**Abstract**

Electrospun fibers are of interest in a number of applications due to their small size, simplicity of fabrication, and ease of modification of properties. Piezoelectric polymers such as Polyvinylidene Fluoride (PVDF) can be charged when formed in the electrospinning process. This chapter discusses fabrication of PVDF fiber mats and fiber yarns and the measurement of their charge using a custom-made Faraday bucket. The results show the measured charge per mass of fiber mats was greater than the values measured for the yarns of the same mass. The measured charges may be related to both mass and external surface areas of the mats and yarn samples. It was observed the area/mass ratios of the fiber yarns were more than 30% less than the fiber mats.

**Keywords:** PVDF, Faraday bucket, electrospinning, yarns, fibers

### **1. Introduction**

In recent years, nanotechnology has been used to develop novel materials including nano and submicron scaled materials such as nanorods, nanofoams, nanotubes, nanofilms, and nanofibers. These materials find use in various industrial applications and are the topics of many contemporary academic research efforts. Of these materials, the polymer electrospun fibers have found broad uses for catalysis, drug delivery, semiconductors and filtration [1–3].

Many polymers have been electrospun into nonwoven fiber mats. The polymer materials can have intrinsic piezo, thermal, and mechanical properties. When the polymers are formed into fiber structures such as thin mats, the high porosities and high specific surface areas of the mats can enhance the mat structural properties compared to similar mats of microfibers. The material and structural properties of these mats are ideally suited for filter media for air filtration and face masks.

Less common in the literature are discussions of the fabrication of yarns from electrospun fibers. The fabrication of yarns requires a mechanical method to entangle and interlock the intrinsic fibers, often by twisting, to form a self-supporting assembly of the fibers of an overall cylindrical shaped structure that can be characterized by a structure diameter.

Prior to electrospinning, the submicron fibers were often synthesized by techniques such as drawing, templating, solution casting, and phase separating. Most of these techniques had shortcomings including deformation failures, inability to produce continuous fibers, inability to scale-up, low production rates, or significant by-product wastes. The electrospinning method overcomes some of these shortcomings, and because of its simplicity, is a highly popular synthesis method.

Electrospinning is well-documented, established, and cost-effective, and is applied commercially. **Figure 1** shows numbers of publications, by publication year, as determined from the Scifinder ™ data base for the past 25 years. The plot shows a steady rise in numbers of papers since about 2000 when Reneker [4] published a seminal paper on electrospinning. The data search was conducted in August 2020 hence the final year was incomplete.

Electrospinning has been used to spin fibers for a wide range of polymers. One of these polymers, polyvinylidene fluoride (PVDF), is well known for its electrical properties. PVDF exhibits five known crystalline phases- α, β, γ, δ and ε. Amongst them, the β-phase has the highest permanent dipole moment due to its trans, TTT, planar zig-zag configuration. The β-phase is considered most responsible for the piezoelectric response obtained from the PVDF materials. A goal of enhancing the beta-phase contents in PVDF materials is an ongoing research pursuit [5, 6].

Electrospun fibers have been used as electrets in several applications. Electrets have a surface charge which can be exploited in capturing charged particles. Nanofibers can be converted into electrets by various methods such as corona discharging, surface fluorination and nafion functionalization. Several research groups developed custom made bench-scale procedures to produce polarized fibers which involved simultaneous stretching, heating and electrical poling. Similarly, Lolla *et al.* [6, 7] produced polarized PVDF fiber mats and tested them for aerosol filtration. The polarized fibers were observed to have higher surface charges, better capture efficiencies and lower pressure drops compared to as-spun fibers. The study was limited by measurement of localized surface potential via a hand-held electrostatic field meter [6]. **Table 1** lists several instruments reported in literature used to measure surface potential and charge. All of these instruments make localized measurements (do not measure properties over a large area of a mat) and may be impractical to use for production scale processes due to complexity and cost of operations. Measurements of the surface potential or electrical field are related to electrical charges but methods to calculate charges from the measurements are not always apparent.

Gade *et al.* [12] fabricated a custom-made Faraday bucket and a procedure to calculate the charges of fiber mat samples. The Faraday bucket overcomes some limitations or challenges of using the methods listed in **Table 1**, namely: it is nondestructive, measures large sample sizes, is easy to scale-up, and has a tractable mathematical model to convert voltage to charge value. In this chapter, the Faraday

**197**

**Table 2.**

*Polarization of Electrospun PVDF Fiber Mats and Fiber Yarns*

Du *et al*. Kelvin Probe force microscopy (open

performance

Takahashi and Yoshita

*List of instruments and materials tested.*

**Table 1.**

Fredrick Brown

Romay *et al.* Walsh *et al.* Wang *et al.*

Liu *et al.* Khalid *et al.* Jing *et al.*

and closed loop techniques)

Fatihou *et al*. Electrostatic voltmeter Electrospun PVDF

Lolla *et al*. Electrostatic field meter Polarized electrospun

Choi *et al*. Aluminum coatings applied to micro and nanoscale fibers, modified surface charge to control filter performance

affinity to fine particulate matter in aerosols

Li *et al.* Fibrous filters hybridized with carbon nanotubes (CNT) exhibiting slip flow effects at the CNT surfaces

*A sampling of literature on topics of fiber surface charge, fiber coatings, and additives.*

bucket is used to measure and compare charges between electrospun fiber mats and electrospun (continuous twisted fiber) yarns. Layers of fibers mats and yarns were stacked together to explore whether the Faraday bucket was more sensitive to bulk (mass) charge or more sensitive to surface charge. The charges on polarized fibers

Many publications discuss methods to charge fibers or to modify fibers surfaces (with coatings or additives such as carbon nanotubes) to enhance performances of fiber filter media. The subject matter is broad, and the numbers of publications are too numerous for a complete list. **Table 2** lists a sample of some of the publications. The electrospinning processes typically produce nonwoven, randomly oriented,

**Researcher Instrument Materials tested Reference** Collins *et al.* Scanning Probe microscopy Various dielectric surfaces [8]

**Researcher(s) Description Reference(s)**

Filters made of highly polar polymer fibers showing high binding

Quasi-permanent charges on dielectric polymer fibers [16–19]

Fundamental physics of electrical and charge effects on filter

Inversion algorithmic methods DC basin-type insulator [10]

Single and multi-layer graphene structures

nanofibers

nanofibers

[9]

[11]

[6]

[13, 14]

[20–22]

[23, 24]

[15]

fiber mats. These fiber mats typically have low mechanical strength (compared to microfiber mats). The electrospinning processes have a low mass production rates per nozzle that limits commercial applications from an economic standpoint. Researchers have studied various approaches to increase the mass production by increasing the number of electrospinning jets in the process [25, 26]. To overcome some of the limitations, researchers have studied electrospun yarns to improve the alignment of fibers and to increase the mechanical strength. Production of highly twisted PVDF – HFP electrospun fiber yarns using a novel ring collector was reported by Shuakat *et al.* [27]. Afifi *et al.* [28] and Teo *et al.* [29] studied methods to continuously produce electrospun yarns. In this chapter the yarns were produced by twisting and drawing the fibers in flight and the twisted yarn were wound onto

*DOI: http://dx.doi.org/10.5772/intechopen.96305*

and yarns are also compared.

**Figure 1.** *Number of publications on "electrospinning" versus year of publication.*

#### *Polarization of Electrospun PVDF Fiber Mats and Fiber Yarns DOI: http://dx.doi.org/10.5772/intechopen.96305*

bucket is used to measure and compare charges between electrospun fiber mats and electrospun (continuous twisted fiber) yarns. Layers of fibers mats and yarns were stacked together to explore whether the Faraday bucket was more sensitive to bulk (mass) charge or more sensitive to surface charge. The charges on polarized fibers and yarns are also compared.

Many publications discuss methods to charge fibers or to modify fibers surfaces (with coatings or additives such as carbon nanotubes) to enhance performances of fiber filter media. The subject matter is broad, and the numbers of publications are too numerous for a complete list. **Table 2** lists a sample of some of the publications.

The electrospinning processes typically produce nonwoven, randomly oriented, fiber mats. These fiber mats typically have low mechanical strength (compared to microfiber mats). The electrospinning processes have a low mass production rates per nozzle that limits commercial applications from an economic standpoint. Researchers have studied various approaches to increase the mass production by increasing the number of electrospinning jets in the process [25, 26]. To overcome some of the limitations, researchers have studied electrospun yarns to improve the alignment of fibers and to increase the mechanical strength. Production of highly twisted PVDF – HFP electrospun fiber yarns using a novel ring collector was reported by Shuakat *et al.* [27]. Afifi *et al.* [28] and Teo *et al.* [29] studied methods to continuously produce electrospun yarns. In this chapter the yarns were produced by twisting and drawing the fibers in flight and the twisted yarn were wound onto


#### **Table 1.**

*Nanofibers - Synthesis, Properties and Applications*

hence the final year was incomplete.

Electrospinning is well-documented, established, and cost-effective, and is applied commercially. **Figure 1** shows numbers of publications, by publication year, as determined from the Scifinder ™ data base for the past 25 years. The plot shows a steady rise in numbers of papers since about 2000 when Reneker [4] published a seminal paper on electrospinning. The data search was conducted in August 2020

Electrospinning has been used to spin fibers for a wide range of polymers. One of these polymers, polyvinylidene fluoride (PVDF), is well known for its electrical properties. PVDF exhibits five known crystalline phases- α, β, γ, δ and ε. Amongst them, the β-phase has the highest permanent dipole moment due to its trans, TTT, planar zig-zag configuration. The β-phase is considered most responsible for the piezoelectric response obtained from the PVDF materials. A goal of enhancing the beta-phase contents in PVDF materials is an ongoing research pursuit [5, 6].

Electrospun fibers have been used as electrets in several applications. Electrets

Gade *et al.* [12] fabricated a custom-made Faraday bucket and a procedure to calculate the charges of fiber mat samples. The Faraday bucket overcomes some limitations or challenges of using the methods listed in **Table 1**, namely: it is nondestructive, measures large sample sizes, is easy to scale-up, and has a tractable mathematical model to convert voltage to charge value. In this chapter, the Faraday

have a surface charge which can be exploited in capturing charged particles. Nanofibers can be converted into electrets by various methods such as corona discharging, surface fluorination and nafion functionalization. Several research groups developed custom made bench-scale procedures to produce polarized fibers which involved simultaneous stretching, heating and electrical poling. Similarly, Lolla *et al.* [6, 7] produced polarized PVDF fiber mats and tested them for aerosol filtration. The polarized fibers were observed to have higher surface charges, better capture efficiencies and lower pressure drops compared to as-spun fibers. The study was limited by measurement of localized surface potential via a hand-held electrostatic field meter [6]. **Table 1** lists several instruments reported in literature used to measure surface potential and charge. All of these instruments make localized measurements (do not measure properties over a large area of a mat) and may be impractical to use for production scale processes due to complexity and cost of operations. Measurements of the surface potential or electrical field are related to electrical charges but methods to calculate charges from the measurements are not

**196**

**Figure 1.**

*Number of publications on "electrospinning" versus year of publication.*

always apparent.

*List of instruments and materials tested.*


#### **Table 2.**

*A sampling of literature on topics of fiber surface charge, fiber coatings, and additives.*

a spool, which differs from typical electrospinning equipment that collect the fiber mats on a solid grounded surface. The resulting yarns had lengths up to tens of meters long and exhibited mechanical properties different from the electrospun mats.
