Materials Letters 63 (2009) 1199–1202

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Materials Letters

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Crosslinking of electrospun polyacrylonitrile/hydroxyethyl cellulosecomposite nanofibers

Haitao Zhang a, Huali Nie a, Shubai Li a, Christopher J. Branford White b, Limin Zhu a,⁎a College of Chemistry, Chemical Engineering and Biotechnology, Donghua University, Shanghai, 201620, Chinab Institute for Health Research and Policy, London Metropolitan University, 166-220 Holloway Road, London, UK

⁎ Corresponding author. Tel.: +86 21 67792659; fax:E-mail address: lzhu@dhu.edu.cn (L. Zhu).

0167-577X/$ – see front matter © 2009 Elsevier B.V. Adoi:10.1016/j.matlet.2009.02.035

a b s t r a c t

a r t i c l e i n f o

Article history:

The electrospinningof polyac Received 4 June 2008Accepted 17 February 2009Available online 25 February 2009

Keywords:ElectrospunNanomaterialsCrosslinkingHydrophilicitySurface

rylonitrile (PAN)/hydroxyethyl cellulose (HEC)wasperformedwith glutaraldehydeas a cross linker to fabricate highly hydrophilenanofibers. The concentration of the spinning solution and the ratioof HEC/PANwere varied and adjusted to get smooth nanofibers. The nanofiberswere characterized by SEM, FT-IRand contact angle. SEM images showed that the scope of the diameters was 100–300 nm. The nanofibers becamethick with the ratio of the HEC/PAN increasing. FT-IR indicated that there could be interactions between HEC andPAN. Contact angle measurement revealed that the increased ratio of HEC and the crosslinking led improvementin the hydrophilicity of PAN/HEC composite nanofibers.

© 2009 Elsevier B.V. All rights reserved.

1. Introduction

Electrospinningnanofibers areof interest inmanyapplications, theseinclude biomedical applications, composite materials, ultrafiltrationmedia and so on [1–3]. Ultrafiltration process, using porousmembranesto separate pure water and microsolutes from macromolecules andwaste water, has been applied to industries such as beverages, wastewater treatment and electro-coating paint mixtures [4]. However, theapplication of ultrafiltration is limited by inevitable phenomena offouling on membrane surfaces or pores. The fouling phenomenon isexplained by the hydrophilic–hydrophobic interaction between mem-brane surfaces andpermeates [5]. It has beenwidelyaccepted that oneofthe common methods to suppress fouling is to make the surface hy-drophilic. Surface hydrophilicity can be improved by a number of meth-ods such as coating, grafting, surface treatment and so on. In order toincrease the influence of hydrophilic, the method which sophisticatesthe membrane material (polymer blends) is commonly used [6].

PAN is apolymer thathas been intensively studiedowning to its goodmechanical properties desirable for electrospinning and its good filmforming ability, but it is hydrophobic with too low density [7]. Thehydrophobicity of the PAN nanofibers could be reduced by addition ofhydrophilic polymers. Many water-soluble polymers such as poly (vinylalcohol) (PVA) [8], poly (vinyl pyrrolidone) (PVP) [9] and poly (ethyleneglycol) (PEG) [10] are used as the additives to be spun successfully.However, there is still no report about the preparation and character-ization of PAN/HEC composite nanofibers, whichmay combine both theadvantages of PAN such as light weight, flexibility [11], and of HEC such

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as high hydrophilicity, processability, biocompatibility, stabilizer andgood chemical resistance [12]. When the electrospun PAN/HEC com-posite nanofibers were immersed in water, these would dissolve be-cause HEC is a water soluble polymer. Therefore, it is necessaryto crosslink theHEC polymer and to stabilize the electrospun nanofibersin wet condition. All multifunctional compounds capable of reactingwith the hydroxyl group such as dialdehydes, dicarboxylic acids, dian-hydrides may be used as a crosslinker of HEC [13].

In this study, ultrafine PAN/HEC nanofibers were first prepared bythe one-step electrospinning method using N,N-dimethylacetamide(DMAC) as solvent. Glutaraldehyde vapor was used to crosslink thesecomposite nanofibers. Both structures of composite nanofibers beforeand after crosslinking were characterized. The hydrophilicity of com-posite nanofibers was found to have improved greatly.

2. Experimental

PAN with a molecular weight of 50,000 was purchased from Shang-hai Chemical Fibers Institute. HEC having a viscosity average molecularweight of 150,000 was purchased from Shanghai Chemical Co.. Elec-trospinning solutionswere prepared by dispersing controlled amount ofHEC (0, 0.5, 1.0 and 1.5 wt.% to PAN) into 10 wt.% PAN solution in DMAC.Mechanical stirring was applied for 24 h at 70 °C in order to obtainhomogeneous HEC-dispersed PAN solutions. The electrospinning solu-tion was placed in a 5-ml syringe with a metal needle of 0.3 mm indiameter. A power supply (ES40P-20 W/DAM) was used to provide ahigh voltage of 15 kV to the syringe needle tip and ametal collector. Thetip-to-collector distance of 15 cm, and a solution flow rate of 1 mL/h.After the electrospinning process, the electrospun PAN/HEC compositenanofiberswere immersed inmethanol for anhouranddried completely.

Fig. 2. SEM images of glutaraldehyde crosslinked nanofibers with different HEC contents(a) 0.5 wt.%, (b) 1.5 wt.%.

1200 H. Zhang et al. / Materials Letters 63 (2009) 1199–1202

Subsequently, the methanol-treated nanofibers were kept in a chambersaturated with glutaraldehyde vapor for 24 h for crosslinking formationand dried in a vacuum oven for a day. Then, the composite nanofibersbefore and after crosslinking were characterized by SEM (JSM-5600LV&DXS-10A) and FT-IR (NEXUS-670, Thermo Nicolet Co., USA).

The hydrophilicity of composite nanofibers before and after cross-linking was characterized on the basis of pure water contact anglemeasurement. Using a sessile drop method, static water contact anglewas measured at room temperature on a contact angle goniometer(KRUSS DSA10-MK) equipped with video capture. A total of 50 µL ofde-ionized water was dropped onto a dry membrane with a microsyringe in an atmosphere of saturated water vapor. At least 10 contactangles were averaged to get a reliable value.

3. Results and discussion

3.1. Morphology of composite nanofibers

Typically representative SEM photographs of all the nanofibers withdifferent HEC contents are shown in Fig. 1. Based on these SEM photos,the diameters of all nanofibers are measured by SEM micrograph. Anoticeable change in uniformity occurs when the concentration of HECis increased from 0 wt.% to 1.5 wt.%, fiber diameters range from 100 nmto 300 nm. The morphology of electrospun fibers is influenced byvarious parameters. In fact, one of the most significant parametersinfluencing fiber morphology is the solution viscosity [14]. When thesolution viscosity is low, the jets from the needle are unstable and likelyto break up and form beads on fiber surface, and it will be difficult to geta continuous nanofiber on the collector. For high viscosity solutions, thejets would not break up but rather travel and split into filaments andform fibers with increased diameter. Because the aqueous solutions ofHEC are highly viscous, increased HEC content is associated withincreased viscosity of the spinning solution, which in turn is associatedwith increased electrospun fiber diameter. Once the contents of theHECis up to 2.0 wt.%, highly viscous fluid balls will gradually gather outside

Fig. 1. SEM images of PAN/HEC nanofibers with different HEC contents. (a) 0 wt.% (pure PAN), (b) 0.5 wt.%, (c) 1.0 wt.%, and (d) 1.5 wt.%.

Table 1Water contact angle (θ) of nanofibers with different HEC contents (mean±std).

HEC contents (wt.%) Contact angle (θ)a Contact angle (θ)b

0 129.3±0.6 53.5±0.30.5 116.3±0.2 22.2±0.11.0 104.2±0.3 b101.5 93.4±0.7 b10

a Before crosslinking.b After crosslinking.

1201H. Zhang et al. / Materials Letters 63 (2009) 1199–1202

the tip of the needle after electrospinning has started for a while nomatter how high an electric voltage has been applied. So only long andstraight fiber with HEC contents ranging from 0 wt.% to 1.5 wt.% wasgained as shown in Fig. 1.

Fig. 2 showsSEM images of glutaraldehydevapor treated electrospunnanofiberswithHEC contents of 0.5wt.% and 1.5wt.%. Crosslinking doesnot change the mean diameters of nanofibers, it only influences thephysical properties of nanofibers and makes the nanofibers crimped.The effectof glutaraldehyde enhances thepore interconnectivityand thepore size of the blend membrane diminishes gradually.

3.2. FT-IR spectra

As can be seen in Fig. 3A, the spectrum of pure PAN contains prom-inent peaks at 2930, 2240, and 1470 cm−1 due to stretching vibration ofmethylene (–CH2–), stretching vibration of nitrile groups (–CN–), andbending vibration of methylene, respectively [15]. Compared spectra ofpure PANandPAN/HEC composite nanofiberswith variousHEC contents,PAN/HECcompositenanofiberspresent apeakaround2880cm−1,whichis the stretching vibration of – CH3 groups in HEC. It is also seen that thepeak at 3400–3500 cm−1, which is the characteristic band of –OH group,becomes broader with the increase of HEC content.

Fig. 3B denotes the FT-IR spectra of crosslinked composite nanofi-bers. After glutaraldehyde vapor activation, the intensity of the –OH

Fig. 3. FT-IR spectra of nanofibers before crosslinking (A) and after crosslinking (B) withdifferent HEC contents: (a) 0 wt.% (pure PAN), (b) 0.5 wt.%, (c) 1.0 wt.%, (d) 1.5 wt.%,(e) 0.5 wt.%, (f) 1.5 wt.%.

band on the membrane decreased due to the reaction took place be-tween –OH and aldehyde groups. In this spectrum, –OH band peakdecreased while C=O band at 1740 cm−1 increased compared to non-crosslinkednanofibersdue to reactionof glutaraldehydewith–OHgroupsto yield C=O band [16].

3.3. Contact angle measurement

Table 1 shows the contact angles of pure water on the compositenanofiber surface before and after crosslinking. It is found that thecontact angle decreases gradually from 129.3±0.6 to 93.4±0.7 withincreasing the HEC content in the copolymer from 0 to 1.5 wt.%. This isdue to the contribution of the hydroxyl groups in HEC moieties [17].The decreasing of the contact angle indicates that PAN-based com-posite nanofibers with hydrophilic surface are obtained. After cross-linking by glutaraldehyde vapor, the contact angle decreases greatly,compared with the untreated composite nanofibers. Note that thecontact angle of composite nanofiber is hardly to be detected whenthe content of HEC in composite nanofibers is over 1.0 wt.%. Theseresults are very surprising. We presume it is attributed to two reasons:Firstly, intermolecular interactions may be formed between HEC, PANand glutaraldehyde. The reaction between HEC and the aldehyde isbetween the hydroxyl groups, carboxyl groups of HEC and the alde-hyde to form polar parts of acetal ring and ether linkage interact withwater and this interaction contributes to the decrease of the interfacialfree energy of the fiber surface. Polar groups can be isolated on thesurface and the contact angle is decreased greatly resulting in greatsurface hydrophilicity improvement. Secondly, surface contact anglecorrelates with the surface roughness, and the modification throughcrosslinking by glutaraldehyde vapor caused the surface roughness todecrease.

4. Conclusions

The PAN/HEC composite nanofibers containing different amountsof HEC were prepared using a one-step electrospinning method. SEMimages showed that the scope of the diameters was 100–300 nm, andthe spun fibers became thick with the ratio of HEC to PAN increasing.FT-IR indicated that there may be interactions between HEC and PAN.The addition of HEC could greatly improve the hydrophile propertiesof PAN/HEC composite nanofibers. The crosslinking had also greatlyenhanced the hydrophilicity of PAN/HEC composite nanofibers. Thesecrosslinked composite nanofibers with high hydrophilicity could besuitable for a variety of applications like for industrial filtration, tissueengineering, biomaterials, and so on.

Acknowledgement

This work was supported by National Science Foundation of China(Grant No. 50773009).

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