Effect of electrospinning parameters and polymer concentrations on mechanical-to-electrical energy conversion of randomly-oriented electrospun poly(vinylidene fluoride) nanofiber mats

Abstract

Poly(vinylidene fluoride) (PVDF) nanofiber mats prepared by an electrospinning technique were used as an active layer for making mechanical-to-electric energy conversion devices. The effects of PVDF concentration and electrospinning parameters (e.g. applied voltage, spinning distance), as well as nanofiber mat thickness on the fiber diameter, PVDF β crystal phase content, and mechanical-to-electrical energy conversion properties of the electrospun PVDF nanofiber mats were examined. It was interesting to find that finer uniform PVDF fibers showed higher β crystal phase content and hence, the energy harvesting devices had higher electrical outputs, regardless of changing the electrospinning parameters and PVDF concentration. The voltage output always changed in the same trend to the change of current output whatever the change trend was caused by the operating parameters or polymer concentration. Both voltage and current output changes followed a similar trend to the change of the β crystal phase content in the nanofibers. The nanofiber mat thickness influenced the device electrical output, and the maximum output was found on the 70 μm thick nanofiber mat. These results suggest that uniform PVDF nanofibers with smaller diameters and high β crystal phase content facilitate mechanical-to-electric energy conversion. The understanding obtained from this study may benefit the development of novel piezoelectric nanofibrous materials and devices for various energy uses.

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1. Introduction

Piezoelectric films from polyvinylidene fluoride (PVDF) have been widely used in areas such as force/vibration sensors, transducers, actuators, speakers, microphones and power generators,^1–4 owing to excellent piezoelectricity converting mechanical deformation into electric signals and mechanical flexibility. In the solid-state, PVDF is a semicrystalline polymer with five different crystal phases: α, β, γ, δ and ε.⁵ Because of the all trans (TTTT) planar zigzag structure, the β crystal phase has the highest dipolar moment per unit,⁶ accounting for the piezoelectricity. Conventionally, piezoelectric PVDF films are prepared by a series of processes including (1) melt-casting of a PVDF film, which is dominated with the α crystal phase because of its high thermodynamical stability,^7,8 (2) mechanically stretching the film to convert the α crystal phase to the β crystal phase, and (3) poling treatment at an elevated temperature in a high electric field to align the dipole moment in the film. Each step needs precise control of the processing conditions.^9,10

Recently, nanofibrous PVDF prepared by electrospinning has been reported to show mechanical-to-electric energy conversion ability. Chang et al.¹¹ first reported the mechanical-to-electric energy conversion property of single PVDF nanofibers. By using a near-field electrospinning technique (i.e. spinning distance < 1 cm), they directly deposited freshly-electrospun single PVDF nanofiber between two metal electrodes. The nanofiber under small mechanical vibrations can generate an electric output around 30 mV and 3 nA.

Aligned PVDF nanofibers were also reported to have energy harvesting ability. Hansen et al.¹² prepared an energy harvesting device using aligned PVDF nanofiber mat with two parallel metal electrodes which were placed on one side of the fiber mat along the fiber length. However, aligned PVDF nanofibers had to be subjected to a poling treatment before it showed piezoelectricity. This is different to near-field electrospun single PVDF nanofiber which shows piezoelectricity without poling treatment although the electricity in both the devices is generated along the fiber length. In addition, aligned poly(vinylidene fluoride-trifluoroethylene) [P(VDF-TrFE)] nanofiber mats were reported by Mandal et al.¹³ to show mechanical-to-electric energy conversion across the mat thickness direction and potential to detect compressive impact.

Fang et al.³ in our group reported that randomly-oriented electrospun PVDF nanofiber mats can convert mechanical energy into electricity. They proved this finding by sandwiching a thin piece of PVDF nanofiber mat between two metal foils. Without poling treatment, the nanofibers in the simple device can produce two opposite-polarity voltage outputs when it received a mechanical deformation (e.g. compressive impact, padding or bending). Repeatedly impacting the nanofiber device led to a continuous alternation of positive and negative voltage signals, which is a typical characteristic of AC power. The unique feature of this device is that it can produce as high as 6.5 V outputs and as high as 4 μA current from a small mechanical action. An electrical generation is between the two sides of nanofiber mat. The energy conversion performance of this PVDF nanofiber membrane is much higher than that of a commercial piezoelectric PVDF film under the same compressive impact condition. Since the finding of piezoelectricity, randomly-oriented PVDF nanofiber mats have been studied for mechanical-to-electrical energy conversion under different conditions (e.g. different forces, deformation conditions, device structures).^1,14,15 The formation of β crystal phase in randomly-oriented PVDF nanofibers was attributed to high mechanical stretching during electrospinning.^3,13,16

In terms of electrospinning, the jet initiation, fiber stretching and deposition are driven by a high electric field. Electrospinning parameters (e.g. applied voltage and spinning distance), polymer concentration, and additives have considerable effects on fiber morphology, diameter, and fibrous structure of nanofibers.^17–25 Although several papers have reported the effect of additives (e.g. BaTiO₃ nanoparticles, organically modified montmorillonite, carbon nanotubes, graphene) on β crystal phase content of PVDF nanofibers,^26–28 systematic study on how electrospinning conditions affect the energy conversion behavior has not been reported in research literature.

In this study, we examine the effect of applied voltage, spinning distance, PVDF concentration in spinning solution and fiber mat thickness on PVDF β crystal phase content in the nanofibers and mechanical-to-electrical energy conversion of randomly-orientated PVDF nanofiber mats. It was interesting to find that finer uniform PVDF fibers showed higher β crystal phase content hence the energy harvesting devices having higher electrical output regardless the change of electrospinning parameters and PVDF concentration. The voltage output always changed in the same trend to the change of current output whatever the change trend was caused by operating parameter or polymer concentration. Both voltage and current output changes followed a similar trend to the change of the β crystal phase content in nanofibers. Nanofiber mat thickness also plays a role in determining the level of electrical outputs.

2. Experimental

2.1 Materials

Poly(vinylidene fluoride) (PVDF) pellets (M_w = 275 000), acetone and N,N-dimethylformamide (DMF) (≥99%) were purchased from Sigma-Aldrich and used as received.

2.2 Electrospinning

The electrospinning solutions with different PVDF concentrations were prepared by dissolving PVDF pellets into a mixture solvent of DMF/acetone (v/v 4/6) at 100 °C. A needle based electrospinning setup was employed to prepare PVDF nanofiber mats.³ During electrospinning, the flow rate was controlled at 1 mL h⁻¹ using a syringe pump (KD Scientific). This flow rate condition was chosen because the electrospinning process at this condition maintained stably regardless the variation of other parameters studied. A grounded aluminum rotating drum (diameter: 5 cm; length: 10 cm; rotating speed: 100 rpm) was used as the fiber collector. The thickness of fiber mats were controlled through the deposition time of the electrospinning process. The electrospinning process was conducted at room temperature and all the electrospun PVDF nanofiber mats were then dried in an oven at 40 °C for 5 hours to remove the residual solvent.

2.3 Characterizations

Nanofiber morphology was observed under a scanning electron microscopy (SEM, Joel Neoscope). Fiber diameter was calculated using image processing software (Image J). X-ray diffraction (XRD) patterns were obtained on a Panalytical X-ray diffractometer with Cu radiation of 1.54 Å. The samples were scanned in the 2θ range of 5° to 30° with the step size of 0.05°. Fourier transform infrared (FTIR) spectra were obtained on a Bruker Optics spectroscopy in ATR mode. The power generator devices were fabricated using the process reported in our previous paper.¹⁶ The electrical outputs of all the samples (4 cm²) were recorded by an e-Corder 401 electrochemistry working station with a compression force of 10 N at 1 Hz, using a purpose built testing platform reported previously.^3,16 The thickness of the nanofiber mats was measured using a digital micrometer.

3. Results and discussion

PVDF nanofiber mats were prepared using a needle-based electrospinning technique under different parameters (e.g. polymer concentration, applied voltage, spinning distance). Detailed effects of the parameters on nanofiber characteristics and mechanical-to-electrical conversion are described below.

3.1 Polymer concentration

Fig. 1 shows the typical SEM images of PVDF fibers prepared from different PVDF solutions. All fibers in the as-spun mats had a randomly-oriented fibrous morphology. As expected, the PVDF concentration affected fiber morphology (see SEM images for each concentration condition in ESI†).

Fig. 1 (a)–(c) SEM images of the PVDF nanofibers electrospun from PVDF solutions of different concentrations (a-16%, b-20%, c-26%), and (d) effect of PVDF concentration on fiber diameter (applied voltage 15 kV; spinning distance 15 cm; nanofiber mat thickness 70 μm).

Beaded PVDF fibers resulted when the PVDF concentration was below 20%. The bead morphology and numerical density were also affected by the polymer concentration. When the PVDF concentration increased from 16% to 19%, the bead numerical density decreased and the beads turned to change from round to oval shape. The average fiber diameter calculated based on the fiber sections increased slightly from 230 nm to 284 nm. When the PVDF concentration was above 20%, non-beaded PVDF fibers were electrospun. With increasing the PVDF concentration from 20% to 26%, the average fiber diameter increased from 284 nm to 810 nm.

The effect of polymer concentration on the morphology and diameter of electrospun nanofibers has been widely examined.^29,30 With increasing the polymer concentration, electrospun product may show different morphologies, such as beads, beaded fibers, and uniform fibers. This was explained by the entanglement of polymer macromolecular chains in the solution. When the polymer concentration is above the entanglement concentration (C_e), beaded fibers are prepared, and when the solution concentration is 2–2.5 times of C_e, the chain entanglement is sufficient to allow forming uniform fibers.³¹

The formation of β crystal phase in electrospun PVDF nanofibers has been studied by several groups.^7,32–35 It was mainly attributed to the unique formation mechanism of electrospun fiber and molecular structure of PVDF. During electrospinning, the polymer solution jet is highly stretched (stretching ratio up to 10⁵), making polymer chain orientate along the fiber length.^36,37 Such intensive stretching also leads to drawing of polymer chains. On the other hand, the PVDF molecular conformation corresponding to the β crystal phase has the longest chain length among all possible crystal phases (see the chain conformation and dimension data in ESI†). Therefore, intensive stretching facilitates the conversion of PVDF into β crystal phase, and the solvent evaporation contributes to retain the β crystal phase in the solidified fibers.

Fig. 2a shows XRD curves of PVDF nanofibers electrospun from different PVDF solutions. The XRD peaks at 2θ = 17.8° and 26.6° corresponded to the (110) and (021) crystal planes of α phase. The peak at 2θ = 20.4° was the characteristic of β crystal phase, which is the sum of the diffraction at (110) and (200) planes.³⁸ The XRD results indicated that PVDF nanofibers mainly contained α and β phases. With increasing the PVDF concentration in the electrospinning solution, the peak intensity at 20.4° increased gradually until the concentration reached 20%, and the peak decreased with further increasing the concentration.

Fig. 2 (a) XRD curves, (b) FTIR spectra of PVDF nanofibers electrospun from different PVDF solutions, (c) calculated β crystal phase contents based on the FTIR spectra, and (d–f) electrical outputs of the nanofiber mats (applied voltage 15 kV; spinning distance 15 cm; nanofiber mat thickness 70 μm).

Fig. 2b shows the Fourier Transform Infrared (FTIR) spectra of the nanofibers. The α crystal phase shows vibration characteristic bands at 761 cm⁻¹ (CH₂ in-plane bending or rocking), 877 cm⁻¹ (CH₂ out-of-plane bending or rocking) and 976 cm⁻¹ (CH₂ twisting). The peaks at 840 cm⁻¹ and 1274 cm⁻¹ corresponded respectively to CH₂ rocking/CF₂ asymmetrical stretching and C–F stretching vibrations of β phase.^39,40 Based on the FTIR spectra, the β phase content, F(β), in the PVDF nanofibers was calculated using the eqn (1):⁸

F(β) = X_β/(X_α + X_β) = A_β/[(K_β/K_α)A_α + A_β] (1)

where A_α and A_β are the absorbance at 761 cm⁻¹ and 840 cm⁻¹, respectively. K_α and K_β are the absorption coefficient at the respective wavenumber, which is 6.1 × 10⁴ and 7.7 × 10⁴ cm² mol⁻¹ in value.⁸ Fig. 2c shows the calculation result. The β crystal phase content increased from 78% to 85.9% when the PVDF concentration increased from 16% to 20%. Further increasing the PVDF concentration from 20% to 26% resulted in gradual decrease of the β crystal phase content to 82.5%. The effect of polymer concentration on β crystal phase content can be explained by that solution with higher polymer concentration is harder to be stretched due to the higher viscosity and stronger macromolecular chain entanglement.

The mechanical-to-electric energy conversion property of the nanofiber mat was evaluated by repeatedly pressing the energy harvesting devices prepared in a controlled manner. The voltage and current outputs of the PVDF nanofiber mats under 1 Hz compressive impact (force 10 N) are shown in Fig. 2d and e. The insets in the figures show the typical outputs from a single compressive impact, which always generate two signals with opposite polarities. The first signal was caused by the compressive deformation of nanofiber mat and the second was related to the recovery deformation.³

The average positive voltage and current outputs were calculated and presented in Fig. 2f. Both voltage and current outputs had a similar change trend. The PVDF concentration changed from 16% to 20% during electrospinning led to the change of voltage output from 1.3 V to 2.2 V, and the current output change followed a similar trend, from 1.4 μA to 2.3 μA. The voltage and current increased by 69% and 64%, respectively. However, the β phase content in the nanofibers only increased by 10%.

When the PVDF concentration increased from 20% to 26%, the voltage and current outputs of the resulting nanofiber mat changed to 1.6 V and 1.7 μA, decreasing by 27% and 26%, respectively. However, the corresponding β phase content decreased just by 3.9%. These results indicate a significant effect of β crystal phase content in PVDF nanofibers on the mechanical-to-electrical energy conversion performance. Since the nanofiber mat prepared from 20% PVDF solution generated the highest electrical outputs, 20% solution was used in the following experiments.

3.2 Applied voltage

Electrical field is the driving force to jet initiation and fiber thinning during electrospinning. In our study, PVDF solution was electrospun at a constant flow rate (1 mL h⁻¹). Uniform nanofibers were prepared when the applied voltage was set in the range of 9–21 kV (see the corresponding SEM images in ESI†). Fig. 3a shows the fiber diameter of the PVDF nanofibers prepared from a 20% PVDF solution at different applied voltages. All the fibers electrospun were uniform without bead. With increasing the applied voltage from 9 kV to 15 kV, the average fiber diameter decreased from 630 nm to 284 nm, and further increasing the applied voltage to 21 kV led to increasing the diameter to 580 nm.

Fig. 3 Effect of (a) applied voltages, (b) spinning distances, and (c) electric field intensity on PVDF fiber diameters, β crystal phase contents and electrical outputs of PVDF nanofiber mats (PVDF concentration 20%; nanofiber mat thickness 100 μm).

Increasing the applied voltage results in higher electrostatic force to stretch the jet and filament during electrospinning, therefore decreasing fiber diameter. This is in good agreement with the trend of fiber diameter change when the applied voltage increased from 9 kV to 15 kV. The increase in fiber diameter when further increasing the applied voltage was attributed to the intensive bending instability, which increased the diameter distribution (see the standard deviation in Fig. 3a). The formation of more coarse fibers when the applied voltage was above 15 kV (see Fig. S2, ESI†) could be the reason leading to the large diameter distribution.

The XRD and FTIR results indicated that PVDF nanofibers electrospun at different applied voltages showed similar characteristic peaks. The calculated β crystal phase content based on the FTIR spectra is shown in Fig. 3a. With increasing the applied voltage from 9 kV to 15 kV, the β phase content increased from 76.7% to 85.9%, and the β phase content decreased with further increasing the applied voltage. For the PVDF nanofibers electrospun at 21 kV, the β phase content was 81.6%. Fig. 3a also shows the mechanical-to-electrical energy conversion properties of the nanofiber mats (see the voltage and current output curves in the ESI†). Both voltage and current outputs showed a similar trend to the β phase content, with output maximum of 1.5 V and 1.6 μA (PVDF nanofibers were electrospun at 15 kV). These results suggest that uniform PVDF nanofibers with higher β phase content have higher mechanical-to-electric conversion ability.

3.3 Spinning distance

Spinning distance (nozzle tip-to-collector distance) in electrospinning affects fiber stretching and deposition.⁴¹ When electric field force is sufficient to maintain the electrospinning process, increasing spinning distance provides larger space for jet stretching and longer time for solvent evaporation from the fibers. A spinning distance shorter than a critical length often leads to insufficient fiber stretching and solvent evaporation. As a result, wet coarse fibers, and even porous films, result. To examine the effect from spinning distance, a PVDF solution (concentration, 20%) was electrospun at a constant applied voltage (15 kV). At the same applied voltage, altering the spinning distance also leads to change in electric field intensity, which affects electrospinning process. To exclude the effect from electric field intensity, we adjusted the applied voltage when different spinning distances were employed, so that the average electric field intensity was maintained at a constant value (1 kV cm⁻¹).

3.3.1 At constant voltage. When electrospinning was conducted at a constant applied voltage, while the spinning distance changed from 9 cm to 21 cm, PVDF fibers produced showed a similar fibrous morphology (see the SEM images in Fig. S6 in ESI†). Fig. 3b shows the average diameter of the PVDF fibers prepared at different electrospinning distances. The average fiber diameter decreased when the spinning distance increased from 9 cm to 15 cm. With further increasing the spinning distance, the average diameter increased.

At a constant applied voltage, the electric field intensity reduces with increasing the spinning distance. Increasing the spinning distance initially provide more time and space for the jets to be stretched. Short spinning distance (<15 cm) in our experiment resulted in coarse fibers with large diameter distribution. When the spinning distance changed from 13 cm to 15 cm, the average fiber diameter decreased significantly from 458 nm to 284 nm. The fiber diameter had a slight increase to 341 nm when the spinning distance was further increased to 21 cm. Again, β crystal phase content was calculated based on the FTIR spectra (Fig. 3b). The highest β crystal phase content of 85.9% was obtained at the spinning distance 15 cm. The electrical outputs of the PVDF nanofiber mats collected at different spinning distances are shown in Fig. 3b as well. Both voltage and current followed a similar change trend to the fiber β phase content. When the spinning distance increased from 9 cm to 15 cm, the voltage and current outputs reached the maximum level of 1.5 V and 1.6 μA.

3.3.2 At constant electric field intensity. Under constant electric field intensity, all PVDF nanofibers electrospun at different distances had a uniform fibrous morphology (see the SEM images in ESI†). Fig. 3c shows the average fiber diameter calculated based on the SEM images. The average fiber diameter decreased from 392 nm to 284 nm when the spinning distance was increased from 9 cm to 15 cm. A spinning distance of 17 cm could prepare PVDF nanofibers with a similar average diameter (283 nm) to the fibers collected at a 15 cm spinning distance. The fiber diameter had a slight increase when the spinning distance was longer than 17 cm.

Fig. 3c also shows the β crystal phase content and electric outputs of the PVDF nanofibers. The β crystal phase content increased first with increasing the spinning distance until 17 cm (β crystal phase content, 86.2%). Similar to the nanofibers produce at a constant applied voltage, further increasing the spinning distance also led to a decrease in the β crystal phase content. When the spinning distance increased from 9 cm to 15 cm, the voltage and current outputs increased respectively from 1.1 V to 1.5 V and 1.1 μA to 1.6 μA, respectively. The outputs were almost the same when the spinning distance was in the range of 15–17 cm. Further increasing the spinning distance decreased both the voltage and current outputs.

The above results suggest that electric field intensity in electrospinning process plays a dominating role in determining the fiber diameter and β crystal phase content. The finer fibers may result from more effective stretching, hence having higher β crystal phase content.

3.4 Nanofiber mat thickness

PVDF nanofibers of different thicknesses were prepared under the same electrospinning condition except for that electrospinning time was varied to control the mat thickness. It was reasonable to anticipate that nanofibers electrospun under the same condition should contain the same level of β crystal phase within the fibers. When the fiber mate were prepared into energy harvesting devices, the difference in the device performance should come from the mat thickness.

Fig. 4 shows the effect of PVDF nanofiber mat thickness on the energy conversion performance. When the mat was thinner than 20 μm, a short circuit occurred occasionally during the compressing process. With increasing the mat thickness from 20 μm to 70 μm, both voltage and current outputs increased significantly, to reach 2.2 V and 2.3 μA, respectively. However, with further increasing the mat thickness, electrical outputs reduced. The increase in the electrical output with increasing the mat thickness can be explained by that increasing mat thickness leads to increase in active material therefore the charge generation. However, increasing the mat thickness also increases the resistance for charge transfer across the mat. Increasing mat thickness could also decrease the strain level of the deformed fibers under the same compressive force, which reversely affect the power-generating performance of the nanofiber web.

Fig. 4 (a) Voltage, and (b) current outputs of PVDF nanofiber mats with different mat thicknesses (solution concentration 20%; applied voltage 15 kV; spinning distance 15 cm).

4. Conclusions

In this study, the effects of PVDF concentration, applied voltage, spinning distance, and nanofiber mat thickness on fiber diameter, PVDF β crystal phase content, and mechanical-to-electrical energy conversion properties of electrospun PVDF nanofiber mats have been examined systematically. It was interesting to find that finer uniform PVDF fibers showed higher β crystal phase content hence the energy harvesting having higher electrical outputs regardless the change of electrospinning parameters and PVDF concentration. The voltage output change always follows the same trend to the change of current output whatever the change is caused by operating parameter or polymer concentration. Both electric outputs follow a similar change trend to the change of β crystal phase content in nanofibers. Nanofiber mat thickness affects the mechanical-to-electric energy conversion. Uniform nanofibers with small diameter and high β crystal phase content facilitate to mechanical-to-electric energy conversion. These novel understanding may benefit to develop piezoelectric nanofibrous devices for various uses in energy field.

Acknowledgements

Funding support from Australian Research Council (ARC) through Future Fellowship project (FT120100135) is acknowledged.

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Footnote

† Electronic supplementary information (ESI) available: Fiber morphology, XRD, FTIR and electrical outputs. See DOI: 10.1039/c4ra16360e

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