PDMS/PVDF Nanofiber Membrane with Hydrophobic Property by Electrospinning for Ultrafast Oil-Water Separation

Abstract

Herein, a hydrophobic polydimethylsiloxane-incorporated polyvinylidene fluoride (PDMS/PVDF) nanofiber membrane was prepared by electrospinning method under a certain air humidity. The properties and separation performance of PDMS/PVDF nanofiber membrane were characterized by scanning electron microscopy (SEM), Fourier-transformed infrared spectroscopy attenuated total reflectance (FTIR-ATR), X-ray diffraction (XRD) and the oil-water separation test. The results demonstrate that the PDMS/PVDF membrane have better properties and performance than PVDF nanofiber membrane; with the increase in air humidity, electrospinning membrane had superior properties and separation performance. Additionally, when the PDMS concentration was 5.0 wt.%, the membrane had the best hydrophobicity. Moreover, for oil-water separation performance, the maximum flux could reach 1827.8 kg m^-2 h^-1. In addition, it has a water contact angle of 145.2º and an oil contact angle of 0º. Simultaneously, the membrane prepared under low humidity had continuous structure and little bead structure, which might bring higher crystallinity. Also, after 20 cycles, the separation efficiency could still reach about 95.0%, which showed excellent reusability and long-term stability.

Keywords:
PDMS; PVDF; oil-water separation; fibrous coat; electrostatic spinning

Introduction

Nowadays, a large amount of oily wastewater is produced in petrochemical, food, textile, galvanized steel, metal and other industries, which lead to serious ecological problems.¹ To avoid the secondary environmental pollution caused by direct discharge of oily wastewater, oil-water separation technology has received unprecedented attention. Oily wastewater can be treated by adsorption, photocatalytic treatment, electrocoagulation, Fenton method, gravity separation method, membrane separation and other methods.²-⁸ Among them, membrane separation is a technology with low energy consumption, environmentally friendly and high separation efficiency.⁹

So far, there have been some studies on oil-water separation using hydrophobic polymeric membrane. Hydrophobic membranes are mainly composed of polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE) and polypropylene (PP),¹⁰, ¹¹, ¹² with PTFE showing the best hydrophobic properties. However, it is difficult to prepare PTFE membranes because there is no suitable solvent.¹³ In contrast, PVDF is an optional material for preparing hydrophobic membranes. Xu et al.¹⁴ prepared MXene (a family of two-dimensional (2D) transition metal carbides, nitrides, or carbonitrides with a general chemical formula of M_n+1X_nT_X) and SiO₂ nanoparticles modified PVDF membrane by using solvent phase inversion method, and the separation efficiency of oil-water mixture was more than 99.0%, also demonstrating excellent performance in various pollutants (including heavy metals, dyes and phenol) in oily wastewater. Zhao et al.¹⁵ modified PVDF membrane with dopamine and hydrophilic hydroxyethyl methacrylate by one-step co-deposition method. The water flux of the modified membrane reached 9668 L m^-2 h^-1 bar^-1, and the separation efficiency of various oil-water emulsions exceeded 98.0%. Wang et al.¹⁶ used tannic acid and 3-aminopropyltriethoxysilane to rapidly hydrophilically modify one side of the PVDF membrane to obtain a Janus PVDF modified separation membrane. The separation efficiency of the membrane for oil-in-water emulsion is 99.5%, and the separation efficiency for water-in-oil emulsion is as high as 98.6%. However, its hydrophobicity is lower than that of PTFE, so it is necessary to improve PVDF hydrophobicity by adding hydrophobic polymer, filler and additives.¹⁷, ¹⁸, ¹⁹ Polydimethylsiloxane (PDMS) is the most commonly polymer used for the preparation of dense films. PDMS is also used as active layer in nanofiltration processes.²⁰

Electrospinning is a unique and effective technology for the design and preparation of nanofibers with different diameters.²¹ Electrospinning nanofiber membranes have attracted wide attention due to their tunable porosity, large specific surface area, adjustable morphology, low cost, and functionality.²² The environmental humidity in the electrospinning process will affect the morphology of the fiber membrane. In addition, different morphologies and structures endow them with special properties and functions.²³ Therefore, the control of humidity should be focused on during the preparation of nanofiber membranes. In this study, hydrophobic PDMS/PVDF electrospun nanofiber membranes were prepared by electrospinning technology by adjusting the ambient humidity. The morphology and properties of these membranes were characterized by scanning electron microscopy (SEM), Fourier-transformed infrared spectroscopy attenuated total reflectance (FTIR-ATR), X-ray diffraction (XRD), pore size, porosity, water contact angle, and mechanical properties. The effect of relative humidity and PDMS content on the separation performance of PDMS/PVDF electrospinning nanofibers was investigated by oil-water separation test.

Experimental

Materials

PVDF (SOLEF®6020/1001) was procured from SuWei Co., Ltd. (Shanghai, China), DMAC (AR) and polyvinylpyrrolidone K30 (PVP, weight-average molecular weight 3000) were obtained from TianTai Fine Chemicals Co., Ltd. (Tianjin, China), ethanol (AR) and THF (tetrahydrofuran, AR) were purchased from Fuyu Fine Chemical Co., Ltd (Tianjin, China). TEP (triethyl phosphate, AR) was obtained from Shanghai Sinopharm Chemical Reagent Co., Ltd. PDMS 107 (20000 cP) was procured from Yadong New Material Co., Ltd. (Hangzhou, China), ethyl silicate (AR) and dibutyltin dilaurate (AR) were purchased from Fuchen Chemical Reagent Co., Ltd (Tianjin, China), methyl orange (AR) was purchased from Imoc Chemical Products Co., Ltd. (Shenyang, China).

Preparation of PDMS/PVDF nanofiber membrane

PVDF powder was dissolved in DMAC with PVP and TEP by stirring in the water bath under 60 ºC to form 15.0 wt.% PVDF solution. After 2 h, the casting solution was defoamed. With ethyl silicate as crosslinking agent, 5.0 wt.% PDMS was dissolved in THF for 1 h. The PVDF solution and PDMS solution were blended under mechanical stirring with dibutyltin dilaurate (DBTB) as catalyst. Then, the prepared solution was defoamed for 12 h to make it homogeneous and uniform for electrospinning. To fabricate electrospinning fiber membrane, a device with high voltage power supply, syringe with 0.3 mm inner diameter needle and a stainless-steel roller as a collector was carried out. 8.0 mL casting solution were put in the syringe, and the feeding speed was 0.20 mm min^-1 under 15 kV voltage, needle-to-collector distance 15 cm, and room temperature. Different humidity is controlled by the humidifier. Then, the membrane was placed in a drying oven at 60 ºC. According to relative humidity of 40 ± 5% (low humidity), 60 ± 5% (medium humidity), 80 ± 5% (high humidity), the membranes were named as L-HM, M-HM, H-HM, respectively. The casting solution was composed by different amount of PDMS and 15.0 wt.% PVDF solution. According to the PDMS concentration of 1.0, 3.0, 5.0, 7.0 wt.%, the fiber membranes were designed as PM1, PM3, PM5 and PM7, respectively. Pure PVDF membrane was named as PM0. The diagram scheme of preparation of PDMS/PVDF nanofiber membrane was shown in Figure 1.

Figure 1
The diagram scheme of (a) preparation of PDMS/PVDF membrane casting solution; (b) preparation of PDMS/PVDF nanofiber membrane.

Characterization

The morphology and thickness of the membranes were examined by scanning electron microscope (FESEM, S-4800 Hitachi, Japan). The samples were immersed in liquid nitrogen for a few minutes then were fractured and gold-coated for SEM testing. The contact angles were measured by contact angle meter (DSA30, KRuss GmbH, Germany) at room temperature. Deionized water droplets and different types of oil droplets were dropped onto the membrane surface at three different locations of the same sample, then the results were averaged. The X-ray diffraction (XRD) spectrograms for the membranes were performed on D/MAX 2000/PC (Japan) under 10 mA and 40 kV with Cu Kα radiation.

Pore size and porosity test

Membrane pore size was tested by capillary flow porometer (POROLUX500, Germany). The maximum, minimum and mean pore size of the membrane were obtained by testing the wet and dry samples. Each sample was measured three times and the results were averaged.

Porosity test was carried out by placing the membrane in the Porefil. Each sample was measured three times and the average was calculated. The porosity (ε) was calculated by equation 1:

(1) $\text{ε}\left(\%\right)=\frac{{\text{M}}_{\text{W}}-{\text{M}}_{\text{D}}}{\text{ρAδ}}\times 100$

where M_w (g) and M_D (g) were the weight of the membrane immersed in the Porefil and dry membrane, respectively; p was the density of Porefil which was 1.87 (g cm^-3); A was the membrane area (cm²); δ was the thickness (cm).

Oily water separation test

The picture of oil-water separation test was shown in Figure 2. The prepared electrospinning membranes were put into the membrane module with a diameter of 25.18 mm. The oil-water mixture is composed of kerosene and water in a volume ratio of 3:4, and the water was stained with methyl orange for differentiation. The water-oil mixture was poured from the upper of the device and separated by liquid gravity. A beaker was placed under the membrane to receive the separated solution. The retained oil layer was dumped and then fed to investigate the reusability of the separation membrane. The separation performance was evaluated by separation efficiency (η) and flux (F). They were calculated by equations 2 and 3, respectively.

Figure 2
Picture of oil-water separation test.

(2) $\text{η}\left(\%\right)=\frac{{\text{M}}_1}{{\text{M}}_0}\times 100$

(3) $\text{F}=\frac{\text{W}}{\text{At}}$

where, M₀ (g) was the weight of feed solution and M₁ (g) was the weight of permeation solution.

where, W (g) was the weight of permeation solution; A (m²) was the effective membrane area; and t (h) was the separation time.

Results and Discussion

The effects of air humidity on nanofiber membrane

Morphology analysis

Pore structure of hydrophobic microporous membranes affects the separation performance of the membranes. The morphologies of nanofiber membranes were shown in Figure 3. It could be seen that the fineness of the fibers was affected by humidity. The membrane prepared under high humidity conditions (H-HM) was uneven in fineness and produced beaded structure. However, L-HM and M-HM have little bead structure. The number of beads per unit area was used to count the change of bead formation. The nanofiber membrane prepared under L-HM has almost no bead formation, 3 per 1000 μm² under L-HM and 8 per 1000 μm² under H-HM, because increasing humidity could lead to thermally induced/vapor induced phase separation, which had an impact on morphology and was more prone to formation of bead structures. Besides, it meant that L-HM had a smoother surface than H-HM, which might be related to buckling instability and stretching by electrical force.²⁴, ²⁵, ²⁶

Figure 3
SEM images of nanofiber membranes prepared under different air humidity.

As shown in Table 1, increasing the humidity, both pore size and porosity of the membrane decreased due to the beaded structure decreasing the pore size. At the same time, the increase in humidity promoted the gel reaction of the fibers, which could decrease the pore size and porosity.

Table 1
Pore size and porosity of membranes prepared under different air humidity

Membrane	Maximum / nm	Mean / nm	Minimum / nm	Porosity / %
L-HM	4550	4180	2010	91
M-HM	3780	2390	2010	88.72
H-HM	3220	2110	750	67.15

• L-HM: relative humidity of 40 ± 5% (low humidity); M-HM: relative humidity of 60 ± 5% (medium humidity); H-HM: relative humidity of 80 ± 5% (high humidity); maximum: maximum membrane pore size; mean: mean membrane pore size; minimum: minimum membrane pore size; porosity: porosity of membrane.

Physical properties

The hydrophobicity of these membranes was measured by water contact angle (as shown in Figure 4). It could be seen that with the increase of humidity, water contact angle, and hydrophobicity decreased. The contact angle depends on pore size and porosity, and larger size and higher porosity bring higher hydrophobicity.²⁷

Figure 4
Water contact angles of membranes prepared under different air humidity.

To illustrated the effect of relative humidity on crystal form and crystallinity, XRD was measured and the spectra of nanofiber membranes are shown in Figure 5. The peaks at 2θ about 17.6º, 18.3º, 20.2º and 29.0º corresponded to α-phase of PVDF.²⁸ When the humidity increased, the intensity of peaks at 20.2º decreased because the α-crystallites content decreased, reducing the crystallinity.²⁹

Figure 5
XRD spectra of membranes prepared under different air humidity.

For oil-water separation, the mechanical properties of the membrane reflected the durability of the membrane. The tensile stress-strain curves of nanofiber membranes were shown in Figure 6. The mechanical properties of membranes and compressive resistance were greatly affected by the crystallinity of the polymer. The tensile stress and strain decreased as the humidity increased, which was caused by the decreased crystallinity.³⁰ For morphology, the membrane prepared under low humidity had a continuous structure and little bead structure, which might bring higher crystallinity. For vapor-induced phase separation (VIPS) theory, the rate of phase separation enhance as the water vapor concentration increase which brought that more water molecules diffused into electrospinning fibers before solidification under higher humidity. Additionally, phase separation resulted in the formation of a skin layer, which led to the weak adhesion of fiber.³¹

Figure 6
Strain-strength curves of membranes prepared under different air humidity.

Separation performance

The oil-water mixture was treated with nanofiber membranes under room temperature. From Figure 7, L-HM had the highest flux of 1834.0 L m^-2 h^-1 compared with the other membranes. Besides, each membrane had above 99.0% separation efficiency. L-HM had the highest efficiency of 99.6% because the hydrophobicity decreased with decrease in humidity, causing fewer permeated oil droplets. Besides, porosity decreased, which could bring fewer channels for oil permeation.

Figure 7
Separation performance of membranes prepared under different air humidity.

The effects of the PDMS concentration on nanofiber membrane

The infrared spectra of PDMS/PVDF nanofiber membranes with different amounts of PDMS are shown in Figure 8. The peak at the wavenumber of 1180 and 870 cm^-1 was stretching vibration of -CF₂ bond and skeletal vibration of C-C bond in PVDF, respectively.

Figure 8
FTIR-ATR spectra of PDMS/PVDF membrane and PDMS: (a) wavenumber range 3500 to 500 cm^-1, (b) wavenumber range 920 to 720 cm^-1.

The absorption peak at 1400 cm^-1 was the symmetrical stretching of C–F in PVDF and deformation asymmetric vibration of –CH₃ on Si atom of PDMS. The vibration peak of Si–OH of PDMS was at the wavenumber of 1020 cm^-1. The peak at the wavenumber of 790 cm^-1 represented –Si(CH₃)₂ rocking. By comparing the spectra, it proved that PDMS/PVDF nanofibers composite membranes had PVDF and PDMS. The peak at the wavenumber of 840 cm^-1 was β phase of the PVDF crystal. It could be seen that the transmittance of this peak decreased with the increase in PDMS content the spectra of PDMS/PVDF membranes, there is no new peak, which means that PDMS and PVDF are physically mixed.

SEM images of PDMS/PVDF membranes were shown in Figure 9. The surface of the membrane was composed of random oriented continuous fibers with porous structure and uniform pore size distribution. However, compared with PM0, PDMS/PVDF membranes had a beaded structure. With PDMS concentration increased, more beaded structures were formed because the ratio of PDMS to PVDF in the casting solution increased, and PVDF content decreased, which caused the jet to be unstable and the solvent evaporation was incomplete in the electrospinning process. The high volatility of THF (vs. DMAC) leads to the rapid volatilization of THF, which accelerates the phase separation between PDMS and PVDF at high humidity, resulting in the formation of beads. From the cross section, the membrane nanofiber structure is consistent with the membrane surface. With the increase of PDMS concentration, the membrane thickness is basically the same, about 70 μm.

Figure 9
SEM images of membranes prepared under different PDMS concentration.

Table 2 shows the porosity and average pore size of PM0 to PM7. When the PDMS content increased from 0 to 5.0 wt.%, the pore size and porosity increased. When the PDMS content increased to 7.0 wt.%, the pore size and porosity decreased slightly. With the increase of PDMS content, fiber diameter shows an increasing trend in the SEM images. The single-layer fibers accumulated more, and the fiber distribution in the same area was sparse, resulting in larger average pore size. Additionally, porosity increased with the pore size increased. However, when PDMS are larger than a certain amount, the beaded structure increased, resulting in a decrease in pore size and porosity.

Table 2
Pore size and porosity of membranes prepared under the different PDMS concentration

Membrane	Maximum / nm	Mean / nm	Minimum / nm	Porosity / %
PM0	3140	1600	910	73
PM1	4360	3850	2010	84
PM3	4520	3850	3150	87
PM5	4550	4180	2010	91
PM7	4060	2880	1790	86

• PM0: PDMS concentration of 0 wt.%; PM1: PDMS concentration of 1.0 wt.%; PM3: PDMS concentration of 3.0 wt.%; PM5: PDMS concentration of 5.0 wt.%; PM7: PDMS concentration of 7.0 wt.%; maximum: maximum membrane pore size; mean: mean membrane pore size; minimum: minimum membrane pore size; porosity: porosity of membrane.

As shown in Figure 10, from PM0 to PM5, water contact angle (WCA) increased from 110.9º to 145.2º. However, PM7 had lower WCA than PM5. This is because PDMS 107 is a high hydrophobic material. Furthermore, it is well known that, the hydrophobicity of microporous membranes could be improved by the increase of surface pore size and porosity. From Table 2, the pore size and porosity had the same trend as contact angle. Additionally, oil contact angle (OCA) was tested and the results were shown in Figure 10, where it can be seen that all membranes had an angle of about 0^o, which showed excellent hydrophobicity.

Figure 10
Water/oil contact angle (WCA/OCA) of membranes prepared under different PDMS concentration.

Figure 11 showed the XRD patterns of PM0, PM1, PM3, PM5 and PM7 membranes. The XRD results showed that these membranes had similar diffraction peaks at 2θ of 20º, 22º and 29º. The characteristic peaks of 20.2º and 29.2º corresponding to α (110) and α (020) crystal planes, respectively. With the increase of PDMS content, the diffraction peak intensity at 20.2º became more weaken and the crystallinity decreased. This result was consistent with FTIR results, possibly because the PDMS chains might be evenly distributed among the PVDF polymer chains in low PDMS content. With the increase of PDMS content, an amorphous peak appeared at 10-15º, which could be indicating excessive PDMS loading that leads to precipitation.³²

Figure 11
XRD spectra of PM0 to PM7.

With the increasing of PDMS concentration, stress increased (Figure 12). The long molecular chain of PDMS has a strong flexible structure, which could enhance the toughness of the membrane. For strain, due to the addition of PDMS, the PDMS/PVDF membranes had lower strain than PM0, probably due to the interpenetrating of siloxane chains between PVDF and PDMS, resulting in strong interfacial interaction in the membrane.

Figure 12
Strain-strength curves of membranes.

Separation performance

As shown in Figure 13, the permeability flux and separation efficiency of the membrane first increased and then decreased with the increase of PDMS content. This was because the separation performance depended on the morphology of the membrane and the composition of the casting liquid.³³ Membranes with larger pore sizes and greater hydrophobicity will result in better separation performance.

Figure 13
Separation performance of PM0 to PM7.

To test the stability and reusability of the nanofiber membrane, PM5 was used for cycle test. As shown in Figure 14, after 20 runs, the permeability flux of PM5 membrane decreased from 1827.8 to 1309.6 L m^-2 h^-1. Additionally, the separation efficiency could still reach 95.0% after 20 runs. The flux decline may stem from membrane fouling by oil adhesion and reduced chemical stability under prolonged exposure. In a word, PDMS/PVDF membrane had higher oil-water separation performance than PVDF membrane, and PM5 membrane has excellent reusability and durability.

Figure 14
Reusability of PM5 (a) permeation flux; (b) separation efficiency.

Conclusions

In summary, PDMS-incorporated PVDF membranes were fabricated by electrospinning method under different humidity. From the results, the membrane prepared under low humidity had larger pore diameter, higher porosity, and stronger hydrophobicity. Therefore, it had a talent for oil water separation with the highest flux and separation efficiency compared with the two other membranes. According to XRD results and FTIR spectra of the membranes, PDMS and PVDF are physically mixed. Compared with PVDF membranes (PM0), PDMS/PVDF membranes showed higher pore size, porosity, and water contact angle. With the increase of PDMS concentration, the pore size, porosity, and water contact angle first increased and then decreased. PDMS/PVDF fiber membranes had better mechanical property than PVDF membrane. For oil-water mixture separation, the flux and separation efficiency first increased and then decreased, which has the same trend as the properties. When PDMS content was 5.0 wt.%, it showed excellent separation performance, which can reach flux of 1827.8 L m^-2 h^-1 and separation efficiency of 99.6%. For reusability, after 20 runs, the flux and separation efficiency are slightly decreased to 1305.6 L m^-2 h^-1 and 95.0%, respectively. It is durable and reusable in the separation of oil-water mixture. Besides, it provides an environment friendly solution for the treatment of oily water.

Acknowledgments

The authors thank National Engineering Research Center for High-Speed EMU, China Railway Rolling Stock (CRRC) for financial support. The authors would also like to thank Prof Shaohua Liu of East China Normal University for providing the samples and use of their instruments for characterization. This study was financed in part by Dr Huangyue Cai from Shanghai Jiaotong University for providing support with regard to English language corrections.

Data Availability Statement

The data are available from the corresponding author upon reasonable request.

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Edited by

• Editor handled this article:
  Adriana Nunes Correia (Associate)

Publication Dates

• Publication in this collection
  22 Sept 2025
• Date of issue
  2025

History

• Received
  19 May 2025
• Accepted
  14 Aug 2025