Factors Influencing Conductivity of Electrospun membranes

Electrospun membrane is composed of individual fibers and its conductivity is influenced by the ease at which electron is being transported across it. Although electrospun nanofibers may be made out of conductive materials, composites comprising of conductive material in polymer matrix is also frequently used due to its greater flexibility. Unlike homogeneous and solid conductive material, the physical arrangement and quality of the fibers that form the membrane will also have an impact on its conductivity.

Loading factor in fiber composite

Most conductive materials are non-electrospinnable and it requires a carrier which is electrospinnable to form fibers. The most direct method of fabricating conductive fibers is to load an electrospinnable polymer with a conductive material to form a composite fiber. However, as the conductive material is separated by non-conductive polymer matrix, a minimum loading of conductive material may be needed before there is a detectable increase in its conductivity. This was demonstrated by Wang et al (2012) where multi-walled carbon nanotube (MWCNT) loading of less 3% in an electrospun polystyrene membrane showed no significant effect on the conductivity of the membrane until it reaches 5% to 10% increase where there is a detectable increase in conductivity. The same threshold loading factor is also found in core-shell fibers where a minimum loading is necessary to ensure contacts between the conductive core materials [Miyauchi et al 2010]. Given that the conductive materials are constrained and more localized in a core-shell fiber, it can be anticipated that the threshold loading factor for conduction is lower. In a study by Wang et al (2014) on core shell fibers with MWCNT in the core surrounded by polyvinylidene fluoride (PVDF) shell, a conductivity of 1 x 10^-14 S/cm was measured with just 0.6 wt% loading of MWCNT. A maximum conductivity of 1 x 10^-5 S/cm was recorded at 1.4 wt% MWCNT loading and further increase in MWCNT loading did not lead to any increase in conductivity. This is much lower than the 5% loading in blended fibers.

Conductive material arrangement

In composite conductive electrospun fibers, there are several ways in which the conductive materials may be incorporated with the electrospun fibers. One of the most commonly used and simplest methods is through blending. However, in blended system, the conductive materials may be separated by non-conductive material that forms the matrix. A more effective method may be to coat the fibers with a conductive material or forming core-shell fibers. Comparing core-shell fibers and blended fibers, the conductive materials are constrained and more localized in a core-shell fiber. Comparing the conductivity of the MWCNT/polymer core-shell fibers from Wang et al (2014) and the blended MWCNT/polymer fibers from Wang et al (2012), the former was able to achieve a conductivity of 1 x 10^-5 S/cm at 1.4 wt% MWCNT loading while the latter has a conductivity of just 1.86^-8 S/cm with 10% loading. This showed that concentrating the MWCNT for better contacts more critical for better conduction than loading factor. In coated fibers, the surface coated conductive material will be in touch with one another which facilitate electron transport through the fibers. Kang et al (2007) simply coated electrospun silk membrane with MWCNT by dipping the membrane into a MWCNT suspension. The uniform coating and presence of numerous junctions for electron conduction gives the membrane a conductivity of 2 x 10^-4 S/cm which is even higher than the core-shell MWCNT fibers from Wang et al (2014). Polymerization of conducting polyaniline (PANi) on the surface of electrospun nanofibers is also a straight forward method of improving electrical conductivity of the fibers. Jia et al (2017) used this method to improve the electrical conductivity of electrospun poly(methyl methacrylate) (PMMA) fibers. Electrospun PMMA fibers alone showed very low conductivity of 4.3x10^-8. Following in situ polymerization of PANi on the surface of electrospun PMMA fibers, conductivity of the electrospun PMMA/PANi composite fibers were increased to 2.0x10^-1. With the improved electrical conductivity, the composite fibers were shown to be sensitive to low concentration of NH₃ at ppm level. The electrical resistance of the composite also showed reversible changes with repeated bending and relaxing which makes it suitable for use as strain sensor.

Crystallinity and molecular arrangement of conductive polymers may influence its conductivity. Chen et al used conductive polymer poly(3-hexylthiophene-2,5-diyl) (P3HT) in a core-shell electrospun fiber with P3HT as the core material and poly(methyl methacrylate) (PMMA) as the shell. By using different solvents for P3HT, the crystallinity and the molecular arrangement of electrospun P3HT can be affected. Where P3HT has a lower solubility in a solvent, there exist crystalline aggregates in the solution. P3HT dissolved in better solvent showed low crystallinity after it is electrospun into fibers. However, the electrical conductivity of the fibers are more dependent on the molecular arrangement then its crystallinity. P3HT electrospun using better solvent of chloroform (CF) showed electrical conductivity that were three orders of magnitude greater than P3HT dissolved in poorer solvent of 1,2,4-trichlorobenzene (TCB) with conductivity of 3.57x10^-1 and 1.48x10^-4cm². V^-1s^-1 respectively. Better conductivity of electrospun P3HT dissolved in CF may be attributed to higher ordered orientation between smaller crystalline grains which facilities charge-carrier hoping between polymer chains. In a blended system, having multiple conductive materials mixed into the carrier polymer may not always lead to an improvement in conductivity. For electrospun PVAc/PANI blended mat, Fotia et al (2021) showed that the addition of graphene and iron oxides which are both conductive, led to a decrease in the conductivity of PVAc/PANI blended mat. This has been attributed to the formation of agglomerates in the fibers which may have a negative effect on the conduction pathway.

Carrier Polymer Selection

Conductive and transparent film consisting of copper nanofiber network has been constructed on a glass substrate. Generally, precursor of copper is blended with carrier polymers for electrospinning. Sintering was next carried out to remove the organic component and reduce the nanofiber to CuO. Lastly, the CuO nanofibers are reduced to form Cu nanofiber. Kim et al (2015) compared the optical transmission and electrical conductivity of randomly oriented copper nanofiber prepared using polyvinyl alcohol (PVA) and polyvinyl butyral (PVB) as carrier polymers for electrospinning of its precursor. Their studies showed that the optical transmittance was influenced by the amount of copper nanofibers and not the carrier polymers used but the electrical resistance were affected by the carrier polymers (See table 1). The poorer electrical conductivity of Cu nanofiber derived from PVA carrier was attributed to high reduction temperature used compared to PVB and this led to discontinuity of the nanofibers on the substrate.

Optical and electrical properties of the Cu nanofibers fabricated with different amounts of Cu source. [Kim et al 2015. Journal of Nanomaterials 2015; 2015: 518589. This work is licensed under a Creative Commons Attribution 3.0 Unported License.]

With conductive composite fibers, the selection of the carrier polymer matrix may also influence the overall conductivity of the electrospun membrane. Fotia et al (2021) electrospun mat of polymethyl methacrylate (PMMA) and polyvinyl acetate (PVAc) blended with conductive polyaniline (PANI) doped with camphor-10-sulfonic acid (HCSA). PVAc/PANI blended mat exhibited electrical conductivity value that was two orders of magnitude higher than PMMA/PANI electrospun fibers. Significantly higher electrical conductivity of PVAc/PANI has been attributed to the intrinsic conductivities of the two carrier polymers with pure PVAc (10^-6 S/cm) and PMMA (10^-10 S/cm) and compatibility with PANI. However, the conductivity of PVAc/PANI (1:1) (36 µS/cm) is still much lower than pure PANI (2x10⁴ µS/cm).

Junctions

Carbonized polyamic acid fiber membrane with fiber diameter 1 to 2 µm was found to give conductivity of 2.5 S/cm. For smaller diameter fibers of 80 nm and subjected to compression, the conductivity rises to 16 S/cm. The author hypothesized that this sharp increase in conductivity was due to formation of more fiber junctions in the membrane with fiber of smaller diameter containing more junctions [Xuyen et al 2007]. Hyun et al [2010] constructed a highly conductive SrRuO₃(SRO)-RuO₂ composite nanofibre membrane with single nanofiber conductivity of 476 S/cm and membrane conductivity of 40.8 S/cm. This was the result of the presence of SRO which facilitated the transfer of proton through the amorphous SRO phases. The membrane was also heat compressed prior to calcination which may have created more fiber junctions.

Similarly, the thickness of the membrane also has an effect on the conductivity of the membrane. Pan et al (2015) was able to construct highly aligned silver nanowires (AgNWs) using near field electrospinning. In their study, the resistance of the AgNWs film decreases significantly when the thickness increased from 1.12 µm to 1.6 µm. This may be related to the number of junctions between the nanofibers with a threshold quantity at thickness of 1.12 µm.

Relation between sheet resistance and thickness of AgNws film [Pan et al. Journal of Nanomaterials, vol. 2015, Article ID 494052, 5 pages, 2015. This work is licensed under a Creative Commons Attribution 3.0 Unported License.]

The presence of junctions between overlapping electrospun fibers may hinder charge transfer. DiGregorio et al (2022) constructed a transparent conducting electrode (TCE) using a combination of metallization and plating on electrospun fibers. Active silver ink was first blended with water soluble polymers for electrospinning into fibers. The deposited fibers were calcined at 300 °C to give rise to silver nanowires. An UV-ozone treatment was used to activate the silver nanowires followed by electrodeless copper deposition so that the intersecting junctions between fibers were covered to form a continuous conductive surface. At the initial plating, the electrical resistance of the sheet reduces significantly. However, as all the junctions are fused by the copper plating, further increase in plating duration only brings modest drop in resistance. The TCE with optimum plating showed a sheet resistances of 0.33 Ω sq^-1 and visible light transmittance of 86% with a Haacke figure of merit of 652x10^-3Ω^-1.

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