Hydrophobic polyvinylidene fluoride fibrous membranes with simultaneously water/windproof and breathable performance

Abstract

Water/windproof and breathable membranes are important in many applications due to their resistance to water and wind penetration as well as permissibility of vapor transmitting; nevertheless, fabrication of such membranes remains a serious challenge. Herein, we have prepared a novel kind of hydrophobic fibrous membrane with good water/windproof and breathable performance via electrospinning by using polyvinylidene fluoride (PVDF) as a raw polymer. The fibrous and porous structure of the membranes have been finely regulated by tuning the DMAc/acetone ratios and NaCl concentrations in the PVDF solutions. The resultant fibrous membranes obtained from the solutions with a DMAc/acetone ratio of 7/3 and NaCl concentration of 0.003 wt% possessed an optimized porous structure with maximum pore size of 1.2 μm, mean pore size of 0.8 μm and porosity of 76.2%. The synergetic effect of hydrophobicity and porous structure endowed the membranes with simultaneously good waterproof (hydrostatic pressure of 110 kPa), windproof (air permeability of 6.1 mm s⁻¹) and breathable (water vapor transmission rate of 11.5 kg m⁻² d⁻¹) performance. Meanwhile, the PVDF resultant membranes also exhibited robust mechanical performance with high strength (breakage stress of 11.1 MPa) and excellent toughness (energy to break of 6.4 mJ m⁻³). Therefore, the as-prepared fibrous membranes with good water/windproof and breathable performance, as well as robust mechanical property, would become a key material in many areas, especially in fabricating protective clothing.

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Introduction

Porous membranes with resistance to water and wind penetration, as well as acceptance of steam permeation have gained great attention in both daily life and industrial manufacturing, such as separators in sea water desalination,^1,2 refiners in drug purification,³ protective coatings of buildings,⁴ and especially the crucial functional layer in protective clothing.^5–7 According to the present studies, the water/windproof and breathable properties of membranes would be generated from the integration of surface hydrophobicity and an interconnected porous structure.^8,9 Hence, many methods have been developed to prepare this kind of membranes, including biaxial stretching,¹⁰ phase separation,^11,12 template-based strategy.^13,14 However, these methods are suffered from either limitation of raw polymers or difficulty in regulating the porous structure, which made them unable to obtain membranes with inadequate water/windproof and breathable performance.

Nowadays, electrospinning have been regarded as a sufficient scale-up procedure for preparing membranes with densely packed fibers and interconnected pores.^15–17 Consequently, many researchers have focused on the fabrication of water/windproof and breathable membranes via electrospinning using various kinds of polymer, such as polyurethane (PU),¹⁸ polyacrylonitrile,¹⁹ and polypropylene.²⁰ However, subjecting to the poor hydrophobicity of these raw polymers, the obtained electrospun fibrous membranes exhibit inferior waterproof (hydrostatic pressure ≤ 10 kPa), poor windproof (air permeability ≥ 20 mm s⁻¹), and limited breathable (water vapor transmission rate (WVT rate) ≤ 5 kg m⁻² d⁻¹) performance.^18,21 In our previous study, we used a kind of self-synthesized fluorinated polyurethane as a hydrophobic modification component and prepared PU fibrous membranes with improved hydrostatic pressure (39.3 kPa), air permeability (8.5 mm s⁻¹) and WVT rate (9.2 kg m⁻² d⁻¹).²² Subsequently, we have introduced carbon nanotubes (CNTs) to alter the porous structure of the PU fibrous membranes and obtained membranes with enhanced hydrostatic pressure of 108 kPa.²³ However, the CNTs are hard to be dispersed in polymer solution and easy to aggregate during electrospinning, thus the performance of the membranes would be easily degraded. Therefore, it remains a challenge to develop a sufficient and facile strategy to fabricate membranes with outstanding water/windproof and breathable performance.

According to the previous studies, raw polymers with good surface hydrophobicity is demanded to fabricate membranes with outstanding water/windproof and breathable performance.⁵ Fortunately, polyvinylidene fluoride (PVDF) is a kind of polymer with inherently good hydrophobicity,^24,25 outstanding mechanical properties,^26,27 as well as good processibility,^28,29 which made it an appropriate candidate for fabricating water/windproof and breathable membranes. Up to now, scarcely effort have focused on the fabrication and application of water/windproof and breathable membranes using PVDF as raw polymer.

In this research, we demonstrate a fabrication process of water/windproof and breathable fibrous membranes via electrospinning using PVDF as raw polymer. The fibrous and porous structure inner the membranes have been finely regulated by tuning the DMAc/acetone ratios and the NaCl concentrations of the PVDF solutions. Benefiting from the hydrophobicity of PVDF and the interconnected pores with optimized structure, the fibrous membranes possesses good water/windproof and breathable performance. Meanwhile, the as-prepared fibrous membranes with sticking structure and orientated molecular along the fiber exhibits robust mechanical performance. These properties ensure the membranes as a kind of good candidate in many applications, especially in fabricating of proactive clothing.

Experimental

Materials

Polyvinylidene fluoride powder (PVDF, Kynar 720, Arkema), N,N-dimethyl acetamide (DMAc, >99.5%, Sinopharm), acetone (>99.5%, Sinopharm), sodium chloride (NaCl, >99.5%, Sinopharm) were used as received. The PVDF solutions were obtained by dissolving PVDF powder in component solvents with various DMAc/acetone ratio (w/w) of 1/9, 3/7, 5/5, 7/3, and 9/1, respectively. The concentration of PVDF in solutions were kept 20 wt%. The PVDF solutions with different conductivity were obtained by dissolving NaCl in the as-prepared PVDF solutions. The detailed compositions and properties of the PVDF solutions with various DMAc/acetone ratio and NaCl concentrations were presented in Table S1.†

Fabrication of PVDF fibrous membranes and casting films

The electrospinning process (as illustrated in Fig. S1†) of the PVDF fibrous membranes were proceeded with polymer solutions feed rate of 5 mL h⁻¹ and high voltage of 30 kV. The relative humidity and ambient temperature during electrospinning was kept 70 ± 5% and 25 ± 1 °C, respectively. After electrospinning process, the as-prepared membranes were dried in vacuum oven for 1 h at 80 °C. Thickness of all the fibrous membranes were kept within 40 ± 2 μm. The PVDF casting films were prepared by pouring the polymer solutions on a glass plate then drying at ambient temperature for 24 h.

Characterization of PVDF fibrous membranes

The morphology of the membranes was observed via SEM (Vega 3, Tescan). The advancing water contact angle was measured on a contact angle goniometer (SL200B, Kino). Pore size was characterized by a capillary flow porometer (CFP-1100AI, PMI). Porosity was calculated using the following equation:where ρ₀ and ρ₁ are density of raw polymer and PVDF fibrous membranes, respectively. The waterproof performance was evaluated on a hydrostatic pressure test equipment (YG812C, NBFY) with pressurization rate of 6 kPa min⁻¹. The windproof property was appraised using an air permeability tester (YG461, NBFY) under pressure drop of 100 Pa. The breathable performance was measured on a water vapor transmission test equipment (YG 601, NBFY) with relative humidity of 50% and temperature of 38 °C. The mechanical performance were tested on a tensile tester (XQ-1C, Xinxian).

Results and discussion

PVDF fibrous membranes obtained from solutions with various DMAc/acetone ratio

In order to obtain membranes with outstanding water/windproof, breathable and mechanical performance, hydrophobic PVDF fibrous membranes had been prepared via electrospinning. By regulating the DMAc/acetone ratios of the polymer solutions, the fibrous and porous structure of the PVDF fibrous membranes had been facilely regulated. The representative SEM images of the PVDF fibrous membranes exhibited nonwoven structure formed by random accumulating of fibers. As shown in Fig. 1a, the PVDF fibrous membranes obtained from solution with DMAc/acetone ratio of 1/9 shows fibers with large average diameter of 627 nm (Fig. 1f), which would be related to the rapid evaporation of acetone (low boiling point) that led to solidification of the solution jet before effective stretching.³⁰ However, with the increasing of DMAc (high boiling point) ratio, the solidification rate would be slowed down and the jet could be stretched more efficiently. Therefore, the fibrous membranes obtained from DMAc/acetone ratio of 3/7, 5/5, and 7/3 possess thinner fibers with average diameter of 474, 378, and 297 nm, respectively. However, the polymer solution jet with DMAc/acetone ratio of 9/1 were over-stretched to fracture (as shown in Fig. 1e), thus resulted in break fibers with average diameter of 203 nm. In addition, in the image of fibrous membranes obtained from solutions with low DMAc/acetone ratio as 1/9 (Fig. 1a), sticking structure among the adjacent fibers would be observed obviously. But with the increasing of DMAc/acetone ratio, the sticking structure disappeared gradually (as shown in Fig. 1b–e), which would be ascribed to the different solubility of PVDF in DMAc and acetone.³¹ These results indicated that the fibrous structure of the PVDF fibrous membranes could be facilely regulated by tuning the DMAc/acetone ratio.

Fig. 1 SEM images of PVDF fibrous membranes obtained from solutions with various DMAc/acetone ratios: (a) 1/9, (b) 3/7, (c) 5/5, (d) 7/3, and (e) 9/1. (f) Average fiber diameter of PVDF fibrous membranes obtained from solutions with various DMAc/acetone ratios.

Resulting from the differential of DMAc/acetone ratios, the surface wettability of the membranes would be altered as well, which had been investigated by measuring the advancing water contact angle of the PVDF casting films and PVDF fibrous membranes. Fig. 2 presents the advancing water contact angle of PVDF casting films and PVDF fibrous membranes obtained from solutions with various DMAc/acetone ratios. It could be seen that the advancing water contact angle of the casting films decreased slightly from 122° to 114° as DMAc/acetone ratio increased. This would be interpreted that the solutions with higher acetone ratio would solidified rapidly and resulted in rough surface, but the solutions with higher DMAc ratio would solidified slowly and is more likely to flow to flat. This opinion also had been confirmed by SEM images as show in Fig. S2.†³² However, the PVDF fibrous membranes show higher advancing water contact angle over 140°, which would be related to the incremental macro-sized surface roughness that generated from accumulation of fibers. Moreover, with the reduction of sticking structure (as shown in Fig. 1a–e), the advancing water contact angle raised up from 142° to 150° as DMAc/acetone ratio increased. According to the previous studies, the increasing of hydrophobic property would be favourable for improving the resistance to water penetrating.

Fig. 2 Advancing water contact angle of PVDF casting films and PVDF fibrous membranes obtained from solutions with various DMAc/acetone ratio.

In the meantime, the transformation of fibrous structure also had effect on the interconnected porous structure inner the fibrous membranes, which had been evaluated by measuring the pore size and calculating the porosity. As shown in Fig. 3a, with DMAc/acetone ratio increasing from 1/9 to 7/3, the maximum pore size decreased from 3.4 to 1.6 μm and the mean pore size dropped from 2.4 to 1.1 μm, which would be related to the reduction of fiber diameter.³³ However, the fibrous membrane obtained from solution with DMAc/acetone ratio of 9/1 shows slightly increased maximum and mean pore size of 2.2 and 1.3 μm, respectively. This would be attributed to the fracture of fibers that resulted in large pores in the membranes. Meanwhile, porosity of the PVDF fibrous membranes decreased slightly from 76.9% to 72.1% with DMAc/acetone ratio increasing from 1/9 to 7/3 (Fig. 3b), which would be related to the denser packing of fibers with reduced diameter. But further increasing of DMAc/acetone ratio led to a sharp drop of porosity to 56.8%, which would related to the more compacted accumulating of the fractured fibers.

Fig. 3 (a) Maximum and mean pore size, and (b) porosity of the PVDF fibrous membranes obtained from solutions with various DMAc/acetone ratios.

Benefiting from the good surface hydrophobicity and minor pore size, the PVDF fibrous membranes had been provided with resistance to water penetrating. As shown in Fig. 4a, a regular increasing of hydrostatic pressure from 44.1 to 82.3 kPa could be seen as the DMAc/acetone ratio raised from 1/9 to 7/3, but it dropped to 54.2 kPa when the ratio further increased to 9/1. This phenomenon would be explained by the Young–Laplace equation, which is usually used to evaluate the required pressure for liquid entering into a cylindrical capillary:

where γ is the surface tension of water, θ_adv is the advancing water contact angle of the PVDF casting film, d_max is the maximum pore size of the PVDF fibrous membranes.^34–36 Herein, the advancing water contact angle of PVDF casting films represent the hydrophobicity of the raw materials and avoid the influence of surface roughness of the macro-sized fibrous structure. In the inset in Fig. 4a, the theoretical value calculated via Young–Laplace equation (red curve) is finely match with the actual hydrophobic pressure (blue points). This indicated that the increasing of hydrophobicity and reduction of maximum pore size would bring about improvement of waterproof performance.

Fig. 4 (a) Hydrostatic pressure, and (b) air permeability and WVT rate of PVDF fibrous membranes obtained from component solvents with various DMAc/acetone ratio. The inset in (a) is a fitted relationship among maximum pore size, surface hydrophobicity, and hydrostatic pressure.

On the other hand, the windproof performance of the fibrous membranes had been evaluated by measuring the air permeability. As shown in Fig. 4b, the air permeability of the membranes fell from 24 to 8.1 mm s⁻¹ as DMAc/acetone ratio increased from 1/9 to 7/3, and then slightly raised up to 10.8 mm s⁻¹ with further increasing of DMAc/acetone ratio. According to the previous investigation, lower air permeability would represent better windproof performance.⁸ Therefore, the PVDF fibrous membranes obtained from DMAc/acetone ratio of 7/3 would exhibit best windproof performance.

Meanwhile, the electrospun PVDF fibrous membranes with interconnected porous structure would be permissible for water vapour transmitting, which had been investigated by measuring WVT rate. As shown in Fig. 4b, the WVT rate of the fibrous membranes decreased from 11.8 to 11.5 kg m⁻² d⁻¹ as DMAc/acetone ratio increased from 1/9 to 7/3, and dropped to 10.8 kg m⁻² d⁻¹ with further increasing of DMAc ratio. This would be explained that water vapor transmission through the fibrous membranes would mainly occur from the Fickian diffusion driven by vapor pressure difference, thus the membranes with higher porosity would provide more transmitting channel and present better breathable performance.^37,38

Ideal water/windproof and breathable should also exhibit robust mechanical performance in practical applications, which would be diversified enormously by the transforming of the fibrous structure. In this study, the mechanical performance of the membranes had been evaluated by tensile testing. Fig. 5a shows the typical stress–strain curves of the PVDF fibrous membranes obtained from component solvents with various DMAc/acetone ratios. It should be noticed that when the elongation is smaller than 6%, the fibrous membranes shows a Hookean elastic behavior with a stable initial Young's modulus (inset in Fig. 5a), but when the elongation increased to larger than 6%, the membranes show a nonlinear stress–strain behavior.³⁹ The initial Young's modulus increased from 10.7 to 22.9 MPa with the increasing of DMAc/acetone ratio, which would be attributed to the efficient stretching at higher DMAc/acetone ratio that brought about better orientation of PVDF molecular along the fibers.⁴⁰ Analogously, the breakage stress increased from 5.6 to 8.9 MPa with DMAc/acetone ratio raising up from 1/9 to 7/3. But the breakage stress intensely decreased to 4.5 MPa with further increasing of DMAc/acetone ratio to 9/1, which would be due to the fracture of the fibers (as shown in Fig. 1e). Meanwhile, the elongation at break decreased from 240.8% to 51.9% gradually with the DMAc/acetone ratio increasing from 1/9 to 9/1, which would be ascribed to the reduction of sticking structure that could afford less elongation.^41,42 Furthermore, the toughness of the fibrous membranes were appraised by calculating the energy to break, as shown in Fig. 5b. The energy to break of the fibrous membranes decrease gradually from 8.7 to 6.3 mJ m⁻³ as the DMAc/acetone ratio increased to 7/3, and sharply fall to 1.6 mJ m⁻³ with further increasing of DMAc/acetone ratio to 9/1. By contrast, the PVDF fibrous membranes obtained from component solvents with DMAc/acetone ratio of 7/3 would present optimum mechanical performance with high strength (breakage stress of 8.9 MPa), large elongation (strain at break of 109.8%) and good toughness (energy to break of 6.2 mJ m⁻³).

Fig. 5 (a) Stress–strain curves, and (b) initial Young's modulus and energy to break of PVDF fibrous membranes obtained from solutions with various DMAc/acetone ratios. The inset in (a) shows the start region of the stress–strain curves.

According to the above results, the PVDF fibrous membranes obtained from solutions with DMAc/acetone ratio of 7/3 would exhibit high water/windproof and breathable performance, as well as good mechanical property. Therefore, the solvent with DMAc/acetone ratio of 7/3 could be selected to be used in the following investigation.

PVDF fibrous membranes obtained from solutions with different conductivity

In the aim of further improving the performance of the PVDF fibrous membranes, we had regulated the fibrous structure by tuning the conductivity of the polymer solutions using different concentrations of NaCl. As presented in Fig. 6a, the conductivity of PVDF solutions with NaCl concentrations of 0, 0.003, 0.006, and 0.009 wt% were 10.8, 22.2, 36.1, and 48.1 μs cm⁻¹, respectively. Meanwhile, the fibrous membranes obtained from solutions with higher conductivity possessed thinner fibers (average fiber diameter of ∼250 nm, as shown in Fig. 6b–d) in comparison to the fibrous membranes obtained from solutions without NaCl addition. Moreover, the sticking structure among the adjacent fibers had been further reduced with the increasing of conductivity (as shown in Fig. 6b–d).

Fig. 6 (a) Conductivity of PVDF solutions containing different concentrations of NaCl. SEM images of PVDF fibrous membranes obtained from solutions with different concentrations of NaCl: (b) 0.003 wt%, (c) 0.006 wt%, and (d) 0.009 wt%, respectively.

In the meantime, the increasing of conductivity brought about diversification of the porous structure. As shown in Fig. 7a, with the increasing of NaCl concentration to 0.003 wt%, the maximum pore size decreased to 1.2 μm and the mean pore size decreased to 0.8 μm, respectively, which would be attributed to the reduced fiber diameter. However, further increasing of conductivity brought about slight increasing of maximum and mean pore size, which would be related to the reduced sticking structure among adjacent fibers. Meanwhile, as shown in Fig. 7b, the porosity increased from 72.1% to 76.2% with introduction of 0.003 wt% NaCl, and further increasing of NaCl concentration had little effect on the increasing of porosity.

Fig. 7 (a) Maximum and mean pore size, and (b) porosity of PVDF fibrous membranes prepared from solutions containing different concentrations of NaCl.

According to the above results, the transformation of porous structure would have great influence on the performance of the PVDF fibrous membranes. Fig. 8a presents the hydrostatic pressure of PVDF fibrous membranes with different concentrations of NaCl. The fibrous membranes with 0.003 wt% NaCl exhibits a hydrostatic pressure of 110 kPa, which is much higher than the fibrous membranes without NaCl addition (82.3 kPa). But the hydrostatic pressure decreased with further increasing of NaCl concentrations. This is mainly related to the transformation of the maximum pore size, which had also been explained by the Young–Laplace equation in the inset of Fig. 8a. Meanwhile, as shown in Fig. 8b the fibrous membranes with 0.003 wt% NaCl possesses the best windproof performance with air permeability of 6.1 mm s⁻¹, which would be due to the minimum mean pore size. Moreover, WVT rate of the PVDF fibrous membranes remained ∼11.5 kg m⁻² d⁻¹, revealing persistently good breathable performance.

Fig. 8 (a) Hydrostatic pressure, and (b) air permeability and WVT rate of PVDF fibrous membranes obtained from polymer solutions with different concentrations of NaCl. The inset in (a) is a fitted relationship among maximum pore size, surface hydrophobicity, and hydrostatic pressure.

Moreover, the mechanical performance of PVDF fibrous membranes had been reinforced by the increasing of conductivity of polymer solutions. As shown in Fig. 9a, with the increasing of NaCl concentrations, the Young' modulus increased from 17.4 to 22.4 MPa and the stress at break raised up from 8.9 to 12.2 MPa, respectively. This would be ascribed to the better orientation structure of PVDF molecular along the fibers, which is caused by the more effective stretching of polymer solution jet with higher conductivity.⁴³ However, the fibrous membranes show a slight shortening of elongation from 109.8% to 83.1% as conductivity increased, as shown in Fig. 9b. Therefore, the energy to break increased from 6.3 to 6.4 mJ m⁻³ with addition of 0.003 wt% NaCl, but reduced to 5.8 mJ m⁻³ with further addition of NaCl. It could be seen that the addition of 0.003 wt% NaCl would result in fibrous membranes with optimized mechanical performance.

Fig. 9 (a) Initial Young's modulus and breakage stress, and (b) breakage elongation and energy to break of the PVDF fibrous membranes obtained from solutions containing different concentrations of NaCl.

Consequently, the PVDF fabricated from the component solutions with DMAc/acetone ratio of 7/3 and NaCl concentration of 0.003 wt% would possess optimized maximum pore size of 1.2 μm, mean pore size of 0.8 μm and porosity of 76.3%. The as-prepared fibrous membranes would simultaneously exhibit good waterproof (hydrostatic pressure of 110 kPa), windproof (6.1 mm s⁻¹) and breathable (WVT rate of 11.5 kg m⁻² d⁻¹) performance, which would be used in many kinds of applications, especially in fabricating of protective clothing.

Conclusions

In conclusion, we have successfully fabricated a novel kind of hydrophobic fibrous membranes with water/windproof and breathable via electrospinning of PVDF. The fibrous and porous structure of the membranes had been finely regulated by tuning DMAc/acetone ratios and NaCl concentrations in the PVDF solutions. The fibrous membranes obtained from the solutions with DMAc/acetone ratio of 7/3 and NaCl concentration of 0.003 wt% possessed optimized porous structure with maximum pore size of 1.2 μm, mean pore size of 0.8 μm and porosity of 76.2%. The integration of good hydrophobicity and optimized porous structure endowed the membranes with simultaneously good waterproof (hydrostatic pressure of 110 kPa), windproof (6.1 mm s⁻¹) and breathable (WVT rate of 11.5 kg m⁻² d⁻¹) performance. Moreover, benefiting front the sufficient stretching and orienting of PVDF molecular, the as-prepared fibrous membranes exhibited good mechanical performance with high strength (breakage stress of 11.1 MPa) and excellent toughness (energy to break of 6.4 mJ m⁻³). Therefore, the resultant PVDF fibrous membranes with good water/windproof, permeable and mechanical performance would be used as key materials in many areas, especially in fabricating of protective clothing.

Acknowledgements

This work is supported by the National Natural Science Foundation of China (No. 51503030, 51473030 and 51322304), Shanghai Sailing Program (No. 15YF1400600), the Fundamental Research Funds for the Central Universities, and the “DHU Distinguished Young Professor Program”.

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Footnotes

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra17565a

‡ These authors have contributed equally to this work.

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