Stable Electrospinning of Core-Functionalized Coaxial Fibers Enabled by the Minimum-Energy Interface Given by Partial Core–Sheath Miscibility

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.

perturbations by quickly relaxing into its equilibrium quasi-spherical shape, indicating that 9 mN/m is indeed a significant (albeit generally low) interfacial tension.

RO-TN 651-(PAA/ethanol) system
To perform interfacial tension experiments with the RO-TN 651-(PAA/ethanol) system, we used a stainless steel needle to create a pendant drop of the LC phase inside a bath of a PAA/ethanol solution (10.0% w/w). The experiments were performed at 21 C. SI Movie 7 shows the production of LC drops inside the polymer solution bath. Even by employing a very low flow rate (0.05 µL/s), a stable drop cannot be formed. Instead, a series of small drops connected by a transient LC jet are produced. The LC jet seems to progressively dissolve in the PAA/ethanol solution, leading to individual RO-TN 651 drops that move towards the bottom of the sample cell due to gravity. Interestingly, drops that come into contact do not coalesce (within the time of the experiment), indicating some extent of stabilization, presumably by PAA chains adsorbing onto the LC-polymer solution interface. In addition, the ejected drops seem to slightly increase in size as they sediment.
This suggests that swelling of the LC drop due to ethanol diffusing into its interior might take place. This is further supported by the observation that the drop-solution interface becomes less well-defined, macroscopically, as seen from the decreasing sharpness of the fluid boundary. This qualitative picture is in accordance with the phase behavior of the RO-TN 651-(PAA/ethanol) system ( Fig.4) that shows that the two phases are fully miscible at very low ethanol concentrations; the pendant drop experiment described here is analogous to this case, considering that we have droplets with volumes on the order of microliters inside a bath with volume on the order of a few milliliters.
Even if we stop dosing the LC after a first drop is formed, this drop does not remain stable; the LC keeps flowing, even though there is no external pressure imposed on the syringe. The inability to make a stationary drop prevents us from measuring the equilibrium interfacial tension of the RO-TN 651-(PAA/ethanol) system. SI Movie 6 shows the early S2 stage of the formation of a LC drop, under the optimum conditions we identified (target drop volume 1 µL, flow rate 0.1 µL/s); higher flow rates led to a pronounced ejection of LC drops connected by a LC jet, whereas lower flow rates resulted in the case where the LC drop is not in full contact with the whole orifice of the needle. Under these experimental conditions, a well-defined drop can be formed, which is followed by the formation of a second drop that is however highly non-spherical (due to the broken jet created in its front). the needle, a soft, solid-like phase of irregular shape emerges first. We believe this is highly concentrated (and perhaps gelled) PAA/ethanol solution that was previously left at the S3 opening of the needle. This can be due to drying of a portion of this solution that was left at the tip of the needle while it is removed from the solution bath in order to be cleaned.
Without this cleaning cycle, we are not able to form a pendant drop at all. Furthermore, as more LC is flushed through the needle, an irregularly shaped LC drop emerges which surrounds the solid-like polymer solution phase. At longer times, and with more LC emerging from the orifice, more drops are formed that are initially separated from the first LC drop before they eventually merge to form a large, irregularly-shaped LC drop. The difficulty in merging, in conjunction with the irregular shape of the LC-polymer solution interface (which is nevertheless well-defined), suggests that the LC phase is stabilized by the polymer solution, presumably by PAA chains adsorbing onto the fluid interface. While interesting, these observations clearly show that pendant drop tensiometry is not an appropriate method to measure the interfacial tension of this highly complex system. Figure S1: Snapshots of the interfacial tension measurements using pendant drop technique of ROTN 651 to a solution of 10% w/w PAA in anhydrous ethanol. These still frames are extracted from SI movie 6, the outer diameter of the stainless steel needle used for the measurements is 0.51 mm. Table S1 gives the electrospinning parameters used in the experiments. and an aqueous solution of 11.5% w/w PAA as sheath. The outer diameter of the spinneret needle is 1.7 mm and the electrospinning parameters and conditions are listed in the Table   S1. This movie corresponds to Figure 3.  Table S1. This movie corresponds to Figure5.

S4
SI Movie 3: Movie of the Taylor cone recorded during electrospinning of 10% w/w anhydrous ethanol in RO-TN 651 core and a solution of 10% w/w PAA in anhydrous as sheath.
The outer diameter of the spinneret needle is 1.7 mm and the electrospinning parameters and conditions are listed in the Table S1. This movie corresponds to Figure 6.