Journal of Colloid and Interface Science 368 (2012) 273–281

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Journal of Colloid and Interface Science

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Effect of pullulan/poly(vinyl alcohol) blend system on the montmorillonitestructure with property characterization of electrospun pullulan/poly(vinylalcohol)/montmorillonite nanofibers

Md. Shahidul Islam a, Jeong Hyun Yeum b, Ajoy Kumar Das a,⇑a Department of Applied Chemistry and Chemical Engineering, University of Dhaka, Dhaka 1000, Bangladeshb Department of Advanced Organic Materials Science and Engineering, Kyungpook, National University, Daegu 702-701, South Korea

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

Article history:Received 17 September 2011Accepted 3 November 2011Available online 19 November 2011

Keywords:PullulanPoly(vinyl alcohol)MontmorilloniteNanofiber

0021-9797/$ - see front matter Crown Copyright � 2doi:10.1016/j.jcis.2011.11.007

⇑ Corresponding author. Fax: +880 2 8615583.E-mail address: ajoykddu@gmail.com (A.K. Das).

Nanofibers of the composite of pullulan (PULL), poly(vinyl alcohol) (PVA), and montmorillonite clay(MMT) were prepared using electrospinning method in aqueous solutions. Pullulan is an interesting nat-ural polymer for many of its merits and good properties. Because of biocompatibility and non-toxicity ofPVA, it could be used in numerous fields. Scanning electron microscopy (SEM), transmission electronmicroscopy (TEM), Fourier transform infrared (FTIR), X-ray diffraction (XRD), and thermal gravimetricanalysis (TGA) were done to characterize the PULL/PVA/MMT nanofibers morphology and properties.XRD patterns and FTIR data demonstrated that there were good interactions between PULL and PVAcaused by possibly hydrogen bonds. Moreover, XRD data and TEM images indicated that intercalatedand exfoliated MMT nanoplatelets can be obtained within the PULL/PVA/MMT nanofibers dependingon the PULL/PVA blend ratios. Furthermore, the thermal stability and mechanical property (tensilestrength) of PULL/PVA/MMT nanofibers could be enhanced more by exfoliated MMT nanoplatelets thanintercalated structures of that nanoplatelets.

Crown Copyright � 2011 Published by Elsevier Inc. All rights reserved.

1. Introduction

The electrospinning technique has attracted great interestamong academic and industrial scientists because it is very simpleand effective approach to produce nanofibers, which have beenfound to be attractive for various applications in biomedical engi-neering, filtration, protective clothing, catalysis reactions, and sen-sors [1–5]. Doshi and Reneker [6] reported the main principle ofthe electrospinning technique. In an electrospinning process, apolymer solution, held by its surface tension at the end of a capil-lary tube, is subjected to an electric field. Charge is induced on theliquid surface by an electric field. Mutual charge repulsion causes aforce directly opposite to the surface tension. As the intensity ofthe electric field is increased, the hemispherical surface of the solu-tion at the tip of the capillary tube elongates to form a conicalshape known as the Taylor cone [7]. When the electric field reachesa critical value at which the repulsive electric force overcomes thesurface tension force, a charged jet of the solution is ejected fromthe tip of the Taylor cone. Because this jet is charged, its trajectorycan be controlled by an electric field. As the jet travels in air, thesolvent evaporates, leaving behind a charged polymer fiber, whichlies randomly on a collecting metal screen.

011 Published by Elsevier Inc. All r

Polymer blending constitutes a very useful method for theimprovement or modification of the physicochemical propertiesof polymeric materials. An important property of a polymer blendis the miscibility of its components because it affects the mechan-ical properties, the morphology, and the permeability and degrada-tion. Polymer blends are physical mixtures of structurally differentpolymers or copolymers that interact with secondary forces withno covalent bonding [8], such as hydrogen bonding, dipole–dipoleforces, and charge-transfer complexes for homopolymer mixtures[9–11].

Poly(vinyl alcohol) (PVA) is a semi-crystalline hydrophilic poly-mer with good chemical and thermal stability. It is a highly bio-compatible and non-toxic polymer and it can be processed easilyand has high water permeability. PVA can form physical gels invarious types of solvents which lead to the use of PVA in a widerange of applications in medical, cosmetic, food, pharmaceutical,and packaging industries [12–19]. Its flexibility and toughnessare good, so it is a typical synthetic polymer that is used to improvethe physical properties through mixing with other materials thathave poor physical properties. PVA with functional groups is usefulin practical investigations of functional polymers because of itseasy preparation as a bulk material, films, and fibers.

Pullulan is a natural polymer and recent interest in the use of nat-ural polymers; for example, as proteins in biotechnological materi-als and biomedical applications as well as their biocompatibility

ights reserved.

Fig. 1. SEM images of PULL/PVA blend nanofibers that electrospun with various total polymer concentrations of (a) 12 wt.%, (b) 14 wt.%, and (c) 16 wt.% (Appliedvoltage = 15 kV, TCD = 15 cm, and Blend ratio (PULL/PVA) = 60/40).

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[20]. Pullulan is an extracellular microbial polysaccharide producedby the fungus-like yeast, Aureobasidium pullulans [21,22]. The basicstructure is a linear a-glucan one, made from three glucose unitslinked a-(1,4) in maltotriose units which are linked in a a-(1,6)way. Due to its excellent properties, pullulan is used as a low-calorieingredient in foods, gelling agent, coating and packaging material forfood and drugs, binder for fertilizers, and as an oxidation-preventionagent for tablets. Other applications include contact lenses manu-facturing, biodegradable foil, plywood, water solubility enhancer,

Fig. 2. SEM images of electrospun PULL/PVA blend nanofibers with various mass ratios ofApplied voltage = 15 kV, and TCD = 15 cm).

and for enhanced oil recovery [23–25]. It is water soluble, insolublein organic solvents, and non-hygroscopic in nature. Its aqueous solu-tions are stable and show a relatively low viscosity as compared toother polysaccharides. It decomposes at 250–280 �C. It is moldableand spinnable, being a good adhesive and binder. It is also non-toxic,edible, and biodegradable.

Montmorillonite (MMT) is one of useful inorganic materials. Ithas been attracting great attention due to its remarkableimprovement in mechanical, thermal, flame-retardant, and barrier

(a) 100/0, (b) 90/10, (c) 80/20, and (d) 60/40 (Total polymer concentration = 14 wt.%,

Md.S. Islam et al. / Journal of Colloid and Interface Science 368 (2012) 273–281 275

properties of polymeric composites with small amounts (1–10 wt.%) of MMT fillers added. It is regularly used for packagingand medical applications [26,27]. From the point of view of struc-ture, two idealized polymer/layered silicate structures are possi-ble: intercalated and exfoliated ones [28].

Although several types of pullulan containing nanofibers wereprepared using electrospinning technique, nanoclay enhancedpullulan/poly(vinyl alcohol) blending system nanofibers have notbeen prepared yet using the same technique. Since nanoclay iswidely used in various composites to improve the physical proper-ties of polymer, the aim of this study is to evaluate the influences ofvarious PULL/PVA blend ratios on the MMT structures and to findout the effect of those MMT edifices on the thermal stability ofelectrospun PULL/PVA/MMT composite nanofibers prepared fromaqueous solutions.

2. Experimental

2.1. Materials

PVA with number-average degree of polymerization of 1700(fully hydrolyzed, degree of saponification = 99.9%) is obtained fromDC Chemical Co., Seoul, Korea, and pullulan is a food grade prepara-tion (PF-20 grade) from Hayashibara Biochemical Laboratories Inc.(Okayama, Japan). Montmorillonite (MMT) is purchased from

Fig. 3. SEM images of electrospun PULL/PVA /MMT nanofibers with various blend ratios (concentration = 14 wt.%, MMT concentration = 5 wt.%, Applied voltage = 15 kV, and TCD

kunimine Industries Co. Ltd., Japan. Doubly distilled water (DDW)is used as a solvent to prepare all solutions.

2.2. Preparation of PULL/PVA/MMT blend solutions

MMT powder was dispersed in doubly distilled water undermagnetic stirring for 1 h at room temperature and then PVA wasadded in the solution. The solution was heated at 80 �C under mag-netic stirring for 2 h followed by cooling to room temperature.PULL powder was dissolved in doubly distilled water separatelyunder magnetic stirring for 2 h at room temperature. The PULL/PVA/MMT blend solution was prepared by mixing of PVA/MMTand PULL solutions at total solid concentration of 14 wt.% with100/0, 90/10, 80/20, and 60/40 (PULL/PVA) mass ratios and5 wt.% MMT content.

2.3. Electrospinning of PULL/PVA/MMT blend solutions

During electrospinning, high-voltage power (model CPS-60K02VIT, Chungpa EMT Co., Ltd., Seoul, Korea) was applied to thePULL/PVA/MMT solution in a syringe via an alligator clip attachedto the syringe needle. The applied voltage was adjusted at 15 kV.The solution was delivered to the blunt needle tip via a syringe pumpto control the solution flow rate. Fibers were collected on electrically

PULL/PVA) of (a) 100/0, (b) 90/10, (c) 80/20, (d) 60/40, and (e) 0/100 (Total polymer= 15 cm).

Fig. 4. TEM images of electrospun PULL/PVA/MMT nanofibers with various blend ratios (PULL/PVA) of (a) 100/0, (b) 90/10, (c) 80/20, and (d) 60/40 of those two polymers(white and black arrows indicate intercalated and exfoliated clay nanoplatelets, respectively) (Total polymer concentration = 14 wt.%, MMT concentration = 5 wt.%, Appliedvoltage = 15 kV, and TCD = 15 cm).

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grounded aluminum foil placed at a 15 cm vertical distance to theneedle tip.

2.4. Characterizations

The morphology and property characterization of the electrospunPULL/PVA/MMT blend nanofibers were conducted with scanning

Fig. 5. A schematic presentation of (a) polymer matrix, (b) layered silicate clay fillerand polymer/clay nanocomposite, where layered silicate clay filler present in thematrix in (c) aggregated, (d) intercalated, (e) partially exfoliated, and (f) completelyexfoliated form.

electron microscope (model JSM-6380, JEOL) after gold coating, trans-mission electron microscopy (TEM) (HITACHI, model H-7600) with anaccelerating voltage of 100 kV, Fourier transform infrared (FTIR) (IFS120HR, Bruker), and X-ray diffraction (X’Pert APD, Philips). The thermalstability of that electrospun nanofibers was studied with TGA (modelQ-50; TA Instruments, USA) technique. The tensile strength was deter-mined with a Zwick (Germany) Z005 material testing machine.

Fig. 6. FTIR data of electrospun nanofibers of (a) pure PULL (b) PULL/PVA blend, and(c) PULL/PVA/MMT blend (Applied voltage = 15 kV, TCD = 15 cm, Blend ratio, PULL/PVA = 60/40 by wt., Total polymer concentration = 14 wt.%, MMT concentration =5 wt.%).

Fig. 8. XRD patterns of electrospun PULL/PVA/MMT nanofibers with various blendratios (PULL/PVA) of (a) 100/0, (b) 90/10, (c) 80/20, and (d) 60/40 of those twopolymers (Total polymer concentration = 14 wt.%, MMT concentration = 5 wt.%,Applied voltage = 15 kV, and TCD = 15 cm).

10 20 30 40 50

(a)PULL/PVA=100/0(b)PULL/PVA=90/10(c)PULL/PVA=80/20(d)PULL/PVA=60/40

(a)

(b)

(c)

(d)

0

100

200

300

400

500

Intensity

Fig. 7. XRD data of PULL/PVA blend nanofibers that electrospun with variousweight ratios of those two polymers: (a) 100/0, (b) 90/10, (c) 80/20, and (d) 60/40(Applied voltage = 15 kV, TCD = 15 cm, and Total polymer concentration = 14 wt.%).

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3. Results and discussion

3.1. Morphology of PULL/PVA/MMT composite nanofibers

Morphology of electrospun nanofiber can be affected by theelectrospinning instrument parameters including electric voltage,tip to collector distance, and solution parameters such as polymerconcentration, feed mass ratio, and surface tension. Changing thetotal polymer concentration could alter the fiber diameter andmorphology very effectively, as shown in Fig. 1.

In a fixed applied voltage (15 kV) and tip to collector distance(15 cm), it has been used 12, 14, and 16 wt.% of total polymer con-centration at 60/40 blend ratio of PULL to PVA. It is found that14 wt.% total polymer concentration of those two polymers is idealcondition to obtain thinner and uniform nanofibers (Fig. 1b).

Fig. 2 displays the morphology of PULL/PVA blend nanofiberswith various mass ratios of 100/0, 90/10, 80/20, and 60/40 at totalpolymer concentration of 14 wt.%. at the same electrospinningconditions. Among those blend ratios, 60/40 blend ratio can pro-duce uniform nanofibers (Fig. 2d). At the operation voltage of15 kV and tip to collector distance (TCD) of 15 cm, a series ofnanofibers was made at a total polymer concentration of 14 wt.%with various mass ratios of PULL to PVA (100/0, 90/10, 80/20,and 60/40) and 5 wt.% MMT content. Fig. 3 shows dramatic mor-phological changes of fibers such as bead formation, diameterswith changing the blend ratios of those two polymers at 5 wt.%MMT concentration.

By carefully comparing the SEM images shown in Fig. 3, it is seenthat the number of beads decreases with increasing fiber diameterswhen the mass ratios of PVA augment at 5 wt.% MMT content for thefabrication of electrospun PULL/PVA/MMT nanofibers. It is knownthat the diameter and formation of beads are strongly influencedby the viscoelasticity of the solution [29]. It can be concluded thatless beaded with uniform diameter PULL/PVA/MMT nanofiberscould be obtained with increasing the PVA mass ratios at 14 wt.% to-tal polymer concentration and 5 wt.% MMT content. Moreover, TEMobservations reveal the distribution of the MMT clay in the nanofibermatrix. The TEM images in Fig. 4 indicate the nanosize MMT in thenanofibers electrospun from the solutions of various PULL/PVAblends of 100/0, 90/10, 80/20, and 60/40 mass ratios at 14 wt.% totalpolymer concentration with 5 wt.% MMT content.

It can be clearly observed that each silicate platelet forms a darkline in the nanofibers (Fig. 4). The TEM results also reveal that theMMT platelets can be intercalated or exfoliated within the fiber ma-trix depending on the PULL/PVA blend ratio. White and black arrowsindicate the intercalation as well as exfoliation of MMT clay

nanoplatelets, respectively, in the electrospun PULL/PVA/MMTnanofibers (Fig. 4a–d). These structures are schematically presentedin Fig. 5.

3.2. FTIR data

FTIR spectra give information about the structure of the blendnanofibers studied. In Fig. 6, examples of spectra of electrospunpristine PULL, PULL/PVA, and PULL/PVA/MMT blend nanofibers at500–4000 cm�1 range are shown. Pure PULL nanofibers exhibitidentical bands as shown in Fig. 6a. Strong peak at 850 cm�1 ischaracteristic of the a-glucopiranosid units. Peak at 755 cm�1 indi-cates the presence of a-(1,4) glucosidic bonds, and spectra in932 cm�1 proves the presence of a-(1,6) glucosidic bonds. Besides,in the areas for reference and evaluated samples the frequenciesare analogous [30].

Bands at 2850–3000 cm�1 are due to stretching vibrations of CHand CH2 groups and bands attribute to CH/CH2 deformation vibra-tions are present at 1300–1500 cm�1 range. Also very intensive,broad hydroxyl band occurs at 3000–3600 cm�1 and accompany-ing C–O stretching exists at 1000–1260 cm�1. With the additionof PVA, some absorption peaks of PULL become lower in intensity,whereas some peaks at 1096 and 1447 cm�1 appear (Fig. 6b). Thissuggests that hydrogen bonds between hydroxyl groups in PULLand the same groups in PVA could possibly play a role in the shiftof the peaks. Thus, the FTIR spectroscopy supports the interactionsbetween PULL and PVA [31], which are suggested by XRD data. The

Fig. 10. TGA data of electrospun PULL/PVA/MMT blend nanofibers with PULL/PVAmass ratio of 100/0 (Total polymer concentration = 14 wt.%, MMT concentra-tion = 5 wt.%, Applied voltage = 15 kV, and TCD = 15 cm).

Fig. 9. TGA data of electrospun PULL/PVA blend nanofibers with various PULL/PVAmass ratios of (a) 100/0, (b) 90/10, (c) 80/20, and (d) 60/40 (Total polymerconcentration = 14 wt.%, Applied voltage = 15 kV, and TCD = 15 cm).

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spectrum of MMT shows the characteristic bands at 1035 cm�1 dueto Si–O stretching, 916 cm�1 and 842 cm�1 from Al–O stretching,and 519 cm�1 and 467 cm�1 due to Si–O bending [32]. Neighboringbands situated at 1447 and 1096 cm�1 in the PULL/PVA blend spec-trum are shifted to 1382 and 1061 cm�1, respectively, in PULL/PVAblend with 5 wt.% MMT content. Analyzing the above consider-ations, FTIR spectroscopy supplied the evidence of possible interac-tions between the PULL/PVA blend matrix and MMT clay, which issupported by XRD data.

3.3. XRD data

Fig. 7 illustrates the XRD patterns of electrospun pristine PULLand PULL/PVA blend nanofibers of different mass ratios of 100/0,90/10, 80/20, and 60/40 at 14 wt.% total polymer concentration.There is a broad peak appearing near 19.4�, corresponding to a d-spacing of 4.52 angstrom for pure PULL (Fig. 7a) [33].

With the addition of PVA of 90/10, and 80/20 mass ratios in thePULL/PVA blend nanofibers, the intensity of the diffraction peak atabout 19.4� of pristine PULL becomes lower, while 60/40 mass ratioof PULL/PVA exhibits a significant crystalline peak at about 19.3�,which agrees very well with crystalline peak of pure PVA [17](Fig. 7b–d). The enhancement of the crystallinity of the electrospunPULL/PVA blend nanofibers can probably be attributed to thehydrogen-bonding interaction between PULL and PVA macromole-cules. XRD indicated that PULL could possibly interact with PVAthrough hydrogen bonding between hydroxyl groups in PULL andthe hydroxyl groups in PVA. Subsequently, the electrospinnabilityof PULL with PVA is greatly improved. The spacing between clayplatelets, or gallery spacing, is an indicator of the extent of interca-lation/exfoliation of clay platelets within a polymer matrix and canbe observed using X-ray diffraction. Generally, intense reflection inthe range of 3–9� (2h) indicates an ordered intercalated nanocom-posite. In exfoliated nanocomposites, on the other hand, where sin-gle silicate layers (1 nm thick) are homogeneously dispersed in thepolymer matrix, and XRD patterns with no distinct diffraction peakin the range of 3–9� (2h) could be observed [34,35]. Fig. 8 shows theXRD patterns of electrospun PULL/PVA/MMT nanofibers with

Fig. 12. TGA data of electrospun PULL/PVA/MMT blend nanofibers with PULL/PVAmass ratio of 80/20 (Total polymer concentration = 14 wt.%, MMT concentra-tion = 5 wt.%, Applied voltage = 15 kV, and TCD = 15 cm).

Fig. 11. TGA data of electrospun PULL/PVA/MMT blend nanofibers with PULL/PVAmass ratio of 90/10 (Total polymer concentration = 14 wt.%, MMT concentra-tion = 5 wt.%, Applied voltage = 15 kV, and TCD = 15 cm).

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various blend ratios of those two polymers containing 5 wt.% MMT.Here, the XRD patterns of 100/0, 90/10, and 80/20 blend ratios ofPULL to PVA show an intense diffraction peak in 3–9� for electro-spun PULL/PVA/MMT nanofibers, indicating the possibility of havingintercalated silicate layers of clay dispersed in PULL/PVA blend ma-trix (Fig. 8a–c). For PULL/PVA/MMT nanofibers with 60/40 blend ra-tio (PULL/PVA) and 5 wt.% MMT content, there is no diffraction peakin 3–9� (2h) range, suggesting the predominance of having exfoli-ated silicate layers of clay dispersed in PULL/PVA blend matrix(Fig. 8d). There is a broad peak appearing near 19.4�, correspondingto a d-spacing of 4.52 angstrom for bulk PULL [33]. Moreover, theXRD patterns of PULL/PVA/MMT nanofibers signify the formationof intercalation as well as exfoliation structures due to the variousPULL/PVA blend ratios (Fig. 8). The basal spacing of MMT clay in-creases with increasing PVA mass ratios for the fabrication ofPULL/PVA/MMT electrospun nanofibers and exfoliation structuresof MMT clay can be obtained in case of 60/40 blend ratio (Fig. 8d).

Thermal stability of electrospun PULL/PVA blend nanofibers ismeasured using TGA in nitrogen atmospheres. Fig. 9 shows TGA

thermograms of different decomposition temperature with blendratios of 100/0, 90/10, 80/20, and 60/40 of PULL to PVA. Meanwhile,three weight loss peaks were observed in the TGA curve for purePULL (Fig. 9a). The first peak at 25–95 �C was due to moisturevaporization, the second peak at 230–270 �C was due to the ther-mal degradation of PULL, and the third peak at 300–400 �C wasdue to the byproduct formation of PULL during the TGA thermaldegradation process. Fig. 9b–d shows similar thermogram trendfor PULL/PVA blend nanofibers of 90/10, 80/20, and 60/40 mass ra-tios, respectively. Clearly, it can be said that higher thermal stabil-ity in the mid-point temperature of the degradation of PULL couldbe obtained with a higher mass ratios of PVA in the PVA/PULLblend electrospinning nanofibers. Moreover, with a higher mass ra-tios of PULL in the PVA/PULL blend nanofibers, superior thermalstability could be obtained at higher temperature (above 350 �C).Figs. 10–13 show TGA thermograms of various blend ratios ofPULL/PVA with 5 wt.% MMT content. The thermal stability of100/0, 90/10, and 80/20 blend ratios of PULL to PVA with 5 wt.%

Fig. 13. TGA data of electrospun PULL/PVA/MMT blend nanofibers with PULL/PVAmass ratio of 60/40 (Total polymer concentration = 14 wt.%, MMT concentra-tion = 5 wt.%, Applied voltage = 15 kV, and TCD = 15 cm).

Table 1Tensile strength of PULL/PVA/MMT blend nanofibers with various PULL/PVA blendratios by weight.

Blend ratioby wt.(PULL/PVA)

Totalpolymerconcentration

Tensilestrength(MPa)

Tensile strengthafter adding5 wt.% MMT(MPa)

Enhancedtensilestrength(MPa)

100/0 14 wt.% 5.70 ± 0.10 6.9 ± 0.10 1.20 ± 0.1090/10 5.90 ± 0.10 7.2 ± 0.10 1.30 ± 0.1080/20 6.10 ± 0.10 7.4 ± 0.10 1.30 ± 0.1060/40 6.60 ± 0.10 8.5 ± 0.10 1.90 ± 0.10

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MMT content is almost same due to the identical MMT structure(intercalation) within the aforementioned mass ratios of nanofi-bers (Figs. 10–12). On the other hand, nanofibers of 60/40 blend ra-tio of PULL to PVA with 5 wt.% MMT exhibit dramatic enhancementof thermal stability because of the exfoliated MMT structure in thatnanofibers (Fig. 13).

3.4. Tensile strength

The tensile strength of the PULL/PVA blend nanofibers increaseswith augmenting the weight percentage of PVA, as shown in Table1. Table 1 also shows the tensile strength of the PULL/PVA/MMTnanofibers that were electrospun with 100/0, 90/10, 80/20, and60/40 mass ratios of PULL to PVA at 14 wt.% total polymer concentra-tion and 5 wt.% MMT content. It is found that the tensile strength of100/0, 90/10, and 80/20 blend ratios of PULL to PVA with 5 wt.%MMT content is almost same due to the identical MMT structure(intercalation) within the aforementioned mass ratios of nanofibers(Table 1). On the other hand, nanofibers of 60/40 blend ratio of PULLto PVA with 5 wt.% MMT exhibit dramatic enhancement of tensilestrength because of the exfoliated MMT structure in the nanofibers(Table 1).

4. Conclusions

PULL/PVA/MMT nanofibers could be fabricated successfully byelectrospinning method out of aqueous solutions. The total polymerconcentration with various mass ratios of PULL to PVA at 5 wt.%MMT content is an important factor influencing the electrospinna-bility of the PULL/PVA/MMT solutions as well as thermal stabilityof the electrospun nanofibers. XRD and FTIR data exhibit good inter-actions between PULL and PVA caused by hydrogen bonds [9–11]. Itis evident from XRD data and TEM images that intercalated and exfo-liated MMT nanoplatelets can be obtained within the PULL/PVA/MMT nanofibers depending on the PULL/PVA blend ratios. More-over, the thermal stability and tensile strength of PULL/PVA/MMTnanofibers could be enhanced more by exfoliated MMT nanoplat-elets than intercalated structures of that nanoplatelets.

Acknowledgments

The support of this research by the Kyungpook National Univer-sity, South Korea, is gratefully appreciated. A.K. Das gratefullyacknowledges supports from the University of Dhaka, Bangladesh.

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