Switchable Nanofiber Sensing Platform

Polymer fibers/nanofibers are processed by four techniques: coaxial flow systems, wet-spinning, melt-spinning, and electrospinning.48 Electrospinning is the commonly used method due to its simplicity and compatibility with all polymers 49 A simple electrospinning setup consists of a pump, a power source, and a conductive collector. A solution of the desired product is drawn into a syringe, and then it is placed in a pump connected to the device. The pump allows adjustable flow through the nozzle from the syringe. When the solution comes to the nozzle, a drop of the solution accumulates at the tip of the nozzle. Afterward, an increasingly high electrical field is applied. Up to a critical voltage, the drop can be held by its own surface tension.50 The electrical field electrostatically charges the surface of the drop. When the voltage attains the critical value, the liquid drop begins to elongate and transforms into a conical shape. The liquid polymer solution elongates through the collector, and in the meanwhile, the solvent evaporates.51 Finally, solid polymeric fibers are collected on the collector. A high yield of nanofibers is obtained by low- cost processing steps. Textiles, substrates, and scaffolds made of nanofibers are becoming common building blocks of cutting-edge-technology devices.

In recent years, several studies have been performed to characterize PVDF fiber formation. Fiber formation relies on parameters such as solution concentration and applied potential. Figure 4.15 illustrates the typical effect of solution concentration and applied potential; as the concentration increases to 25%, fibers of defined shapes are obtained.52 A concentration decrease causes bead

Morphology of PVDF nanofibers

Figure 4.15 Morphology of PVDF nanofibers (polymer flow rate, 1.5 mL/min; tip-to-collector distance, 15 cm) showing the presence of beads and inhomogeneities. Nanofibers displaying minimum defects were obtained at 20 kV, 25 wt%.

formation. The applied potential causes a marginal change in the fiber diameter. Pellerin and Richard-Lacroix have discussed the emergence of characterization techniques that enable study at the single-fiber level and, therefore, study of the structures and properties of electrospun nanofibers with their molecular orientations.53 PVDF fibers are readily fabricated by electrospinning from their viscous solution. In our studies, a bicomponent solvent system (acetone- DMF) was used for fiber processing.54 Scanning electron microscopy (SEM) characterization of PVDF nanofibers is shown in Fig. 4.16. The fiber diameters range from 60 nm to 150 nm, and the average fiber diameter (AFD) was found to be 65 nm. The fiber density of the final electrospun product was calculated to be approximately 87.44%. The diameter and density of fibers are vital properties for the fabrication of polymer sensor platforms. The elasticity of the fiber mat is a critical parameter to control the switchability

SEM images of PVDF fibers magnified (a) 500 times, (b) 1000 times, (c) 5000 times, and (d) 15,000 times and (e) AFD distribution of fibers from (d)

Figure 4.16 SEM images of PVDF fibers magnified (a) 500 times, (b) 1000 times, (c) 5000 times, and (d) 15,000 times and (e) AFD distribution of fibers from (d).

Table 4.2 Properties of (1) pristine PVDF fiber and (2) PVDF fiber exposed to acetone and (3) PVDF fiber exposed to toluene

Normalized extension work

Normalized Young’s modulus

Relative

stiffness

Relative

deformation

1

1.00

1.00

1.00

1.00

2

0.33

0.44

0.79

1.02

3

0.66

0.69

0.86

1.09

properties. As the elastic modulus of the fiber increases, the volume and shape of the fiber changes drastically, which causes displacements and entanglements. Reversible transformations in the conformation and position of fibers by a stimulus govern the switchability as well as response of fibers.

The elasticity, deformation, adhesion and peak forces, stiffness, and Young’s modulus of the polymer fiber were characterized by the peak force mode of AFM. Figure 4.17

Micrographs of (a) PVDF fiber, (b) acetone-exposed PVDF fiber, and

Figure 4.17 Micrographs of (a) PVDF fiber, (b) acetone-exposed PVDF fiber, and (c) toluene-exposed PVDF fiber and force-separation curves of (d) PVDF fiber, (e) acetone-exposed PVDF fiber, and (f) toluene-exposed PVDF fiber.

illustrates force separation curves and AFM micrographs under ambient conditions as well as acetone and toluene atmospheres.

AFM micrographs exhibits marginal shape variations contrary to peak force analysis, indicating substantial deviations in Young’s modulus as well as stiffness. Exposure to acetone lowers Young's modulus of PVDF by 56%, while exposure to toluene causes ~30% decrease in Young’s modulus. The decrease in Young's modulus of the single fiber indicates that there is a much steeper deformation in PVDF when acetone is applied, due to polymer-solvent molecule interactions. The PVDF-acetone interaction causes a softening on the fiber surface, lowering stiffness. As acetone is removed, the polymer solidifies rapidly to the initial conditions. The reversible softening-

Typical resistivity response (А Д//?ь) of a PVDF sensor to (a) acetone vapor and (b) toluene vapor

Figure 4.18 Typical resistivity response (А Д//?ь) of a PVDF sensor to (a) acetone vapor and (b) toluene vapor.

solidifying processes provide switchable shape transformation to PVDF. The same behavior was not observed for PVDF-toluene due to the low extent of interaction of polymer and solvent.

Switchable polymer-solvent interactions hold great promise for gas sensors. PVDF-carbon nanotube (CNT) composites were validated for acetone and toluene sensing. Figure 4.17 illustrates the typical resistivity response of a PVDF-based sensor platform at the conductive interface. Here the platform was exposed to acetone vapor and toluene vapor separately. When the gas molecules are adsorbed on the sensor, resistivity increases and electron flow in the CNT network is obstructed by molecules. As discussed above, PVDF was subjected to substantial swelling and conformational changes due to acetone exposure. The conductive layer (CNT) was displaced significantly by PVDF-acetone interactions. As illustrated in Fig. 4.18a, acetone causes a resistivity change of ~20% while toluene causes a change of ~10%. The initial conductivity was restored when gas was removed from the system. The switchable behavior of PVDF enables long-lasting gas exposure monitoring. The functionalization of fibers by gas- selective agents provides a selective gas sensing interface. We anticipate that switchable response of PVDF nanofiber- based electrode will enable continuous monitoring as well as long-lasting measurements in the future.

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