**2.4 Polarization procedure**

Mats and yarns were polarized by the treatments described below. The treatments were not applied to stacked layers of mats in the layered mat experiments described later. **Figure 4** (a) shows a photograph of the sample holder made of PTFE (Teflon®) for the main frame, brass bars for the clamps, and thin aluminum plates for the electrodes. The PTFE was chosen over other materials as it was easy to machine and had many desired properties such as low electrical conductivity, low dielectric constant, and relatively high melt temperature. **Figure 4** (b) shows the sample holder inside of a Fischer Scientific iso-temp oven. The aluminum plates were 19 cm × 11 cm and 1 mm in thickness. One aluminum plate was grounded and

#### **Figure 4.**

*(a) End view of fabricated sample holder showing the two planar electrodes used to apply the electric field for poling the sample. (b) Photo of sample holder inside of oven and high voltage power supply for charging the electrode above the oven.*

**201**

**Figure 5.**

or charge.

*Polarization of Electrospun PVDF Fiber Mats and Fiber Yarns*

including simultaneous heating, stretching and electrical poling.

when the oven was turned off at the end of the soak time.

the other was electrically charged to produce an electric field between the plates of 2.5 kV/cm. The distance between the electrodes was 6 cm. The fiber mat samples and yarn samples were placed in the holder to perform all polarization treatments

Heat treatments were applied to change the sample temperature from room temperature to 150 °C with a temperature ramp-up rate of about 10C per min up to the soak temperature (150 C). The sample was held at the soak temperature for 5 minutes and then allowed to cool at a temperature ramp-down at rate of about 10C/min. The oven did not have ramp-rate control, so the ramp rates are estimates

The electric field poling was applied at field strength of 2.5 kV/cm during the heating of the oven. The poling started at the same time as the oven and stopped

Uniaxial stretched mats and yarns were obtained by clamping the mats and yarns into the holder positioned parallel to and between the aluminum electrodes. The moveable clamp was moved to create a 10% stretch of the samples. The stretch time of the sample started when the sample was placed in the holder and stretched. The stretch time included time to place the holder into the oven, temperature rampup, temperature soak, temperature ramp-down, time to remove the holder, and ended when the sample was removed from the stretching mechanism in the holder, for a total of about 52 min. The as-spun and polarized samples were stored in the static shielding bags immediately after fabrication to avoid any dissipation of ions

*SEM images of (a) fiber yarn and (b) fibers as seen on the surface of the fiber yarn with average fiber diameter of 1139 nm ± 654 nm, and average fiber yarn diameter of 900* μ*m ± 300* μ*m and c) fiber size distribution curve.*

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

based on observed temperature readings.

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

*Nanofibers - Synthesis, Properties and Applications*

the mat into the shape of an inverted cone.

yarns of uniform twist and uniform outer diameter.

edge of the collector.

used in these experiments.

**2.4 Polarization procedure**

metal needles and the collector with an 8 cm distance between the needles and the

Charged polymer jets launched from drops of polymer solution at the tips of the needles and followed the electric field gradient towards the wide neck of the conical-shaped collector. Once a substantial mat of fibers collected over the open end of the collector, the center of mat was hooked onto a wire and pulled to stretch

The metal collector was rotated by the motor to twist the fiber structure into a twisted continuous yarn. The yarn gradually increased in length and was stretched and attached to the take-up reel for collection onto a spool. The rotation speeds of the metal collector and the take-up reel were adjusted by trial and error to produced

In the case of electrospun mats, replicate samples were obtained at consistent basis weights by adjusting the time of fiber accumulation on the mats, so that the resulting fiber mat had uniform thickness and mass over the area of the sample. But in the case of fiber yarns a suitable length of sample was considered from each replicate run and compared for consistency by comparing the mass to length ratio of each sample. Results showed ±3% variation in mass/length for each of sample

Mats and yarns were polarized by the treatments described below. The treatments were not applied to stacked layers of mats in the layered mat experiments described later. **Figure 4** (a) shows a photograph of the sample holder made of PTFE (Teflon®) for the main frame, brass bars for the clamps, and thin aluminum plates for the electrodes. The PTFE was chosen over other materials as it was easy to machine and had many desired properties such as low electrical conductivity, low dielectric constant, and relatively high melt temperature. **Figure 4** (b) shows the sample holder inside of a Fischer Scientific iso-temp oven. The aluminum plates were 19 cm × 11 cm and 1 mm in thickness. One aluminum plate was grounded and

*(a) End view of fabricated sample holder showing the two planar electrodes used to apply the electric field for poling the sample. (b) Photo of sample holder inside of oven and high voltage power supply for charging the* 

**200**

**Figure 4.**

*electrode above the oven.*

the other was electrically charged to produce an electric field between the plates of 2.5 kV/cm. The distance between the electrodes was 6 cm. The fiber mat samples and yarn samples were placed in the holder to perform all polarization treatments including simultaneous heating, stretching and electrical poling.

Heat treatments were applied to change the sample temperature from room temperature to 150 °C with a temperature ramp-up rate of about 10C per min up to the soak temperature (150 C). The sample was held at the soak temperature for 5 minutes and then allowed to cool at a temperature ramp-down at rate of about 10C/min. The oven did not have ramp-rate control, so the ramp rates are estimates based on observed temperature readings.

The electric field poling was applied at field strength of 2.5 kV/cm during the heating of the oven. The poling started at the same time as the oven and stopped when the oven was turned off at the end of the soak time.

Uniaxial stretched mats and yarns were obtained by clamping the mats and yarns into the holder positioned parallel to and between the aluminum electrodes. The moveable clamp was moved to create a 10% stretch of the samples. The stretch time of the sample started when the sample was placed in the holder and stretched. The stretch time included time to place the holder into the oven, temperature rampup, temperature soak, temperature ramp-down, time to remove the holder, and ended when the sample was removed from the stretching mechanism in the holder, for a total of about 52 min. The as-spun and polarized samples were stored in the static shielding bags immediately after fabrication to avoid any dissipation of ions or charge.

#### **Figure 5.**

*SEM images of (a) fiber yarn and (b) fibers as seen on the surface of the fiber yarn with average fiber diameter of 1139 nm ± 654 nm, and average fiber yarn diameter of 900* μ*m ± 300* μ*m and c) fiber size distribution curve.*

#### **2.5 Characterization methods**

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 measured potential was converted to charge.

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