**3. Results and discussions**

#### **3.1 Effect of stacking of fiber mats on charge measurement**

Evaluation of the effects on charge measurments of stacked of mats was conducted only with as-spun mats (not with polarized mats). The purpose of this was to assess whether the Faraday bucket measurements were more sensitive to surface area or to mass of the samples.

**Figure 6a** shows a photograph of a single 4 × 4 cm fiber mat. **Figure 6b** shows five as-spun mats stacked on top of each other. All mats were cut to size 4 cm × 4 cm and had a 1 × 1 cm tab at one edge. **Figure 6c** shows a bar chart of calculated charges per unit mass of individual and stacked layer samples. The measurements of the five individual samples are labeled as 1 to 5. The stacked samples are labeled A to D where A was formed by stacking the mats 1 + 2 (i.e., individual mats 1 and 2 stacked), B was three mats 1 + 2 + 3, C was four mats 1 + 2 + 3 + 4, and D was five mats 1 + 2 + 3 + 4 + 5.

All the single mats in **Figure 6c** had approximately the same measured charges of about 130 nC/g. All the mats had the same basis weight (20 g/m2 ), hence had the same masses.

If the Faraday bucket detected charge in bulk (i.e. per mass) then stacking the mats should not show a difference in charge/mass. If the Faraday bucket detected charge based on charge on the external mat surface area, and the charges of the mats do not transfer between the mats, then we would expect the measured charge/mass to decrease as mass increased and the surface area remained the same.

The results in **Figure 6c** shows the charge/mass linearly increased proportional to the number of mats in the stack. The charge/mass of stacked sample D (with five individual mats) was approximately double that of a single mat. Numerically this indicates that the measured charge per total mass increased over the single mat charge by about 25% for each additional mat in the stack. The increase in charge per mass indicates the

**203**

or

**Figure 6.**

*Polarization of Electrospun PVDF Fiber Mats and Fiber Yarns*

bulk charge mechanism alone is unlikely. The increase in charge also strongly indicates that the measurement is not that of the charges on the external surfaces of the stacked mats assuming the charges do not migrate to the surface. Hence the mechanism is more complex. It is interesting to note that each subsequent mat added to the stack to

*(a) Photograph of example of a single fiber mat of size 4 × 4 cm with a 1 × 1 cm tab on one edge, (b) photograph of five mats stacked on top of each other. (c) Bar chart of charge/mass of various samples (1–5 = measured charge/mass of five individual samples) (a = charge of stacked mats 1 + 2, B = stacked mats 1 + 2 + 3, C = stacked mats 1 + 2 + 3 + 4, D = stacked mats 1 + 2 + 3 + 4 + 5). The error bars in (c) represent average of three charge* 

linearly increased the measured charge by 25%. This gives the relationship

*M M C M*

*CM M CM M* <sup>=</sup> <sup>+</sup> 

11 1

where *C* is the measured charge of the stack and *M* is the mass of the stack.*C*<sup>1</sup> is the measured charge and *M*1 is the mass of one mat. The ratio *M M*<sup>1</sup> / equals the number of mats in the stack. An interpretation of the meaning of the two terms on the right side of Eq. (2) is not apparent. Future experiments should be conducted by varying the surface areas of the mats to determine if the terms are related to area

2

 = + <sup>1</sup> <sup>1</sup>

0.25 0.75 (1)

0.25 0.75 (2)

( )

*C*

*measurements of same mats and error is one standard deviation.*

and possible migration of charges between the stacked mats.

( )

*M*

1

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

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

#### **Figure 6.**

*Nanofibers - Synthesis, Properties and Applications*

measured potential was converted to charge.

**3. Results and discussions**

area or to mass of the samples.

mats 1 + 2 + 3 + 4 + 5.

same masses.

The morphology characteristics of the electrospun fiber mats and yarns were observed using a scanning electron microscopy (SEM, TM3000 and TM3030 Plus, and Hitachi, Japan). SEM images were analyzed by FibraQuant 1.3 software (nano Scaffold Technologies, LLC, Chapel Hill, NC) to measure the fiber diameter distributions. **Figure 5** shows SEM images and fiber size distributions for PVDF fibers and yarns. Electric charges on the fiber mat were measured using a Faraday Bucket. A detailed description of the Faraday Bucket is given in reference [12]. The fiber mats were cut to the size needed for the measurement (4 cm by 4 cm) otherwise the measurements were non-destructive. Based on the electrostatic principles, as a sample lowered into the interior of the Faraday Bucket, the inner metallic "bucket" acquired an electric potential that was detected as a change in voltage relative to the surroundings (ground). By an appropriate circuit model of the Faraday bucket the

Fiber yarns produced using setup in **Figure 3** were characterized as-spun and after polarization discussed in Section 2.4. The as-spun and polarized yarn samples were wrapped on a 'U' shaped copper wire and lowered into the Faraday bucket for measurement. The calculated charges were normalized with respect to mass of sample as discussed by Gade *et al.* [12]. The influence of U-shaped wire holding the yarn on the measured charge was found to be negligible when the wire without yarn

Evaluation of the effects on charge measurments of stacked of mats was conducted only with as-spun mats (not with polarized mats). The purpose of this was to assess whether the Faraday bucket measurements were more sensitive to surface

**Figure 6a** shows a photograph of a single 4 × 4 cm fiber mat. **Figure 6b** shows five as-spun mats stacked on top of each other. All mats were cut to size 4 cm × 4 cm and had a 1 × 1 cm tab at one edge. **Figure 6c** shows a bar chart of calculated charges per unit mass of individual and stacked layer samples. The measurements of the five individual samples are labeled as 1 to 5. The stacked samples are labeled A to D where A was formed by stacking the mats 1 + 2 (i.e., individual mats 1 and 2 stacked), B was three mats 1 + 2 + 3, C was four mats 1 + 2 + 3 + 4, and D was five

All the single mats in **Figure 6c** had approximately the same measured charges

If the Faraday bucket detected charge in bulk (i.e. per mass) then stacking the mats should not show a difference in charge/mass. If the Faraday bucket detected charge based on charge on the external mat surface area, and the charges of the mats do not transfer between the mats, then we would expect the measured charge/mass to decrease as mass increased and the surface area remained the same. The results in **Figure 6c** shows the charge/mass linearly increased proportional to the number of mats in the stack. The charge/mass of stacked sample D (with five individual mats) was approximately double that of a single mat. Numerically this indicates that the measured charge per total mass increased over the single mat charge by about 25% for each additional mat in the stack. The increase in charge per mass indicates the

), hence had the

of about 130 nC/g. All the mats had the same basis weight (20 g/m2

was lowered into the Faraday bucket and produced zero measured voltage.

**3.1 Effect of stacking of fiber mats on charge measurement**

**2.5 Characterization methods**

**202**

*(a) Photograph of example of a single fiber mat of size 4 × 4 cm with a 1 × 1 cm tab on one edge, (b) photograph of five mats stacked on top of each other. (c) Bar chart of charge/mass of various samples (1–5 = measured charge/mass of five individual samples) (a = charge of stacked mats 1 + 2, B = stacked mats 1 + 2 + 3, C = stacked mats 1 + 2 + 3 + 4, D = stacked mats 1 + 2 + 3 + 4 + 5). The error bars in (c) represent average of three charge measurements of same mats and error is one standard deviation.*

bulk charge mechanism alone is unlikely. The increase in charge also strongly indicates that the measurement is not that of the charges on the external surfaces of the stacked mats assuming the charges do not migrate to the surface. Hence the mechanism is more complex. It is interesting to note that each subsequent mat added to the stack to linearly increased the measured charge by 25%. This gives the relationship

$$\frac{\left(^{C}\text{\textdegree C}\_{M}\right)}{\left(^{C}\text{\textdegree M}\_{1}\right)} = 0.25 \left(\frac{\text{M}}{^{M}\text{\textdegree C}}\right) + 0.75\tag{1}$$

or

$$\frac{C}{C\_1} = 0.25 \left(\frac{M}{M\_1}\right)^2 + 0.75 \frac{M}{M\_1} \tag{2}$$

where *C* is the measured charge of the stack and *M* is the mass of the stack.*C*<sup>1</sup> is the measured charge and *M*1 is the mass of one mat. The ratio *M M*<sup>1</sup> / equals the number of mats in the stack. An interpretation of the meaning of the two terms on the right side of Eq. (2) is not apparent. Future experiments should be conducted by varying the surface areas of the mats to determine if the terms are related to area and possible migration of charges between the stacked mats.
