Static liquid yarn

Having water as a collector allows deposited nanofibers to be lifted off the surface without any adhesion to the it. If the membrane is sufficiently thin, the mesh of collected nanofibers will consolidate to form a yarn under its own weight. A continuous strand of yarn can be drawn if the yarn take-up speed matches with the deposition rate of fibers on the water surface. The nanofibers that made up the yarn were found to be highly aligned which is probably due to the tension exerted on yarn with the water between the fibers functioning as lubricants. Several electrospinning nozzles can be used to increase the fiber deposition rate on the water surface. The fibers may be spun with opposing polarity to encourage adhesion between the electrospinning jets [Yao C et al 2007].

Other non-solvent to the electrospinning solution may also be used as the collector bath although water is more commonly used. Nevertheless, Medina E F et al (2013) used ethanol as the coagulation bath for the collection and drawing of polyvinyl alcohol yarn due to its solubility in water. The drawn yarn also showed similar highly aligned nanofiber arrangement as water coagulation bath. With a yarn diameter of about 15 um and average nanofiber diameter of 548 nm, the maximum strength and modulus was found to be 90 MPa and 2.55 GPa respectively. Franco et al (2018) used this technique to produce polyamide 6/66 (PA 6/66) nanofibrous yarn and distilled water as coagulation bath. A PA 6/66 solution concentration of 17%wt with formic acid as solvent was found to be optimal for higher productivity (1.39±0.15 mg/min), draw ratio (9.0±1.2), and tensile strength (29.64±7.40 MPa). The electrospun PA 6/66 nanofibers bundle has a crystallinity and Tg of 17.8% and 40°C respectively which is much lower than its original pellets form with respective crystallinity and Tg at 39.45% and 51°C. Electrospun fibers are known to exhibit lower crystallinity compared to their pellet form and this has been attributed to faster rate of vaporization during electrospinning which limits the time available for the molecules to arrange themselves. With lower crystallinity, the Tg of the electrospun nanofibers bundle is lower due to less molecular chain flow obstruction.

In a slight modification of the setup, Pan et al had the yarn drawn off the reservoir to pass through a heated chamber before collecting on the mandrel. By varying the speed of the collector, they showed that the tensile strength of the polyamide 6/66 nanofibrous yarn increases with the take-up speed.

Polymer	Solvent	Take-up speed	Reference
Polyacrylonitrile (PAN)	dimethyl formamide	0.05 m/s	Smit et al 2005
Polyamide 6/66	88% formic acid	0.15 m/s	Pan et al 2008
Zein/poly(lactide-co-glycolide)	Dimethyl formamide	0.17 m/s	Yao C et al 2007

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▼ Credit and Acknowledgement