Intermolecular Hydrogen Bonding

By investigating the formation process and structures of nylon-6 (PA-6) and methoxypolyethylene glycol (MPEG) oligomer/nylon-6 nanofiber/net membranes, Kim et al. attributed the formation of nanonets to the hydrogen bonds between the nanonets and the nylon-6 nanofibers, and proposed the relevant intermolecular hydrogen bonding mechanism. Fig. 8.2 presents a schematic illustration of the hydrogen bond formation mechanism. In the high-voltage electric field, the electronegativity difference between hydrogen and oxygen/nitrogen atoms would be further enhanced and result in the high polarity of the molecules due to the more charges provided by the electric field. The protonated amide groups of ionic molecules would form strong hydrogen bonds with oxygen atoms of the nylon-6 molecules in scaffold nanofibers, and oxygen atoms of the nylon-6 molecules could connect with hydrogen atoms of the amide groups of the nanofiber as well, to form the interconnected spiderweb-like nanofibers/nets. Moreover, the intermolecular hydrogen bonding between oxygen and hydrogen atoms of MPEG molecules and amide groups of nylon-6 molecules was also proposed to reveal the formation of MPEG oligomer/nylon-6

Basic Setup For Electronetting

As electronetting and electrospinning are similar electrohydrodynamics techniques and the former accompanies the traditional electrospinning process, the basic setup for electronetting is almost the same as the electrospinning setup. The typical basic setup for electronetting includes two standard apparatuses of vertical and horizontal forms, of which the horizontal one is widely used, as shown in Fig. 8.1A. It is clearly shown that the typical electronetting setup consists of four parts, a high-voltage power supply, syringe pump, spin neret, and collecting receiver. The power supply with high voltage applied to the needle is used to induce the formation of charged liquids in the form of jets and/or droplets. A direct current power supply is usually employed for electronetting, while an alternating current supply can also be used as the spinning power. The spinneret with designed needle is attached to the syringe pump, which can control the flow rate of the precursor solution. Generally, the collecting receiver, such as a metal plate, screen, or rotating roller, is used to collect the nanofiber/net assemblies by virtue of the electric field between the needle of the spinneret and the receiver.

CENTRIFUGAL FORCE

Using PEO as a model polymer, Peterson et al. produced nanofibers at a production rate of 6.5 mg/h cm2(Fig. 7.19). Important parameters that affect the nanofiber production rate include voltage, spinneret rotation speed, solution feed rate, distance between spinning head and collector, and solution concentration. In this way, polyacrylonitrile fibers were produced successfully. This technique is also compatible with various collectors, such as a moving belt for collecting large membranes, parallel collector electrodes for aligned fibers, and parallel collector electrodes with one rotating electrode for producing short yarns.

A SUMMARY OF NEEDLE-LESS ELECTROSPINNING SPINNERETS

Moreover, asymmetrical spinnerets, such as a coil (Fig. 7.18), could also produce uniform nano- fibers and nanofiber mats. Unlike the symmetrical spinnerets, the coil spinneret shows an uneven electric field distribution.

Disk

A slowly rotating disk, which is partially immersed in polymer solution, eliminates the need for self replenishment of the solution at the fiber-spinning edge (Fig. 7.15). It also ensures a fresh coating of solution on the disk surface and prevents the solution from solidifying at the disk edge during electrospinning. Rotating cylinder and disk spinnerets have been analyzed by Niu et al. The rim radius of a cylinder spinneret can reduce discrepancies in electric field intensity and influence the fiber productivity. Thinner disk spinnerets increase the electric field intensity, leading to finer nanofibers and higher throughput.

WIRE SPINNERETS

During the electrospinning process, the high voltage was applied to the stationary wires and a polymer solution was loaded on the surface of the wires by moving the wire through the polymer solution. Then, numerous jets were generated from the wire surface. It has a better spinning performance than the previous version, which use a rotating drum as spinneret.

CLEFT SPINNERET

Lukas et al. reported an electrospinning setup that used linear clefts as the spinneret (Fig. 7.6). The authors also developed a one-dimensional electrohydrodynamic theory to explain the process of electrospinning conductive liquids from an open plane surface (Fig. 7.6). During the electrospinning process, the amplitude of characteristic wavelength grew faster because of the electrical force. The fastest-growing stationary wave marked the onset of electrospinning from a free liquid surface. This theory not only predicated the critical value of the electrospinning process but also explained the upward needle-less electrospinning.