Development of superhydrophobic textiles via polyvinylidene fluoride phase separation in one-step process

Abstract

The purpose of this study was to develop a polyester fabric having superhydrophobicity and piezoelectric properties that could be formed in a one-step process. Using a method of non-solvent induced phase separation and immersion precipitation, polyvinylidene fluoride (PVDF) was applied to polyester fabrics, to reduce the surface energy and form hierarchical roughness with micro- and nano-scale structures, and the surface morphology, chemical composition, surface wettability, piezoelectric properties, and breathability of the produced fabric were investigated. When the polyester fabric was coated with only the PVDF solution, a hydrophobic surface with micro-scale roughness was obtained. However, when the fabric was coated with the PVDF solution and subsequently immersed in an n-octyl alcohol bath, nano-scale surface roughness was formed with spherulites of diameter 600 to 870 nm. The specimen coated with 40 mg/ml PVDF solution and subsequently immersed in an n-octyl alcohol bath exhibited a water contact angle of 159.9°, a water shedding angle of 6.3°, and improved water vapor transmission, but did not demonstrate piezoelectric properties. This study is meaningful in that a superhydrophobic and breathable textile, with reduced surface energy and nano-scale surface roughness, was formed in a one-step process by phase separation of PVDF.

Keywords

superhydrophobicity piezoelectricity breathability polyvinylidene fluoride immersion precipitation method non-solvent induced phase separation

A superhydrophobic surface, with a water contact angle greater than 150° and a water shedding angle smaller than 10°, can be achieved by reducing the surface energy and forming binary surface structures with micro- and nano-scale roughness. Nano-scale roughness has been developed using nanoparticles, such as silicon dioxide, zinc oxide, and titanium dioxide, on a fabric with intrinsic micro-scale roughness, or plasma etching.^1–3 However, when nanoparticles were used, the superhydrophobicity might deteriorate owing to the weak adhesion between fabrics and inorganic substances. Moreover, the toxicity due to the inflow into the human body and influx of the nano-sized inorganic materials into the ecosystem is yet to be considered.⁴ In addition, considerable time and costs are required to stabilize the nanomaterial and adhere it stably to the surface of the clothing material. Surface roughness achieved through plasma etching creates the problem of durability deterioration due to external forces. Furthermore, limitations, such as special machine requirements for large-scale production, exist. To reduce the surface energy, a fluorine-based polymer with a low surface energy has been widely applied to surfaces by dip coating, spin coating, spray coating, plasma enhanced chemical vapor deposition, etc.⁵ These approaches also have limitations in terms of special equipment and multi-step processes requiring large amounts of chemicals.

Polyvinylidene fluoride (PVDF) intrinsically has a low surface energy of 25 mN/m, owing to the C–F bond. Therefore, it was expected that the hydrophobicity would be further improved by the porous PVDF surface, and a non-solvent induced phase separation method was used to control the surface morphology.^6,7 This is a method for fabricating polymer membranes, which is described as a demixing process, wherein a homogeneous polymer solution is transferred from a liquid to a solid state. The polymer solution is immersed in a non-solvent bath (precipitation bath), where the exchange of the solvent and non-solvent takes place; immersion precipitation occurs when the composition of the polymer solution changes and reaches the binodal curve, which divides the region of component concentration.⁸ Kuo et al. expected that PVDF membranes with porous surface structures would enhance hydrophobicity.⁶ The water contact angle was approximately 84° for the dense surface of a single water-bath-precipitated PVDF membrane and increased to 148° for PVDF membranes obtained after immersion in a single n-propanol bath. Peng et al. also predicted that PVDF membranes with porous surface structures would have enhanced hydrophobicity, and attempted to achieve a hydrophobic surface through non-solvent induced phase separation.⁷ The water contact angle was 85° for the smooth surface of the PVDF membrane when water was used as the precipitation bath, and was 140° for the sponge-like porous structure of the PVDF membrane obtained by phase inversion using water/N,N-dimethylacetamide as the soft precipitation bath.

Polyvinylidene fluoride has five crystalline phases, α, β, γ, δ, and ɛ, according to chain conformation.^9,10 The α and β phases are the most common. The α phase has no piezoelectric properties because the packing of the dipole moment is anti-parallel within the unit cell. However, the β phase possesses all-trans conformations with parallel chain packing, yielding a polar structure.¹¹ Therefore, piezoelectricity can be induced by spontaneous polarization in the β phase.¹² Piezoelectricity is the result of an interaction between the mechanical and electrical states in crystalline materials, and the piezoelectric effect is the generation of electrical charge in response to an applied mechanical force.¹³ Therefore, it is possible to harvest energy from human motion by using the piezoelectric properties of PVDF from its crystalline structure; piezoelectric PVDF materials have the advantage of being flexible compared with piezoelectric ceramics.¹⁴ Using the piezoelectric properties of PVDF, Granstrom et al. devised a method harvesting energy from a backpack instrumented with piezoelectric PVDF shoulder straps.¹⁵ Shenck and Paradiso developed a method of energy scavenging from shoe-mounted piezoelectrics.¹⁶

When smart textiles that exhibit electrical characteristics have a self-cleaning ability owing to superhydrophobicity, they can have functional durability against various liquids in practical usage and, ultimately, a prolonged lifespan. This kind of fabric has potential for application to smart clothing material, which can generate electricity from human body motion and have durable function from self-cleaning properties. The objective of this study was to develop a polyester fabric having superhydrophobicity and piezoelectric properties using a non-solvent induced phase separation method in the immersion precipitation of PVDF, since PVDF nanostructures formed on the surface are also expected to exhibit self-cleaning and piezoelectric properties. To determine the optimum condition by varying the PVDF concentration and n-octyl alcohol treatment, the surface morphology and chemical composition were investigated. Surface wettability was measured based on the static contact angle and shedding angle using distilled water. Piezoelectric properties were verified by measuring the voltage output under repeated bending.

Experimental details

Materials

Polyester fabric (yarn count: 75d/72f × 150d/144f in inch × inch, density: 144 × 80 in inch, weave type: plain weave, weight: 108 g/m²; Youngpoong Filltex, Republic of Korea) was scoured before use. All reagents, such as PVDF pellets (M_w ∼ 275,000, M_n ∼ 107,000; Sigma Aldrich Co. LLC, USA), 99.5% N,N-dimethylacetamide (Daejung chemicals & metals, Republic of Korea), and 99.9% n-octyl alcohol (Daejung chemicals & metals, Republic of Korea) were commercially available and used without further purification.

Treatment

Preparation of PVDF solution

The PVDF was dissolved at 70℃ using N,N-dimethylacetamide as the solvent,^7,17 and the concentration was adjusted in steps of 10 mg/ml to give a range of concentrations between 10 and 70 mg/ml. To obtain a completely dissolved and homogenous solution, the dissolution time was set differently; 1 h for 10 and 20 mg/ml, 2 h for 30, 40, and 50 mg/ml, and 2.5 h for 60 and 70 mg/ml. Prior to its application to the fabric, the temperature of the PVDF solution was reduced to 25℃.

Surface hydrophobization

Prior to the hydrophobization treatment, polyester fabric was scoured in a solution consisting of 5 g/l of 60% sodium dodecylbenzenesulfonate and 5 g/l of 99% sodium carbonate anhydrous at 90℃ with a liquor ratio of 1:30. After scouring for 1 h, fabric was sufficiently rinsed with distilled water and dried at room temperature. To investigate the effect of each step on the superhydrophobicity, the samples were prepared through immersion in the PVDF solution or by immersing in the PVDF solution and non-solvent bath sequentially. When treated with only the PVDF solution, the polyester fabric was dip-coated in the PVDF solution for 2 s, and then air-dried at room temperature. When treated with both the PVDF solution and non-solvent, the sample was immersed in n-octyl alcohol for 2 s after dip coating with PVDF solution, and then air-dried at room temperature. Sample codes according to the treatment conditions are presented in Table 1.

Table 1.

Specimen code and description of the experimental process

Code	Description
UT	Untreated
SP10–70	Specimen coated with only PVDF solution of 10–70 mg/ml concentration
SP10–70NS	Specimen coated with PVDF solution of 10–70 mg/ml concentration and continuously treated with n-octyl alcohol

Characterization

Surface morphology

A field emission scanning electron microscope (SUPRA 55VP, Carl Zeiss, Germany) was used to observe the surface morphology. Image J (National Institute of Health, USA), an image analysis software, was used to determine the surface roughness and crystallite size of the PVDF on the surface of the sample. Five positions were selected from one image and the average value was used.

Chemical composition

The chemical composition was analyzed using energy dispersive X-ray spectroscopy (Aztec, Oxford Instruments, UK). Chemical components detected on the surface of the sample were represented using different colors, and the content was expressed as wt.%.

Wettability

Using an optical tensiometer (Theta Lite optical tensiometer, KSV Instruments, Sweden), the water contact angle and water shedding angle were examined. The water contact angle was measured 3 s after 3.5 µl of distilled water was dropped from a height of 1 cm above the sample in the vertical direction. The water shedding angle was determined by the angle at which water droplets moved over 2 cm when 12.5 µl of water was dropped from a height of 1 cm above the sample.¹⁸ For samples that the droplets did not roll off at 85°, the angle was taken to be 90°. The mean values of five measurements at different locations were used.

Piezoelectric properties

The output voltage of the sample was measured to confirm the piezoelectric performance. As shown in Figure 1, indium tin oxide coated polyester film and aluminum foil were used as electrodes, and copper wires were connected to each electrode. The sample was sealed with polyimide tape and then pressed. The wires attached to the electrodes were connected to a nanovoltmeter (2182A, Keithley, USA) and the output voltage was measured for 60 s with manual bending once per second.

Figure 1.

Sample preparation for measurement of output voltage.

Breathability

Air permeability and water vapor transmission rate were measured to evaluate clothing compatibility. Air permeability was determined according to ASTM D 737.¹⁹ Using the Frazier air permeability tester (TEXTEST, FX3300, Switzerland), the amount of air permeated was measured when a constant pressure (125 Pa) was applied to the tested area (38.3 cm²). After five measurements, the mean value was used.

The water vapor transmission rate was determined according to ASTM E96-80.²⁰ Samples were fixed at 3 mm above a cup containing calcium chloride and stabilized at 40 ± 2℃ and 90 ± 5% relative humidity in a chamber for 1 h, and the initial weight (a₁) was measured; thereafter, the samples were placed again in the chamber for 1 h and then weighed (a₂). The amount of water vapor transmission per unit area for 24 h was calculated as

P = \frac{a_{2} - a_{1}}{S} \times 24

(1)

wherein P is the water vapor transmission rate (g/m²·24 h), (a₂ − a₁) is the weight change after 1 h (g), and S is the area (m²).

Results and discussion

Surface morphology

The surface morphology of the samples coated with only the PVDF solution and treated with both the PVDF solution and n-octyl alcohol was analyzed using field emission scanning electron microscopy. The results are shown in Figures 2 and 3. The size of the spherulite structures formed on the surface of the sample was determined using the Image J program, and the results are shown in Figure 4.

Figure 2.

Scanning electron micrographs depicting the change in surface morphology of polyester fabrics coated with a PVDF solution under various solution concentrations.

Figure 3.

Scanning electron micrographs depicting the change in surface morphology of polyester fabrics coated with a PVDF solution under various solution concentrations and treated with n-octyl alcohol.

Figure 4.

Average diameter of PVDF spherulites on polyester fabrics when coated with only PVDF solution (PVDF) and when treated with both PVDF solution and n-octyl alcohol (PVDF + NS).

When the sample was coated only with the PVDF solution, the individual fibers could be clearly observed at concentrations of 10 and 20 mg/ml in the low-magnification images, and it was confirmed that the PVDF solution did not sufficiently adhere to the surface of the sample. For concentrations ≥30 mg/ml, the PVDF solution began to adhere to the surface of the sample uniformly. As the concentration increased, the solution densely filled up to the spaces between the fibers. At a high magnification, a smooth surface morphology similar to the untreated sample was observed at a concentration of 10 mg/ml. Bumps were observed on the fiber surface at 20 mg/ml, though not in the form of a complete spherulite. At concentrations over 40 mg/ml, spherulites of diameter 2.39–6.90 µm were formed densely on the fiber surface. This may be because the polymer chains tend to be extended and entangle with neighboring chains in contact with a good solvent, forming micro-scale spherulites.^21,22 Meanwhile, in the presence of non-solvent PVDF, molecule entanglement is reduced because of phase separation.

When the sample was coated with the PVDF solution and sequentially treated with n-octyl alcohol, a structure similar to the untreated sample at concentrations of 10 and 20 mg/ml was observed in the low-magnification images. The PVDF began to adhere uniformly to the surface of the sample at concentrations ≥30 mg/ml. At 30–50 mg/ml, it was observed that the spherulite structures covering the surface of the sample were significantly smaller than those covering the sample coated with only the PVDF solution. The fabric was coated with the polymer solution, and then immersed in a precipitation bath containing the non-solvent, whereby the solvent and the non-solvent are exchanged. At this moment, the nanostructure is formed as the phase separation occurs after overpassing the limit of solubility.^22,23 This is because the nucleation and growth of PVDF starts and subsequently the precipitation of PVDF fixes on the fabric surface.²⁴ When observing concentrations of 10 and 20 mg/ml at high magnification, spherulites having a diameter of approximately 200 nm were formed on the surface of the sample. However, the spherulites were aggregated and a smooth surface was observed between them. At concentrations greater than 30 mg/ml, the sample was completely covered with PVDF. At concentrations of 30 and 40 mg/ml, a net-like surface of interconnected PVDF spherulites with diameter 600–700 nm was observed. As the concentration increased further, the diameter of the spherulites increased, and the connections between them decreased. At concentrations of 60 and 70 mg/ml, the diameter of the PVDF spheres was approximately 2.2 µm, and the surface roughness, which was previously nano-scale, changed to a micro-scale roughness. In the non-solvent induced phase separation, polymer concentration is a significant parameter affecting the diameters of the spherulites.²³ Increased polymer concentration in the coating solution results in increased polymer concentration at the non-solvent interface.²⁵ This resulted in increases in the volume fraction of polymer and consequent increases in the diameters of the spherulites.

The average diameter of PVDF spherulites on polyester fabric might also have affected the add-on after each treatment, as the add-on for SP40 was 7.03% while that for SP40NS was 2.57%. When the polyester fabric was coated with only the PVDF solution (SP40), the surface was covered with spherulites having an average diameter of 6904 nm. However, when the polyester fabric was coated with the PVDF solution and subsequently immersed in an n-octyl alcohol bath (SP40NS), the surface was covered with spherulites having an average diameter of 691 nm.

Chemical composition

The chemical composition of the SP40NS sample was examined using energy dispersive X-ray spectroscopy. The results are shown in Figure 5. After treatment with the PVDF solution and n-octyl alcohol, fluorine peaks were observed, in addition to carbon and oxygen peaks in the polyester. The weight ratios of fluorine, carbon, and oxygen were 26.7 wt.%, 55.8 wt.%, and 4.1 wt.%, respectively. On the right side of Figure 5, the yellow color indicating the fluorine component was found on the entire surface, and it was confirmed that the PVDF used to reduce the surface energy was evenly coated on the whole surface.

Figure 5.

Energy dispersive X-ray spectroscopy mapping images showing chemical composition and element distribution of a polyester fabric coated with 40 mg/ml PVDF solution and treated with n-octyl alcohol.

Wettability

Figure 6 shows the water contact angle and water shedding angle according to PVDF concentration. The untreated polyester fabric exhibited a water contact angle of 75.8° and the water droplet on the sample was completely absorbed within 35 s.

Figure 6.

Water contact angle and water shedding angle of polyester fabrics coated with only PVDF solution (PVDF) and treated with both PVDF solution and n-octyl alcohol (PVDF + NS).

When the sample was coated with only the PVDF solution at concentrations of 10 and 20 mg/ml, the water contact angle the water contact angles were 85.8° and 96.5°. At 30 mg/ml, the sample showed a water contact angle of 130.1° without the droplet being absorbed afterwards. This finding is attributed to the fact that PVDF is uniformly coated on the sample surface at concentrations ≥30 mg/ml. The water contact angle was 141.0° at a concentration of 40 mg/ml, and the water contact angle did not increase with increasing concentration. When the sample was coated with only the PVDF solution, the water shedding angle was greater than 10° under all conditions and did not meet the criteria of superhydrophobicity.

When the sample was coated with PVDF solution at a concentration of 10 or 20 mg/ml and sequentially treated with n-octyl alcohol, it showed a hydrophobic property with water contact angles of 113.5° and 125.3°, respectively. This property was attributed to the surface roughness caused by spherulites with a diameter of approximately 200 nm. At a concentration of 30 mg/ml, the sample exhibited a water contact angle of 152.8° and a water shedding angle of 9.0°, showing superhydrophobic properties. For concentrations ≥30 mg/ml, the surface began to be evenly covered with the PVDF solution, and the diameter of the spherulites connected to the net-like structure was approximately 600 nm. It is believed that successful hierarchical roughness was formed on the surface with the inherent micro-scale roughness of fibers and yarns in polyester fabrics, resulting in superhydrophobicity. The samples prepared at a concentration of 40 mg/ml exhibited a water contact angle of 159.9° and water shedding angle of 6.3°, and had the most superhydrophobic surface. At concentrations of 60 and 70 mg/ml, however, the samples having a spherulite diameter of approximately 2.2 µm did not show the superhydrophobicity.

Piezoelectric properties

It has been reported that the PVDF solution precipitated into spherical crystallites with the β-phase crystal structure at lower temperatures (25℃), whereas the membrane became predominant in the α phase crystal structure at higher temperatures (60 or 65℃).^26,27 Therefore, in this study, the piezoelectric properties were examined for the β-phase crystal structure by reducing the temperature of the PVDF solution, which was dissolved for 2 h at 70℃ using N,N-dimethylacetamide as a solvent, to 25 ° C before applying it to the fabric. However, as shown in Figure 7, piezoelectric properties did not appear after coating the PVDF solution on the polyester fabric. It was considered that the piezoelectric properties were not demonstrated because the experimental conditions, such as time, temperature, and solvent systems, were inappropriate for the isothermal crystallization of PVDF, thereby yielding no β-phase PVDF.^28,29 It is necessary to impart piezoelectric properties with superhydrophobicity to a fabric by optimizing the experimental conditions. This will be investigated through further research.

Figure 7.

Voltage output of the studied fabrics by bending movements.

Breathability

Air permeability and water vapor transmission rates were measured to confirm that the SP40NS sample selected under optimum conditions was suitable as a clothing material. The untreated sample and sample coated with only PVDF solution (SP40) were set as controls, and the results are shown in Figure 8.

Figure 8.

Breathability of the studied fabrics at optimal condition.

Air permeabilities of the untreated sample, SP40, and SP40NS were 17.1, 13.9, and 14.0 cfm, respectively. Samples SP40 and SP40NS were considered to have lower air permeabilities than the untreated sample because their surfaces were covered with PVDF and some pores were clogged by the coating material. However, water vapor transmission rates of the untreated sample, SP40, and SP40NS were 7992, 8342, and 8863 g/m²·24 h, respectively. The treatment for surface hydrophobization caused the pores of the fabric to be clogged with the coating material, thereby reducing the air permeability. However, owing to the superhydrophobicity, the interaction between the fabric surface and water molecules was weakened and the water vapor transmission through the pores was accelerated, resulting in the greatest water vapor transmission rate for SP40NS.³⁰ In addition, the decreased add-on by the treatment of non-solvent because of the phase separation might be another reason why SP40NS had somewhat higher water vapor transmission rate and air permeability than SP40. For the comfort of the wearer, the clothing material is required to have a water vapor transmission rate of 8,000–10,000 g/m²·24 h.³¹ From this viewpoint, SP40NS offers sufficient comfort as a clothing material.

Conclusions

Polyvinylidene fluoride is a fluorine-based polymer with low surface energy of 25 mN/m and demonstrates piezoelectric properties owing to the all-trans conformation of the crystalline β phase. In this study, polyester textiles having superhydrophobicity and sufficient breathability were developed by reducing the surface energy and forming the surface roughness through PVDF coating and continuous immersion in an n-octyl alcohol bath.

When the polyester fabric was coated with only the PVDF solution, micro-scale surface roughness was formed and a hydrophobic surface was implemented. However, when the polyester fabric was coated with the PVDF solution and subsequently immersed in an n-octyl alcohol bath, nano-scale surface roughness was formed in the process of PVDF crystallization by desolvation. At concentrations ranging from 30 to 50 mg/ml, a superhydrophobic surface with a water contact angle greater than 150° and a water shedding angle less than 10° was achieved. Under these conditions, spherulites with an average diameter of 600–870 nm were formed on the surfaces of the fabrics. The specimen coated with 40 mg/ml PVDF solution and subsequently immersed in an n-octyl alcohol bath showed the most outstanding superhydrophobicity exhibiting a water contact angle of 159.9° and a water shedding angle of 6.3°.

Using PVDF that can achieve low surface energy and piezoelectricity, this study makes a significant contribution toward the development of superhydrophobic and piezoelectric textiles via a simple one-step process by reducing the surface energy and simultaneously forming nano-scale surface roughness. Therefore, the newly developed textile has a potential of being utilized for flexible and breathable smart wear, such as apparel-based energy harvesting products. Further studies are required to evaluate and improve the durability of superhydrophobicity and to enhance the piezoelectricity.

Footnotes

Declaration of conflicting interests

The authors declared no potential conflicts of interest with respect to the research, authorship and/or publication of this article.

Funding

The authors disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: This work was supported by the National Research Foundation of Korea and the Ministry of Science and ICT of the Korean government (Grant Numbers NRF-2016M3A7B4910940 and 2018R1A2B6003526).

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