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Superhydrophilic and underwater superoleophobic PVDF membranes via plasma-induced surface PEGDA for effective separation of oil-in-water emulsions
Accepted Manuscript Title: Superhydrophilic and underwater superoleophobic PVDF membranes via plasma-induced surface PEGDA for effective separation of oil-in-water emulsions Author: Junping Ju Tingmei Wang Qihua Wang PII: DOI: Reference:
S0927-7757(15)00069-2 http://dx.doi.org/doi:10.1016/j.colsurfa.2015.01.041 COLSUA 19688
To appear in:
Colloids and Surfaces A: Physicochem. Eng. Aspects
Received date: Revised date: Accepted date:
9-12-2014 20-1-2015 21-1-2015
Please cite this article as: J.J. Tingmei Wang, Q. Wang, Superhydrophilic and underwater superoleophobic PVDF membranes via plasma-induced surface PEGDA for effective separation of oil-in-water emulsions, Colloids and Surfaces A: Physicochemical and Engineering Aspects (2015), http://dx.doi.org/10.1016/j.colsurfa.2015.01.041 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Graphical abstract
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Highlights The surface grafting of poly (vinylidene fluoride) (PVDF) membrane with poly (ethylene
glycol) diacrylate
(PEGDA) was
obtained
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plasma-induced graft polymerization.
via low-pressure
The PVDF-g-PEGDA membranes became superhydrophilic and underwater
as-prepared
PVDF-g-PEGDA
membranes can
effectively
separate
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The
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superoleophobic, and showed also excellent mechanical properties.
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oil-in-water emulsions.
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Superhydrophilic and underwater superoleophobic PVDF membranes via plasma-induced surface PEGDA for effective separation of oil-in-water emulsions Junping Jua,b Tingmei Wanga Qihua Wanga ※
State Key Laboratory of Solid Lubrication, Lanzhou Institute of Chemical Physics,
University of Chinese Academy of Sciences, Beijing 100039, P. R. China.
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b
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Chinese Academy of Sciences, Lanzhou 730000, P. R. China.
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a
Corresponding author: E-mail: [email protected]; Phone: +86 (0)931 4968252;
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Fax: t86 (0)931 4968252. Abstract
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This work describes the surface grafting of poly (vinylidene fluoride) (PVDF) membrane with poly (ethylene glycol) diacrylate (PEGDA) via low-pressure
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plasma-induced graft polymerization. The chemical composition and microstructure of the surface modified PEGylated PVDF membranes were characterized by fourier
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transform infrared spectroscopy (FTIR), X-rayphotoelectron spectroscopy (XPS), scanning lectron microscopy (SEM), atomic force microscopy (AFM) and contact angle measurements. By tuning polymerization conditions, a superhydrophilic and underwater superoleophobic PVDF-g-PEGDA membrane was successfully prepared. Results show that the as-prepared PVDF-g-PEGDA membranes can effectively separate oil-in-water emulsions with high separation efficiency and high fluxes under ultralow pressure. Importantly, the membrane showed excellent mechanical properties, which was also important parameter for practical application. Keywords: superhydrophilic; underwater superoleophobic; PVDF membranes;
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separation; oil-water emulsions. 1 Introduction Oil contamination of water is a significant global environmental problem that
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mankind currently faces, which puzzles the survival and development of human
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society. To date, a variety of techniques such as oil skimmers, magnetic separations,
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settling tanks, centrifuges and depth filters for oil/water separation have been
developed in the last few years [1-3], but broadly applicable approach for oil/water
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separation, especially for processing emulsified oil/water mixtures, are highly desired and still challenging. Filtration membranes are being actively explored as advanced
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technology for emulsified oil/water separation [4, 5]. As a microfiltration and ultrafiltration material, poly (vinylidene fluoride) (PVDF) has earned lots of interest
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due to its excellent thermal stability, chemical stability and radiation resistance [6-10]. Recently, polyvinylidene fluoride (PVDF) has been widely used for preparing
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ultrafiltration and microfiltration membranes to separate various types of oil/water mixtures. Jin and co-workers demonstrated a facile modified-phase inversion approach for fabricating superhydrophobic-superoleophilic PVDF membranes for effective separation of a wide range of water-in-oil emulsions with high flux [11]. However, oil removing membranes, such as superhydrophobic-superoleophilic materials, are easily fouled by oils because of their intrinsic oleophilic property. So considerable attentions have been focused on the underwater superoleophobic materials, which is inspired by the antiwetting behavior of oil droplets on fish scales and these membranes show high separation efficiency and low oil fouling. For
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example, a novel superhydrophilic and underwater superoleophobic zwitterionic polyelectrolyte grafted PVDF membrane was successfully attained via a SI-ATRP technique for oil/water separation [12]. The oil contents after one time separation of a
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selection of oil/water mixtures are all less than 10 ppm and some of them are even
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lower than 2 ppm. In addition, salt-induced fabrication of superhydrophilic and
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underwater superoleophobic PAA-g-PVDF membrane for effective separation of oil-in-water emulsions had been prepared [13]. At the same time, liu and co-workers
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provided a novel superamphiphilic PVDF membrane with multi-scale surface structure, which shows under-water ultralow adhesive superoleophobicity and
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under-oil low adhesive superhydrophobicity in oil/water/solid three phase systems and made it a promising candidate for oil/water emulsion separation [14]. Their group
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also reported a methodology to fabricate mussel-inspired PVDF membrane via solution-immersion for oil/seawater emulsion separation [15]. The hydrophilicity and
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superoleophobicity under seawater membrane was achieved when immersed in dopamine solution. However, a facile and inexpensive method towards designing superwetting materials for effective separation is indeed needed. Polyethylene glycol diacrylate (PEGDA, CH2=CHCO(OCH2CH2)nOCOCH=CH2)
is a derivative of PEG with repeated ethylene oxide (EO) units and active end groups [16]. It is considered as one of the most promising candidates for preparation of antifouling surface due to their high hydration capacity and excellent resistance to non-specific protein and other macromolecule adhesion. A plenty of studies have been reported about polyethylene glycol diacrylate (PEGDA), as functional material for
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oil/water separation [17, 18]. Herein, we reported a simple and effective method to prepare superhydrophilic and underwater superoleophobic PVDF membrane by using polyethylene glycol diacrylate. Firstly, we fabricated PVDF microfiltration
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membranes by non-solvent induced phase separation. Then PVDF microfiltration
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membranes grafted with hydrophilic poly (ethylene glycol) diacrylate (PEGDA) via
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low-pressure plasma-induced surface copolymerization were studied [19, 20]. Plasma induced grafting has become a major interest of research to modify the membrane
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surfaces by fast, safe, and low cost ways. The results showed that PVDF-g-PEGDA membranes exhibited superhydrophilic and underwater superoleophobic PVDF
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character that can effectively separate oil-in-water emulsions with high separation efficiency and high fluxes under ultralow pressure. Importantly, the membrane
application.
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showed good mechanical properties, which was also important parameter for practical
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2 Experimental section 2.1 Materials
PVDF powders (Mn= 238 000) and poly (ethylene glycol) diacrylate (PEGDA)
were purchased from Sigma-Aldrich. N, N-dimethylacetamide (DMAc, 99.7%), methanol, ethanol and dichloromethane were used as received. Tween 80 was yellow and purchased from Tianjin Chemical Reagents Company. Deionized water was prepared by our own lab. colza oil, lubricating oil, soybean oil were used as received.
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Fig.1.Schematic illustration of the preparation process of the PVDF-g-PEGDA membranes via low-pressure plasma-induced surface copolymerization.
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2.2 PVDF microfiltration membrane preparation
Membranes were prepared via the non-solvent induced phase separation [21].
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PVDF (18%), Tween80 (3%) and water (3%) were dissolved in the DMAc by vigorous stirring until a clear homogeneous solution was obtained. The solution was
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cast on glass plates with a steel knife and then immediately immersed in a coagulation bath of ethanol at room temperature for 5 min. The obtained membranes were kept in
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deionized water for 24 h and then dried for at least 24 h in a vacuum oven before use. 2.3 Surface PEGDA copolymerization of PVDF membrane As illustrated in Figure 1, the plasma-induced surface copolymerization of PEGDA
was performed using a low-pressure plasma source with aradio frequency of 13.56 MHz. The as-prepared membrane first was immersed into methanol solution containing different concentrations PEGDA (from 10% to 30%) for 1h. Afterward, the PEGDA-coated membrane was dried at room temperature for 2 h. Subsequently, the membranes were treated by a low pressure plasma source. Then the modified PVDF membranes were immersed in methanol and enthanol for 60 min using an
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ultrasonic device to remove any trace of adhering homopolymer to the membranes surface. Then membranes were dried at room temperature under vacuum. 2.4 Emulsion separation experiments
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Surfactant-free oil-in-water emulsions were prepared by mixing three types of oil:
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colza oil, lubricating oil and soybean oil. 10% (w/w) oil was stirred with water for 2 h.
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The PVDF membranes were placed between one vertical glass tube with a diameter of 40 mm and one conical flask. Oil-water emulsion separation experiment was achieved
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driven under low pressure. The flux was calculated on the permeated volume of an emulsion through the membrane. . Oil content in permeates was tested by gravimetric
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analysis [9]. A sample taken from permeate solutions was heated to 80 °C and maintained isothermally until all moisture evaporated. Oil content was calculated
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from permeate mass before and after heating. Both the feed and permeate solutions were examined to obtain oil rejection ratio
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using the following equation:
R(%)
C feed C filtrate 100 C feed
Where Cfeed and Cfiltrate are oil concentrations of feed and permeate solutions,
respectively, R is the separation efficiency. 2.5 Instruments and characterization Attenuated total reflectance Fourier transform infrared spectroscopy (FTIR-ATR) was measured by BrukerIFS 66v/s IR spectrophotometer (Bruker optics, Germany). FE-SEM images were obtained with a field emission scanning electron microscope (JSM-6701F). Scanning electron microscopy (SEM) was performed on a JSM-5600
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(JEOL, Japan) operated at 20 kV, in order to obtain the cross sectional structure. All samples were coated with gold by sputtering prior to observation. AFM study was tested at a Nanoscope IIIa multimode atomic force microscope (AFM, Digital
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Instruments) at the tapping mode. The chemical composition of the prepared
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membrane was determined by X-ray photoelectron spectroscopy (XPS), which was
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conducted on a PHI-5702 electron spectrometer (Perkin-Elmer, USA) using an AlKa line excitation source. The mechanical property of PVDF membranes were evaluated
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at room temperature by tensile tests performed on a ShimadzuAG-X with a strain rate of 2 mm/min. Samples were cut to standard dimensions according to ISO 527-2/1BB.
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Water contact angles of membrane surfaces were measured at ambient temperature on a DSA100machine (Krüss, Germany) and at least five measurements were taken at
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different positions on each sample. For underwater oil contact angles, the membrane was placed in a transparent and cubic quartzose vessel filled with ultrapure water first.
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Then oil droplet (1, 2-dichloroethane about 5μL) was directly placed onto the
membrane surfaces and keep 1 min to attain underwater oil contact angle. Mercury intrusion porosimetry was performed on a Micromeritics Autopore 9500 apparatus. 3 Results and discussion
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Fig.2. FTIR spectra of the raw PVDF membrane (A) and PVDF-g-PEGDA
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membranes via plasma treatment monomer concentration of 10%, 20%, 30% (B, C, D).
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A schematic illustration of the plasma-induced surface copolymerization of PEGDA on PVDF membranes is shown in Fig.1. The PEGDA-grafted layer on PVDF
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membranes can be first regulated by the amount of uniformly coated PEGMA monomer and then followed by low-pressure plasma-induced surface copolymerization. The evidence of PEGDA polymer grafting onto the surface of PVDF membranes were confirmed by comparing the FTIR spectra. Fig.2 shows the FTIR spectra of virginal and plasma treated PVDF membranes. It can be seen from Fig.2A that the raw PVDF membrane shows three typical strong absorption peaks at 1400, 1180 and 875 cm−1 due to -CH2-, -CF2- and C-C groups, respectively. Compared
with the raw PVDF membranes, the PEGDA polymer grafted PVDF membranes (Fig.2B-D) exhibit a new characteristic peaks near 1727 cm−1, indicating the presence
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of O-C=O from PEGDA. In addition, it can be found that both the intensity of the O-C=O adsorption at and the [O-C=O]/[C-F] ratio increased obviously as the plasma treatment monomer concentration increased from 10% to 30%. The result indicates
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that the growth of the grafted PEGDA polymer is dependent on the increasing plasma
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concentration. The details of the changes in surface chemical structure of the
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membrane were further ascertained by XPS study. Fig.3 shows the XPS survey
spectra of raw PVDF and PVDF-g-PEGDA membranes. The strong peaks of C1s, F1s
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and a small peak of O1s are observed in the wide scan spectrum of the virginal PVDF membrane. After plasma treatments, the intensity of the O1s peaks (Fig.3B-D)
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increased significantly, indicating a higher concentration of oxygen introduced on the surface of membrane. Nevertheless, the F1s peak of PVDF-g-PEGDA membranes
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decreases greatly and last disappears with increasing plasma treatment monomer concentrations. These results indicate that the PEGDA have been grafted onto the
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surface of PVDF membrane, which is in good agreement with the results of FTIR-ATR.
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Fig.3. XPS survey spectra of the raw PVDF membrane (A) and PVDF-g-PEGDA
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membranes via plasma treatment monomer concentration of 10%, 20%, 30% (B, C, D).
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Table1 Physicochemical characteristics of raw PVDF and PVDF-g-PEGDA membranes. Membrane
Vrigin PVDF PVDF-g-PEGDA-10 PVDF-g-PEGDA-20 PVDF-g-PEGDA-30
PEGDA concentrations (%) 0 10 20 30
Plasma treatment time (s) 0 60 60 60
Grafting yield (mg/cm2) 0 0.61 1.97 3.06
Tensile strength (MPa) 0.47 1.01 2.40 3.70
The grafting yield is calculated by (W2-W1)/A. Where W 2 and W1 are the weights of the dried membrane before and after PEGDA grafted, respectively, and A is the area of the membrane. As shown in Table1, the grafting yield of grafted PEGDA layers on the surface of PVDF membranes increased as the plasma treatment
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monomer concentration increased from 10 % to 30 %.
Fig.4. Top surface structure (T), bottom surface structure (B) and cross sectional
structure (C) images of the original PVDF membrane (M-0) and the modified PVDF with grafted PEGDA (M-10, 10%).
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Fig.5 Top surface structure (T), bottom surface structure (B) and cross sectional structure (C) images the modified PVDF membranes with grafted PEGDA (M-20, 20%, M-30, 30%)
Fig.4 and Fig.5 give surface and cross section SEM images of raw PVDF and
PVDF-g-PEGDA membrane. All the membranes show the classic structure of the polymer microfiltration membrane prepared by non-solvent induced phase separation [22, 23], which were induced by the delayed demixing in congulant containing pure ethanol. The morphology surface of the membrane is obviously porous, with interconnected holes in the networks constructed by PVDF small globules connecting with each other. The cross sectional exhibits a uniform
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microporous structure composed of spherical particles of approximately the same size. Such structure affirms the domination of crystallization during the precipitation process, where in all crystalline particles were nucleated and grown in a similar
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concentration field and finally fused together to form a bi-continuous structure [24].
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The surface coverage of the PVDF-g-PEGDA membrane revealed an obvious
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change as the plasma treatment monomer concentration increased from 10% to 30%. When the surface plasma treatment monomer concentration is 30%, the porous
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structure on the membrane surface was almost covered with the grafted PEGDA layer. AFM analysis was performed to determine the roughness changes caused by
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grafting PEGDA polymer. Fig.S1 shows three dimensional AFM images and Ra values of the raw PVDF and PVDF-g-PEGDA membranes. The Ra roughness of
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membranes was studied by the tapping mode. As shown in Fig.S1, both raw PVDF and PVDF-g-PEGDA membrane have very high and much the same RMS roughness
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value, indicting the surface of all the membrane is rather rough. The RMS roughness did not distinctly vary with the different PEGDA concentrations. In our case, we think that the RMS roughness of the surface was controlled by the casting procedure, so the grafting PEGDA polymer has minor influence on the global shape of the porous structure.
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Fig.6. Variation of the water contact angle in air and underwater oil contact angle of
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the membrane with increasing the PEGDA concentration.
Fig.7. Water contact angle and underwater oil contact angle of the PVDF-g-PEGDA-20 membrane heated at different temperature.
After plasma treatment, the hydrophilicity changes for both treated and untreated
membranes were measured. As shown in Fig.6, the raw PVDF membrane shows a hydrophobic property with a contact angle of 110. After plasmas treatment, the contact angles of the PVDF-g-PEGDA membranes decreased from about 110°to 0
°instantly. From the Wenzel model [25], By introducing the hydrophilic PEGDA into porous rough surface induced by phase separation，a superhydrophilic surface is
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achieved. In order to test the performances of the membranes in oil/water separation, underwater oil wettability of membrane was further measured. Fig.6 also shows the
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underwater oil contact angle of the membrane. The raw PVDF membrane shows a
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hydrophobic property with a contact angle of 110°and underwater superoleophilic.
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After PEGDA polymer grafting, all the PVDF-g-PEGDA membranes become
superhydrophilic and underwater superoleophobic, as shown in Fig.6. It is known to
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all that the wettability of a surface is mainly governed by its chemical composition and roughness. The particular wettability achieved in oil/water/solid three-phase
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system is mainly attributed to the hierarchical surface structure combined with superhydrophilic of the membrane. When PVDF-g-PEGDA membranes are
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immersed into water, the hydrophilic PEGDA are hydrated to transform an extended conformation. Then water can be readily trapped in the rough micro/nanostructures
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to form an oil/water/solid three-phase interface. And these trapped water molecules will greatly decrease the contact area between oil and membrane surface, resulting in a large oil contact angle in water, which is a fluoride-free strategy. The good mechanical property of the membrane is important parameter for
practical application. The tensile strength of raw PVDF membrane and PVDF membranes with a different grafted PEGDA layer are also shown in Table1. The raw PVDF membrane is brittle and exhibits small tensile strength. By comparison, the improvements in tensile strength of PVDF-g-PEGDA membrane are also clearly visible. Stress strength for PVDF-g-PEGDA membranes steadily increase from 1.0
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MPa to 3.8 MPa. The first reason is that PEGDA itself has desirable and intrinsic good mechanical properties, which can alter the processing conditions during polymerization or combine various polymers to achieve a desired mechanical
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characteristic [26]. The second reason may be that defect becomes small after a
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grafted PEGDA layer on the PVDF membrane surface using the plasma technique.
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PVDF-g-PEGDA-10 has the lowest stress strength and it is prone to break up during filter process. Although PVDF-g-PEGDA-30 has the highest stress strength, it has the
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lowest flux due to the fact that most large pores become blocked. So the optimal as-prepared membrane is PVDF-g-PEGDA-20 to study oil-water mixtures separation
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property. Thermal stability of the membrane is one of important factors for practical application. The optimal as-prepared membranes (PVDF-g-PEGDA-20) were heated
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at different temperature for 1 h. As shown in Fig.7, the membrane maintains still stable underwater superoleophobicity by increasing the drying temperature.
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Video S1 (Supplementary) shows a very simple experiment to study underwater
oil-adhesion behavior. Where a 5 μL oil droplet was dropped onto the membrane
surface, an additional force was applied on the oil droplet to make it in sufficient contact with the membrane surface and then allowed to relax. We can see that oil droplets easily roll off from membrane surface. This result indicates that the membrane has the low oil-adhesion characteristics in oil/water/solid system and is important for achieving anti-fouling performance of the membrane.
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Fig.8. Pore size distribution and porosity of PVDF-g-PEGDA-20 membrane.
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To determine the effective separation size of PVDF-g-PEGDA-20 membrane
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membrane, pore size distribution of the membrane was also measured. Fig.8 presents the pore size distribution of PVDF-g-PEGDA-20 membranes. The
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PVDF-g-PEGDA-20 membrane has a mean pore size of 1.33 μm and the porosity of
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57%. The porosity, mean pore size and pore size distribution are very important
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parameters to determine the separate performance of membranes. Surfactant-free oil/water emulsions are typically made of much larger oil droplets ( >20 μm), so most pore in the membrane is expected to block oil droplets.
Fig.9. Photograph of the lubricating oil-water emulsion (10% (w/w)) separation process from PVDF-g-PEGDA-20 membrane.
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Table2 Flux and separation efficiency of oil/water emulsions through PVDF-g-PEGDA-20 membrane. Density
Viscosity
Flux
Separation
(g/mL)
(mPa.s)
(L m-2 h-1)
Efficiency (%)
0.919
7.11
2464.3
99
lubricating oil
0.88
22
3773.7
97
soybean oil
0.9375
14
2414.5
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The as-prepared emulsions were then poured onto PVDF-g-PEGDA-20 membranes to carry out filtration separation under a pressure difference of 5 kPa as shown in
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Fig.9 to give a photograph of the lubricating oil-water emulsion. All dispersed oil-water mixtures can be successfully separated in one time. The results indicate that
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the PVDF-g-PEGDA-20 membranes can effectively block the oil drops with high
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water permeation flux and oil rejection. As shown in Table2, the water permeation
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flux are 2464.3 L/m2 h, 3773.7 L/m2 h and 2414.5 L/m2 h, respectively, for those oil-in water emulsions containing colza oil, lubricating oil, soybean oil, and shows high separation efficiency. After each filtration of the feed emulsion, the membranes can be washed with water and ethanol to recover the flux. 4 Conclusion
A superhydrophilic and underwater superoleophobic PVDF-g-PEGDA membrane has been fabricated by phase inversion method and low-pressure plasma-induced surface copolymerization. The as-prepared membranes can effectively separate oil-in-water emulsions with high separation efficiency under ultralow pressure and much higher
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fluxes. These results indicate that the PVDF-g-PEGDDA membrane promising for practical applications for treating wastewater produced in industry and daily life. Acknowledgments
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The authors would like to acknowledge the financial supportof the National Basic
national young scientist foundation of china (51403219).
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References
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Research Program of China (973 Program, Grant No. 2015CB057502) and the
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[13]W. Zhang, Y. Zhu, X. Liu, D. Wang, J. Li, L. Jiang, J. Jin, Salt-Induced Fabrication of Superhydrophilic and Underwater Superoleophobic PAA-g-PVDF Membranes for Effective Separation of Oil-in-Water Emulsions, Angewandte Chemie International Edition 53 (2014) 856-860. [14]M. Tao, L. Xue, F. Liu, L. Jiang, An Intelligent Superwetting PVDF Membrane Showing Switchable Transport Performance for Oil/Water Separation, Advanced Materials (2014) 2943-2948. [15]Y. Xiang, F. Liu, L. Xue, Under seawater superoleophobic PVDF membrane inspired by polydopamine for efficient oil/seawater separation, J. Membr. Sci. 476 (2015) 321-329. [16]G.D. Kang, Y.M. Cao, H.Y. Zhao, Q. Yuan, Preparation and characterization of crosslinked poly(ethylene glycol) diacrylate membranes with excellent antifouling and solvent-resistant properties, J. Membr. Sci. 318 (2008) 227-232. [17]T. Yuan, J. Meng, T. Hao, Y. Zhang, M. Xu, Polysulfone membranes clicked with poly (ethylene glycol) of high density and uniformity for oil/water emulsion purification: Effects of tethered hydrogel microstructure, J. Membr. Sci. 470 (2014) 112-124. [18]H. Ju, B.D. McCloskey, A.C. Sagle, Y.-H. Wu, V.A. Kusuma, B.D. Freeman, Crosslinked poly(ethylene oxide) fouling resistant coating materials for oil/water separation, J. Membr. Sci. 307 (2008) 260-267. [19]Y. Chang, Y.-J. Shih, C.-Y. Ko, J.-F. Jhong, Y.-L. Liu, T.-C. Wei, Hemocompatibility of Poly(vinylidene fluoride) Membrane Grafted with Network-Like and Brush-Like Antifouling Layer Controlled via Plasma-Induced Surface PEGylation, Langmuir 27 (2011) 5445-5455. [20]A. Venault, Y. Chang, H.-H. Hsu, J.-F. Jhong, H.-S. Yang, T.-C. Wei, K.-L. Tung, A. Higuchi, J. Huang, Biofouling-resistance control of expanded poly(tetrafluoroethylene) membrane via atmospheric plasma-induced surface PEGylation, J. Membr. Sci. 439 (2013) 48-57. [21]P.Y. Zhang, H. Yang, Z.L. Xu, Y.M. Wei, J.L. Guo, D.G. Chen, Characterization and preparation of poly(vinylidene fluoride) (PVDF) microporous membranes with interconnected bicontinuous structures via non-solvent induced phase separation (NIPS), J. Polym. Res. 20 (2013) 1-13. [22]M.G. Buonomenna, P. Macchi, M. Davoli, E. Drioli, Poly(vinylidene fluoride) membranes by phase inversion: the role the casting and coagulation conditions play in their morphology, crystalline structure and properties, Eur. Polym. J. 43 (2007) 1557-1572. [23]L.P. Cheng, T.H. Young, L. Fang, J.J. Gau, Formation of particulate microporous poly(vinylidene fluoride) membranes by isothermal immersion precipitation from the 1-octanol dimethylformamide poly(vinylidene fluoride) system, Polymer 40 (1999) 2395-2403. [24]T.-H. Young, L.-P. Cheng, D.-J. Lin, L. Fane, W.-Y. Chuang, Mechanisms of PVDF membrane formation by immersion-precipitation in soft (1-octanol) and harsh (water) nonsolvents, Polymer 40 (1999) 5315-5323. [25]R.N. Wenzel, RESISTANCE OF SOLID SURFACES TO WETTING BY
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WATER, Industrial & Engineering Chemistry 28 (1936) 988-994. [26]J.P. Mazzoccoli, D.L. Feke, H. Baskaran, P.N. Pintauro, Mechanical and cell viability properties of crosslinked low- and high-molecular weight poly(ethylene glycol) diacrylate blends, Journal of Biomedical Materials Research Part A 93A (2010) 558-566.
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S0927-7757(15)00069-2 http://dx.doi.org/doi:10.1016/j.colsurfa.2015.01.041 COLSUA 19688
To appear in:
Colloids and Surfaces A: Physicochem. Eng. Aspects
Received date: Revised date: Accepted date:
9-12-2014 20-1-2015 21-1-2015
Please cite this article as: J.J. Tingmei Wang, Q. Wang, Superhydrophilic and underwater superoleophobic PVDF membranes via plasma-induced surface PEGDA for effective separation of oil-in-water emulsions, Colloids and Surfaces A: Physicochemical and Engineering Aspects (2015), http://dx.doi.org/10.1016/j.colsurfa.2015.01.041 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Graphical abstract
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Highlights The surface grafting of poly (vinylidene fluoride) (PVDF) membrane with poly (ethylene
glycol) diacrylate
(PEGDA) was
obtained
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plasma-induced graft polymerization.
via low-pressure
The PVDF-g-PEGDA membranes became superhydrophilic and underwater
as-prepared
PVDF-g-PEGDA
membranes can
effectively
separate
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The
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superoleophobic, and showed also excellent mechanical properties.
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oil-in-water emulsions.
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Superhydrophilic and underwater superoleophobic PVDF membranes via plasma-induced surface PEGDA for effective separation of oil-in-water emulsions Junping Jua,b Tingmei Wanga Qihua Wanga ※
State Key Laboratory of Solid Lubrication, Lanzhou Institute of Chemical Physics,
University of Chinese Academy of Sciences, Beijing 100039, P. R. China.
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b
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Chinese Academy of Sciences, Lanzhou 730000, P. R. China.
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a
Corresponding author: E-mail: [email protected]; Phone: +86 (0)931 4968252;
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Fax: t86 (0)931 4968252. Abstract
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This work describes the surface grafting of poly (vinylidene fluoride) (PVDF) membrane with poly (ethylene glycol) diacrylate (PEGDA) via low-pressure
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plasma-induced graft polymerization. The chemical composition and microstructure of the surface modified PEGylated PVDF membranes were characterized by fourier
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transform infrared spectroscopy (FTIR), X-rayphotoelectron spectroscopy (XPS), scanning lectron microscopy (SEM), atomic force microscopy (AFM) and contact angle measurements. By tuning polymerization conditions, a superhydrophilic and underwater superoleophobic PVDF-g-PEGDA membrane was successfully prepared. Results show that the as-prepared PVDF-g-PEGDA membranes can effectively separate oil-in-water emulsions with high separation efficiency and high fluxes under ultralow pressure. Importantly, the membrane showed excellent mechanical properties, which was also important parameter for practical application. Keywords: superhydrophilic; underwater superoleophobic; PVDF membranes;
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separation; oil-water emulsions. 1 Introduction Oil contamination of water is a significant global environmental problem that
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mankind currently faces, which puzzles the survival and development of human
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society. To date, a variety of techniques such as oil skimmers, magnetic separations,
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settling tanks, centrifuges and depth filters for oil/water separation have been
developed in the last few years [1-3], but broadly applicable approach for oil/water
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separation, especially for processing emulsified oil/water mixtures, are highly desired and still challenging. Filtration membranes are being actively explored as advanced
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technology for emulsified oil/water separation [4, 5]. As a microfiltration and ultrafiltration material, poly (vinylidene fluoride) (PVDF) has earned lots of interest
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due to its excellent thermal stability, chemical stability and radiation resistance [6-10]. Recently, polyvinylidene fluoride (PVDF) has been widely used for preparing
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ultrafiltration and microfiltration membranes to separate various types of oil/water mixtures. Jin and co-workers demonstrated a facile modified-phase inversion approach for fabricating superhydrophobic-superoleophilic PVDF membranes for effective separation of a wide range of water-in-oil emulsions with high flux [11]. However, oil removing membranes, such as superhydrophobic-superoleophilic materials, are easily fouled by oils because of their intrinsic oleophilic property. So considerable attentions have been focused on the underwater superoleophobic materials, which is inspired by the antiwetting behavior of oil droplets on fish scales and these membranes show high separation efficiency and low oil fouling. For
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example, a novel superhydrophilic and underwater superoleophobic zwitterionic polyelectrolyte grafted PVDF membrane was successfully attained via a SI-ATRP technique for oil/water separation [12]. The oil contents after one time separation of a
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selection of oil/water mixtures are all less than 10 ppm and some of them are even
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lower than 2 ppm. In addition, salt-induced fabrication of superhydrophilic and
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underwater superoleophobic PAA-g-PVDF membrane for effective separation of oil-in-water emulsions had been prepared [13]. At the same time, liu and co-workers
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provided a novel superamphiphilic PVDF membrane with multi-scale surface structure, which shows under-water ultralow adhesive superoleophobicity and
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under-oil low adhesive superhydrophobicity in oil/water/solid three phase systems and made it a promising candidate for oil/water emulsion separation [14]. Their group
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also reported a methodology to fabricate mussel-inspired PVDF membrane via solution-immersion for oil/seawater emulsion separation [15]. The hydrophilicity and
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superoleophobicity under seawater membrane was achieved when immersed in dopamine solution. However, a facile and inexpensive method towards designing superwetting materials for effective separation is indeed needed. Polyethylene glycol diacrylate (PEGDA, CH2=CHCO(OCH2CH2)nOCOCH=CH2)
is a derivative of PEG with repeated ethylene oxide (EO) units and active end groups [16]. It is considered as one of the most promising candidates for preparation of antifouling surface due to their high hydration capacity and excellent resistance to non-specific protein and other macromolecule adhesion. A plenty of studies have been reported about polyethylene glycol diacrylate (PEGDA), as functional material for
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oil/water separation [17, 18]. Herein, we reported a simple and effective method to prepare superhydrophilic and underwater superoleophobic PVDF membrane by using polyethylene glycol diacrylate. Firstly, we fabricated PVDF microfiltration
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membranes by non-solvent induced phase separation. Then PVDF microfiltration
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membranes grafted with hydrophilic poly (ethylene glycol) diacrylate (PEGDA) via
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low-pressure plasma-induced surface copolymerization were studied [19, 20]. Plasma induced grafting has become a major interest of research to modify the membrane
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surfaces by fast, safe, and low cost ways. The results showed that PVDF-g-PEGDA membranes exhibited superhydrophilic and underwater superoleophobic PVDF
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character that can effectively separate oil-in-water emulsions with high separation efficiency and high fluxes under ultralow pressure. Importantly, the membrane
application.
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showed good mechanical properties, which was also important parameter for practical
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2 Experimental section 2.1 Materials
PVDF powders (Mn= 238 000) and poly (ethylene glycol) diacrylate (PEGDA)
were purchased from Sigma-Aldrich. N, N-dimethylacetamide (DMAc, 99.7%), methanol, ethanol and dichloromethane were used as received. Tween 80 was yellow and purchased from Tianjin Chemical Reagents Company. Deionized water was prepared by our own lab. colza oil, lubricating oil, soybean oil were used as received.
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Fig.1.Schematic illustration of the preparation process of the PVDF-g-PEGDA membranes via low-pressure plasma-induced surface copolymerization.
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2.2 PVDF microfiltration membrane preparation
Membranes were prepared via the non-solvent induced phase separation [21].
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PVDF (18%), Tween80 (3%) and water (3%) were dissolved in the DMAc by vigorous stirring until a clear homogeneous solution was obtained. The solution was
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cast on glass plates with a steel knife and then immediately immersed in a coagulation bath of ethanol at room temperature for 5 min. The obtained membranes were kept in
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deionized water for 24 h and then dried for at least 24 h in a vacuum oven before use. 2.3 Surface PEGDA copolymerization of PVDF membrane As illustrated in Figure 1, the plasma-induced surface copolymerization of PEGDA
was performed using a low-pressure plasma source with aradio frequency of 13.56 MHz. The as-prepared membrane first was immersed into methanol solution containing different concentrations PEGDA (from 10% to 30%) for 1h. Afterward, the PEGDA-coated membrane was dried at room temperature for 2 h. Subsequently, the membranes were treated by a low pressure plasma source. Then the modified PVDF membranes were immersed in methanol and enthanol for 60 min using an
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ultrasonic device to remove any trace of adhering homopolymer to the membranes surface. Then membranes were dried at room temperature under vacuum. 2.4 Emulsion separation experiments
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Surfactant-free oil-in-water emulsions were prepared by mixing three types of oil:
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colza oil, lubricating oil and soybean oil. 10% (w/w) oil was stirred with water for 2 h.
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The PVDF membranes were placed between one vertical glass tube with a diameter of 40 mm and one conical flask. Oil-water emulsion separation experiment was achieved
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driven under low pressure. The flux was calculated on the permeated volume of an emulsion through the membrane. . Oil content in permeates was tested by gravimetric
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analysis [9]. A sample taken from permeate solutions was heated to 80 °C and maintained isothermally until all moisture evaporated. Oil content was calculated
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from permeate mass before and after heating. Both the feed and permeate solutions were examined to obtain oil rejection ratio
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using the following equation:
R(%)
C feed C filtrate 100 C feed
Where Cfeed and Cfiltrate are oil concentrations of feed and permeate solutions,
respectively, R is the separation efficiency. 2.5 Instruments and characterization Attenuated total reflectance Fourier transform infrared spectroscopy (FTIR-ATR) was measured by BrukerIFS 66v/s IR spectrophotometer (Bruker optics, Germany). FE-SEM images were obtained with a field emission scanning electron microscope (JSM-6701F). Scanning electron microscopy (SEM) was performed on a JSM-5600
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(JEOL, Japan) operated at 20 kV, in order to obtain the cross sectional structure. All samples were coated with gold by sputtering prior to observation. AFM study was tested at a Nanoscope IIIa multimode atomic force microscope (AFM, Digital
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Instruments) at the tapping mode. The chemical composition of the prepared
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membrane was determined by X-ray photoelectron spectroscopy (XPS), which was
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conducted on a PHI-5702 electron spectrometer (Perkin-Elmer, USA) using an AlKa line excitation source. The mechanical property of PVDF membranes were evaluated
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at room temperature by tensile tests performed on a ShimadzuAG-X with a strain rate of 2 mm/min. Samples were cut to standard dimensions according to ISO 527-2/1BB.
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Water contact angles of membrane surfaces were measured at ambient temperature on a DSA100machine (Krüss, Germany) and at least five measurements were taken at
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different positions on each sample. For underwater oil contact angles, the membrane was placed in a transparent and cubic quartzose vessel filled with ultrapure water first.
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Then oil droplet (1, 2-dichloroethane about 5μL) was directly placed onto the
membrane surfaces and keep 1 min to attain underwater oil contact angle. Mercury intrusion porosimetry was performed on a Micromeritics Autopore 9500 apparatus. 3 Results and discussion
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Fig.2. FTIR spectra of the raw PVDF membrane (A) and PVDF-g-PEGDA
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membranes via plasma treatment monomer concentration of 10%, 20%, 30% (B, C, D).
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A schematic illustration of the plasma-induced surface copolymerization of PEGDA on PVDF membranes is shown in Fig.1. The PEGDA-grafted layer on PVDF
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membranes can be first regulated by the amount of uniformly coated PEGMA monomer and then followed by low-pressure plasma-induced surface copolymerization. The evidence of PEGDA polymer grafting onto the surface of PVDF membranes were confirmed by comparing the FTIR spectra. Fig.2 shows the FTIR spectra of virginal and plasma treated PVDF membranes. It can be seen from Fig.2A that the raw PVDF membrane shows three typical strong absorption peaks at 1400, 1180 and 875 cm−1 due to -CH2-, -CF2- and C-C groups, respectively. Compared
with the raw PVDF membranes, the PEGDA polymer grafted PVDF membranes (Fig.2B-D) exhibit a new characteristic peaks near 1727 cm−1, indicating the presence
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of O-C=O from PEGDA. In addition, it can be found that both the intensity of the O-C=O adsorption at and the [O-C=O]/[C-F] ratio increased obviously as the plasma treatment monomer concentration increased from 10% to 30%. The result indicates
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that the growth of the grafted PEGDA polymer is dependent on the increasing plasma
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concentration. The details of the changes in surface chemical structure of the
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membrane were further ascertained by XPS study. Fig.3 shows the XPS survey
spectra of raw PVDF and PVDF-g-PEGDA membranes. The strong peaks of C1s, F1s
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and a small peak of O1s are observed in the wide scan spectrum of the virginal PVDF membrane. After plasma treatments, the intensity of the O1s peaks (Fig.3B-D)
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increased significantly, indicating a higher concentration of oxygen introduced on the surface of membrane. Nevertheless, the F1s peak of PVDF-g-PEGDA membranes
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decreases greatly and last disappears with increasing plasma treatment monomer concentrations. These results indicate that the PEGDA have been grafted onto the
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surface of PVDF membrane, which is in good agreement with the results of FTIR-ATR.
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Fig.3. XPS survey spectra of the raw PVDF membrane (A) and PVDF-g-PEGDA
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membranes via plasma treatment monomer concentration of 10%, 20%, 30% (B, C, D).
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Table1 Physicochemical characteristics of raw PVDF and PVDF-g-PEGDA membranes. Membrane
Vrigin PVDF PVDF-g-PEGDA-10 PVDF-g-PEGDA-20 PVDF-g-PEGDA-30
PEGDA concentrations (%) 0 10 20 30
Plasma treatment time (s) 0 60 60 60
Grafting yield (mg/cm2) 0 0.61 1.97 3.06
Tensile strength (MPa) 0.47 1.01 2.40 3.70
The grafting yield is calculated by (W2-W1)/A. Where W 2 and W1 are the weights of the dried membrane before and after PEGDA grafted, respectively, and A is the area of the membrane. As shown in Table1, the grafting yield of grafted PEGDA layers on the surface of PVDF membranes increased as the plasma treatment
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monomer concentration increased from 10 % to 30 %.
Fig.4. Top surface structure (T), bottom surface structure (B) and cross sectional
structure (C) images of the original PVDF membrane (M-0) and the modified PVDF with grafted PEGDA (M-10, 10%).
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Fig.5 Top surface structure (T), bottom surface structure (B) and cross sectional structure (C) images the modified PVDF membranes with grafted PEGDA (M-20, 20%, M-30, 30%)
Fig.4 and Fig.5 give surface and cross section SEM images of raw PVDF and
PVDF-g-PEGDA membrane. All the membranes show the classic structure of the polymer microfiltration membrane prepared by non-solvent induced phase separation [22, 23], which were induced by the delayed demixing in congulant containing pure ethanol. The morphology surface of the membrane is obviously porous, with interconnected holes in the networks constructed by PVDF small globules connecting with each other. The cross sectional exhibits a uniform
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microporous structure composed of spherical particles of approximately the same size. Such structure affirms the domination of crystallization during the precipitation process, where in all crystalline particles were nucleated and grown in a similar
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concentration field and finally fused together to form a bi-continuous structure [24].
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The surface coverage of the PVDF-g-PEGDA membrane revealed an obvious
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change as the plasma treatment monomer concentration increased from 10% to 30%. When the surface plasma treatment monomer concentration is 30%, the porous
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structure on the membrane surface was almost covered with the grafted PEGDA layer. AFM analysis was performed to determine the roughness changes caused by
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grafting PEGDA polymer. Fig.S1 shows three dimensional AFM images and Ra values of the raw PVDF and PVDF-g-PEGDA membranes. The Ra roughness of
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membranes was studied by the tapping mode. As shown in Fig.S1, both raw PVDF and PVDF-g-PEGDA membrane have very high and much the same RMS roughness
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value, indicting the surface of all the membrane is rather rough. The RMS roughness did not distinctly vary with the different PEGDA concentrations. In our case, we think that the RMS roughness of the surface was controlled by the casting procedure, so the grafting PEGDA polymer has minor influence on the global shape of the porous structure.
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Fig.6. Variation of the water contact angle in air and underwater oil contact angle of
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the membrane with increasing the PEGDA concentration.
Fig.7. Water contact angle and underwater oil contact angle of the PVDF-g-PEGDA-20 membrane heated at different temperature.
After plasma treatment, the hydrophilicity changes for both treated and untreated
membranes were measured. As shown in Fig.6, the raw PVDF membrane shows a hydrophobic property with a contact angle of 110. After plasmas treatment, the contact angles of the PVDF-g-PEGDA membranes decreased from about 110°to 0
°instantly. From the Wenzel model [25], By introducing the hydrophilic PEGDA into porous rough surface induced by phase separation，a superhydrophilic surface is
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achieved. In order to test the performances of the membranes in oil/water separation, underwater oil wettability of membrane was further measured. Fig.6 also shows the
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underwater oil contact angle of the membrane. The raw PVDF membrane shows a
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hydrophobic property with a contact angle of 110°and underwater superoleophilic.
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After PEGDA polymer grafting, all the PVDF-g-PEGDA membranes become
superhydrophilic and underwater superoleophobic, as shown in Fig.6. It is known to
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all that the wettability of a surface is mainly governed by its chemical composition and roughness. The particular wettability achieved in oil/water/solid three-phase
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system is mainly attributed to the hierarchical surface structure combined with superhydrophilic of the membrane. When PVDF-g-PEGDA membranes are
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immersed into water, the hydrophilic PEGDA are hydrated to transform an extended conformation. Then water can be readily trapped in the rough micro/nanostructures
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to form an oil/water/solid three-phase interface. And these trapped water molecules will greatly decrease the contact area between oil and membrane surface, resulting in a large oil contact angle in water, which is a fluoride-free strategy. The good mechanical property of the membrane is important parameter for
practical application. The tensile strength of raw PVDF membrane and PVDF membranes with a different grafted PEGDA layer are also shown in Table1. The raw PVDF membrane is brittle and exhibits small tensile strength. By comparison, the improvements in tensile strength of PVDF-g-PEGDA membrane are also clearly visible. Stress strength for PVDF-g-PEGDA membranes steadily increase from 1.0
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MPa to 3.8 MPa. The first reason is that PEGDA itself has desirable and intrinsic good mechanical properties, which can alter the processing conditions during polymerization or combine various polymers to achieve a desired mechanical
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characteristic [26]. The second reason may be that defect becomes small after a
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grafted PEGDA layer on the PVDF membrane surface using the plasma technique.
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PVDF-g-PEGDA-10 has the lowest stress strength and it is prone to break up during filter process. Although PVDF-g-PEGDA-30 has the highest stress strength, it has the
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lowest flux due to the fact that most large pores become blocked. So the optimal as-prepared membrane is PVDF-g-PEGDA-20 to study oil-water mixtures separation
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property. Thermal stability of the membrane is one of important factors for practical application. The optimal as-prepared membranes (PVDF-g-PEGDA-20) were heated
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at different temperature for 1 h. As shown in Fig.7, the membrane maintains still stable underwater superoleophobicity by increasing the drying temperature.
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Video S1 (Supplementary) shows a very simple experiment to study underwater
oil-adhesion behavior. Where a 5 μL oil droplet was dropped onto the membrane
surface, an additional force was applied on the oil droplet to make it in sufficient contact with the membrane surface and then allowed to relax. We can see that oil droplets easily roll off from membrane surface. This result indicates that the membrane has the low oil-adhesion characteristics in oil/water/solid system and is important for achieving anti-fouling performance of the membrane.
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Fig.8. Pore size distribution and porosity of PVDF-g-PEGDA-20 membrane.
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To determine the effective separation size of PVDF-g-PEGDA-20 membrane
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membrane, pore size distribution of the membrane was also measured. Fig.8 presents the pore size distribution of PVDF-g-PEGDA-20 membranes. The
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PVDF-g-PEGDA-20 membrane has a mean pore size of 1.33 μm and the porosity of
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57%. The porosity, mean pore size and pore size distribution are very important
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parameters to determine the separate performance of membranes. Surfactant-free oil/water emulsions are typically made of much larger oil droplets ( >20 μm), so most pore in the membrane is expected to block oil droplets.
Fig.9. Photograph of the lubricating oil-water emulsion (10% (w/w)) separation process from PVDF-g-PEGDA-20 membrane.
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Table2 Flux and separation efficiency of oil/water emulsions through PVDF-g-PEGDA-20 membrane. Density
Viscosity
Flux
Separation
(g/mL)
(mPa.s)
(L m-2 h-1)
Efficiency (%)
0.919
7.11
2464.3
99
lubricating oil
0.88
22
3773.7
97
soybean oil
0.9375
14
2414.5
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The as-prepared emulsions were then poured onto PVDF-g-PEGDA-20 membranes to carry out filtration separation under a pressure difference of 5 kPa as shown in
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Fig.9 to give a photograph of the lubricating oil-water emulsion. All dispersed oil-water mixtures can be successfully separated in one time. The results indicate that
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the PVDF-g-PEGDA-20 membranes can effectively block the oil drops with high
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water permeation flux and oil rejection. As shown in Table2, the water permeation
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flux are 2464.3 L/m2 h, 3773.7 L/m2 h and 2414.5 L/m2 h, respectively, for those oil-in water emulsions containing colza oil, lubricating oil, soybean oil, and shows high separation efficiency. After each filtration of the feed emulsion, the membranes can be washed with water and ethanol to recover the flux. 4 Conclusion
A superhydrophilic and underwater superoleophobic PVDF-g-PEGDA membrane has been fabricated by phase inversion method and low-pressure plasma-induced surface copolymerization. The as-prepared membranes can effectively separate oil-in-water emulsions with high separation efficiency under ultralow pressure and much higher
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fluxes. These results indicate that the PVDF-g-PEGDDA membrane promising for practical applications for treating wastewater produced in industry and daily life. Acknowledgments
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The authors would like to acknowledge the financial supportof the National Basic
national young scientist foundation of china (51403219).
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References
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Research Program of China (973 Program, Grant No. 2015CB057502) and the
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