Nanofibrous Smart Fabrics from Twisted Yarns of Electrospun

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Nanofibrous Smart Fabrics from Twisted Yarns of Electrospun Piezo Polymer Enlong Yang, Zhe Xu, Lucas Chur, Ali Behroozfar, Mahmoud Baniasadi, Salvador Moreno, Jiacheng Huang, Jules Gilligan, and Majid Minary-Jolandan ACS Appl. Mater. Interfaces, Just Accepted Manuscript • Publication Date (Web): 23 Jun 2017 Downloaded from http://pubs.acs.org on June 24, 2017

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ACS Applied Materials & Interfaces

Nanofibrous Smart Fabrics from Twisted Yarns of Electrospun Piezo Polymer Enlong Yangab, Zhe Xub, Lucas K Churb, Ali Behroozfarb, Mahmoud Baniasadib, Salvador Morenob, Jiacheng Huangb, Jules Gilliganb+, and Majid Minary-Jolandanb* a

College of Material and Textile Engineering, Jiaxing University, Jiaxing, Zhejiang 314001, China b Department of Mechanical Engineering and Alan G. MacDiarmid NanoTech Institute, The University of Texas at Dallas, Richardson, Texas 75080, United States + Currently at Northwestern University *To whom correspondence may be addressed. Email: [email protected]. ABSTRACT: Smart textiles are envisioned to make a paradigm shift in wearable technologies to directly impart functionality into the fibers rather than integrating sensors and electronics onto conformal substrates or skin in wearable devices. Among smart materials, piezoelectric fabrics have not been widely reported, yet. Piezoelectric smart fabrics can be used for mechanical energy harvesting, for thermal energy harvesting through the pyroelectric effect, for ferroelectric applications, as pressure and force sensors, for motion detection, and for ultrasonic sensing. We report on mechanical and material properties of the plied nanofibrous piezoelectric yarns as a function of post-processing conditions including thermal annealing and drawing (stretching). In addition, we used a continuous electrospinning setup to directly produce P(VDF-TrFE) nanofibers and convert them into twisted plied yarns, and demonstrated application of these plied yarns in woven piezoelectric fabrics. The results of this work can be an early step toward realization of piezoelectric smart fabrics.

Keywords: Smart fabrics, Nanostructured materials, multifunctional materials, P(VDFTrFE), electrospun nanofiber, plied yarn, stretching, annealing, crystallization, piezoelectric properties, mechanical properties.

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1. INTRODUCTION Textiles are one of the most basic aspects of human life, which have not changed from their “passive” form for many decades. Conventional fabrics are made of passive materials, such as nylon and cotton. Smart textiles are envisioned to make a paradigm shift in wearable technologies to directly impart functionality into the fibers rather than integrating sensors and electronics onto conformal substrates or skin in wearable devices. Smart fabrics are defined as textiles with the ability to react to different physical stimuli (mechanical, electrical, thermal, etc.) and as such can interact (sense, respond, communicate, and/or adapt) with their environment

1, 2, 3, 4, 5, 6, 7, 8, 9

. Applications of smart textiles could include power generation

and storage, personal protection, sports, fashion, communication, medical and physiological monitoring applications, and the internet of things

1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13

. Various types of

smart fabrics have been reported in the literature, including conductive, shape memory and phase change fabrics, and chromic and thermoelectric fabrics 1, 2, 3, 9, 10, 11, 12, 13, 14, 15, 16. Among smart materials, piezoelectric fabrics have not been widely reported, yet. Piezoelectric materials are a class of smart materials that convert the mechanical energy into electrical energy and vice versa

17, 18, 19

. This property makes their usage for sensors, actuators, and

energy harvesting applications inevitable

18, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36

. In

particular, piezoelectric smart fabrics can be used for mechanical energy harvesting, for thermal energy harvesting through the pyroelectric effect, for ferroelectric applications, as pressure and force sensors, for motion detection, and for ultrasonic sensing. Piezoelectric ceramics are often brittle and are not suitable for fabrics. Piezoelectric 2


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polymers PVDF (polyvinylidene fluoride) and in particular its co-polymer P(VDF-TrFE) (poly[(vinylidenefluoride-co-trifluoroethylene]) are the strongest candidates for smart piezo fabrics. Advantages of P(VDF-TrFE) for smart fabrics applications include: (i) processability into fibers and nanofibers using relatively low-cost processes; (ii) lightweight (ρ ~ 1.87 g/cm3); (iii) largest piezoelectric constant among polymers; (iv) biocompatible; and (v) soft and conformal elasticity. For realization of piezoelectric polymer fabrics critical developments are required: processing and scale-up manufacturing capabilities for continuous production of weavable, knittable, and sewable piezoelectric polymer yarns should be developed; strategies for integration of piezoelectric yarns with conventional and conductive threads should be designed; additionally, improvement in the electromechanical conversion efficiency of the piezoelectric polymer is needed in order to enhance their energy harvesting and sensing performance. For piezoelectric materials, the electromechanical coupling factor k2, which is the ratio of stored electrical energy (‫ܧ‬෠௘ ) to input mechanical మ

ௗ ௒ energy (‫ܧ‬෠௠ ), is an important performance metrics. It can be shown that ݇ ଶ = ఌ , in which Y

is the elastic modulus, d is the piezoelectric constant, and ε is the dielectric constant

19, 37

.

Therefore, the performance of piezoelectric materials depends on both the piezoelectric constants and also the elastic properties. Since PVDF and P(VDF-TrFE) are semicrystalline, both mechanical and electro-mechanical properties strongly depend on polymer morphology 38, 39, 40, 41

. Therefore, enhancement of degree of crystallinity and effective orientation of the

crystallites may result in improvement of the electromechanical coupling in these polymers. Post-processes such as thermal annealing and drawing affect the morphology (crystallinity, orientation and size of the crystallites, and alignment of the amorphous chains, etc.) of 3



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polymers. As such, for smart piezoelectric fabrics to be realized, there is a need for understanding the quantitative inter-relation between thermomechanical post-processing (thermal annealing and drawing), morphology (crystallinity), mechanical and material properties, and electromechanical conversion efficiency of semicrystalline piezoelectric polymer nanofibers. In this article, we investigated mechanical and material properties of the plied nanofibrous piezoelectric yarns as a function of post-processing conditions including thermal annealing and drawing (stretching). In addition, we used a continuous electrospinning setup to directly produce P(VDF-TrFE) nanofibers and convert them into twisted yarns. We converted the single yarns to 2-ply and 3-ply yarns, and demonstrated application of these plied yarns in woven piezoelectric fabrics. The results of this work can be an early step toward realization of piezoelectric smart fabrics.

2. RESULTS AND DISCUSSIONS Uniaxially aligned nanofiber mats were fabricated using electrospinning process, as is shown in Figure 1A. After 3-hour fabrication, the paper substrate with aligned nanofiber membrane was detached from the collector for further processing. Briefly, the mats were cut into ribbons and the ribbons were twisted into single ply yarns. Subsequently, the single ply yarns were twisted into 2-ply and 3-ply yarns (Fig. 1B), using a costume made twisting device (Fig. 1C).

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Fig. 1 (A) Schematic of the electrospinning process for fabrication of P(VDF-TrFE) nanofiber mats. (B) Five millimeter-wide ribbons were cut from the mat and twisted to a single yarn. (C) To fabricate a 2-ply yarn, two single yarns were twisted in the direction opposite to the direction of the single yarn twist using the setup shown. (D) and (E) As-made yarn and the drawn (stretched to 80% strain) yarn, respectively, prepared for mechanical testing.

We investigated the effect of post-processing conditions (thermal annealing and drawing (stretching)) on the mechanical behavior and materials properties of singe yarns, 2-ply, and 3ply yarns. Addition of TrFE co-polymer to PVDF shifts the Curie temperature to lower than the melting point, which allows for thermal annealing at the paraelectric phase

39, 40

. In

addition, the glass transition temperature of P(VDF-TrFE) is -35 °C, which allows for roomtemperature drawing. The twisted and plied yarns were annealed at 135 °C for two hours.

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For drawing, the yarns were subjected to 80% strain before annealing (Fig. 1 D and E). This strain value was obtained from earlier experiments as the lower bound of the maximum strain before failure of the as-made yarns. Figure 2 shows SEM micrographs of single, 2-ply, and 3ply P(VDF-TrFE) yarns. In addition, the SEM micrographs obtained after annealing or stretch-annealing process for each yarn are shown.

Fig. 2 SEM images of P(VDF-TrFE) yarns. (A), (D) and (G) single yarn, (B), (E) and (H) 2ply yarn, and (C), (F) and (I) 3-ply yarn. (A)-(C) are as-made, (D)-(F) are annealed, (G)-(I) are stretched-annealed yarns. Insets in H and I show the schematic of the cross-section of 2ply and 3-ply yarns, respectively. Inset in A shows an entangled single yarn, scale bar is 500 µm. Insets in B and C show larger magnification view from the yarns to show the direction of the surface nanofibers with respect to the axis of the yarn.

The twist angle of yarns has been shown to have significant effect on the mechanical properties of yarns 42. We obtained the twist angle distribution of the yarns by analyzing SEM images of the yarns (n = 10), as shown in Fig. 3. Annealing process, as expected, did not change the twist angle of the yarns. Drawing process, however, resulted in reduction of the 6


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twist angle compared to the as-made and annealed yarns. In addition, single yarns had the largest twist angle, and the 2-ply yarns had the smallest twist angle. Inset in 2A shows an entangled single yarn. Single yarns tend to entangle due to the internal torque generated in the yarns as a result of twisting process. However, in the 2-ply and 3-ply yarns, these torques were eliminated or minimized since the yarns were plied in the opposite direction of the original twist in the single yarns. Hence, as-made 2-ply and 3-ply yarns did not entangle. The internal torque as a result of twisting process was also eliminated during subsequent annealing process. The surface nanofibers of the strand were parallel to axis of the as-made and annealed 2-ply and 3-ply yarns, as shown in insets of Figure 2B and C. The surface nanofibers were slightly inclined to the ply yarn axis in 2-ply and 3-ply yarns after stretching process.

As Made Annealed Stretched-then-Annealed

50 40

o

Twist Angle ( )

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30 20 10 0

Single Yarn

2-Ply Yarn

3-Ply Yarn

Fig. 3 Twist angle of yarns.

Figure 4 A-C shows the representative specific stress-strain responses of the yarns. The units of the stress are expressed in MPa/g/cm3 in accordance with analysis of stress in textile materials to account for the effect of the porosity in the yarns 43. For the single ply yarns, the catastrophic failure happened immediately as soon as it reached its ultimate specific stress. On the other hand, stress-strain response of 2-ply and 3-ply yarns showed one and two additional drops (respectively) in force after the point of maximum stress, pointed by arrows 7



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in the Fig. 4. Each failure peaks is indication of the failure of one of the strands in plied yarns.

Fig. 4 Representative specific stress-strain responses for yarns with different post-processing conditions for (A) single yarn, (B) 2-ply yarn, and (C) 3-ply yarn.

Mechanical properties from the tensile test are summarized in Fig. 5. We defined the failure strain as the strain in which the first drop in stress was observed. The as-made yarns 8


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showed several times larger ductility compared to the annealed and drawn yarns. In addition, the failure strain of drawn yarns was smaller than the annealed yarns. On the other hand, the drawn yarns showed larger strength (by more than two times) and elastic modulus compared to the as-made and annealed yarns. The specific modulus was calculated from the slope of the stress-strain response up to 2% strain. The strength of the as-made and annealed yarns were similar within the margin of error. Tensile toughness (energy to failure in tension per unit mass), defined as the area under the stress-strain response divided by mass, represents amount of energy that a material can absorb before final rupture in tension. Tensile toughness of the yarns was much higher for as-made yarns, compared to the annealed and the drawn

160

As Made Annealed Stretched-then-Annealed

120 80 40 0

C

Single Yarn 2-Ply Yarn

B

3-Ply Yarn

300 250 200 150 100 50 0


3-Ply Yarn


3-Ply Yarn

D

2.5

100 2.0

Toughness (J/g)

Failure Strain (%)

200

3

A

Specific Strength (MPa/g/cm )

yarns, which mainly resulted from their large ductility.

Specific Modulus (GPa)

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1.5 1.0 0.5 0.0

80 60 40 20


0

3-Ply Yarn

Fig. 5 Mechanical properties of the yarns from tensile test. (A)-(D) Failure strain, specific strength, specific modulus, and tensile toughness of the yarns for different processing conditions. Error bars are standard deviation (N = 5). All graph have the same legend.

P(VDF-TrFE) is a semi-crystalline polymer, with crystalline forms including α, β, γ and δ phases. The β-phase, having the anti-polar arrangement of fluorine and hydrogen atoms

9



is the most piezoelectrically responsive phase

18, 44, 45, 46

. One advantage of P(VDF-TrFE)

over PVDF is that, regardless of the processing method, it always contains β-crystalline phase. This is attributed to the steric hindrance of TrFE in the co-polymer, which forces PVDF into all-trans (TTTT) configuration (β-phase). XRD spectra for yarns with different post-processing conditions clearly show the main peak at 2θ ≈ 19.6°, which corresponds to the (110) and (200) planes of the β-phase, Fig. 6A

41, 47

. A small shoulder at lower angles

appeared for the as-made yarns, which is often attributed to γ-phase

39, 40

. This shoulder did

not exist for annealed and stretched-then-annealed yarns. In return, the intensity of the main peak assigned to β-phase increased for the annealed and stretched yarns. The degree of crystallinity for different post-processing conditions was estimated based on the deconvolution of the XRD spectra into Gaussian peaks for crystalline phase and amorphous halo. As-made yarns had ∼32% crystalline phase. The degree of crystallinity increased after annealing process to ∼46% for annealed yarns. Stretched-then-annealed yarns showed slightly larger percentage of crystallinity (∼50 %) compared to the annealed yarns.

A

B Degree of Crystallinity (%)

β (110) (200)

Intensity (a.u.)

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Stretched-then-Annealed Annealed As Made 10

20

30

40

50 45 40 35 30

50

As Made

2 θ (o)

Annealed

Stretched-then -Annealed

Fig. 6 (A) X-ray diffraction (XRD) spectra of as-made, annealed, and stretched-thenannealed yarns. The inset shows a sample of the deconvolution of the peaks to crystalline and non-crystalline peaks. (B) Degree of crystallinity of the yarns estimated from the XRD spectra.

Figure 7A shows differential scanning calorimetry (DSC) thermogram of as-made, 10


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annealed, and stretched-then-annealed yarns. For as made yarns, the endothermic peak for ferroelectric to paraelectric transition appeared at ~95 °C. Upon annealing and drawing, this peak shifted to ~104-106o C (Fig. 7B). The enthalpy of ferroelectric to paraelectric phase transfer increased from ~8 J/g for as made yarn to 26.4 J/g for annealed, and 29.4 J/g for stretched-then-annealed yarn (Fig. 7C). The significant jump in the Curie temperature and Curie enthalpy for annealed and stretched-then-annealed yarns is an indication of larger crystallinity content with more stable ferroelectric phase, which would require more energy to transfer from the ferroelectric phase to paraelectric phase. The melting temperature of asmade yarn was 152 °C, while this value increased to ~155 °C for annealed yarns and stretched-then-annealed yarns (Figure 7D). The melting enthalpy for as made, annealed, and stretched-then-annealed yarns was obtained to be 26.9 J/g, 27.2 J/g and 28.4 J/g, respectively (Fig. 7E). Higher melting point and melting enthalpy shows that higher energy is needed to break down the bonds between the crystallites, which is the result of higher crystallinity content developed by thermal post-processing and stretching processes.

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As-Made Annealed Stretched-then-Annealed Endothermic

Heat Flow (a.u.)

A

80

B

100

120 140 o Temperature ( C)

C

160

35 30

105

Curie Enthalpy (J/g)

CurieTemperature (oC)

108

102 99

25 20 15 10

96

5 As Made

Annealed


155

154

153

As Made

Annealed


As Made

Annealed


E 28.5 Melting Enthalpy (J/g)

D Melting Temperature (oC)

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152

28.0

27.5

27.0 As Made

Annealed


Fig. 7 (A) DSC spectra of as-made, annealed, and stretched-then-annealed yarns. (B) The Curie temperature, (C) The Curie enthalpy, (D) melting temperature, and (E) melting enthalpy for yarns with different processing conditions.

The DSC spectra indicated that crystallization occurred in annealed and stretched-thenannealed yarns. The higher melting enthalpy of the annealed and stretched-annealed yarns compared to the as-made yarns is in agreement with the increase in the crystallinity of these yarns, indicated by the XRD spectra (Fig. 6B). The results are also consistent with the improvement of strength and elastic modulus and reduction of ductility for the annealed and drawn yarns. The annealing process results in an increase in the degree of crystallinity of the yarns. The drawing process can align the polymer chains along the axis of the nanofibers in the yarns. This alignment may in turn result in alignment-induced crystallization 12


48

.

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Enhancement of crystallinity from annealed to drawn yarns is much smaller compared to enhancement of crystallinity from as-made to the annealed yarns (Fig 6B). This enhancement alone cannot explain the significant enhancement in strength and modulus and reduction of ductility in drawn yarns compared to the annealed ones. Therefore, effect of structural properties of the drawn yarns must be also significant. More specifically, in a drawn yarn the nanofibers are more aligned along the axial direction of the yarns. Hence, they carry more load under tension compared to the nanofibers in non-drawn yarns. This results in enhancement of elastic modulus and strength. In addition, ductility of the drawn yarns decreases, since the aligned nanofibers would fail once the failure was initiated in few of the nanofibers.

It has been shown that piezoelectric constants of the P(VDF-TrFE) nanofibers and films also enhance by annealing and drawing processes

39, 40

.

Therefore, by combined

enhancement of the elastic properties and piezoelectric constant of the yarns their electromechanical conversion efficiency would increase. This is in agreement with the observed behavior from the woven fabric in the following, in which the annealed fabric exhibited larger voltage generation compared to the as-made fabric. Fig. 8A shows schematic of the continuous electrospinning apparatus that was used for fabrication of continuous twisted yarns for development of piezo fabrics. In this apparatus, the electrospun nanofibers collect at the large end of the funnel and form a thin nanofiber web (Fig. 8B). Continuous bundle of nanofibers were stretched and twisted from this web and were collected on spools (Fig. 8C).

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Fig. 8 (A) Schematic of the apparatus for manufacturing of continuous twisted yarns. (B) The 3D fibrous cone and twisted yarn from this cone. (C) The twisted yarns collected on spools. The twisted yarns on spools were used for fabrication of plied yarns (2-ply and 3-ply) and woven fabrics for demonstration. Fig. 9A shows a nanofibrous 2-ply piezo yarn stitched on a cotton fabric. The 2-plied yarn was fed into a sewing machine on a spool and was stitched on the fabric. Fig. 9B shows the false-colored SEM image of the piezo yarn on the cotton fabric. This process shows that the yarn are mechanically strong enough to be sewed with a sewing machine. Fig. 2C shows an SEM image of a woven piezo fabric made from 2ply yarns. This woven fabric was weaved manually.

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Fig. 9 P(VDF-TrFE) nanofibrous piezo yarns. (A) Piezo yarn stiches on a cotton fabric. The stiches were made using a conventional sewing machine. (B) False-color SEM image of the stitch on the cotton fabric (colored red). (C) SEM image of a woven piezoelectric fabric.

As another example, the piezo yarns were weaved with 2-ply conductive threads (316L stainless steel) and 2-ply conventional yarns (polyester) (Fig. 10A). P(VDF-TrFE) 2ply yarns were used as weft in the woven structure. Figure 10B shows the false-colored SEM image of the fabric. The conductive threads in this woven fabric function as electrodes for charge collection from piezoelectric yarns. In the design of the fabric, one set of the conductive threads was in contact with the top surface of the piezo yarns and the other set was in contact with the bottom surface of the piezo yarns (Fig. 10C). As such, when the piezo fabric deforms, one set of threads collect positive charges and the other set of threads collect negative charges. To satisfy this conditions and prevent electrically shorting of the collected charges, for this special design, two conventional yarns were used in between each conductive threads as separator, as shown in Fig. 10C.

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Fig. 10 (A) Photo of a weaved piezo-fabric made of P(VDF-TrFE) yarns, conductive threads, and conventional threads. (B) False-colored SEM image of the fabric. Blue color is the conventional polyester thread, and red color is the conductive thread and the grey color is the P(VDF-TrFE) yarns. (C) Schematic shows the weave pattern of the piezo-fabric. 16


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As a demonstration for the application of weaved piezo fabric, the electric voltage generation from the fabrics under applied pressure was measured (Fig. 11). Before measurement, the fabric was electrically poled using a corona poling apparatus.

Polarization direction in electrospun PVDF nanofibers is in the radial direction of each nanofiber. During twisting process to yarns and the subsequent knitting process of fabric, the radial orientation of the nanofibers changes from point to point. Therefore, corona poling is necessary to align all the dipoles in one direction after twisting of yarns or after knitting of the fabrics. Otherwise, all the dipoles will be random. Odd and even conductive threads formed two electrodes on top and bottom of braided yarns in the fabric structure. The conductive threads were connected to a copper strip using colloidal Silver. The copper strips were connected to a digital multimeter. A customized LabVIEW interface was developed to record the generated voltage from the multimeter. To measure the electric potential generated by the fabric, a PDMS block with the side length of ~20 mm was placed on the fabric, then a periodic pressure was exerted on the PDMS block. The conductive threads were connected to a Cu strip, which was connected to a multimeter (Fig. 11A). Figure 11B and 11C show the generated voltage from the as-made fabric in forward and reverse connection. In forward and reverse connections, the connection of electrodes to the poles of the multimeter were switched. The reversal of the generated voltage in forward and reverse connections for press and un-press cycles is an indication that the voltage originates from the piezoelectric charges in the piezo yarns. The as-made fabric generated an average voltage of ~3.6 mV (Fig. 11B and C). It has been shown that annealing can improve the electromechanical performance of P(VDF-TrFE) films 17


40

. We examined the


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voltage generation from the fabrics after they were thermally annealed at 135 °C for two hours. The annealing duration and temperature was based on our previous work and reports in the literature, in which annealing at the paraelectric phase has been shown to improve the electromechanical performance of P(VDF-TrFE). However, such data are not available for piezo yarns or piezo fabrics. Therefore, initially we investigated the effect of annealing on the twisted yarns and the annealing on fabric was built on these results. The annealed fabric generated an average of ~ 16.2 mV under the same condition as the as-made fabric, a 4.5time improvement (Fig. 11D and E).

Fig. 11 (A) Output voltage measurements from the piezo fabrics. In each cycle, the first and second peaks represent pressing and un-pressing, respectively. (B) and (C) as-made fabric. (D) and (E) annealed fabric.

Figures S1 and S2 in the Supplementary Information present variation of the generated voltage and current in the fabric and yarn vs. frequency of the applied load. An increasing trend was observed vs. frequency for both the yarn and the fabric, although quantitative comparison of the absolute values cannot be made using these experiments and additional experiments are required. In this study, for demonstration purposes, we fabricated the 18


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fabrics manually, and therefore, there was variations from sample to sample in terms of generated voltage and current. However, for each sample we consistently obtained similar results within experimental variations. Duration of sustained piezo effect in fabrics is expected be the same as piezoelectric nanofibers or films. Often times, the polarization in the piezo materials last for a long time. We obtained similar results from the same fabrics within a year long period, for fabrics which were stored in sealed bags during this period. Longer term evaluation of performance can be an interesting study by itself.

Above results show the promise for fabrication of weaved piezo fabrics out of electrospun nanofibers. For future studies, we plan to investigate the effect of twisting angle on the properties of the yarns. In addition, yarns and fabrics with a larger variation of processing conditions will be examined to optimize the properties for best performance. We plan to fabricate weaved fabrics from drawn yarns and investigate their properties. Various nanostructured materials such as nanoparticles and nanowires can be added to the nanofibers during electrospinning to further enhance their properties and performance. There are some foreseeable technical challenges that can be addressed which include: (i) Continuous yarn spinning, stretching, and annealing system for large area piezo-fabric; (ii) Optimum design of piezo-fabric such as fabric structure and density for different applications; (iii) As the textiles are intended for use as wearable smart textiles, the important factors such as air permeability, wicking properties, and stretchability need to be tested and controlled to provide a high level of comfort to the user; and finally (iv) the effects of wear and tear, washing and regular use also need to be verified to ensure reproducibility of the piezoelectric response and provide a certain lifetime value for the fabric. Collectively, the advancement in yarns twisted from P(VDF-TrFE) nanofibers and device design mechanics will continue to offer new opportunities for smart fabrics. 19



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3. EXPERIMENTAL SECTION

Materials: Polyvinylidene fluoride trifluoroethylene, P(VDF-TrFE), copolymer with a 70/30% molar ratio was purchased from Piezotech (Piezotech S.A.S, France). N,Ndimethylformamide (DMF) and acetone purchased from Sigma Aldrich (USA) were used as received.

Preparation of P(VDF-TrFE) solution: To obtain 10 g of 20 wt% of P(VDF-TrFE) solution, 2 g of P(VDF-TrFE) powder was dissolved in 5.6 g of DMF using a magnetic stirrer on a hot plate at 70° C for 6 hours. Then, 2.4 g acetone was added into the mixture and stirred at room temperature for at least 4 h until the solution became uniform and clear.

Conventional electrospinning: P(VDF-TrFE) solution was loaded into a 1 mL syringe fitted with an 18 gauge needle. The syringe was then placed on a syringe pump (New Era Pump systems Inc., Farmingdale, NY) and the solution pumping rate was set to 170 µL·h−1. The fiber mats were collected on a rotating drum collector. The rotational speed was set to 5500 rpm based on our previous work to obtain fairly aligned nanofibers. A DC electric voltage of 28 kV was applied between the needle tip and the collector that was placed 20 cm away from needle tip. Syringe pump system was moved laterally with the stroke of 13 cm and 10 cycles per minute in order to obtain uniform nanofiber mats.

Yarn Fabrication and processing: The electrospun nanofiber mat was cut into several ribbons with identical size of 20 mm × 5 mm, as is shown in Figure 1B. The ribbons were twisted into yarns using a home-made twisting device. To obtain a uniform yarn, 20 clockwise twists per centimeter was applied to a ribbon. Using a custom 3D printed 20


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ropewalk setup with planetary gears as shown in Figure 1, individual ribbons were attached to each gear and all ribbons were attached to a proper weight at the other end that prevented that end from rotation without removing the applied tension. For 2-ply yarn, the setup used two gears attached to a stepper motor, and for 3-ply yarn, 3 gears were used. For 1-ply, only the stepper motor was used. Using planetary gears, the 1st stage twisted the nanofiber mats into 1-ply yarns, and then moving the gears in the opposite direction, the 2nd stage twisted the yarns into a 2-ply yarn.

Stretching (drawing) and thermal annealing process: Yarns with different post-processing conditions were fabricated, including as-made yarn, 2 hr annealed, and 180% stretched then annealed. For thermal annealing the yarns were annealed at 135 °C for two hours, following our previous reports 39, 40. During annealing process, both ends of the yarns were fixed at the constant length, while their surfaces remained free of any solid material. After thermal annealing, the yarns were cooled down to room temperature at ambient condition. For drawing process, the yarns were stretched to 180% of their original length (80% strain). The direction of stretching was the same as orientation of the nanofibers.

Continuous electrospinning process and fabrication of yarns: For fabrication of yarns for the fabric, we used an electrospinning apparatus that fabricates continuous twisted yarns of nanofiber piezoelectric polymer

49, 50

. The setup is composed of two nozzles fed by two

syringe pumps at the rate of 0.270 ml/hr and two high voltage power supplies with opposing polarity (positive/negative) 24 kV and a rotating funnel at 6 rpm. The two nozzles were placed at the controlled angle of 30° with 16.5 cm horizontal distance with respect to the central axis of funnel. To prepare a twisted yarn, nanofibers were electrospun and collected 21



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at the large end of the funnel to form a thin nanofiber web, which formed into a 3D fibrous cone. Polyester thread were added to facilitate drawing P(VDF-TrFE) nanofibers from the metal funnel. An oriented nanofiber bundle was continuously stretched from the apex of the nanofiber cone, and twists were simultaneously inserted in the bundle through rotation of the funnel collector to form a twisted yarn. The continuous twisted yarn was collected with a winder on a spool at the rate of 40 cm/hr. 2-ply yarns were fabricated from two single yarns that were ply-twisted in the opposite direction at twist rate of 10 twists/cm. Similarly, 3-ply yarns were processed in the same way while the twist rate was 9 twists/cm 42.

Weaving of fabrics from plied yarns: 2-ply yarns were used as weft in the woven structure and two sets of the conductive threads were used as charge collectors. The nanofiber fabrics were made on a standard wood frame loom modified for smaller diameter fibers using 3D printed parts. Depending on the fabric pattern, the loom was strung along the length of the frame using polyester yarn, stainless steel yarn, and P(VDF-TrFE) nanofiber yarn in an alternating over/under pattern as warp thread that was held in tension. P(VDF-TrFE) nanofiber yarn and polyester yarn was used as weft thread in the horizontal direction using a needle. A custom heddle bar/rotating stick was used to create separation between the upper/lower warp threads allowing the needle to pass through. An additional 3D printed comb was used to make sure the weft thread was compact. Shorter P(VDF-TRFE) nanofiber yarns were held in place using tension or a knot.

Corona poling of the fabrics and voltage measurement: To induce dipole orientation in the nanofibers of the yarns, the fabrics were electrically poled using a homebuilt corona apparatus. The poling was performed in air using the following parameters: needle voltage 22


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VN = 10 kV, grid voltage VG = 5 kV. The distance between needle tip and grid was 5 mm, and the distance between grid and fabric was 10 mm. The fabrics were poled for 2 h at 80 oC.

Tensile tests: Tensile tests were performed on an Instron tensile machine (Series 5969, Instron, MA) equipped with a 50 N load cell and mechanical thumb-screw action type grippers. The crosshead speed was set to 0.3 % (of the initial gauge length 10 mm) per second. Yarn samples were wrapped three times and fixed onto U-shape cardboard frames using epoxy (ITW Devcon, Danvers, MA).

Scanning electron microscopy: Prior to SEM imaging, samples with different postprocessing conditions were coated with ∼7 nm Palladium/Gold using Hummer-VI sputtering tool (Anatech USA,Union City, CA) to reduce charging artifact. SEM images were captured using ZEISS SUPRA-40 FE-SEM (Carl Zeiss Microscopy GmbH, Germany) with EHT set to 5 kV.

DSC (differential scanning calorimetry) analysis: Differential scanning calorimetry was performed using Q2000 Differential Scanning Calorimeter (TA Instruments, New Castle DE). DSC measurements were performed in a complete temperature cycle, ranging from 45 °C to 250 °C with the rate of 10 °C per min. The cycle started from room temperature, sample was cooled down to -45 °C and kept at that temperature for 3 min. Subsequently, it was heated up to 250 °C and remained in that temperature for 5 min. Finally, it was cooled down to room temperature.

X-ray diffraction analysis: Yarns were cut into 10 mm pieces and placed side-by-side together (8 mm wide) on a glass slide. The XRD analysis was conducted using a Rigaku Ultima III XRD (40 kV, Rigaku Co. X-ray diffraction p., Tokyo, Japan) with Cu Kα source 23



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(wavelength, 0.15418 nm). The diffractogram was recorded between angles 2θ = 10˚ and 2θ = 50˚ with a scan rate of 3° per minute at room temperature.

4. CONCLUSIONS We report on twisted yarns of electrospun P(VDF-TrFE) nanofibers for smart piezo fabric application. Our results show that using continuous electrospinning process long twisted yarns can be obtained, which makes it feasible to fabricate weaved fabrics. In addition, results of post-processing shows that mechanical properties of the yarns can be improved by thermal annealing and drawing. As a demonstration of energy harvesting, our results showed that annealed fabric had 4.5-fold output voltage of as-made fabric. The spectroscopy analysis showed that this improvement is the result of enhanced crystallinity in the semicrystalline morphology of P(VDF-TrFE). The DSC spectra indicated that crystallization occurred in annealed and stretched yarns. The higher melting enthalpy of the annealed and stretched yarns compared to the as-made yarns was in agreement with the increase in the crystallinity of these yarns, indicated by the XRD spectra. The results are also consistent with the improvement of strength and elastic modulus for the annealed and drawn yarns. The annealing process results in an increase in the degree of crystallinity of the yarns. The drawing process can align the polymer chains along the axis of the nanofibers in the yarns. This alignment may in turn result in alignment-induced crystallization. In addition, in a drawn yarn the nanofibers are more aligned along the axial direction of the yarns. Hence, they carry more load under tension compared to the nanofibers in non-drawn yarns. This results in enhancement of elastic modulus and strength.

Acknowledgements 24


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Funding for this work was provided by College of Material and Textile Engineering, Jiaxing University, Jiaxing, Zhejiang 314001, China, and the US National Science Foundation by NSF-ECCS award number 1549965. The Eugene McDermott Graduate Fellowship (Grant No. 201502) and the NSF Graduate Research Fellowship Program (Grant No. DGE 1147385) provided additional funding. The authors would like to extend thanks to Dr. Gyuho Kim for the advice and discussion regarding fabrication. Thanks to Dr. Jae Ah Lee for advice and help with measurement of piezoelectric test of fabrics. The authors would also like to extend thanks to Andrea Rivera for the design and fabrication of the setup to twist ropes and Nithisha Maghanalli for helping with the sewing machine. Supporting Information. Following materials are provided in the Supporting Information: - Peak voltage and peak current generated in the fabric and in the yarn under three different frequencies. - Impedance vs. frequency behavior of a single yarn. References (1)

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candidate cover image 137x166mm (144 x 144 DPI)


Nanofibrous Smart Fabrics from Twisted Yarns of Electrospun

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