Extension Rate and Bending Instability of Electrospinning Jets: the Role of the Electric Field

Transient Viscosity Adjustment Using a Coaxial Nozzle for Electrospinning Nanofibers from Non-Spinnable Pure m-Poly(hydroxyamide)

In this study, a transient viscosity adjustment method using a coaxial nozzle was explored to fabricate nanofibers from non-spinnable m-poly(hydroxyamide) (m-PHA). Unlike conventional electrospinning methods that often require additives to induce fiber formation, this approach relies on a sheath-core configuration, introducing tetrahydrofuran (THF) to the sheath to temporarily adjust solution viscosity. The diffusion of THF into the core m-PHA solution resulted in momentary solidification at the interface, promoting nanofiber formation without compromising polymer solubility. SEM and rheological analyses confirmed that optimized sheath-to-core flow ratios yielded nanofibers with significantly reduced particle formation. Notably, increasing the THF flow rate facilitated a faster solidification rate, enhancing jet elongation and resulting in uniform nanofibers with diameters of approximately 180-190 nm. Although complete nanofibers without beads were not achieved in this study, this coaxial electrospinning approach presents a possible pathway for fabricating nanofibers from polymers with limited spinnability, potentially expanding the application scope of electro-spun materials in high-performance fields.

Triboelectric Nanogenerator-Based Near-Field Electrospinning System for Optimizing PVDF Fibers with High Piezoelectric Performance

Electrospinning is an effective method to prepare polyvinylidene fluoride (PVDF) piezoelectric fibers with a high-percentage β phase. However, as an energy conversion material for micro- and nanoscale diameters, PVDF fibers have not been widely used due to their disordered arrangement prepared by traditional electrospinning. Here, we designed a near-field electro-spinning (NFES) system driven by a triboelectric nanogenerator (TENG) to prepare PVDF fibers. The effects of five important parameters (PVDF concentration, needle inner diameter, TENG pulse DC voltage (TPD-voltage), flow rate, and drum speed) on the β phase fraction of PVDF fiber were optimized one by one. The results showed that the electrospun PVDF fibers had uniform diameter and controllable parallel arrangement. The β phase content of the optimized PVDF fiber reached 91.87 ± 0.61%. For the bending test of a single PVDF fiber piezoelectric device, when the strain is 0.098%, the electric energy of the single PVDF fiber device of NFES reaches 7.74 pJ and the energy conversion efficiency reaches 13.5%, which is comparable to the fibers prepared by the commercial power-driven NFES system. In 0.5 Hz, the best matching load resistance of a PVDF single fiber device is 10.6 MΩ, the voltage is 6.1 mV, and the maximum power is 3.52 pW. Considering that TENG can harvest micromechanical energy in the low frequency environment, the application scenario of the NFES system can be extended to the wild or remote mountainous areas without traditional high-voltage power supply. Therefore, the electrospun PVDF fibers in this system will have potential applications in high-precision 3D fabrication, self-powered sensors, and flexible wearable electronic products.

Light Scattering of Electrospinning Jet with Internal Structures by Flow-Induced Phase Separation

Herein, the direct morphological evidence of the extension-induced phase-separated structures in the electrospinning jet observed by high-speed video imaging and by light scattering technique is reported. Model solutions of poly(vinyl alcohol) (PVA)/water are electrospun. Two types of internal structures, that is, long strings and short ellipsoids, are found. A light scattering model is derived for the V_v scattering configuration to account for the scattered intensities contributed from the liquid jet itself and those from the internal structures. For the severely stretching jet of PVA/water, the Vv intensity profile is dominant by the internal structures to mask the scattering contribution from the jet itself. Moreover, the H_v intensity profile reflects the anisotropy of the oriented chains parallel to the jet axis. For the 7 wt% solution, the derived extension rate in the vicinity of the Taylor cone apex is about 3420 s^-1 , which is higher than the Rouse relaxation rate measured by rheometer. It is concluded that extension-induced phase separation of the single-phase PVA solution is likely to occur in Taylor-cone apex to trigger the self-assembly process for producing strings (and/or bulges) in the flowing jet, which eventually transform to become the nanofibers, after solvent removal, to be collected on the grounded collector.

Electrospinning of Aqueous Solutions of Atactic Poly(N-isopropylacrylamide) with Physical Gelation

The phase diagram of a given polymer solution is used to determine the solution’s electrospinnability. We constructed a phase diagram of an aqueous solution of atactic poly(N-isopropylacrylamide) (a-PNIPAM) based on turbidity measurements and the rheological properties derived from linear viscoelasticity. Several important transition temperatures were obtained and discussed, including the onset temperature for concentration fluctuations T1, gel temperature Tgel, and binodal temperature Tb. On heating from 15 °C, the one-phase a-PNIPAM solution underwent pronounced concentration fluctuations at temperatures above T1. At higher temperatures, the thermal concentration fluctuations subsequently triggered the physical gelation process to develop a macroscopic-scale gel network at Tgel before the phase separation at Tb. Thus, the temperature sequence for the transition is: T1 < Tgel < Tb~31 °C for a given a-PNIPAM aqueous solution. Based on the phase diagram, a low-temperature electrospinning process was designed to successfully obtain uniform a-PNIPAM nanofibers by controlling the solution temperature below T1. In addition, the electrospinning of an a-PNIPAM hydrogel at Tgel < T < Tb was found to be feasible considering that the elastic modulus of the gel was shown to be very low (ca. 10−20 Pa); however, at the jet end, jet whipping was not seen, though the spitting out of the internal structures was observed with high-speed video. In this case, not only dried nanofibers but also some by-products were produced. At T > Tb, electrospinning became problematic for the phase-separated gel because the enhanced gel elasticity dramatically resisted the stretching forces induced by the electric field.