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Colloids and Surfaces A: Physicochem. Eng. Aspects 362 (2010) 117–120

Contents lists available at ScienceDirect

Colloids and Surfaces A: Physicochemical andEngineering Aspects

journa l homepage: www.e lsev ier .com/ locate /co lsur fa

reparation of superhydrophobic membranes by electrospinning of fluorinatedilane functionalized pullulan

d. Shahidul Islama,b, Nusnin Akterc, Mohammad Rezaul Karima,d,∗

Department of Applied Chemistry and Chemical Engineering, University of Dhaka, Dhaka 1000, BangladeshDepartment of Advanced Organic Materials Science & Engineering, Kyungpook National University, Daegu 702-701, South KoreaDepartment of Chemistry, Hunter College and the Graduate Center, The City University of New York, 695 Park Avenue, NY 10021, USACenter of Excellence for Research in Engineering Materials, College of Engineering, King Saud University, Riyadh 11421, Saudi Arabia

r t i c l e i n f o

rticle history:eceived 17 December 2009eceived in revised form 28 March 2010

a b s t r a c t

Superhydrophobic pullulan (PULL) membrane with a contact angle larger than 150◦ is prepared by theelectrospinning of the fluorinated silane functionalized PULL. The morphologies of the membranes arecharacterized using scanning electron microscopy (SEM). Interaction occurring between PULL and perflu-

ccepted 2 April 2010vailable online 10 April 2010

eywords:uperhydrophobic

orooctyltriethoxysilane (PFOTES) of the membranes is analyzed using differential scanning calorimetry(DSC), and Fourier transform infrared (FT-IR) and the contact angles and water drops on the surface ofthe membrane are measured using video microscopy.

© 2010 Elsevier B.V. All rights reserved.

lectrospinningullulanembrane

. Introduction

The wettability of a solid surface is an interesting property of aaterial and is described as the contact angle between a liquid andsolid surface. When the contact angle between water and a solid

urface is larger than 150◦, this solid surface is called a superhy-rophobic surface [1]. Superhydrophobic surfaces are widely found

n nature. For example, the surface of a lotus leaf is observed toe an array of nanoscale buds [2]. Water drops on the surface ofhe leaf tend to slide down, rendering its self-cleaning property.he surfaces which mimic “lotus effect” have triggered extensiventerests for their potential applications involving water repellency,elf-cleaning and anti-fouling properties [3–6].

Generally, the hydrophobicity of surfaces depends on bothheir chemical composition and surface geometrical structure [7].n terms of chemical composition, hydrophobicity can only bencreased by introducing of component with low surface energyuch as fluorinated methyl groups. However, this method to

ncrease hydrophobicity is limited.

The maximum contact angle that can be reached by coating flu-rinated methyl groups onto a flat solid surface is only 120◦, whichan be hardly called superhydrophobic [8]. Therefore, a hierarchical

∗ Corresponding author at: Center of Excellence for Research in Engineering Mate-ials, College of Engineering, King Saud University, Riyadh 11421, Saudi Arabia.el.: +966 14678929; fax: +966 14670199.

E-mail address: mkarim@ksu.edu.sa (M.R. Karim).

927-7757/$ – see front matter © 2010 Elsevier B.V. All rights reserved.oi:10.1016/j.colsurfa.2010.04.004

structure is introduced into the solid surface to achieve super-hydrophobicity. A number of methods have been used to makea hierarchical superhydrophobic surface including phase separa-tion [9], electrochemical deposition [10], chemical vapor deposition[11], crystallization control [12], photolithography [13], assembly[14], sol–gel methods [15], solution-immersion methods [16], andarray of nanotubes/nanofibers [17,18].

Electrospinning is a simple but versatile method to producecontinuous fibers with diameters ranging from nanometer to sub-micron scale. Superhydrophobic surfaces can be obtained throughthis process by controlling the surface roughness under appropri-ate conditions [19,20]. Superhydrophobic polystyrene nanofiberswere electrospun by either using various solvents [8] or by addingroom temperature ionic liquid [21]. Cellulose triacetate fibrous mat[22] and poly(3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBV)fibrous surface [23] were fabricated using electrospun fibers fol-lowed with plasma treatment.

Pullulan is a natural polymer and recent interest in the useof natural polymers; for example, as proteins in biotechnologicalmaterials and biomedical applications as well as their biocom-patibility [24]. However, only few reports are available in theliterature regarding superhydrophobic PULL membrane preparedby electrospinning. Pullulan’s solubility can be controlled or pro-

vided with reactive groups by chemical derivatization. Due to itsexcellent properties, pullulan is used as a low-calorie ingredient infoods, gelling agent, coating and packaging material for food anddrugs, binder for fertilizers and as an oxidation-prevention agentfor tablets. Other applications include contact lenses manufactur-

1 Physicochem. Eng. Aspects 362 (2010) 117–120

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Fig. 1. DSC data of membranes prepared from (a) PULL and (b) PULL/PFOTES usingelectrospinning method (applied voltage = 15 kV, TCD = 15 cm, polymer concentra-tion = 9 wt.%, and PFOTES concentration = 1 wt.%).

18 Md.S. Islam et al. / Colloids and Surfaces A:

ng, biodegradable foil, plywood, water solubility enhancer and fornhanced oil recovery [25–27]. In this study, a pioneering work iseported about the preparation of superhydrophobic PULL mem-ranes putting to some purpose the electrospinning of PULL by itsunctionalization and interaction with fluorinated silane.

. Experimental

.1. Materials

Pullulan is a food grade preparation (PF-20 grade) fromayashibara Biochemical Laboratories Inc. (Okayama, Japan) anderfluorooctyltriethoxysilane (PFOTES) is purchased from Aldrichhemical Co. Doubly distilled water is used as a solvent to preparell solutions.

.2. Preparation of PULL/PFOTES blend solutions

The PULL solutions (9 and 12 wt.%) were prepared in doublyistilled water at room temperature under magnetic stirring for–3 h. The PULL/PFOTES blend solutions were prepared by mixingf bulk PULL (9 and 12 wt.% respectively) with 1 wt.% of PFOTES inqueous solutions with gently stirred for further 24 h. The formedolutions were then used for the preparation of membranes usinglectrospinning technique.

.3. Membrane preparation by electrospinning

During electrospinning, each solution was continuously sup-lied using a syringe pump with a speed of 0.04 ml/h through a 25Geedle producing a Taylor cone. A high power voltage (15 kV) wasupplied to the solution with a tip-to-collector distance of 15 cm.

.4. Characterization of membranes

The water contact angles of the membranes were measuredsing microscopy. Using a micro syringe, 5 �l deionized water wasropped perpendicularly to each surface of the membranes placedn a horizontal glass sheet. Then, the images of water drops onhe surface of the membranes were observed using Scalar Videooupe (VL-11s) and analyzed using the Sigma TV II program. Mem-ranes were also characterized with DSC (model Q-10) from TA

nstruments, USA and FT-IR (Bruker IFS 120HR). The morphology ofembranes was observed with a field-emission scanning electronicroscope (JEOL, model JSM-6380) after gold coating.

. Results and discussion

.1. DSC data of membranes

The interactions between PULL and PFOTES of membranesre investigated by DSC. Fig. 1 shows the DSC thermogram ofULL/PFOTES membrane with 1 wt.% of PFOTES contents at a poly-er concentration of 9 wt.%. Pure PULL shows a large thermogram

eak of melting transition (Tm) at ∼95 ◦C (Fig. 1a). This peak ishifted to ∼84 ◦C with the addition of 1 wt.% PFOTES (Fig. 1b). TheSC thermograms for the membranes show clearly the melting

ransition of the PULL, in which there are significant effects of theFOTES content. The dramatic change of Tm of the composite mem-ranes can be attributed by the introduction of CF3 groups intoetero atom (O atom) containing hydrophobic carbon ring of PULL.

.2. FT-IR spectra of membranes

FT-IR spectra give additional information about the structuref PULL membrane studied. In Fig. 2, examples of spectra of elec-

Fig. 2. FT-IR data of membranes prepared from (a) PULL and (b) PULL/PFOTES usingelectrospinning method (applied voltage = 15 kV, TCD = 15 cm, polymer concentra-tion = 9 wt.%, and PFOTES concentration = 1 wt.%).

trospun PULL and PULL/PFOTES membranes at 500–4000 cm−1

range are shown. Pure PULL exhibits identical bands as shownin Fig. 2a. In the specific area (1500–650 cm−1) which is char-acteristic for the pullulan molecule as a whole, the spectra forcommercial pullulan as well as those for PULL/PFOTES electro-spun membrane sample exhibited similar features (Fig. 2a andb). Such results confirm the identical chemical structure of thesamples. Strong absorption in 850 cm−1 is characteristic of the�-d-glucopiranosid units. Absorption in 755 cm−1 indicates thepresence of �-(1,4) glucosidic bonds, and spectra in 932 cm−1

proves the presence of �-(1,6) glucosidic bonds. Besides, in theareas for reference and evaluated samples the frequencies areanalogous [28]. Bands at 2850–3000 cm−1 are due to stretch-ing vibrations of CH and CH2 groups and bands attribute toCH/CH2 deformation vibrations are present at 1300–1500 cm−1

range. Also very intensive, broad hydroxyl band occurs at3000–3600 cm−1 and accompanying C–O stretching exists at1000–1260 cm−1. Moreover, the absorption peaks which appeared

at 1145 cm−1, 1235 cm−1 and 1430 cm−1 are attributed to the vibra-tions of CF2 and CF3 groups in PFOTES. The absorption peaks at700–800 cm−1 are due to the vibration of Si–O groups in the silanes.Thus, the FT-IR spectroscopy supplies also evidences of possi-

Md.S. Islam et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 362 (2010) 117–120 119

F LL, (c) 9 wt.% PULL/PFOTES and (d) 12 wt.% PULL/PFOTES using electrospinning method(

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ig. 3. SEM images of membranes prepared from (a) 9 wt.% PULL, (b) 12 wt.% PUapplied voltage = 15 kV, TCD = 15 cm, and PFOTES concentration = 1 wt.%).

le interactions between PULL and PFOTES, which are suggestedbove.

.3. Water contact angles of membranes

Membranes prepared by electrospinning method have muchigher contact angle because of the high surface area of the formedbers that ranges from nanometer to submicron scale. As shown

n Fig. 3a, nanofibers electrospun using 9 wt.% PULL contain a largeumber of beads in nanometer size. Whereas, less number of beadontaining nanofibers can be electrospun using 12 wt.% PULL, 9 wt.%nd 12 wt.% PULL coupled with PFOTES applying the same param-ters as shown in Fig. 3b–d. The reason for different bead contentsn each concentration is the Raleigh forces, which assist in beadormation, were able to overcome the viscous forces to enablehe formation of beads [29]. Moreover, strong interaction betweenULL and PFOTES is attributed to the formation of less number ofeads in the fibers. PFOTES is water soluble and chemically sta-le in its aqueous solution because it has a bulky perfluorooctylhain with both hydrophobic and lipophobic properties. Due to itsipophobic properties, it dissolves in water through hydrogen bond-ng without any chemical reaction. The interaction between PULLnd PFOTES can be put forward as a cause to two factors. First,he introduction of hydrophobic CF3 groups on the hetero atom (Otom) containing hydrophobic carbon ring and the second factor ishe interaction obtained from the formation of hydrogen bondingetween polar hydrogen of –OH group of PULL with O of PFOTES.ater contact angles of all samples taken in this experiment are

hown in Fig. 4. Although contact angles of up to 146◦ and 143◦

an be reached by directly electrospinning 9 wt.% PULL and 12 wt.%ULL solutions, respectively, the membranes cannot still be con-idered as superhydrophobic membranes. Therefore, 9 wt.% PULLnd 12 wt.% PULL coupled with PFOTES were used to successfully

Fig. 4. Variation of water contact angles with different composition.

prepare superhydrophobic membranes with high contact angles of155◦ and 151◦, respectively. Higher contact angle of 9 wt.% PULLthan that of 12 wt.% PULL can be explained on the basis of surfaceroughness like lotus leaf [2] due to the more bead formation in caseof 9 wt.% PULL.

4. Conclusions

Superhydrophobic PULL membranes were prepared by electro-spinning of fluorinated silane functionalized PULL. It was observedthat both 9 wt.% and 12 wt.% PULL membranes can only reachcontact angles lower than 150◦ by electrospinning of pure PULL

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[31] N. Teramoto, M. Saitoh, J. Kuroiwa, M. Shibata, R. Yosomiya, Morphology and

20 Md.S. Islam et al. / Colloids and Surfaces A:

olutions. Superhydrophobic membranes with contact angles of upo 155◦ and 151◦ can be achieved by electrospinning 9 wt.% PULLnd 12 wt.% PULL coupled with fluorinated silane, respectively.ince PULL is a hydrophilic natural polymer, its mechanical and bar-ier properties tend to weaken when exposed to elevated relativeumidity. As a result, many studies aim at reducing the moistureensitivity and enhancing the physical properties for this polymeranofibrous membrane, including the incorporation of hydropho-ic components such as lipids into the membrane, blending withther less hydrophilic polymers, chemical modification, and judi-ious use of plasticizer in the membrane formulation [30–32].

cknowledgements

The support of this research by the University of Dhaka,angladesh is gratefully appreciated. M.R. Karim gratefullycknowledges supports from King Saud University and Ministry ofigher Education, Kingdom of Saudi Arabia.

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