Electrospinning helical fibers

The mechanical property of a fibrous membrane comes from the arrangement of the fibers. Therefore, when the membrane is made out of helical fibers instead of straight fibers, a change in mechanical properties can be expected. There are also other potential benefits of having helical fibers, Zhao et al (2017) showed that helical fibers are better at trapping oil compared to straight fibers.

Electrospinning jet is known to travel in a chaotic but largely helical motion in flight. Observation of helical nanofibers on a flat plate collector has been reported by several researchers especially when the collector is moving relative to the nozzle tip. Getting individual helical strand of electrospun fibers is as simple as catching the fibers in a slow moving collector. For some, this is considered an artefact due to buckling of the nanofibers upon impacting the collector when the collector speed is insufficient to lay the nanofiber straight [Zheng et al 2012].

Electrospun helical fibers have been reported by various research groups. In some cases, general buckling of the electrospinning jet on the collector was able to yield helical fibers. Yu et al (2008) was able to collect electrospun helical polycaprolactone (PCL) by having a tilted glass slide as collector. Since the deposition of helical fibers were due to the electrospinning jet impacting the surface of the collector, the gradient of tilt and the distance of the collector away from the electrospinning nozzle determine the formation of the helical fibers.

A side-by-side dual orifice nozzle may be used to extrude two different solutions such that a single electrospun fiber is made out of two materials along its length. When there is differences in the shrinkage or expansion property between the materials, the fiber may coil into a helical form. Such an output has been demonstrated by Zhang et al (2009) using high shrinkage polyethylene terephthalate (HSPE) and polytrimethylene terephthalate (PTT) as the bicomponents. Lin et al (2005) showed that using polyacrylonitrile (PAN) and polyurethane (PU) as the two materials in their bi-component system, they were able to construct curly and helically crimped fibers. This behavior has been attributed to differential shrinkage within the fibers which causes one of the components to compress. Increasing the amount of PAN by increasing its flow rate leads to less curly nanofibers while the converse is true when the PU content was increased. Chen et al (2009) showed that very curly nanofibers may work as a nanospring and this may influence the mechanical properties of the resultant mat. Using poly(m-phenylene isophthalamide) (Nomex) and thermoplastic polyurethane (TPU), they found that the net elastic force of the aligned nanospring fibers from side-by-side bicomponent fiber showed a higher elongation, higher toughness and higher modulus than fibers without nanospring. Fibers with and without the nanospring were both side-by-side bicomponent fibers. To obtain aligned fibers without nanospring, the collector was set at a higher rotation speed while aligned fibers with nanospring were collected with lower rotation speed. Wu et al (2018) did a study on the potential for helical fiber formation of three polymers, poly(m-phenylene isophthalamide) (Nomex), polystyrene (PS) and polyacrylonitrile (PAN) with polyurethane (TPU) in side-by-side bicomponent form. Of the three polymers, only Nomex was found to form consistent helical fibers with TPU. Three possible hypothesis was brought forward from their investigation. Nomex has the highest stiffness compared to PS and PAN and the greatest difference in the material stiffness against TPU. In terms of miscibility of the solution with TPU solution, PS solution is completely immiscible with TPU solution, PAN solution is completely miscible while Nomex solution is partially miscible. Partially miscible solutions may facilitated in the bonding between the two polymer components and would have helped in the helical fiber formation. Lastly, Nomex solution was found to exhibit Coulomb force between the solution and the polymer molecule during electrospinning and this generates a longitudinal interfacial stress between the Nomex and TPU solutions to encourage helical structure formation. Such Coulomb forces were not found in other polymer solutions. Instead of using hydrophobic NOMEX with TPU, Wu et al (2018A) showed that helical fibers may also be formed using hydrophilic cellulose acetate (CA) with TPU. To generate helical electrospun CA/TPU bicomponent fibers, LiCl salt must be added to the solution. The presence of LiCl aided in the stiffening of CA such that the stiffness difference between CA and TPU polymer chains are greater. LiCl also increases the conductivity of the solution and the longitudinal interfacial stress between the molecular chain and the solution. However, the experiment also showed that beyond an optimum concentration of LiCl, less helical fibers were formed which was attributed to excessive conductivity that stretched the helical fiber to form a straight fiber. While it is evident from their studies that not all bicomponent polymer fibers will form helical fibers, more studies will be needed to determine the requirement for helical fiber formation.

Helical bicomponent fibers need not be restricted to side-by-side arrangement. Zhao et al (2017) showed that a core-shield bicomponent electrospun fibers may also form helical fibers. In their study, electrospun polyvinylidene fluoride (PVDF) with a good ductility was utilized as the core and stiffer, porous polystyrene (PS) as the shell. On a stationary collector, helical fibers were formed. A rotating drum collector was used to collect straight fibers.

Sorption of crude oil by the helical porous fibers. a) Photographs showing the crude oil sorption process of the helical porous fibers from oil/water system. b) Photographs showing the crude oil desorption process of the helical porous fibers from pure oil system. c) Sorption efficiency of the various helical fibers (B, C, D, E, F) and their corresponding straight fibers (b, c, d, e, f) for crude oil. d) Schematic showing crude oil sorption mechanism of the helical fibers, compared to the straight fibers. [Zhao et al 2018]

PElectrospun nanofiber after thermal treatment (a) TTIP 9wt%, PVP 8wt%, acetic acid (AA) 53wt% and others (Solution I) (b) TTIP 9wt%, PVP 9wt%, AA 52wt% and others (Sol(a) Schematic illustration of tri-channel off-centered spinneret and the generation process for helical groove nanofibers; (b) Schematic illustration of tri-channel concentric spinneret and the generation process for helical hollow nanofibers; (c) Digital photographs showing a full view of the spinneret; (d, e) front view and side view of the off-centered and concentric spinneret, respectively. (f) High-speed photographs of the tri-fluid electrospinning process, the upper-right insert shows a closer view of the Taylor cone. Compound Taylor cones using (g) tri-channel off-centered spinneret and (h) tri-channel concentric spinneret [Zhao et al 2021].ution II) (c) TTIP 8wt%, PVP 10wt%, AA 51wt% and others (d) TTIP 8wt%, PVP 12wt%, AA 50wt% and others (e) Solution II, higher magnification.

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