Fabrication of superhydrophobic nanofiber fabric with hierarchical nanofiber structure

2.1 Materials

PVDF (Mw=300,000), polyethylene glycol (PEG, Mw=20,000), N-N dimethyl formamide (DMF), tetrahydrofuran (THF), and SiO₂ nanoparticles (30–50 nm) were all supplied by the Shanghai Chemical Reagent Co. Ltd (China).

2.2 Preparation of spinning solution

A 15 wt.% solution of PVDF was prepared by dissolving powdered PVDF in a DMF/THF (W_D/W_A:5/5) solution and heating the mixture at 80°C for 8 h. A 16.5 wt.% solution of PVDF/PEG was prepared by dissolving the PVDF powder and PEG slices in a DMF/THF (W_D/W_A:5/5) solution and heating the mixture at 80°C for 12 h. An 18 wt.% solution of PVDF/PEG/SiO₂ was prepared by dissolving the PVDF powder, PEG slices, and SiO₂ nanoparticles in DMF/THF (W_D/W_A:5/5) solution and heating the mixture at 80°C for 24 h.

2.3 Fabrication of the electrospun nanofiber yarns

A conjugate electrospinning set-up (Figure 1) was used to spin continuous nanofiber yarns, which consisted of a fluid-supply apparatus, liquid-transport tubes, nozzles, a metal funnel collector and a yarn winder. Two sets of liquid-transport tubes were equipped with two nozzles, which were individually connected to the positive and negative terminals of a direct current (DC) power supply, and arranged symmetrically on both sides beneath the funnel collector. The funnel collector was not earthed. The solution was transported to the nozzles through the liquid-transport tubes at a uniform rate via the fluid-supply apparatus. The nanofibers were electrospun from the oppositely charged nozzles and deposited in the rotary funnel to form a nanofiber web that covered the end of the funnel. By drawing out the insulating rod, the nanofiber web was pulled into the fiber bundles. The nanofiber yarn was then obtained by twisting the fiber bundles through the rotary funnel and continuously winding the bundles using the yarn winder.

Figure 1:

Schematic diagram of the conjugate electrospinning set-up used to prepare the nanofiber yarns.

The nanofiber yarns were continuously prepared at an applied voltage of 18 kV, with a spacing of 17.5 cm between the positive and negative nozzles and a nozzle-to-funnel distance of 4 cm (21).

2.4 Weaving

Briefly, parallel warp and weft filaments of the nanofiber yarns were interwoven vertically to form a fabric layer (Figure 2). Warp and weft densities were 30 root/cm and 50 root/cm, respectively, with a thickness of 2.0±0.1 mm.

Figure 2:

Schematic diagram of nanofiber fabric.

2.5 Aftertreatment of nanofiber fabric

The obtained PVDF/PEG nanofiber fabric and PVDF/PEG/SiO₂ nanofiber fabric were soaked in distilled water for 72 h to dissolve the PEG in the nanofiber. Then, they were taken out and washed for removing the residual PEG, and at last, dried to a constant weight at room temperature.

2.6 Characterization

Field emission scanning electron microscopy (FESEM, Hitachi, S-4800, Tokyo, Japan) was used to examine the morphologies of the PVDF, PVDF/PEG, PVDF/PEG/SiO₂ nanofiber fabrics. Prior to SEM examination, the specimens were sputter-coated with gold to prevent charge accumulation. Fourier transform infrared (FTIR) spectra (100 scans) were collected at intervals of 2 cm⁻¹ on a Nicolet Nexus 670 FTIR spectrometer (Thermo Nicolet Corporation, USA) using discs consisting 1 mg of powdered sample and 300 mg of KBr (Aladdin, Shanghai, China). Samples were also analyzed by thermogravimetric analysis (TGA) using a Perkin Elmer TGA (MA, USA). Samples were heated to 1000°C at 10.0°C/min. The tensile mechanical properties were measured using an Instron 5582 (USA) tester at room temperature under room humidity. The gauge length was set to 15 cm and the crosshead speed was 150 mm/min. The reported data of the ultimate tensile strength and failure strain represent the average of 20 tests. The contact angle of the specimens was measured using a Dataphysics OCA20 (Germany) tester at room temperature and humidity. The reported contact angle data represent the average of 10 tests. Redistilled water was used in the test.

3.1 Morphology

Lotus leaf with a better self-cleaning function account for the existence of a hydrophobic wax and the complex nano- and micro-scale structure on the surface of lotus leaf. In this study, nanofibers were fabricated by mixing PEG and SiO₂ nanoparticles in a PVDF spinning solution to obtain a morphology similar to a lotus leaf surface structure. The nanofiber surfaces in the pure PVDF nanofiber fabric are smooth, as observed from the SEM images (Figure 3A–D). However, the PVDF/PEG composite nanofibers after water scrubbing show an uneven topography because of the hydrolysis of PEG in the fiber, which is similar to the coarse structure of a lotus leaf surface, and the diameters of the nanofiber are between 300 and 400 nm (Figure 3E–H). To mimic the lotus leaf surface structure further, SiO₂ nanoparticles were doped in the PVDF/PEG composite nanofiber to obtain the hierarchical nanofiber structure of the lotus leaf surface. The fiber structure of the PVDF/PEG composite nanofiber doped with SiO₂ nanoparticles after water scrubbing consists of many balls, which are formed by the inclusion of the SiO₂ nanoparticles in the polymer. Moreover, the “ball” is not only similar to the “hill” of the lotus leaf surface, but also similar to the “small thorn” on the “hill” because of the embedding of the SiO₂ nanoparticles on the “ball” surface (Figure 3I–L). The diameters of the PVDF/PEG composite nanofiber-doped SiO₂ nanoparticles are also between 300 and 400 nm, and the diameters of the ball are between 50 and 70 nm.

Figure 3:

SEM images showing the (A, E, I) a single nanofiber, (B, F, J) orientational nanofibers in a yarn, (C, G, K) nanofiber yarn, and (D, H, L) nanofiber fabric of pure PVDF, PVDF/PEG and PVDF/PEG/SiO₂, respectively.

3.2 Microstructural analysis

The infrared spectra of the pure PVDF nanofiber fabric, PVDF/PEG nanofiber fabric and the PVDF/PEG/SiO₂ nanofiber fabric are shown in Figure 4A. All the three kinds of nanofiber fabrics show the characteristic bands of the C-F vibration of PVDF at 1403.9, 1190.5, and 882.3 cm⁻¹, and the strongest band is at 882.3 cm⁻¹. Moreover, the β-absorption peak of PVDF appears at 841.9 cm⁻¹, indicating that a conformational change occurred in PVDF because of electrospinning. However, the PVDF/PEG/SiO₂ nanofiber fabric also shows the stretching vibration peak of Si-O-Si at 1062.1 cm⁻¹, and the stretching and bending vibration peak of Si-O at 803.3 cm⁻¹, apart from the abovementioned four characteristic peaks of the C-F bond. The presence of the SiO₂ nanoparticle is further confirmed by the results of TGA analysis (Figure 4B). The SiO₂ content was obtained by subtracting the two curves shown in Figure 4. The decomposition temperature of PVDF increases from 460°C to 490°C upon doping with SiO₂ nanoparticles, indicating that there may be an improvement in the PVDF crystallinity on adding the SiO₂ nanoparticles. SiO₂ nanoparticles are dispersed evenly in the PVDF nanofibers, as evidenced from the energy dispersive X-ray spectroscopy (EDX) mappings of the PVDF/PEG/SiO₂ nanofiber (Figure 4C).

Figure 4:

(A) The infrared spectra and (B) thermogravimetry of pure PVDF nanofiber fabric, PVDF/PEG nanofiber fabric and PVDF/PEG/SiO₂ nanofiber fabric; (C) distribution of elemental Si in the PVDF/PEG nanofiber.

3.3 Mechanical properties

The stress-strain diagram of the pure PVDF nanofiber fabric, PVDF/PEG nanofiber fabric, and the PVDF/PEG/SiO₂ nanofiber fabric are shown in Figure 5. The ultimate tensile strength of the three fabrics increases, whereas the failure strain decreases with the addition of PEG and SiO₂ nanoparticles. The ultimate tensile strength and failure strain of the PVDF/PEG nanofiber fabric are 86.04 MPa and 18.52%, respectively, while ultimate tensile strength is higher than the corresponding values of 84.24 Mpa for the pure PVDF nanofiber fabric and failure strain is less than the corresponding values of 18.82%. The ultimate tensile strength of the PVDF/PEG/SiO₂ nanofiber fabric increases to 92.12 MPa, and the failure strain decreased to 17.53% after doping with SiO₂ nanoparticles. This was because there is an improvement in the PVDF crystallinity upon adding SiO₂ nanoparticles during the electrospinning process, which changes the mechanical properties of these fabrics.

Figure 5:

Stress-strain diagram of the pure PVDF nanofiber fabric, PVDF/PEG nanofiber fabric and PVDF/PEG/SiO₂ nanofiber fabric.

3.4 Infiltration mechanism and performance

It is difficult to wet the fabric surface, and it shows a higher contact angle when the volume of air between the liquid surface and the fabric surface is large because of the surface tension of the liquid and gas. The increased roughness of the fabric surface will decrease the wettability, and the contact angle between the fabric and the liquid surface increases according to the Wenzel equation (22). Cassie and Baxter also proposed a model for describing the surface wettability (23) based on the Wenzel equation where the hydrophobicity was related to the surface area between the water and the material. The liquid contact angle for a surface can be expressed as eq. [1]

where θ' and θ are the liquid contact angles on a rough surface and flat surface, respectively, f₁ and f₂ are the liquid and air/solid interface, respectively, and 1=f₁+f₂. Hence, increasing the ratio of the contact area between the liquid and air will decrease the wettability of the surface.

The contact angles with water and dye liquid droplets on pure PVDF nanofiber fabric, PVDF/PEG nanofiber fabric, and PVDF/PEG/SiO₂ nanofiber fabric are shown in Figure 6. The water and dye liquid droplets are spherical and have a large contact angle on the three sample surfaces. The contact angle of the PVDF/PEG/SiO₂ nanofiber fabric was the maximum at 173.2°, while that of the PVDF/PEG nanofiber fabric and pure PVDF nanofiber fabric was 155.71°and 134.49°, respectively. The main reason may be the difference in the material composition and the roughness on the fiber surface. The pure PVDF nanofiber fabric is highly water repellent owing to the low surface energy of PVDF. The contact angle of the PVDF/PEG nanofiber fabric increases because of the increase in the contact area between water and air due to the increased nanofiber surface roughness. Moreover, several embossments are formed on the surface of the PVDF/PEG/SiO₂ nanofiber fabric because of the SiO₂ nanoparticle doping, which forms a rough nanoscale structure on the nanofiber surface and further increases the hydrophobicity of the fabric because the SiO₂ nanoparticles that are coated on the fiber surface are hydrophobic. The rough surface results in the trapping of a large volume of air between the water and fabric surfaces, which acts likes a cushion to prop up the water under the water interface. As a result, the fabric exhibits non-wettability and a higher contact angle. In addition, the stability of hydrophobicity for these three nanofiber fabrics were also measured. The water droplets on these three nanofiber fabric surfaces all disappeared after 30 days, which was because the gaps among yarns in fabric led to the infiltration of the droplets.

Figure 6:

Water and dye contact angles on pure PVDF nanofiber fabric, PVDF/PEG nanofiber fabric and PVDF/PEG/SiO₂ nanofiber fabric.