Role of Electrical Conductivity of Spinning Solution on Enhancement of Electrospinnability of Polyamide 6,6 Nanofibers

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1 Copyright 13 American Scientific Publishers All rights reserved Printed in the United States of America Journal of Nanoscience and Nanotechnology Vol. 13, 43 42, 13 Role of Electrical Conductivity of Spinning Solution on Enhancement of Electrospinnability of Polyamide 6,6 Nanofibers Su-Yeol Ryu and Seung-Yeop Kwak Department of Materials Science and Engineering, Seoul National University, 599 Gwanak-ro, Gwanak-gu, Seoul 1-744, Korea Optimal conditions for electrospinning of uniform polyamide 6,6 (PA66) nanofibers were determined by the control of various parameters, such as polymer solution concentration, flow rate, tip-tocollector distance (TCD), applied voltage, and electrical conductivity of the polymer solution. An organic salt, benzyl trimethyl ammonium chloride (C 10 H ClN, BTMAC), was added to the solutions for increase of electrical conductivity. When no salt was added to the PA66 solution, the uniform nanofibers were electrospun only at limited conditions, such as flow rate of 0.5 ml/h and electric fields greater than kv/cm. In contrast, by the addition of BTMAC, range of optimal conditions for uniform nanofibers was expanded; uniform nanofibers were obtained at flow rate of ml/h and electric fields greater than kv/cm. This means that the electrospinnability of the nanofibers is improved by increasing electrical conductivity of the solutions. Furthermore, the addition of BTMAC affected the increasing of number average diameters and standard deviation of the nanofibers. On the other hand, the process variables, such as flow rate, TCD, and applied voltage, exerted Delivered little influence by Publishing on thetechnology increase of diameters. to: Dental Library Seoul Natl Univ Keywords: Polyamide 6,6, Copyright: BTMAC, American Nanofiber, Scientific Electrospinning, Publishers Electrospinnability. 1. INTRODUCTION Nanomaterials demonstrate interesting physical and chemical properties compared with conventional materials. Various types of new materials, such as nanoparticle, nanorod, nanosphere and nanofiber, have been investigated. For the synthesis of new functional nanomaterials, materials structures, compositions and orientations in nanoscale have been actively controlled using various nanoscience and nanotechnologies. 1 3 In particular, various methods, such as self-assembly, 4 5 layer-by-layer (LBL) assembly, 6 7 Langmuir-Blodgett (LB) technique, 8 9 template synthesis, 10 vapor-phase deposition 11 and electrostatic processing technique, have been conducted for fabrication of unique nanostructures. The creation of new functional nanomaterials contributes significantly to the innovation of nanoscience and nanotechnologies. A nanofiber, which is fiber with diameter less than 1 m, 14 has many features including small diameter, high surface area-to-volume ratio, light weight and controllable pore structures compared to other conventional Author to whom correspondence should be addressed. fibers. So, nanofibers have been studied for various applications in fields of filtration, tissue engineering, sensor, protective materials, electronic and photonic materials and drug delivery. 27 These nanofibers have been fabricated using various ways, such as self-assembly, template synthesis, phase separation method, drawing method and electrospinning Among them, electrospinning is an electrostatic processing method for fabricating non-woven mats by applying high voltage to the polymer melt or solution. When the electric stretching forces overcome the surface tension of the polymer solution, electrically charged polymer jets are ejected. Electrospinning has many advantages, such as easy process, simple apparatus, and possibility of using various types of polymers in the melt or solution state. Also it is possible to prepare composite materials using various kinds of materials. There are many variables affecting the electrospinning process: (1) material variables, such as polymer solution concentration, solvent composition, viscosity and vapor pressure of the solution, polymer molecular weight, and electric conductivity of the solution, (2) process variables, such as the solution flow rate, tip-to-collector distance (TCD) and applied voltage, and (3) ambient variables, such as temperature and J. Nanosci. Nanotechnol. 13, Vol. 13, No /13/13/43/010 doi:10.16/jnn

2 Role of Electrical Conductivity of Spinning Solution on Enhancement of Electrospinnability of PA66 Nanofibers humidity. Through the control of these parameters, many conductivity meter (CP-500L, istek, Inc., Korea) at room researchers have looked upon the optimization of these temperature. nanofibers In particular, some researchers have studied the optimal 2.2. Electrospinning of PA66 Nanofibers electrospinning conditions for polyamide 6,6 (PA66). Tsai et al. electrospun PA66 nanofibers with an average diameter of 78 nm using formic acid as solvent. 37 Guerrini et al. using electrospinning apparatus (NanoNC, Korea). The PA66 polymer solutions were electrospun to nanofibers also electrospun PA66 nanofibers with average diameters equipment was composed of a rotating drum collector, of nm using formic acid. 38 This work discussed a high DC-voltage supply and a syringe pump basically. the influence of the PA66 molecular weight on the electrospinning. De Vrieze et al. studied the effect of the soling a steel needle with an internal diameter of 0.33 mm. The polymer solution was placed in a 10-mL syringe havvent system on the electrospinning. 39 This work presented A drum-type collector covered with aluminium foil was the steady state electrospinning parameters for PA66 nanofibers. These efforts are closely related to the establishment place 6 12 cm horizontally from the needle tip. The flow rates were controlled at ml/h by the syringe of electrospinning conditions for uniform nanofibers and pump during electrospinning. For preparation of electrospun nanofibers, the syringe needle tip was directly improving of the electrospinnability. charged by a positive voltage from to kv and the Improvement of electrospinnability influences on the collector was grounded by copper wire. This is the typical electrospinning setup that has been studied by many nanofiber productivity. Among the variables affecting the electrospinnability, in particular, electrical conductivity of research workers. All PA66 nanofibers were electrospun at polymer solution is an important variable. Enhancing the electrical conductivity reduces critical voltage applied to C and 45 55% humidity condition in the chamber. The optimized electrospinning conditions for uniform the polymer solution. So, the electric stretching forces PA66 nanofibers were determined based on nanofiber morphology with no beads or drops and stability of the Taylor can overcome the surface tension of the polymer solution under low electric field conditions. In addition, it is possible to produce uniform nanofibers under the faster solu- cone during electrospinning. tion flow rate. The present study focuses on presenting the optimized electrospinning Delivered conditions by Publishing for producing Technology uniform nanofibers using PA66 solution to: 2.3. Dental Characterization Library Seoul ofnatl PA66 Univ Nanofibers IP: with improved On: elec- Mon, 05 Aug 13 06:34:37 Copyright: American Scientific The morphologies Publishersof PA66 nanofibers were investigated trical conductivity. To enhance the conductivity, an organic salt is added to PA66 solution. Then, the solution concentration, applied voltage, tip-to-collector distance (TCD) and the solution flow rate are controlled to determine the optimized electrospinning conditions. The influence of the conductivity on the electrospinnability enhancement is studied. And the influence of variables on the nanofiber morphology is discussed. by using a field-emission scanning electron microscope (FE-SEM, JEOL JSM-6330F) at an accelerating voltage of 5 kv. The samples were placed on an aluminium holder using double-sided adhesive carbon tape and sputterdeposited with a thin platinum layer at ma for 300 s. 3. RESULTS AND DISCUSSION 3.1. Viscosity of PA66 Solutions for Electrospinning 2. EXPERIMENTAL DETAILS 2.1. Preparation of PA66 Solutions Polyamide 6,6 (PA66, average molecular weight 30.5 kda, Sigma Aldrich, USA) was dissolved in formic acid ( 95%, Sigma Aldrich, USA) to prepare polymer solutions of 10, and wt%. Benzyl trimethyl ammonium chloride (BTMAC, 99.0%, Tokyo Chemical Industry, Japan), as organic salt, was added in some of polymer solutions for enhancing the conductivity (1 wt%/pa66). These solutions were stirred for 12 h at 50 C until transparent and homogeneous solutions were obtained. All components were used directly without further purification. The viscosities of the solutions were measured using a Brookfield RVDV-II viscometer at room temperature. The conductivity of the solutions was measured using a Previous studies used sodium chloride (NaCl) as a salt for polyamide nanofibers. 40 In this study, benzyl trimethyl ammonium chloride (C 10 H ClN, BTMAC) was used as an organic salt. It was selected by considering the solubility and functionality. It has excellent solubility characteristics for various solvents such as formic acid, acetic acid, dimethylformamide (DMF), tetrahydrofuran (THF) and deionized water. And, it is studied and used for disinfection applications. The viscosity of the solution of polyamide 6,6 (PA66) in formic acid was measured and is shown in Figure 1. As more and more PA66 concentration increases, the viscosity increases significantly. For PA66/formic acid solution, we have determined that the addition of BTMAC has little effect on the viscosity change. Additionally, as the retention time of the PA66 solutions increased, the viscosities decrease gradually. This is due to the PA66 polymer chain scission during 44 J. Nanosci. Nanotechnol. 13, 43 42, 13

3 Role of Electrical Conductivity of Spinning Solution on Enhancement of Electrospinnability of PA66 Nanofibers Fig. 1. Effect the BTMAC addition on stability over time for fully dissolved PA66 solutions. the retention time. In the case of wt% PA66 solutions with or without BTMAC, the viscosities significantly decreased. As the concentration of PA66 increased, degree of viscosity decrease is increased. Therefore, for efficient electrospinning process, we conducted the electrospinning immediately as soon as the PA66 was completely dissolved in formic acid. polymer solution. It influences the electric stretching force exerted on the polymer jet. PA66 solution concentration affects the solvent evaporation rate during electrospinning. So it influences the nanofiber diameter and morphology. Applied voltage affects the strength of electric field that stretches the polymer jet into nanofiber. Hence it influences the nanofiber morphology. Tip-to-collector distance (TCD) is related to electric field. It affects the stretching duration and traveling distance of the polymer jet. Flow rate of polymer solution is closely connected with the volume of solution applied to the Taylor cone. It influences the nanofiber morphology. We controlled the parameters for electrospinning: whether the addition of the salt, PA66 solution concentration of 10 to wt%, DC voltage of to kv, tip-to-collector distance (TCD) of 6 to 12 cm and flow rate of 0.5 to 1.5 ml/h. The optimized electrospinning conditions for uniform PA66 nanofibers were selected by observing nanofibers morphology. It was verified with FE-SEM images. Also the conditions were selected by observing the shape of the Taylor cones and electrospun jets emitted from Taylor cones during electrospinning. It was confirmed visually. The electrospinning results are summarized in Tables I VI. The optimized conditions are shown in light gray regions, and the non-optimized conditions are shown in dark gray regions (Tables I VI) Establishment ofdelivered Optimal Electrospinning by Publishing Technology to: Dental Library Seoul Natl Univ Conditions We conducted an electrospinning process based on the various parameters, and selected the optimized electrospinning conditions for uniform PA66 nanofibers with no defects. Addition of salt affects electrical conductivity of 3.3. Optimal Electrospinning Conditions for PA66 Nanofiber Without Salt The optimized electrospinning conditions for preparation of PA66 nanofibers without salt are summarized in Table I. Optimal electrospinning conditions for 10 wt% PA66 Solution without salt. 0.5 / / / / / / / / / 6 J. Nanosci. Nanotechnol. 13, 43 42, 13 45

4 Role of Electrical Conductivity of Spinning Solution on Enhancement of Electrospinnability of PA66 Nanofibers Table II. Optimal electrospinning conditions for wt% PA66 Solution without salt. Table III. 0.5 / / / / / / / / / 6 Optimal electrospinning conditions for wt% PA66 Solution without salt. 0.5 / / / / / / / / / 6 Tables I III. For 10 wt% PA66 solution without salt, as shown in Table I, uniform nanofibers were electrospun only at flow rate of 0.5 ml/h. In this case, uniform nanofibers were prepared under electric fields greater than kv/cm. The morphology of electrospun nanofiber in these parameters is shown in Figure 2. Although it was under high electric fields, uniform nanofibers were able to be prepared. The average diameter of the nanofibers is ±.91 nm. In contrast, at flow rate of 1.0 ml/h, Taylor cones were formed unstable at the end of tip. Nanofibers with beads were obtained in this condition. At flow rate of 1.5 ml/h, small droplets were formed and spattered from the end of tip. It did not generated sufficient electrostatic repulsion for the faster flow rate. For wt% PA66 solution without salt, the range of conditions which shows the possibility of preparation for the uniform nanofibers is significantly reduced (Table II). In this case, uniform nanofibers were electrospun under stronger electric field, compared with the results of Table I. At TCD of 6 cm, the nanofibers were electrospun regardless of the flow rate. The morphology of the uniform nanofiber is shown in Figure 3. The average diameter of the nanofibers is 1.86±.73 nm. The nanofibers diameter increases with increasing of PA66 solution 46 J. Nanosci. Nanotechnol. 13, 43 42, 13

5 Role of Electrical Conductivity of Spinning Solution on Enhancement of Electrospinnability of PA66 Nanofibers Table IV. Optimal electrospinning conditions for 10 wt% PA66 Solution with BTMAC. Table V. 0.5 / / / / / / / / / 6 Optimal electrospinning conditions for wt% PA66 Solution with BTMAC. 0.5 / / / / / / / / / 6 concentration from 10 to wt%. During the electrospinning, when a jet is extracted from the Taylor cone by the electrostatic repulsion, the solvent begins to evaporate. For large amount of the PA66 solution concentration, more solvent is needed to keep the PA66 dissolved. So, it quickly reaches a critical amount of solvent to keep the PA66 dissolved in the liquid phase. This phenomenon occurs faster when the concentration is higher. It means that a faster coagulation of PA66 nanofibers and formation of thicker nanofibers occurred. For wt% PA66 solution without salt, we were not able to prepared uniform nanofibers under most conditions. Uniform nanofibers were prepared under limited conditions (Table III). The morphology of the nanofiber is shown in Figure 4. The average diameter of the nanofibers is 2. ±.40 nm. The nanofibers diameter slightly increases with increasing of PA66 solution concentration from to wt%. With increasing concentration of polymer solution, the viscosity of the solution is increased gradually. So, the J. Nanosci. Nanotechnol. 13, 43 42, 13 47

6 Role of Electrical Conductivity of Spinning Solution on Enhancement of Electrospinnability of PA66 Nanofibers Table VI. Optimal electrospinning conditions for wt% PA66 Solution with BTMAC. 0.5 / / / / / / / / / 6 Fig. 2. Morphologies of electrospun PA66 nanofibers without salt at 10 wt%, 0.5 ml/h, 9 cm and kv: (a) low magnification and (b) high 48 Fig. 3. Morphologies of electrospun PA66 nanofibers without salt at wt%, 0.5 ml/h, 9 cm and kv: (a) low magnification and (b) high J. Nanosci. Nanotechnol. 13, 43 42, 13

7 Role of Electrical Conductivity of Spinning Solution on Enhancement of Electrospinnability of PA66 Nanofibers Copyright: American electrospun PA66 nanofibers without salt at Scientific Publishers Fig. 4. Morphologies of wt%, 0.5 ml/h, 6 cm and kv: (a) low magnification and (b) high surface tension of polymer solution droplet, formed at the end of nozzle tip, is increased. However, the electrostatic repulsion was not strong to overcome the surface tension under these electrospinning parameters. When voltage above kv was applied, the electrospinning system was unstable. It was unable to prepare the uniform nanofibers just by changing the strength of the electric field through the control of TCD and applied voltage. In order to produce uniform nanofibers effectively, materials variables such as solution conductivity, had to be controlled rather than process variables. So, benzyl trimethyl ammonium chloride (C10 H ClN, BTMAC) was selected as a salt to increase the electrical conductivity of the PA66 solution Optimal Electrospinning Conditions for PA66 Nanofiber with BTMAC The electrical conductivities of PA66 solutions were increased by BTMAC addition (1 wt%/pa66). Absolute amount of the salt was increased with increasing concentration of PA66 solutions. The solutions with the salt show higher electrical conductivity ( ms/cm) than that of the solutions without the salt ( ms/cm). So, many J. Nanosci. Nanotechnol. 13, 43 42, 13 Fig. 5. Morphologies of electrospun PA66 nanofibers with BTMAC at 10 wt%, 0.5 ml/h, 12 cm and kv: (a) low magnification and (b) high 49 changes of electrospinning conditions for uniform nanofibers were expected because of the increase of electrical conductivity of the solutions. The optimized electrospinning conditions for preparation of PA66 nanofibers with BTMAC are summarized in Tables IV VI. The control parameters for electrospinning were the same as that of PA66 solutions without salt. Effect of the salt addition on electrospinning conditions was confirmed. For 10 wt% PA66 solution with BTMAC, as shown in Table IV, uniform nanofibers were electrospun only at a flow rate of 0.5 ml/h. The morphology of electrospun nanofiber in these parameters is shown in Figure 5. The average diameter of the nanofibers is ±.34 nm. These uniform nanofibers were prepared under electric fields greater than kv/cm. Comparing for 10 wt% PA66 solution without the salt (Table I), the nanofibers were possible to be electrospun under lower electric field. Addition of BTMAC increases the charge density in extracted jets from the Taylor cone. The jet is formed more easily due to the self-repulsion of the excess charges under the electric field. So, uniform nanofibers are able to be electrospun even under lower electric field. For flow rate of 1.0 ml/h, Taylor cones were

8 Role of Electrical Conductivity of Spinning Solution on Enhancement of Electrospinnability of PA66 Nanofibers stable at the end of tip. But nanofibers with some beads were electrospun. At flow rate of 1.5 ml/h, Taylor cones were unstable. Also, small droplets were formed and spattered from the end of tip. For wt% PA66 solution with BTMAC (Table V), at flow rate of 0.5 ml/h, optimal electrospinning conditions showed a similar trend in the case of 10 wt% PA66 solution with BTMAC. At flow rate of 1.0 ml/h, the range of electrospinning conditions for uniform nanofibers is more expanded than that of wt% PA66 solution without salt. Also, electrospinning of uniform nanofibers were enabled at flow rate of 1.5 ml/h. The morphology of the uniform nanofiber is shown in Figure 6. The average diameter of the nanofibers is 3.96 ± nm. When the concentration is increased from 10 wt% to wt%, the increase of nanofiber diameter is larger compared with nanofibers electrospun form PA66 solutions without salt. For wt% PA66 solution with BTMAC, optimal electrospinning conditions are similar to the case of wt% PA66 solution with salt. Under most of the conditions, except for parameters of 1.5 ml/h flow rate and 12 cm TCD, uniform nanofibers were obtained (Table VI). The morphology of the nanofiber is shown in Figure 7. Copyright: American Scientific Publishers Fig. 7. Morphologies of electrospun PA66 nanofibers with BTMAC at wt%, 0.5 ml/h, 9 cm and kv: (a) low magnification and (b) high Fig. 6. Morphologies of electrospun PA66 nanofibers with BTMAC at wt%, 1.5 ml/h, 6 cm and kv: (a) low magnification and (b) high 40 The average diameter of the nanofibers is ± 62. nm. The nanofibers diameter increases with increasing of PA66 solution concentration of 10 to wt%. Phadke et al. studied the effects of salt addition on average diameter of polyacrylonitrile (PAN) nanofibers.45 They found that salt addition increases the viscosity of low-molecularweight PAN (50 kda) solution. But, in our case, the viscosity is not increased with the addition of BTMAC. For PA66 solution with BTMAC, the effect of the salt on nanofiber diameter should be considered as well as the increase of the concentration of the PA66 solutions. We think that entanglement of interpolymer chains of PA66 is increased during electrospinning with interaction between the amide groups of PA66 and BTMAC. So, the results of increasing diameter of the BTMAC-added PA66 nanofibers are result as both addition of the salt and increasing concentration of PA66 solution. Comparing the average diameter of nanofibers electrospun form P66 solution without the salt, we conclude that addition of BTMAC influence on the diameter distribution and number average diameter. PA66 nanofiber distributions are shown in Figure 8. Nanofibers with BTMAC have broader diameter distributions than those of nanofibers J. Nanosci. Nanotechnol. 13, 43 42, 13

9 Role of Electrical Conductivity of Spinning Solution on Enhancement of Electrospinnability of PA66 Nanofibers Fig. 8. Diameter distributions of PA66 nanofibers without salt and with BTMAC. without salt. The number average diameter and standard deviation of PA66 nanofibers were calculated based on the diameter distribution (Fig. 9). The nanofibers diameter increases with increasing of PA66 solution concentration regardless of having or not BTMAC. However, the degree of diameter increases is larger with addition of BTMAC. There are little differences in standard deviations of PA66 nanofibers without salt. For PA66 nanofibers with Fig. 9. The number average diameter and standard deviation of PA66 nanofibers. BTMAC, the standard deviations increase with addition of the salt. 4. CONCLUSIONS The wide establishment of optimal electrospinning conditions for uniform nanofibers is important because electrospinning conditions are affected by materials, process and ambient variables. It allows for comprehensive understanding of the variables for the electrospinning and optimal electrospinning conditions. In this study, electrical conductivity of PA66 solutions was increased by adding BTMAC as an organic salt. The optimal electrospinning conditions were expanded as the conductivity was improved: when no salt was added to the PA66 solution, the uniform nanofibers were electrospun only at flow rate of 0.5 ml/h and electric fields greater than kv/cm. On the other hand, when the salt was added, uniform nanofibers were obtained at flow rate of ml/h and electric fields greater than kv/cm. The expansion of optimal electrospinning conditions by improving conductivity, it means enhancement of electrospinnability and contribute to productivity improvement of uniform nanofibers. The addition of BTMAC affected the change of the average diameter of uniform nanofibers. When the salt was not J. Nanosci. Nanotechnol. 13, 43 42, 13 41

10 Role of Electrical Conductivity of Spinning Solution on Enhancement of Electrospinnability of PA66 Nanofibers added, the nanofibers had an average diameter of ±.91 to 2. ±.40 nm. In contrast, the nanofibers with increased electrical conductivity showed an average diameter of ±.34 to ± 62. nm. Also, addition of BTMAC influenced on the increase of diameter distribution. Acknowledgment: This research was supported by a grant from the Fundamental R&D Program for Technology of World Premier Materials funded by the Ministry of Knowledge Economy, Republic of Korea. References and Notes 1. S. Mandal, M. V. Lee, J. P. Hill, A. Vinu, and K. Ariga, J. Nanosci. Nanotechnol. 10, (10). 2. Y. Wang and W. Zhou, J. Nanosci. Nanotechnol. 10, 63 (10). 3. K. Ariga, M. Li, G. J. Richards, and J. P. Hill, J. Nanosci. Nanotechnol. 11, 1 (11). 4. A. Ajayaghosh, P. Chithra, R. Varghese, and K. P. Divya, Chem. Commun. 969 (08). 5. K. Ariga, J. P. Hill, M. V. Lee, A. Vinu, R. Charvet, and S. Acharya, Sci. Technol. Adv. Mater. 9, (08). 6. C. Y. Jiang and V. V. Tsukruk, Adv. Mater., 829 (06). 7. Y. Wang, A. S. Angelatos, and F. Caruso, Chem. Mater., 848 (08). 8. T. Michinobu, S. Shinoda, T. Nakanishi, J. P. Hill, K. Fujii, T. N. Player, H. Tsukube, and K. Ariga, J. Am. Chem. Soc. 128, (06). 13, (04). 9. S. Acharaya. A. Shundo, J. P. Hill, and K. Ariga, J. Nanosci. Nanotechnol. 9, 3 (09). 10. S. Kronholz, S. Rathgeber, S. Karthäuser, H. Kohlstedt, S. Clemens, and T. Schneller, Adv. Funct. Mater., 46 (06). 11. D. Byrne, E. McGlynn, K. Kumar, M. Biswas, M. O. Henry, and G. Hughes, Cryst. Growth Des. 10, 00 (10). 12. J. Miao, M. Miyauchi, T. J. Simmons, J. S. Dordick, and R. J. Linhardt, J. Nanosci. Nanotechnol. 10, 5507 (10). 13. M. P. Prabhakaran, L. Ghasemi-Mobarakeh, and S. Ramakrishna, J. Nanosci. Nanotechnol. 11, 3039 (11). 14. D. H. Reneker and O. Chun, Nanotechnology 7, 6 (96).. D. Li and Y. N. Xia, Adv. Mater., 11 (04).. Z. M. Huang, Y. Z. Zhang, M. Kotaki, and S. Ramakrishna, Compos. Sci. Technol. 63, (03).. Y. Caik, Q. Wang, Q. Wei, Q. You, F. Huang, L. Song, Y. Hu, and W. Gao, Int. J. Polym. Anal. Charact., 110 (10).. X. Y. Wang, C. Drew, S. H. Lee, K. J. Senecal, J. Jumar, and L. A. Samuelson, J. Macromol. Sci. Part A-Pure Appl. Chem. 39, 11 (02).. J. Kim, H. F. Jia, and P. Wang, Biotechnol. Adv., 296 (06).. S. Ramakrishna, K. Jujihara, W. E. Teo, T. Yong, Z. W. Ma, and R. Ramaseshan, Mater. Today 9, 40 (06).. X. H. Qin and S. Y. Wang, J. Appl. Polym. Sci. 102, 1285 (06).. R. Murugan and S. Ramakrishna, Tissue Eng. 12, 435 (06).. M. W. Jose, B. W. Steinert, V. Thomas, D. R. Dean, M. A. Abdalla, and G. Price, Polymer 42, 9955 (01).. J. Zheng, A. He, J. Li, J. Xu, and C. C. Han, Polymer 47, 7095 (06).. R. Gopal, S. Kaur, and S. Ramakrishna, J. Membr. Sci. 281, 581 (06). 26. E. R. Kenawy, G. L. Bowlin, and K. Mansfield, J. Control. Release 81, 57 (02). 27. Z. Ma, M. Kotaki, R. Inai, and S. Ramakrishna, Tissue Eng. 11, 101 (05). 28. S. G. Zhang and X. J. Zhao, J. Mater. Chem. 14, 82 (04). 29. K. Nakata, K. Fujii, Y. Ohkoshi, Y. Gotoh, M. Nagura, and M. Numata, Macromol. Rapid. Commun. 28, 732 (07). 30. D. H. Reneker and I. Chun, Nanotechnology 7, 6 (96). 31. P. X. Ma and R. Zhang, J. Biomed. Mater. Res. 46, 60 (99). 32. J. D. Hartgerink, E. Beniash, and S. I. Stupp, PNAS 8, 5133 (02). 33. H. Fong, I. Chun, and D. H. Reneker, Polymer 40, 4585 (99). 34. K. H. Lee, H. Y. Kim, H. J. Bang, Y. H. Jung, and S. G. Lee, Polymer 44, 4029 (03). 35. G. T. Kim, J. S. Lee, J. H. Shin, Y. C. Ahn, K. H. Jeong, C. M. Sung, and J. K. Lee, Microsc. Microanal. 10, 554 (04). 36. J. M. Deitzel, J. Kleinmeyer, D. Harris, and N. C. Beck Tan, Polymer 42, 261 (01). 37. P. P. Tsai, W. Chen, and J. R. Roth, International Nonwovens Journal 38. L. M. Guerrini, M. C. Branciforti, T. Canova, and R. E. S. Bretas, Mater. Res.-Ibero-am. J. Mater. 12, 1 (09). 39. S. De Vrieze, P. Westbroek, T. Van Camp, and K. De Clerck, J. Appl. Polym. Sci. 1, 837 (10). 40. S.-W. Park, H.-S. Bae, Z.-C. Xing, O. H. Kwon, M.-W. Huh, and I.-K. Kang, J. Appl. Polym. Sci. 112, (09). 41. C. Yao, X. Li, and T. Song, J. Appl. Polym. Sci. 114, 79 (09). 42. W. Wei, J.-T. Yeh, P. Li, M.-R. Li, W. Li, and X.-L. Wang, J. Appl. Polym. Sci. 1, 3005 (10). 43. L. Tan and S. K. Obendorf, J. Membr. Sci. 305, 287 (07). 44. C. Carrizales, S. Pelfrey, R. Rincon, T. M. Eubanks, A. X. Kuang, M. J. McClure, G. L. Bowlin, and J. Macossay, Polym. Adv. Technol., 1 (08). 45. M. A. Phadke, D. A. Musale, S. S. Kulkarine, and S. K. Karode, J. Polym. Sci. Pt. B-Polym. Phys. 43, 61 (05). Received: 1 December 11. Accepted: January J. Nanosci. Nanotechnol. 13, 43 42, 13