Materials Letters 65 (2011) 493–496
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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 . 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 . 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
R. Nirmala et al. / Materials Letters 65 (2011) 493–496
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 . 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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Electrical Conductivity (S/m)
0.4 a - Polyamide-6 b - Polyamide-6 + chitosan 1 wt% c - Polyamide-6 + Chitosan 1.5 wt% d - Polyamide-6 + Chitosan 2 wt%
Polyamide-6 Polyamide-6 + Chitosan 1% Polyamide-6 + Chitosan 1.5% Polyamide-6 + Chitosan 2%
-0.4 0.45 -0.6 -10.0
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 . 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)
Intensity (a. u.)
Polyamide-6 + Chitosan 2 wt%
Polyamide-6 + chitosan 1.5 wt%
Sheet Resistance (x 109 Ω / )
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.%.
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
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%
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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