chitosan composite nanofibers via electrospinning

chitosan composite nanofibers via electrospinning

Materials Letters 65 (2011) 493–496 Contents lists available at ScienceDirect Materials Letters j o u r n a l h o m e p a g e : w w w. e l s ev i e ...

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Materials Letters 65 (2011) 493–496

Contents lists available at ScienceDirect

Materials Letters j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / m a t l e t

Preparation and electrical characterization of polyamide-6/chitosan composite nanofibers via electrospinning R. Nirmala a, R. Navamathavan b, Mohamed H. El-Newehy c, Hak Yong Kim c,d,⁎ a

Bio-nano System Engineering, College of Engineering, Chonbuk National University, Jeonju, 561 756, South Korea School of Advanced Materials Engineering, Chonbuk National University, Jeonju 561 756, South Korea Petrochemical Research Chair, Department of Chemistry, College of Science, King Saud University, Riyadh 11451, Saudi Arabia d Center for Healthcare Technology and Development, Chonbuk National University, Jeonju, 561 756, South Korea b c

a r t i c l e

i n f o

Article history: Received 18 August 2010 Accepted 21 October 2010 Available online 29 October 2010 Keywords: Electrospinning Polyamide-6 Chitosan Nanofibers Electrical studies

a b s t r a c t We report on the preparation and electrical characterization of polyamide-6/chitosan composite nanofibers. These composite nanofibers were prepared using a single solvent system via electrospinning process. The resultant nanofibers were well-oriented and had good incorporation of chitosan. Current–voltage (I–V) measurements revealed interesting linear curve, including enhanced conductivities with respect to chitosan content. The electrical conductivity of the polyamide-6/chitosan composite nanofibers increased with increasing content of chitosan which was attributed to the formation of ultrafine nanofibers. In addition, the sheet resistance of composite nanofibers was decreased with increasing chitosan concentration. © 2010 Elsevier B.V. All rights reserved.

1. Introduction Electrospinning is a versatile technique to fabricate continuous fibers with diameters ranging from several micrometers to a few nanometers [1]. The high application potential of semiconducting polymers in chemical and biological sensors is one of the main reasons for the intensive investigation and development of these materials. The remarkable high surface area-to-volume ratio, small diameter and high porosity bring electrospun nanofibers highly attractive to ultrasensitive sensors and increasing importance in many technological applications [2,3]. Over the last few years, many synthetic strategies have been derived for the fabrication of one dimensional (1D) semiconducting polymer nanomaterials [4,5]. In particular, polyamide-6 is one such polymer that has been investigated the most due to its good mechanical and physical properties [6,7]. Moreover, the functional properties of these nanofibers can be improved by adding cross-linking agents like chitosan. Chitosan is a natural nontoxic biopolymer derived by the deacetylation of chitin, possessing unique polycationic and chelating properties due to the presence of active amino and hydroxyl functional groups [8]. Therefore, we take the advantage from both polyamide-6 and chitosan by blending them into composite nanofibers.

⁎ Corresponding author. Center for Healthcare Technology and Development, Chonbuk National University, Jeonju, 561 756, South Korea. Tel.: + 82 63 270 2351; fax: + 82 63 270 4249. E-mail address: [email protected] (H.Y. Kim). 0167-577X/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2010.10.066

In this work, we describe a one step preparation of polyamide-6/ chitosan composite nanofibers with a single solvent system via electrospinning. These composite nanofibers exhibited uniform twodimensional ultrafine networks. The morphological and electrical characteristics of these polyamide-6/chitosan composite nanofibers were investigated. We made an attempt to analyze the influence of ultrafine nanofibers on the electrical properties. 2. Experimental Polyamide-6 (KN120 grade, Kolon Industries, South Korea) and chitosan powder (degree of deacetylation = 85%, low molecular weight, Wako Pure Chemical Industries, Japan) were used in making the polymer solution. The composite nanofibers were produced by dissolving polyamide-6 pellets and chitosan powder in 85% formic acid (analytical grade, Showa, Japan). Polyamide-6 (18 wt.%) with different concentrations of chitosan with 0, 1, 1.5 and 2 wt.% was used to prepare the composite nanofiber mats. After that the polymer solution was loaded into a 5 ml plastic syringe equipped with a polystyrene micro-tip (0.3 mm inner diameter and 10 mm length), which was connected with a high-voltage power supply (CPS-60 K02V1, Chungpa EMT, South Korea). Electrospinning was performed at a voltage about 22 kV. A grounded iron drum was rotated at a constant speed by a DC motor to collect the developing nanofibers, which was kept at a distance of 15 cm from the micro-tip. All experiments were conducted at room temperature. The conductivity of polyamide-6/chitosan in solvent solution was measured by using a Brookfield DV-III programmable rheometer and

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an EC meter CM 40 G Ver 1.09 (DKK TOA Co., Japan). The morphology of the as-spun polyamide-6/chitosan composite nanofibers was observed by using field-emission scanning electron microscopy (FESEM, Hitachi S-7400, Hitachi, Japan) and transmission electron microscopy (TEM, JEM-2010, JEOL, Japan). Structural characterization was carried out by X-ray diffraction (XRD, Rigaku, Japan) operated with Cu-Kα radiation (λ = 1.540 Å). Silver metal contacts were made on the nanofiber mat to have good ohmic contact. Then the current– voltage (I–V) characteristic was measured for the polyamide-6/ chitosan composite nanofibers by using a semiconductor parameter analyzer (4200-SCS, Keithley). 3. Results and discussion Fig. 1(a)–(d) shows the FE-SEM images of electrospun polyamide6/chitosan composite nanofibers for the different concentrations of chitosan with 0, 1, 1.5 and 2 wt.%, respectively. These as-spun nanofibers exhibited a smooth surface and uniform diameters along their lengths. As shown in the figure, very clear arrangement of ultrafine mesh-like nanofibers strongly bound with the main fibers was observed. These ultrafine nanofiber structures resulted in a large surface area-to-volume ratio and interconnected porosity. The size of the ultrafine nanofibers (20 to 40 nm) is one order less than those of main fibers (200 to 400 nm). The density of high aspect ratio fibers increases with increasing chitosan content. It is believed that the formation of large surface area-to-volume ratio nanofibers was due to the strong applied voltage that was created between the electrodes. In order to study bonding of these ultrafine nanofibers, we further carried out TEM analysis. The TEM samples were obtained by placing the TEM grid very close to the syringe micro-tip end for very short time during electrospinning. Fig. 2 shows the TEM image of the nanofiber emerging from the syringe micro-tip. It is clearly seen from the TEM image that these ultrafine nanofibers bound in between the main fibers. Fig. 3 shows the electrical conductivity of the polymer solution in the solvent. The electrical conductivity was increased when the polyamide-6/chitosan composites were dissolved in formic acid demonstrating that enhanced amounts of free ions in the solution.

Fig. 2. TEM images of electrospun polyamide-6/chitosan composite nanofibers with chitosan content of 2 wt.%.

The polyamide-6 bearing reactive functional groups may yield reactions of chemical exchange when they are mixed with solvent resulted in a poly-electrolytic behavior of the solution [9]. At this stage, the reactive ions in the polymer solution drive further by the applied strong electric field. Then the solution become highly ionized state and forced to come out from the syringe can be aligned as high aspect ratio structures in between the main fibers by relaxing the electrical stress. The resulting morphology could have been responsible for the high electrical conductivity of the composite nanofibers. The crystalline structures of as electrospun polyamide-6/chitosan composite nanofibers were characterized by XRD, and the result was compared with that acquired from the pristine. The XRD patterns of the pristine and blended polyamide-6/chitosan composite nanofibers are shown in Fig. 4. The diffraction pattern of polyamide-6 nanofibers exhibited a broad peak appeared at 2θ = 20°. As shown in Fig. 4, the XRD data of blended polyamide-6/chitosan composite nanofibers were composed of their characteristic peaks at 2θ = 20 and 24° corresponding to the α1(200) and α2(200) phases, respectively.

Fig. 1. FE-SEM images of electrospun polyamide-6/chitosan composite nanofibers with different wt.% of chitosan (a) 0, (b) 1, (c) 1.5 and (d) 2 wt.%.

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However, for the lower chitosan content (1 and 1.5 wt.%) very feeble peaks (or no peaks) were observed as shown in the XRD data. The intensity was slightly increased with increasing chitosan content in the blended nanofibers. At the same time, another distinct peak at 2θ = 33.8° corresponding to the characteristic of the γ phase also appeared [10]. These results confirmed the successful blending of chitosan in polyamide-6 nanofibers via electrospinning process. Fig. 5(a) shows I–V characteristics of the polyamide-6/chitosan composite nanofibers. Interestingly, when the chitosan content was increased, the current was enhanced compared to that of the pristine polyamide-6 nanofibers. The formation of denser ultrafine nanofibers with addition of chitosan content showed a great improvement in I–V characteristics. The excess chitosan possibly enveloped the ultrafine fiber networks in between polyamide-6/ chitosan composite nanofibers can be enhanced the electrical pathways. Consequently, the electrical conductivity of the polyamide-6/chitosan composite nanofibers prepared with 2 wt.% chitosan exhibited the maximum current of 0.4 pA. We also believe that the enhanced porosity of these composite nanofibers can be utilized for the biosensor applications with improved performance and sensitivity. Sheet resistances were calculated from plots of the measured resistances versus the spacings between the metal α1(200)

α2 (200)

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Polyamide-6 + Chitosan 2 wt%

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Fig. 3. Electrical conductivity of polyamide-6/chitosan composites in formic acid solution with different chitosan concentrations of 0, 1, 1.5 and 2 wt.%.

b

a - Polyamide-6 b - Polyamide-6 + chitosan 1 wt% c - Polyamide-6 + Chitosan 1.5 wt% d - Polyamide-6 + Chitosan 2 wt%

120 100 80 60 40 20 0

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Samples Fig. 5. (a) I–V characteristics and (b) sheet resistance of electrospun polyamide-6/ chitosan composite nanofibers with different wt.% of chitosan.

contacts. Fig. 5(b) shows the sheet resistance of electrospun polyamide-6/chitosan composite nanofibers with different chitosan content. The sheet resistance was determined to be decreased from 120 to 23 × 109 Ω/□ with increasing chitosan content from 0 to 2 wt.%. It is worth noting that increased chitosan content leads to a significant reduction in the sheet resistance compared to that of the pristine polyamide-6 nanofibers. The decrease in sheet resistance of chitosan rich sample can be attributed to the highly denser ultrafine nanofiber structures. Our preliminary results strongly suggested that the formation of ultrafine structures play an important role on the electrical properties. As a next step, we plan to optimize the experimental parameters so as to apply for the device fabrications. 4. Conclusions

Polyamide-6 + Chitosan 1 wt%

Polyamide-6

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2θ θ (degree) Fig. 4. XRD patterns of electrospun polyamide-6/chitosan composite nanofibers with different chitosan concentrations of 0, 1, 1.5 and 2 wt.%.

Chitosan blended in polyamide-6 nanofibers with high aspect ratio structure is successfully prepared using a single solvent by electrospinning process. These as-spun nanofibers were observed to be smooth with uniform diameters along their lengths. High aspect ratio polyamide-6/chitosan composite nanofibers with diameters of about 20 to 40 nm were bound in between main fibers. Electrical conductivity of polymer solution increased with increasing chitosan content. The electrical characteristics of the polyamide-6/chitosan composite nanofibers increased with increasing content of chitosan, which was attributed to the formation of ultrafine nanofibers. The

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sheet resistance was determined to be decreased with increasing chitosan content. The significant enhanced electrical properties of this biodegradable blend can be utilized for quite promising future nanotechnological applications.

Acknowledgements This work was supported by the grant of the Korean Ministry of Education, Science and Technology (The Regional Core Research Program/Center for Healthcare Technology and Development, Chonbuk National University, Jeonju 561-756 Republic of Korea).

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