successfully electrospun.[9−13] Pioneering contributions in demonstrating the potential of dual-phase coaxial electrospinning for making controlled core−sheath fibers were published by Sun et al.,[14] Yu et al.,[15] and Li and Xia.[16]. While the mineral oil used as core liquid by Li and Xia was largely a sacrificial fluid, its presence ensuring tube-like fiber morphology, several subsequent electrospinning studies incorporated more precious liquids, e.g., phase change materials,[17−19] liquid crystals (LC),[20−36] and shear thickening fluids,[37] to remain as a functional core inside the fibers. These specially selected core liquids enhance the composite fibers with dynamic and responsive performance that the sheath polymer itself is incapable of, while the coaxial fiber geometry provides a powerful means of encapsulating the liquids which are unspinnable on their own in a flexible form factor with high surface-to-volume ratio. Several modifications of the fundamental core−sheath electrospinning process have been explored, such as triple-phase coaxial electrospinninga, enabled by adding a third nested capillary, which can yield fibers with an intermediate layer between the innermost core and the outermost sheath.[38−41] With noncoaxial electrospinning using multiple bundled capillaries inside an outer capillary that flows the sheath solution, fibers were produced with multiple core channels, consisting of identical[42] or different[18,27] materials
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