Molecule/ion movement and organisation in electrospinning

During electrospinning, ions and molecules in the electrospinning jet are influenced by the electric field and the shear forces of the thinning jet. Any inhomogenity in the charges and molecular weight in the electrospinning jet may result in specific ordered distribution of the molecules or ions across the fiber. Small cross sectional area of electrospun fibers may also exert confinement effects on the ordering and self-assembly of molecules. Such properties may be used to construct specific structure with unique properties.

Movements of ions have been known to be influenced by electrode polarity. Since electrospinning involves the application of electrical charges, the presence of surface charges on the electrospinning jet may induce migration of any ions present in the solution. Tsaroom et al (2011) observed the formation of core-shell polymer-metal salt fibers with the positively charged metal salt concentrated at the core of the fibers after electrospinning with high positive charge. However, this was only seen when the metal salt is mixed with polyethylene oxide polymer but not with polyacrylic acid. It was hypothesized that the interaction between the negative ions of polyacrylic acid with the positive metal salt restricted any metal-salt ions distribution under the influence of the positive external charge. It is with polyethylene oxide, which is neutral, that core-shell structure was formed with metal salt rich core. However, application of negative high voltage does not see a concentration shift of the positive metal towards the shell. This has been attributed to crystallization of polyethylene oxide from the surface which prevented the aggregation of metal-salt at the surface.

Electrospinning of polymer solution blended with low molecular weight additives have been shown to encourage migration of the additives to the surface. Niu et al (2016) used this concept to produce core-shell fibers with polyvinyl pyrrolidone (PVP) as the core material and Si atoms as the shell layer. For their electrospinning, thiol-ene monomer with Si atom and the initiator were mixed into the PVP solution. During electrospinning, evaporation of the solvent brought the smaller thiol-ene monomer with Si atom and the initiator to the surface of the electrospinning jet. Under UV, the thiol-ene monomer with Si atom polymerizes on the surface of the electrospinning jet prior to deposition on the collector. In another study, Niu et al (2016a) added a photoinitiator containing fluorine into polyacrylonitrile (PAN) solution with dimethylformamide (DMF) as the solvent. During electrospinning, the photoinitiator migrates to the surface of the fiber. This resultant photosensitive fiber was used for initiating the polymerization of tripropylene glycol diacrylate (TPGDA) and hydroxyethyl acrylate (HEA) monomers via UV irradiation on the PAN fiber surface. Note that this reaction is carried out post electrospinning. Post reaction imaging using TEM showed a distinct core-shell fiber structure.

The flow of polymers with different molecular weight within the solution during electrospinning may also lead to separation of the molecules. Niu et al (2015) demonstrated this with by preparing precursor solution with a mixture of low-, middle- and high- molecular weight polyvinyl alcohol. During electrospinning, they hypothesized that the low-, middle- and high- molecular weight polyvinyl alcohol will separate to form three layers with the lowest molecular weight PVA at the core instead of uniform distribution. They supported this hypothesis by replacing low molecular weight PVA with polyvinyl pyrrolidone (PVP) for electrospinning and followed by the removal of PVP to give hollow tube.

(a) Schematic of the process of replacing low-molecular-weight PVA by PVP, then removing PVP with trichloromethane, to prove the layered distribution of low-, middle- and high-molecular-weight PVA. (b) TEM image of low-, middle- and high-molecular-weight PVA composite polymer nanowire after electrospinning with a scale bar at 100 nm. (c) TEM image of PVP and middle-/high-molecular-weight PVA composite polymer nanowire after electrospinning with a scale bar at 50 nm. Low-molecular-weight PVA is replaced by PVP. (d) TEM image of middle-/high-molecular-weight PVA polymer nanotubes with a scale bar at 500 nm, after removing PVP in the inner center using CHCl3. [Niu et al. Nature Communications 2015; 6: 7402. This work is licensed under a Creative Commons Attribution 4.0 International.]

Block copolymer solutions and melt are known to self-assemble as they harden to form various nanoscale morphologies. In electrospinning, the cross-section area of its fibers is smaller than conventional form such as extruded microfibers and films. This results in a confinement effect which influences the chain packing of the block copolymer. Kalra et al (2006) found that the d spacing of electrospun poly(styrene-block-isoprene) (PS-b-PI) submicron fibers are smaller than cast film. The domain structures in the fibers were also not as developed as those in films. This has been attributed to the short residence time and strong elongational deformations during electrospinning. Ma et al (2009) investigated the domain variation of electrospun core-shell fibers using co-axial electrospinning with poly(styrene-b-dimethylsiloxane) (PS-PDMS) block copolymer as the core component and a poly(methacrylic acid) (PMAA) as the shell. Their study found that for a given number of domains, any variation in the core fiber diameter will cause the central domain to vary accordingly while the outer domains remain relatively constant. Under the strong electric field for electrospinning, it has been suggested that dipole molecules may be influenced and form specific crystal structure. There are conflicting results on the influence of voltage on polyvinylidene fluoride (PVDF) polymorphism. Cozza et al (2013) reported that varying electrospinning parameters have no effect on the α/β crystal ratio. However, Sengupta et al (2017) showed with a single strand electrospun PVDF fiber that increasing voltage does lead to a general increase in its piezoelectric d₃₃ coefficient up to a value of -58.77 pm/V. He attributed this to the electric field encouraging dipole alignment of its molecules. Research has shown that altering the electrospinning parameters will have an influence on the β-phase in PVDF. Sengupta et al (2018) tested the influence of increasing electrospinning voltage on the fraction of β-phase in PVDF fibers. At 10kV, a β-phase fraction of 0.749 was recorded. At 12kV, the β-phase fraction was reduced to 0.715 before increasing voltage led to a gradual increase to 0.789 at 18kV. While it can be explained that increasing electrospinning voltage encourages stretching and poling thus favoring the transformation of α-phase to β-phase, there is an unexplained dropped in β-phase from 10kV to 12kV. It may be that the increase in acceleration of the jet due to higher voltage reduces the time for phase transformation before deposition on the collector while that voltage is insufficiently high to induce rapid transformation. A study by Locarno et al (2019) showed that dipole molecules may not always be influenced by the electric field. Electrospinning simple amino acid, short peptide Fmoc-Phe-Gly, they found no orientation with the molecules although they possess high dipole moments. Lacking a long chain, these short molecules should have sufficient mobility for molecular organization during electrospinning. However, the resultant fibers showed an amorphous structure. It is also possible that the dipoles and Π-stacking of the aromatic systems that hold the molecules together during electrospinning may be too strong to allow reorientation by the electric field.

When there are more than one polymers and additives mixed together for electrospinning, self-organization of the molecules in the electrospun fiber may be observed. Avossa et al (2019) constructed a nanofibrous conductive chemical sensor based on combination of two insulating polymers, polystyrene (PS) and polyhydroxybutyrate (PHB), doped with 5,10,15,20-tetraphenylporphyrin (H2TPP) and mesoporous graphene nanopowder (MGC). To prepare the electrospinning solution, a surfactant, hexadecyltrimethylammonium bromide (CTAB) was added to the solution mixture. The resultant electrospun nanofibers were observed to exhibit heterogeneous distribution of nanoaggregates within its matrix. A layer of aggregates just under the surface of the nanofiber was suggested to be MGC. Between PS and PHB, PHB was found to be more concentrated on the surface of the fiber. While Avossa et al (2019) was not able to verify the dispersion of H2TPP, literature survey suggested that affinity between H2TPP and PS, and a weaker bonding between H2TPP and MGC would keep H2TPP at the subsurface level and uniformly dispersed in PS at the core of the fiber.

Annular dark field mode-scanning - transmission electron microscopy (ADFM-STEM) image of a porphyrin doped fiber. The inset shows the corresponding energy dispersive X-ray spectroscopy (EDXR) chemical map from carbon (blue) and oxygen (green) [Avossa et al 2019].