Journal of Colloid and Interface Science 359 (2011) 380–388

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Preparation of a durable superhydrophobic membrane by electrospinning poly(vinylidene fluoride) (PVDF) mixed with epoxy–siloxane modified SiO2

nanoparticles: A possible route to superhydrophobic surfaces with low watersliding angle and high water contact angle

Shuai Wang a, Yapeng Li a, Xiaoliang Fei a, Mingda Sun a, Chaoqun Zhang a, Yaoxian Li a, Qingbiao Yang a,⇑,Xia Hong b,⇑a Department of Chemistry, Jilin University, Changchun 130021, People’s Republic of Chinab Center for Advanced Optoelectronic Functional Materials Research, Northeast Normal University, Changchun 130024, People’s Republic of China

a r t i c l e i n f o a b s t r a c t

Article history:Received 9 January 2011Accepted 1 April 2011Available online 14 April 2011

Keywords:SuperhydrophobicElectrospinningPoly (vinylidene fluoride) (PVDF)Epoxy–siloxane modified SiO2 nanoparticlesDurable

0021-9797/$ - see front matter � 2011 Elsevier Inc. Adoi:10.1016/j.jcis.2011.04.004

⇑ Corresponding authors. Fax: +86 431 88499576 (E-mail address: yangqb@jlu.edu.cn (Q. Yang).

A durable superhydrophobic surface with low water sliding angle (SA) and high water contact angle (CA)was obtained by electrospinning poly (vinylidene fluoride) (PVDF) which was mixed with epoxy–siloxanemodified SiO2 nanoparticles. To increase the roughness, modified SiO2 nanoparticles were introduced intoPVDF precursor solution. Then in the electrospinning process, nano-sized SiO2 particles irregularlyinlayed (it could also be regard as self-assembly) in the surface of the micro-sized PVDF mini-islandsso as to form a dual-scale structure. This structure was responsible for the superhydrophobicity andself-cleaning property. In addition, epoxy–siloxane copolymer was used to modify the surface of SiO2

nanoparticles so that the SiO2 nanoparticles could stick to the surface of the micro-sized PVDF mini-islands. Through the underwater immersion test, the SiO2 nanoparticles cannot be separated from PVDFeasily so as to achieve the effect of durability. We chiefly explore the surface wettability and the relation-ship between the mass ratio of modified SiO2 nanoparticles/PVDF and the CA, SA of electrospun mat. Asthe content of modified SiO2 nanoparticles increased, the value of CA increased, ranging from 145.6� to161.2�, and the water SA decreased to 2.17�, apparently indicating that the membrane we fabricatedhas a perfect effect of superhydrophobicity.

� 2011 Elsevier Inc. All rights reserved.

1. Introduction

Wettability is one of the important properties of a solid surface.A direct expression of the wettability of a surface is the contact an-gle (CA) of a water droplet on the surface. Surfaces with a watercontact angle (CA) larger than 150� and a water sliding angle(SA) lower than 10� are generally considered to be superhydropho-bic surfaces. The phenomenon of superhydrophobicity appearsubiquitously in nature, such as the leaf surfaces of many plants[1–3] and the legs of water striders [4]. These surfaces have at-tracted great attention in recent years due to their special proper-ties such as anti-contamination, self-cleaning and nonstick, whichare widely applied to industry applications and daily life includingself-cleaning coating for automobiles [5], anti-snow/anti-icingwindows, nonstick coating for antennas and power line [6].

ll rights reserved.

Q. Yang).

It was believed that the microscale rough structure on the sur-face of a lotus leaf is the key to the water repellency and the self-cleaning properties [7] until Jiang’s group reported a novel findingof micro- and nanoscale hierarchical structures on the lotus leaf[8]. Their research revealed that the hierarchical structure withthe combination of micro- and nanoscale is the fundamental factorwhich made the surface extremely superhydrophobic. They sug-gested that the nanostructures which were found on the top ofthe micropapillae would effectively prevent the attachment ofwater droplets so that the surface of the leaf cannot be wetted.

Since nature has developed very efficient superhydrophobicsystems, it is useful to learn from and mimic its design. Varioussurfaces of the special multi-scale micro/nanosurface structureswhich mimic ‘‘lotus effect’’ have been designed and synthesizedfor their predominant applications [9–11]. These researches clearlyreveal that surfaces with hierarchical roughness have higher waterrepellency property than the corresponding surfaces with only mi-cro-scale or nano-scale of roughness. It has been suggested that thehierarchical roughness promotes the trapping of air between thewater droplet and surface, which helps to prevent water from

S. Wang et al. / Journal of Colloid and Interface Science 359 (2011) 380–388 381

penetrating the surface [12]. It also increases the tortuosity of thethree-phase contact line and reduces the pinning effects [13,14],which are important to influence the water repellency and theself-cleaning properties.

In recent years, a number of different techniques, includingelectrochemical deposition [15–17], chemical vapor deposition[18], plasma etching [19], phase separation [20], sol–gel methods[21], electrospinning [22–24], assembly [25], array of nanotubes/nanofibers [26,27], and solution–immersion methods [28], havebeen used to make superhydrophobic surfaces. These methodsare mainly based on two ideas: one is making a rough surface froma low surface energy material, the other is modifying a rough sur-face with a material of low surface energy.

Poly (vinylidene fluoride) (PVDF) as a new unusual materialwith extremely low surface energy has received great attentionrecently. PVDF which exhibits flexibility and thermal stability, isa semi-crystalline polymer including an amorphous phase and acrystalline phase. Besides, PVDF is an advanced material formembrane application due to its unique advantages such as goodprocessability, excellent mechanical property, and exceptionalchemical stability. Hence, PVDF membrane is widely used in struc-tural material coating applications, such as the surface materials ofvehicle, boots, raincoat, and the cover of outdoor airconditioner[29,30]. In the future, with the coating layer with self-cleaningproperty, the PVDF membranes will be able to be fabricated, thedusts on which will be easily washed away by rain, thus saving agreat deal of manpower and cost for cleaning services [31–37].

SiO2 nanoparticles play an important role in making a roughand hierarchical structure surface. A large number of superhydro-phobic surfaces have been fabricated by processing various sizesof the silica nanoparticles for years [10,38–42]. Nevertheless, mostof the preparations must have strict conditions (such as harshchemical treatment), expensive materials, and complex processingmethods including plasma etching, chemical vapor deposition,electrodeposition, calcination, and the use of templates. Therefore,a simple and easy method without high cost problem and the lim-itation in large-scale superhydrophobic surfaces production shouldbe widely used.

Electrospinning as a simple and versatile method has beenestablished for producing continuous fibers with diameters rang-ing from nanometer to submicron scale. Recently, superhydropho-bic surfaces can be fabricated through this technique by controllingthe surface roughness under appropriate conditions [43–45]. Agreat number of polymers and inorganic/polymer composites havebeen explored with the development of electrospinning technology[46–48] to exhibit excellent applications in various fields. Thestructures and morphologies of electrospun mats can be effectivelycontrolled by plenty of factors including polymer type, concentra-tion of solutions, environment parameters and electrospinningconditions. Electrospinning is a predominant and promising tech-nique to develop superhydrophobic surfaces from some polymerswith low surface energies. In recent years, several types of super-hydrophobic surfaces, produced by electrospinning PVDF, havebeen reported in the literature [31–33]. Their main task was pro-ducing fiber materials with much higher CA. However, the SAwhich is of the same importance was seldom mentioned. Besides,a durable superhydrophobic surface which could not be easily de-stroyed when water droplet is moving on it frequently, must catchour attention. Therefore, it will be a tough task to obtain a kind ofdurable self-cleaning membrane with low SA and high CA byelectrospinning.

In this research, a durable superhydrophobic membrane withhigh CA and low SA was prepared facilely through the combinationof electrospinning and SiO2 nanoparticles self-assembly withoutcoating any of the low surface energy substance (such as fluori-nated silane) on the membrane surface. In our method, the SiO2

nanoparticles modified by epoxy–siloxane are dispersed in PVDFprecursor solution, and then in the electrospinning process, theepoxy–siloxane modified SiO2 nanoparticles irregularly inlayed inthe substrate (PVDF) surface to generate dual-scale roughness.The size and mass of the modified SiO2 nanoparticles were wellcontrolled. To fully utilize the water-repellent property of the hier-archical structured surfaces, we explored the relationship betweenthe size of micro- and nanoscale structures and the wettability bytheoretical analysis, and investigated in detail the mass ratio effecton the morphology and superhydrophobicity as well as the dura-bility of the surfaces.

2. Materials and methods

2.1. Materials

Poly(vinylidene fluoride) (PVDF, Mw = 400,000–600,000) waspurchased from Shanghai Ofluorine Chemical Technology Co., Ltd.4-Vinyl-1-Cyclohexene 1,2-Epoxide (VCHO) was purchased fromGaomi City Zhongyuan Technological Chemical Co., Ltd. KH-602(N-(b-aminoethyl)-c-aminopropyl Methyldimethoxysilan) waspurchased from Hangzhou Feidian Chemical Co., Ltd. Octa-methylcyclotetrasiloxane (D4), polymethylhydrogensiloxane(PMHS, Mw = 2000), and hexamethyldisiloxane (HMDS) were pur-chased from CNPC Jilin Chemical Industry Co., Ltd. and used withoutfurther purification. Tetraethyl orthosilicate (TEOS), ammoniumhydroxide (25 wt.%), dimethylformamide (DMF), ethanol, toluene,dichloromethane, isopropanol and hydrogen hexachloroplatinate(IV) hydrate (CPA) were from Beijing Chemicals Co. China.

2.2. Epoxy–siloxane modified SiO2 nanoparticles and PVDF precursorsolution preparation

The SiO2 nanoparticles with diameters of about 400 nm weresynthesized using a modified Stöber method [49]. Briefly, 1 mL ofTEOS was added dropwise, under magnetic stirring, to a flask con-taining 3 mL of ammonium hydroxide (25 wt.%) and 30 mL of eth-anol. The reaction was carried out at 30 �C for 6 h, followed byadditional 0.5 mL of KH-602 in 5 mL of ethanol. The stirring wascontinued for 12 h under N2 atmosphere at 60 �C. The nanoparti-cles were separated by centrifugation and the supernatant was dis-carded. The nanoparticles were then washed by ethanol for threetimes. The white powder was vacuum-dried at 30 �C for 12 h.The existence of amine groups on the surface of SiO2 nanoparticleswas examined by the ninhydrin test [50,51]. The amino-function-alized SiO2 particles were added into 5 wt.% ninhydrin aqueoussolution at room temperature. The color of the particles turnedblue within a few minutes, indicating the successful grafting ofamine moieties on the surface of SiO2 particle.

Amino-functionalized SiO2 nanoparticles (2.5 g) were sus-pended in 20 mL of ethanol, and dispersed by ultrasonic for5 min. Afterwards, the epoxy-functionalized PDMS (3.0 g, synthe-sized using an existing successful method [52]) dissolved in a mix-ture of 7.5 mL toluene and 2.5 mL CH2Cl2, and was added dropwiseat 50 �C, under vigorous stirring, into the amino-functionalizedSiO2 nanoparticles suspension. The reaction was refluenced for12 h under N2 atmosphere at 80 �C. The modified SiO2 particleswere then separated by centrifugation and washed with ethanolfor three times. The white powder was vacuum-dried at 40 �C for12 h.

PVDF precursor solutions with the concentration of 20 wt.%were prepared by dissolving certain amount of PVDF to DMF firstly.The PVDF solutions were stirred for 12 h at room temperature to at-tain sufficient viscosity required for the further experiments. Thenseveral groups of precursor solutions with different mass ratios of

Fig. 1. 1H NMR spectrum of the SV copolymer (epoxy–siloxane copolymer).

382 S. Wang et al. / Journal of Colloid and Interface Science 359 (2011) 380–388

modified SiO2 nanoparticles/PVDF (1.00:1, 1.25:1, 1.67:1, 2.50:1,5.00:1) were prepared respectively. The precursor solutions werestirred drastically for 12 h at room temperature so that theepoxy–siloxane modified SiO2 nanoparticles were dispersed evenlyin PVDF solution for electrospinning.

2.3. Superhydrophobic membrane preparation by electrospinning

The electrospinning needle was connected to a high voltagesupply which can generate positive DC voltages of up to 50 kV. Apositive voltage of 15 kV was applied across a fixed collection dis-tance of 14 cm between the tip of the needle and the groundedelectrode. The feed rate of the precursor solution was controlledby a syringe pump. During electrospinning, the applied electricfield overcomes the surface tension of the polymeric solution,thereby ejecting a continuous jet, which produces nanofibers (orbeads) on the collector surface upon subsequent solvent evapora-tion and bending.

2.4. Characterization

1H NMR spectra was performed on a 300 MHz Bruker NMRspectrometer using CDCl3 as solvent and tetramethylsilane asinternal standard. The composition of epoxy–siloxane and modi-fied SiO2 nanoparticles were determined by Fourier transforminfrared (FT-IR) spectra. A KBr method was applied to the FT-IRmeasurement. An FEI XL30 scanning electron microscope was usedto observe the morphologies of electrospun surfaces. The contactangles of the electrospun mats were measured with a drop shapeanalysis system (Krüss DSA100) in the sessile mode at roomtemperature.

3. Results and discussion

The epoxy–siloxane copolymer (SV copolymer) which containsepoxide groups on the side-chain was synthesized from the hydro-silylation reaction of poly (methylhydrosiloxane) copolymer (SH)and 4-Vinyl-1-Cyclohexene 1,2-Epoxide (VCHO) as shown inScheme 1. The SH copolymer was obtained by the acid-catalyzedequilibration of D4 and PMHS with hexamethyldisiloxane as anend-capper (Eq. 1). The SV copolymer was then hydrosilated withVCHO in the presence of a Pt-catalyst to gain the epoxy–siloxanecopolymer (Eq. 2). The 1H NMR spectrum of the SV copolymer(Fig. 1) shows the Si–H strong, characteristic peak for Si–Me at d0.07 ppm which is split into two peaks (d 1.22 and d 1.57) due to

Scheme 1. Reaction to produce SH copolymer (Eq. (1)

the adjacent Si–Me atom. It also shows the complete disappear-ance of the Si–H resonance (d = 4.7 ppm) and the concomitant dis-appearance of the vinyl group in VCHO (d = 5.5 and 6.0 ppm). Inaddition, the peaks at (d = 0.5 and 3.7) ppm are the resonancepeaks of the silypropyl group, while those at (d = 3.1) ppm repre-sent the resonance peaks of the epoxide group. These results obvi-ously show that the hydrosilyation reaction was successful.

We followed the Stöber method to prepare amine-functional-ized SiO2 particles of about 400 nm. The epoxy–siloxane (SVcopolymer) synthesized previously was then covalently graftedonto the amine-functionalized SiO2 particles via the reaction be-tween epoxy and amine groups, leading to core–shell structure.The flow chart to produce epoxy–siloxane modified SiO2 nanopar-ticles is showed in Scheme 2. The FTIR spectra of SV copolymer isshown in Fig. 2b. The peaks of 1000–1130, 1260 and 2960 cm�1

correspond to asymmetric stretching vibration of Si–O–Si, sym-metric bending vibration of Si–CH3 and symmetric stretchingvibration of C–H, respectively. There are CH2 stretching bands at2850 and 2925 cm�1, which indicate that VCHO is bonded to theSi–H of SH copolymer. It is also proved by the absorption band ofC–O–C at 1100 cm�1, which is covered up by characteristic absorp-tion peak of Si–O–Si, and 910 cm�1. From the FTIR spectra ofepoxy–siloxane modified SiO2 nanoparticles in Fig. 2a, we can seethat the asymmetric stretching vibration near 810 cm�1 and bend-ing vibration near 470 cm�1 of Si–O–Si. The peaks of 1100 and1390 cm�1 correspond to stretching vibration of Si–OH and asym-metric stretching vibration of C–H, respectively. Significantly, FTIR

) and reaction to produce SV copolymer (Eq. (2)).

Scheme 2. The flow chart to produce epoxy–siloxane modified SiO2 nanoparticles.

Fig. 2. FTIR spectra of (a) epoxy–siloxane modified SiO2 nanoparticles and (b) SVcopolymer.

S. Wang et al. / Journal of Colloid and Interface Science 359 (2011) 380–388 383

analyses of SiO2 nanoparticles after epoxy-functionalized PDMSmodification showed a disappearance in the intensity of the peakat 910 cm�1 (characteristic absorption peak of C–O–C), indicatingthat epoxy-functionalized PDMS had been successfully grafted onthe surface of SiO2 spheres by the reaction between epoxy andamine groups.

By using the Stöber method, it is very convenient to manipulatethe particle size while maintaining a narrow size distribution,which enables us to readily gain the size we need. Fig. 3 showsSEM images of monodisperse SiO2 nanoparticles and SiO2 nanopar-ticles modified by epoxy–siloxane. They are both spherical, anduniform in size. With the comparison of the Fig. 3b and d, aftermodification by epoxy–siloxane, the size of particles had signifi-cant changes. Besides, a large number of SiO2 nanoparticles modi-fied by epoxy–siloxane stick together because the epoxy–siloxanewith a high viscosity was successfully grafted on the surface of theSiO2 nanoparticles.

Fig. 4a shows typical SEM pictures of beads with morphologyproduced from a 20 wt.% PVDF/DMF solution in this work. Thebead diameter ranges from 1 lm to 4 lm. A huge number of beadswhich stuck together closely like some mini-islands were observed

as demonstrated in Fig. 4a. The water contact angle of this electro-spun mat is as high as 145.6� (Fig. 4b), indicating that the PVDFsurface is hydrophobic. However, the contact angle is less than150� so that it could not be considered a superhydrophobic surface.

The wettability, usually determined by measuring the apparentcontact angle (CA) of a water droplet on a solid surface, is governedby both the chemical composition and the topographic structure ofthe surface [53]. In our work, PVDF is chemically hydrophobic dueto its low surface energy. However, according to the previousstudy, the maximum contact angle is only 120�, which can bereached by the introduction of –CF3 onto the flat solid surface.So, the second factor, the surface structure of electrospun matplays a leading role in determining the superhydrophobicity.Though the surface roughness of membranes could be easily fabri-cated and increased by electrospinning due to the widely dispersedbeads in the size of several micrometers, the PVDF beads we fabri-cated is so big that the water droplet come into contact with a rel-evant large area of the electrospun mat. Therefore, the air is noteasy to be trapped in the apertures as its surface is not rough en-ough. It is suggested that the air trapped in the surface is veryimportant to improve the hydrophobicity, because the water CAof air is considered to be 180� [54].

Confronted with this situation, we actively make some changesin our designs to mimic ‘‘lotus effect’’. It is known that superhy-drophobicity of the lotus leaf is due to the surface roughnesscaused by branchlike nanostructures on the top of the micropapil-lae and the low surface energy epicuticular wax [8]. To preparesuperhydrophobic PVDF membranes, we added the epoxy–silox-ane modified SiO2 nanoparticles synthesized in advance into PVDFprecursor solutions. Nano-sized SiO2 particles were introducedinto the micro-sized PVDF membrane by the technique of electros-pinning. Accordingly, many nano-sized SiO2 particles were inlayedon the surface of the micro-sized PVDF mini-islands and formed adual-scale structure.

Composite films comprised of epoxy–siloxane modified SiO2

nanoparticles and PVDF were prepared, and the mass ratios ofnano-sized SiO2 particles/PVDF varied from 1.00:1 to 5.00:1. Tofind out the effect of the surface structure on the film wettability,we use some SEM pictures (Fig. 5) to describe the film morpholo-gies with different mass ratios of modified SiO2 nanoparticles/PVDF. The introduced nanoparticles with diameter of 500 nm suc-cessfully play the role of building a dual-scale surface roughness. It

Fig. 3. Scanning electron microscope images (SEM) of monodisperse SiO2 nanoparticles of 400 nm (a and b) and SiO2 nanoparticles modified by epoxy–siloxane of 500 nm (cand d) in diameter, respectively.

384 S. Wang et al. / Journal of Colloid and Interface Science 359 (2011) 380–388

can be found that with the increased weight ratio of modified SiO2

nanoparticles, the CA value of PVDF surfaces tend to ascend asshown in Fig. 6, because nano-sized SiO2 nanoparticles are favor-able to increase the roughness and subsequently to form superhy-drophobic surface. It is also revealed that the more epoxy–siloxanemodified SiO2 nanoparticles there are, the more easily the surfacewill not to be wetted. When the weight of modified SiO2 nanopar-ticles is the same as PVDF, affording a CA value of 147.8� < 150� asshowed in Fig. 6, the membrane cannot be identified as superhy-drophobic membranes. When the mass ratios of nano-sized SiO2

particles/PVDF is 5.00:1, the increase of water CA value becomesslow sharply due to the disadvantage of this morphology film, onwhich there are so many SiO2 nanoparticles that micro-sized PVDFare nearly covered entirely by modified SiO2 nanoparticles. Thisleads to the system of dual-scale structure destroyed and less airtrapped in the pocket on the surface.

In fact, the impact of roughness on the wettability of a solid sub-strate is well known. Wetting on rough surfaces may assume either

Fig. 4. (a) SEM image of electrospun film prepared from 20 wt.% PVDF/DM

of the two models: Wenzel (homogeneous) wetting [55], where theliquid completely penetrates the roughness grooves, or Cassie–Baxter (composite) wetting [54], where air is trapped underneaththe liquid inside the roughness grooves. In general, the secondmodel dominates the observed behavior and the transition be-tween these models [56] has a major role in superhydrophobicityand water-repellent property. The Cassie–Baxter model relatesthe surface wettability to the surface roughness

cos hCB ¼ f1 cos hS � f2 ð1Þ

where f1 and f2(=1 � f1) are the fractions of solid–water and air–water contact areas, respectively, and hS and hCB denote the appar-ent contact angles of a water droplet on a smooth surface and arough surface composed of a solid and air, respectively.

We designed an idealized theoretical model to analyze the rela-tionship between local roughness and the wettability and estimatethe apparent contact angle by Cassie–Baxter equation, as shown inFig. 7. The model encompasses the effects of contact area, solid–

F solution. (b) Behavior of a water droplet on its surface. CA = 145.6�.

Fig. 5. SEM images of electrospun composite films comprised of the weight ratio of modified SiO2 nanoparticles/PVDF: (a) 1.00:1, CA = 147.8�, (b) 1.25:1, CA = 150.4�, (c)1.67:1, CA = 156.1�, (d) 2.50:1, CA = 160.7�, (e) 5.00:1, CA = 161.2�.

Fig. 6. Variation of CA value of composite films based on different weight ratios ofmodified SiO2 nanoparticles/PVDF.

S. Wang et al. / Journal of Colloid and Interface Science 359 (2011) 380–388 385

liquid–air composite interface. The fractional contact areas, f1 andf2, were calculated by using this model. In our idealized model,we assume that the micro-sized PVDF are packed closely and thenano-sized epoxy–siloxane modified SiO2 particles, which are in-layed in the PVDF surfaces, arranged in a regular array as shownin Fig. 7. First, we calculated how many micro-sized PVDF are ar-ranged in the projected area of apparent liquid–solid contact. Theblue fraction in Fig. 7a exhibits the projected area of apparent li-quid–solid contact. Let the diameters of the projected area of regionof a 2 lL water droplet contacting with the rough surface and PVDFmicrosphere be D1 and d1 respectively. This quantity N1 is given by

N1 ¼14ð3A2

1 þ 1Þ ð2Þ

where

A1 ¼D1

d1

Then, in the same way, we calculated the quantity of nano-sizedepoxy–siloxane modified SiO2 particles, which are able to contactwith the liquid, on the surface of one PVDF microsphere. Fig. 7bshows the planform of one PVDF microsphere in contact with thedroplet. The blue fraction exhibits the projected area of the round

Fig. 7. (a) The planform of the projected area of apparent liquid–solid contact. (b) The planform of one PVDF microsphere in contact with the droplet. (c) The cross-sectionalview of the droplet sitting on the tops of the nanoparticles.

386 S. Wang et al. / Journal of Colloid and Interface Science 359 (2011) 380–388

which is occupied by modified SiO2 particles in contact with thedroplet on one PVDF microsphere. Let the diameters of this pro-jected area and modified SiO2 nanoparticle be D2 and d2 respec-tively. This quantity N2 is given by

N2 ¼14ð3A2

2 þ 1Þ ð3Þ

where

A2 ¼D2

d2

Here we limit A1 and A2 to be odd. If A1 (or A2) is even, it is defaultedto plus 1. Let the actual wetted area of one nano-sized epoxy–silox-ane modified SiO2 particle with the liquid

S ¼ 18pd2

2

So the actual wetted area of liquid–solid contact Ss�l and f1 are givenby

Ss�l ¼ N1N2S ¼ p128

3D22 þ d2

2

� �3

D1

d1þ 1

� �ð4Þ

f1 ¼Ss�l

14 pD2

1

¼ 132D2

1

ð3D22 þ d2

2Þ 3D1

d1þ 1

� �ð5Þ

Fig. 8. Sliding behavior of electrospun film comprised of the weight ratio of nano-sized modified SiO2 particles/PVDF: 2.50:1 (a) a water droplet on the electrospunfilm which is horizontal. (b) The instantaneous sliding behavior of a water dropleton the electrospun film which is tilted slightly (SA = 2.56�).

Predicted by the Cassie–Baxter model, we estimate the appar-ent contact angle of the electrospun composite film which is com-prised of the weight ratio of modified SiO2 nanoparticles/PVDF:2.50:1 (we think that the electrospun composite film which iscomprised of the weight ratio of modified SiO2 nanoparticles/PVDF: 2.50:1 and the idealized theoretical model we designedare most similar.). D1 is calculated to be 625.27 lm. We define thatthe average diameter d1 is 2.5 lm and D2 = 3

5 d1. The contact angle114.5� of water on the smooth surface (hS) was measured by putt-ing epoxy–siloxane on a silicon wafer. We deduce from the Eq. (1)and (5) that the apparent contact angle hCB corresponds to 159.7�.It is almost as same as the experimentally observed value of theapparent contact angle (160.7�). In addition, from the Eq. (4), wecan see that when D1 and D2 are confirmed, the smaller the scaleof nano-sized SiO2 particles are, the smaller the actual contact areaof liquid–solid contact is. So there must be more air trapped in therough surface. As a result, the air trapped in the surface would pre-vent the surface being wetted, making the apparent contact anglehCB increase as described by Cassie–Baxter model. It could be alsoconsidered that with the decrease of the scale of the nanoparticles,the apparent contact angle becomes larger, which attributes to theincrease of the ratio of the cavities on rough surface [57].

To make a surface superhydrophobic and water-repellent, thewater CA should be high enough (>150�) and, more importantly,the CA hysteresis should be very small, so as to lead to a small

Fig. 9. Variation of SA value of composite films on different weight ratios ofmodified SiO2 nanoparticles/PVDF: (a) 1.00:1, SA = 4.51�, (b) 1.25:1, SA = 3.54�, (c)1.67:1, SA = 3.02�, (d) 2.50:1, SA = 2.56�, (e) 5.00:1, SA = 2.17�.

Fig. 10. The changes of electrospun surfaces on which the SiO2 were unmodified (a) and modified by epoxy–siloxane (b) in water after 24 h. Behaviors of the water droplet onthe electrospun surfaces on which the SiO2 were unmodified (c) and modified by epoxy–siloxane (d) in water after 24 h.

S. Wang et al. / Journal of Colloid and Interface Science 359 (2011) 380–388 387

roll-off angle of the water droplet. Contact angle hysteresis is verysignificant in evaluating the sliding behavior of a water droplet,which is attributed to many factors. Consequently, a surface of highCA does not necessarily indicate a low SA. If a water droplet is onthe rough surface, there will be plenty of cavities as the air pocketsbetween the water droplet and the solid surface. When this waterdroplet slide on the superhydrophobic surface, there must be atransition from the Wenzel model to the Cassie–Baxter model, asthe Cassie–Baxter model describes that, as surface roughness factorfurther increases and passes a critical standard, the water recedingangle increases noticeably at the same time (water does not pene-trate into the surface cavity), thus minimizing the CA hysteresis[54]. In our work, the SA of the electrospun simplex PVDF film ismore than 10� (calculated by the equation of SA = tan�1(h/L),

)determined by placing a 10 lL water droplet on its

surface, which was inclined to tilt gradually until water dropletstarted to roll. The photographs in Fig. 8 respectively exhibit awater droplet on the electrospun film which is horizontal andthe instantaneous sliding behavior of a water droplet on the elec-trospun film which is tilted slightly. The films comprised of someweight ratios of nano-sized modified SiO2 nanoparticles/PVDFwere measured in the same way as showed in Fig. 9. With the in-crease of SiO2 nanoparticles, water droplet no longer requires alarge angle to roll off the surface of electrospun films. When theweight ratio of nano-sized modified SiO2 nanoparticles/PVDF is5.00:1, a water droplet could roll under the condition that the sur-face is tilted to 2.17�. Because the air pockets on the surface of thefilm increase sharply, they are able to effectively trap sufficient airbetween the water droplet and the solid surface, thus enabling thewater droplet not to penetrate into the surface cavities. It is also in-duced that discrete contact is built up between the solid and liquidsurfaces, which impacts the contour, length, and continuity of thetriple contact line around water droplets, consequently leading tothe drastic decrease of the sliding angle [14].

To put the superhydrophobic membrane in an advantageousposition in application, making the surface durable is very signifi-cant. The superhydrophobic membrane could be used frequentlyand could not be destroyed easily if the surface we fabricated byelectrospinning is more durable. The durability of the superhydro-phobic film was tested underwater. The photos in Fig. 10 describethe changes of the various electrospun surfaces in water for 24 h.As shown in Fig. 10a, the electrospun PVDF membrane unmodifiedby anything was placed in the beaker filled with water for 24 h. Themuddy water in the beaker could be seen obviously because someof the SiO2 nanoparticles, which was mixed into precursor solutionpreviously, were separated from PVDF film and washed away.

Therefore, the SiO2 nanoparticles were floating and dispersed inthe water and the water CA of this membrane decreased to149.5� as shown in Fig. 10c. In contrast, the electrospun PVDF filmmixed into epoxy–siloxane modified SiO2 nanoparticles was alsoplaced in the beaker which was filled with water for 24 h as shownin Fig. 10b.There were no muddy phenomena in the beaker be-cause few SiO2 nanoparticles were floating in the water. The waterCA value of this electrospun surface is 159.8� as shown in Fig. 10d,which is almost as same as the water CA value when the superhy-drophobic film has not been placed in the beaker. It was found thatthe immersion of the film in water for hours did not alter the sur-face wetting property. Thus it can be seen that the surface with thePVDF modified by SiO2 nanoparticles is more durable than the sur-face with the PVDF unmodified. Because the long-chain siloxanegrown on the surface of the SiO2 nanoparticles is cross-linked withthe polymer and more viscous, the SiO2 nanoparticles could stick tothe surface of the micro-sized PVDF mini-islands. Accordingly, theSiO2 nanoparticles cannot be separated from PVDF film andwashed away easily when water droplet was moving thus achiev-ing the effect of durability.

4. Conclusion

In our research, durable superhydrophobic PVDF surfaces withhigh CA and low SA, made of epoxy–siloxane modified SiO2 nano-particles and micro-sized PVDF mini-islands, were successfullyprepared by electrospinning. In the process of electrospinning,nano-sized epoxy–siloxane modified SiO2 nanoparticles wereself-assembled homogeneously on the surface of the micro-sizedPVDF to build a dual-scale structure. A novel electrospun mat isfabricated on which the dual-scale structure is created by electros-pinning, and at the same time the surface roughness is increasedfor superhydrophobicity and self-cleaning property. Epoxy–silox-ane copolymer grafted on the surface of SiO2 nanoparticles makesthe superhydrophobic surface more durable. The value of CA andSA of electrospun composite film can be controlled by alteringthe mass ratio of epoxy–siloxane modified SiO2 nanoparticles/PVDF. Therefore, it can be believed that more superhydrophobicsurfaces with various properties can be further explored in the fu-ture by mixing various substances modified nanoparticles.

Acknowledgment

The authors gratefully acknowledge the support of the NationalNatural Science Foundation of China (No. 20874033).

388 S. Wang et al. / Journal of Colloid and Interface Science 359 (2011) 380–388

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