Fabrication of flexible SiO2 nanofibrous yarn via a conjugate electrospinning process

Abstract Nowadays, different kinds of polymers, including ceramics, are electrospun into fibrous materials with different structures by electrospinning. Generally, the as-spun ceramic fibers are randomly oriented membranes and brittle without flexibility. Here, we report the fabrication of flexible SiO2 electrospun yarns using poly(vinyl alcohol) (PVA) as a template through a conjugate electrospinning process and calcination. It was found that the calcined as-spun fibers and yarns are obviously thinned with PVA component removal. Fourier transform infrared spectroscopy and energy-dispersive spectroscopy examinations suggested that the obtained yarn after calcination was SiO2 yarn. The SiO2 yarn showed good flexibility without cracking after 180° bending. The flexible ceramic yarn may have potential application in functional textiles.

Generally, electrospun fibrous materials were collected as a nonwoven mesh with fibers randomly arranged. After a thorough research, it was found that the structures of the as-spun fiber and fibrous materials may play important roles in their properties and applications (30)(31)(32). Consequently, more and more attention was paid to control and design the structures of the electrospun fibrous materials by modifying the spinnerets and collectors (32)(33)(34)(35). Specific to the electrospun yarns, "conjugate electrospinning," with which two spinnerets are connected to two opposite polarity high-voltage supply, was found to be obtained through the electrospun fiber yarn in one step (36)(37)(38)(39). By conjugate electrospinning, a series of polymer yarns as well as inorganic oxide arrays were fabricated (38)(39)(40). However, flexible inorganic yarns were rarely reported due to the complex process and brittleness.
Generally, the electrospun inorganic fibers are too brittle without flexibility. To overcome this disadvantage, researchers had found that using poly(vinyl alcohol) (PVA) or polyvinyl pyrrolidone (PVP) as template materials blended with inorganic salt gel could help to produce flexible ceramic nanofibrous mats via electrospinning process and calcination (41)(42)(43)(44). Nevertheless, flexible inorganic nanofibrous yarn is still a challenge. This study reports on the fabrication of flexible SiO 2 nanofibrous yarn via a conjugate electrospinning process and calcination. The morphology, chemical structures, thermal stability, and flexibility of the prepared yarns are examined.

Preparation of electrospinning solutions
PVA was dissolved in deionized water at 11 wt% within a water bath at 40°C for 4 h and then cooled naturally. TEOS, H 3 PO 4 , and deionized water were mixed with a weight composition of TEOS:H 2 O:H 3 PO 4 = 1:1:0.02 according to previous studies (41,42) and stirred at room temperature for 4 h to form a uniform silica gel, as suggested in Figure 1a. Then, the prepared silica gel was dropped slowly into PVA solutions with a weight ratio of 1:1 and stirred for another 3 h to obtain the spinning precursor (41).

Fabrication of SiO 2 yarn
To produce SiO 2 yarn, conjugate electrospinning was processed with two parallel separated opposite-polarity highvoltage supply and vertically placed rotating trumpet collector as well as a rotating drum collector, as suggested in Figure 1b. During the conjugate electrospinning process, the prepared spinning precursor was loaded into two 5 mL syringes with a flat metal needle (18G, inner diameter: 0.9 mm), and then the two syringes were fixed onto the syringe pumps with a feed rate of 18 mL/min. The needles were connected to two opposite-polarity high-voltage supply (−30 kV-0 and 0-30 kV), respectively. The high voltages were set to be ±16 kV, respectively. The horizontal distance between the high voltage supply to the collector was set to be about 19 cm, and the vertical distance between the two collectors was about 20 cm. The electrospinning process was taken at an ambient temperature of 26°C and a humidity of 56%. The rotating drum collector was static, however, and the average collected yarn length was about 12 cm.
The collected PVA/silica yarns were placed in a ceramic ark and then put into the tube furnace. Afterward, the yarns were calcined in air with a heating rate of 3°C/min to 800°C and then kept at 800°C for 6 h to remove the organic components (41). After natural cooling, the inorganic SiO 2 yarns could be obtained.

Characterization
The morphology and the energy-dispersive spectroscopy (EDS) of the electropsun yarn were examined by a scanning electron microscope (SEM, Phenom Pro; Thermo Fisher Scientific). The average diameter of the as-spun fiber was measured through a soft: nano-measurement of 1.2 with ten yarns. The Fourier transform infrared spectroscopy (FTIR) spectra were characterized by a Thermo Scientific Nicolet iS10 spectrometer. The mechanical property of the fabricated yarns (5 cm length) was examined by a universal testing machine (Instron 3352) with a speed of 0.02 mm/min. The weight-temperature relationships of the yarns were tested through a thermal gravimetric analyzer (TGA/DSC3+, Mettler Toledo).

Results
The morphology of the fabricated yarn is shown in Figure 2. As can be seen in Figure 2a, the electrospun PVA/silica yarn with an average diameter of 199.77 ± 13.86 mm and a linear density of 14.8 tex had a relative uniform twisted structure with a little hairiness. The enlarged image in Figure 2c suggested that the electrospun fibers generally oriented well and had average diameters of about 380 ± 76 nm. After calcination, the yarn showed a more tight and lightly twisted structure without hairiness, and the yarn diameter was reduced to 128.96 ± 2.88 mm with the fiber diameters of about 315 ± 110 nm, as suggested in Figure 2b and d. Based on ten samples, after calcination, the reduction rate of the yarn diameters was found to be about 35.1%, while the fiber diameter reduced to about 17.3%. The reduction rate of the fiber diameter was nearly half of that of the yarns. It could be imagined that both the neighborhood fibers reduced due to the organic component removal after calcination, and then the extra space was filled by the calcined fibers.
We also examined the elements in the as-spun yarns. As shown in Figure 2e, there are mainly three elements in the PVA/silica yarn: carbon, oxygen, and silicon, While, after calcination, there are mainly oxygen and silicon elements (Figure 2f), which indicates that the organic components were removed during the calcination process and then only pure SiO 2 yarn remained. Figure 3a shows the FTIR spectra of the prepared yarns before and after calcination. As can be seen from the spectra, the fabricated PVA/silica yarn showed typical PVA absorption peaks at 3,430 cm −1 (-OH), 2,930 cm −1 (-CH 2 ), and 1,720 cm −1 (C]O). There were also Si-O-Si bond absorption peaks at 1,080 cm −1 , 804 cm −1 as well as the Si-OH bond peaks at 963 cm −1 , suggesting the presence of silica in the as-spun yarn. After calcination, the PVA absorption peaks disappeared; only Si-O-Si absorption peaks could be found at 1,130 and 804 cm −1 , indicating the calcined yarn was SiO 2 yarn. Figure 3b presents the weight-temperature curve of the prepared yarns before and after calcination. For the as-spun PVA/silica yarn, as the temperature increased the yarn weight was reduced accordingly due to the PVA melt volatilization. When the temperature exceeded 500°C, the weight reduction was weakened, since the organic components almost completely evaporated. For the calcined yarn, the weight did not seem to change as the temperature increased.
As presented in Figure 4a, the tensile property of the prepared yarns before and after calcination was tested. It was found that the PVA/silica yarn showed a maximum stress of 3.8 MPa and maximum elongation to 0.32%. Once the PVA/silica yarn was broken, a few continuous fibers still remained, as suggested in the left inset image in Figure 4a. However, after calcination, the tensile property was obviously reduced to 1.5 MPa stress and about 0.03% strain. Once the calcined yarn was broken, one can find all the fibers were broken. The decreased strength of the calcined yarn may result from the brittle individual SiO 2 fiber. Since the organic component PVA was removed after calcination, the remaining SiO 2 formed a fiber with extremely small-sized crystal grains (41). In some sense, the SiO 2 fiber was not a continuous one. Consequently, its strength is weakened. Thankfully, the calcined yarn showed a certain flexibility, as suggested in Figure 4b-d. The calcined SiO 2 yarn could be bent without cracking. After bending, it could recover to the original form. From the SEM images before and after ten-circle bending, only a few fibers were broken, while the whole still retained the  yarn structure. The flexibility was believed to result from the one-dimensional (1D) silica nanofiber with extremely small-sized crystal grains (41).

Conclusions
In summary, we have successfully fabricated PVA/silica nanofiber yarn through a conjugate electrospinning process and then calcined the PVA/silica yarn in air at 800°C for 6 h to obtain the SiO 2 nanofiber yarn. The diameter of the calcined SiO 2 fiber and yarn was reduced up to 17.3 and 35.1%, respectively. The EDS, FTIR as well as the weight-temperature examination suggested that the calcined yarn was mainly SiO 2 yarn. Although the calcined SiO 2 yarn showed a significant weakness in the tensile property, it had good flexibility due to the extremely small size of the crystal grains in the nanofiber. Once the mechanical property was improved, the flexible inorganic yarns could be applied in the functional textiles.