Physical cross-linking of electrospun fibers (Fiber Fusion)

Fused fibers when insufficient time is given for the solvent to vaporise.

Physical cross-linking involves the formation of interconnected junctions between fibers. The presence of these links may improve mechanical strength of membrane and conduction in conductive material. Chavoshnejad et al (2020) used mathematical modeling to show that with inter-fiber bonding, stiffness of the membrane increases by 60% regardless of the mechanical properties of the individual fibers. Increasing elastic modulus of individual fibers led to a linear increase in membrane stiffness while increasing fiber diameter led to a parabolic increase in membrane stiffness. Comparing the stiffness ratio of 100% bonding over unbonded membrane, membranes with lower porosity or greater fiber density showed greater stiffening effect with inter-fiber bonding. For other membrane characteristics such as fiber elastic modulus and diameter, the stiffness ratio did not change significantly. While chemical cross-linking refers to the formation of covalent bonds between the molecules, physical cross-linking may not involve molecular bond formation but instead on the formation of physical bonds at the junctions made by two or more fibers.

There are several ways of forming physical links between fibers, some may be formed in situ during electrospinning and deposition while others may be carried out after the mesh is formed. In some cases, this also involves chemical cross-linking which increases the bonding strength at the junctions.

Junction fusion

The distance between top and collector parameter has an effect on the jet path and traveling time before resting on the collector. In a typical electrospinning setup, this distance range from 10 cm to 15 cm which generally allows sufficient flight time for the solvent to vaporize such that a dry fiber strand is deposited. If the distance is too short such that the solvent is inadequately vaporized, fused fibers may be formed. While it is easily understood that when the distance is too short, fused fibers may be formed due to insufficient solvent vaporization, the same fused fiber has been observed when the distance is beyond the optimal range. Ghelich et al (2015) made this observation when they electrospin PVA/NiO-GDC fibers. At electrospinning tip to collector distance of 8 cm, fused fibers were observed. At 10 cm, distinct individual fibers were collected. The diameter of the fibers collected at both distances was similar. At 15 cm, fused fibers were again observed and the diameter increases significantly. The increase in fiber diameter was attributed to reduced electrostatic field strength which leads to less stretching of the fibers. As there are less stretching, the greater diameter increase the amount of solvent trapped within the fibers. The trapped solvents continue to diffuse out after the fiber has deposited and this causes fusion of the fibers.Kim et al (2018) constructed a separator made of electrospun cross-linked poly(vinylidene fluoride-co-hexafluoropropene) (PVDF-co-HFP) fibers. Cross-linking was achieved by using a less volatile solvent, N-methyl-2-pyrrolidone (NMP) in the solution such that the deposited fibers were sufficiently wet as to cause fusion at contact points.

Heat treatment

Concurrent application of pressure and heat may shorten the treatment duration and improve the connectivity between fiber intersections. Heating of electrospun membrane under pressure can be achieved easily using a hot press machine. It is important to note that the temperature to cause fusion at inter-fiber contact points will be lower under pressure. Na et al (2008) showed that with a hot-pressed temperature of 145 °C, polyvinylidene fluoride fibers have merged together to form a network of large pores. Under normal heating at that temperature, the fibers would just fused together at the junction while retaining individual fiber morphology [Gopal et al 2006 ].

Thermal heating may encouraged melting of polymer matrix at contact points between fibers to create a fused junction. For metal oxide, certain ionic compounds may be added to the precursor solution to encourage mass transfer during sintering. Zhang et al (2018) introduced MoO₂ to facilitate fusion at the junction during carbonization of electrospun polyacrylonitrile (PAN) fibers. The MoO₂ reacts with C during carbonization process to form Mo₂C which are more mobile and migrates at points of contact to form fused junctions. Compared with carbonized electrospun PAN fibers without MoO₂, the pure C fibers showed poor fusions at the contact points. Fused junction may increase ion/electron transport in the membrane as the ion/electron may take the shortest path across fibers.

[+]

[-] comments powered by Disqus

Content [Cross-Linking]