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Macromolecules 2009, 42, 5278–5284 DOI: 10.1021/ma900657y
Electrospun Core-Shell Structure Nanofibers from Homogeneous Solution of Poly(ethylene oxide)/Chitosan Jian-Feng Zhang, Dong-Zhi Yang, Fei Xu, Zi-Ping Zhang, Rui-Xue Yin, and Jun Nie* State Key Laboratory of Chemical Resource Engineering, Key Laboratory of Beijing City on Preparation and Processing of Novel Polymer, Beijing University of Chemical Technology, Beijing, China 100029 Received March 27, 2009; Revised Manuscript Received June 5, 2009
ABSTRACT: The core-shell structure nanofibers of poly(ethylene oxide)/chitosan have been electrospun from the homogeneous solution of chitosan (CS, as shell) and poly(ethylene oxide) (PEO, as core). The preparation process of core-shell structure was quite simple and efficient without any complex electrospinning setup or post-treatment. The core-shell structure and major component of each layer had been characterized by TEM and further supported by SEM, XRD, DSC, and EDS studies. The blending ratio of PEO and CS, molecular weight of chitosan, and temperature of electrospinning were thought to be the key influence factors on the formation of core-shell structure. Because of the chitosan outer layer and shell thickness being controllable, the core-shell structure nanofiber would show a potential application for the biomedical fields involving wound care and tissue engineering.
Introduction Electrospinning is an attractive method to produce continuous micro- to nanoscale fibers. This method can be applied to polymer and polymer composite for special function or unique architecture, for example, core-shell structure. Polymeric nanofibers with core-shell structure have been attracting special attention in the past decade for the unique properties which could potentially be applied in the areas of catalysis, optical and electronical devices, and biomaterial (tissues engineering, drug delivery, and release system).1-4 Up to now, several methods have been developed to prepare core-shell structure of polymeric nanofibers by electrospinning. For example, in the approach of template, polymeric fibers (template) were produced by ordinary electrospinning, and then the fibers were coated with the shell component by chemical vapor deposition (CVD)5-7 or surfaceinitiated atom transfer radical polymerization (ATRP).8 More recently, the coaxial electrospinning was reported as an effective and versatile process for the fabrication of core-shell structure. By this technique, the functional component could be encapsulated as the core or coated as the shell; i.e., ferritin nanoparticles/ poly(2-acrylamido-2-methyl-1-propanesulfonic acid),9 poly(N-isopropylacrylamide)/poly(methyl methacrylate),10 polycarbonate/poly(methyl methacrylate),11 and chitosan/poly(ethylene oxide)12 have been electrospun for special applications. Furthermore, nanofibers with single-channel13-15 or multichannel16 structure could also be prepared by coaxial electrospinning. A comprehensive review on the issue of coaxial electrospinning has been demonstrated by Moghe and Gupta.17 However, novel strategies, electrospun of emulsion and homogeneous solution, could obtain the core-shell structure as well. In the case of emulsion electrospinning, the core-shell structure was generated due to the stretching and coalition of emulsion during the electrospinning.18,19 In the case of homogeneous solution electrospinning, the formation of core-shell structure might attribute to the phase separation of polymer blends on a larger length *Corresponding author: e-mail
[email protected]; Tel þ86 010 64421310; Fax þ86 010 64421310. pubs.acs.org/Macromolecules
Published on Web 06/24/2009
scale, different solubility of the two components, and some other rheological factors.20-22 Chitosan (CS), a derivative from chitin, is one of the most abundant polysaccharides. Chitosan has several biological properties such as biocompatible, biodegradable, antimicrobial, wound acceleration, hemostatic, and immune system stimulation, which make it an attractive biomaterial.23,24 At present, electrospinning is one of the facilitative ways to process chitosan for the application of drug delivery, tissue engineering scaffolds, and wound healing dressings.23-27 In order to improve the inherent poor electrospinnability of chitosan, chitosan derivatives or fiber-forming additives such as poly(ethylene oxide) (PEO) and poly(vinyl alcohol) (PVA) were employed to help the fabrication of chitosan nanofibers.12,25-27 Since the features amphiphilic and unique mechanical properties of PEO, chitosan blended with PEO had been electrospun into nanofibers successfully. Zhang and co-workers utilized ultrahigh-molecular-weight PEO in order to minimize the amount of PEO in nanofibers.25 The nanofibers prepared by this technique were obtained with uniform internal structure. The reason might be due to the intensive polar solvent (dimethyl sulfoxide, DMSO) and low PEO fraction which would go against phase separation of the blending system. However, the PEO/CS core-shell structure nanofibers could be electrospun by the technique of coaxial electrospinning, demonstrated as Ojha et al., which is known as an effective method to produce core-shell structure, while this study is aimed at pure CS nanofiber, which could be obtained after removing the shell layer (PEO).12 Here, a facile way for the PEO/CS core-shell structure nanofiber from homogeneous solution by single nozzle electrospinning was reported. Different from the previous researches,12,25,28,29 no coaxial nozzle was employed; the core-shell structure nanofiber was directly electrospun from homogeneous polymer solution. Most of all, the nanofiber was obtained with chitosan in shell while PEO in core, which was proved by the characterization of TEM, SEM, DSC, XRD, and EDS studies. The core component (PEO) would enhance the mechanical performance of the fibers, whereas the shell component (CS) is biocompatible, which could be utilized directly in biomedical r 2009 American Chemical Society
Article
fields. The advantages of two biocompatible polymers, PEO and CS, were combined well. Moreover, the thickness controllability of shell layer would be of great significance to the biomaterial field. Experimental Details Material. Chitosan oligosaccharide and chitosan (about 88% deacetylated) were obtained from Zhejiang Golden-Shell Biochemical Co. (Yuhuan, Zhejiang, China) with the molecular weights of 3000, 10 000, 50 000, and 200 000 g/mol. Poly(ethylene oxide) (PEO) with molecular weight of 900 000 g/mol was supplied by Acors Organics. The deionized water was used as solvent and acetic acid (HAc, 99.8%) was induced as cosolvent. All of the materials were used without further purification. Electrospun Solution. Chitosan oligosaccharide aqueous solutions (Mw =3000 and 10 000 g/mol) were prepared by dissolving them into deionized water. Chitosan (Mw = 50 000, and 200 000 g/mol) were dissolved by a component solvent (volumedeionized water/volumeHAc= 97/3). PEO was added directly to the chitosan solution and then stirred in room temperature for 24 h. The PEO and chitosan were blended with different weight ratios, but the total polymer concentration was kept at 5.0% (w/v) in all experiments. The detailed information on the electrospinning solution is listed in Table 1. Electrospinning. The electrospinning setup used in the experiments is schematically shown in Figure 1. The electrospun nozzle was connected through PE (polyethylene B. Braun Melsungen AG, length = 1.5 m) tubing to a plastic syringe, and the solution was supplied by a syringe pump which maintained at a constant feed rate of 0.3 mL/h. An aluminum foil with the dimension of 30 30 cm was used as the collector. A constant positive high voltage (15 kV) generated by a power supply (BGG4-21, BMEI CO., Ltd.) was applied between the electrospun nozzle and collector with a nozzle-to-collector distance of 20 cm. All the experiments were carried out at room temperature except one which was carried out at 70 C. These parameters were chosen from an optimization of a series experiments. Characterization. The surface morphology of the electrospun fibers was observed by scanning electron microscopy (SEM, S4700 Hitachi) by using an accelerating voltage of 20.0 kV. The core-shell structure and cross section of the nanofiber were characterized by transmission electron microscopy (TEM, S800 Hitachi, Tecnai G2 20 S-TWIN FEI, JEM-100CX JEOL). The shell layer component was characterized by the TEM (JEM3010 JEOL) with an energy dispersive spectrometer (EDS). Wide-angle X-ray diffraction (WAXD, D/max 2500 VB 2þ/PC, Rigaku) and differential scanning calorimetry (DSC, Q100 TA Instruments) were induced to reveal the crystal structure of the electrospun fibers. The XRD patterns were recorded with Cu KR radiation (λ=0.154 18 nm). The sample was sealed in an aluminum pan and heated in the temperature range 0-165 C in the DSC instrument with a rate of 10 C/min.
Results and Discussion Internal Structure and Surface Morphology of the PEO/CS Nanofiber. Figure 2 displays the internal structure of PEO/ CS nanofibers. The contrast in the TEM images demonstrated the internal structure of the nanofibers. The dark region in the images was core, and the bright region was shell. As shown in Figures 2A to 2C, core-shell structure with sharp interface boundary could be electrospun from the PEO/CS (Mw =10 000 g/mol) mixture with different blending weight ratio of PEO/CS. It could be found that an increase of PEO fraction caused a decrease of the shell layer thickness (Table 2), which was reflected by the core to fiber diameter ratio changed from 0.694 to 0.922. Cross-sectional TEM image of the nanofiber (WPEO/WCS = 1/3), shown in
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Table 1. Electrospinning Solution Parameters of the PEO/CS Blending System component
PEO/CS weight ratio
PEO/CS (Mw = 3000 g/mol) PEO/CS (Mw = 10 000 g/mol) PEO/CS (Mw = 10 000 g/mol) PEO/CS (Mw = 10 000 g/mol) PEO/CS (Mw = 50 000 g/mol)
3/1 1/3 1/1 3/1 3/1
PEO/CS (Mw = 200 000 g/mol)
3/1
solvent deionized water deionized water deionized water deionized water Vdeionized water/ VHAc = 97/3 Vdeionized water/ VHAc = 97/3
Figure 1. Schematic representation of the electrospinning setup.
Figure 2H, also demonstrated a well-defined core-shell structure. However, when the electrospinning temperature was increased to 70 C, the core-shell structure of the nanofiber changed into uniform structure, and the TEM image of the nanofiber presented with single contrast (Figure 2D). Nanofibers with core-shell structure could also be electrospun from the solutions of PEO and CS with the molecular weight of 3000, 50 000, and 200 000 g/mol, as shown in Figures 2E to 2G. As seen from Table 2, the core to fiber diameter ratio of PEO/CS (Mw=3000 g/mol) nanofiber was the smallest, while the date of PEO/CS (Mw =10 000 g/ mol) nanofiber was the largest, and the core to fiber diameter ratio decreased sharply in aqueous acetic acid solutions from 0.922 to less than 0.9. In TEM characterization, the electron beam would be diffracted by crystal structure. Diffraction of the electron beam would represent in the form of contrast in the image, known as diffraction contrast. Chitosan and PEO are known as highly crystalline polymers.29 Crystallization of PEO carried out during the electrospinning process, while crystallization of chitosan was blocked, which could be concluded from DSC results. The above results obtained from TEM images led the authors to the assumptions that the major constituent of core layer (dark region) was PEO, while the major constituent of shell layer (bright region) was CS. The trend of diameter ratio of core to fiber supported the assumption (Table 2). The formation of core-shell structure might be due to the binodal phase separation of the ternary system. Additionally, the influence of chitosan molecular weight on diameter ratio of core to fiber could be explained by the mobility and interaction of the polymer chain. For the lower molecular weight and therefore strong mobility of CS (Mw = 3000 g/mol), phase separation of the blending processed more easily, resulting in a small core to fiber diameter. The entanglement of PEO and chitosan with higher molecular weight blocked the phase separation; the core to fiber diameter ratio would be larger, from 0.826 to 0.922 in aqueous solution and 0.845 to 0.873 in aqueous acetic acid solution. However, the reason for the sharp decrease of the
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Figure 2. TEM images of the PEO/CS nanofibers. (A-D) Nanofibers prepared by PEO and chitosan oligosaccharide (Mw = 10 000 g/mol) with different weight ratios: (A) WPEO/WCS = 1/3, (B) WPEO/WCS = 1/1, (C) WPEO/WCS = 3/1, and (D) WPEO/WCS = 3/1 (electrospun at the temperature of 70 C). (E-G) Nanofibers produced by PEO and chitosan (Mw = 3000, 50 000, and 200 000 g/mol) with the weight ratio of 3/1 (WPEO/WCS = 3/1). (H) Cross-sectional TEM image of the PEO/CS nanofiber (WPEO/WCS = 3/1, with the chitosan molecular weight of 10 000 g/mol). The electrospinning polymer solutions concentrations were kept at 5.0% (w/v). Table 2. Summary of Fiber Diameter Data Obtained from TEM Images component
blend ratio
mean diameter of core layer (nm)
mean diameter of nanofiber (nm)
diameter ratio of core to fiber
PEO/CS (Mw = 3000 g/mol) PEO/CS (Mw = 10 000 g/mol) PEO/CS (Mw = 10 000 g/mol) PEO/CS (Mw = 10 000 g/mol) PEO/CS (Mw = 50 000 g/mol) PEO/CS (Mw = 200 000 g/mol)
3/1 1/3 1/1 3/1 3/1 3/1
492.97 123.48 122.34 199.12 288.72 432.70
597.08 177.92 145.88 216.05 341.74 495.37
0.826 0.694 0.837 0.922 0.845 0.873
core to fiber diameter ratio in aqueous acetic acid solutions might be that the chitosan transform from bundles of rigid rods to flexible polymer chains.29 Therefore, the stronger mobility of chitosan polymer chain would be helpful to phase separation. The reason for the absence of core-shell structure in the nanofibers prepared at 70 C could be that nucleation and growth of PEO crystal could not process because melting temperature of PEO is lower than 70 C.30 So, there was no phase separation of the ternary system during the fiber formation. After the fibers were removed from high-temperature atmosphere, the polymer chains were miscible at molecular level for the evaporation of solvent. In order to determine the major component of the shell layer, the surface morphology of the nanofibers rinsed by alcohol or deionized water was characterized by SEM. Figures 3A and 3B were the nanofibers electrospun from the solution of PEO and CS with the molecular weight of 3000 g/mol. Before being rinsed in alcohol, nanofibers presented a smooth surface and uniform diameter. After soaking the nanofiber in alcohol for 24 h, the separated nanofibers adhered to each other, however, without changing the shape and morphology. Nanofibers prepared by PEO and chitosan (Mw =10 000 g/mol) blending are shown in Figures 3C and 3D. Similar shape and morphology could be observed before and after the immersion of nanofiber in alcohol. However, serious adhesion and swelling occurred when rinsing the PEO/CS (Mw =50 000 g/mol) nanofiber in deionized water (Figures 3E and 3F); similar results were obtained for the PEO/CS (Mw=200 000 g/mol, no picture is shown here). Because PEO is alcohol- and water-soluble; chitosan oligosaccharide could be soluble in water but not in
alcohol, and chitosan cannot be dissolved in deionized water without adding acetic acid. The shape and morphology of the nanofiber maintained after immersion in alcohol or deionized water which meant that the shell layer was CS. However, adhesion and swelling of the PEO/CS (Mw = 50 000 g/mol) nanofiber might be due to the residual acetic acid, which was difficult to remove.29 Therefore, the morphology of the fiber would be destroyed for the chitosan dissolution. The results obtained from SEM could support the assumption that the component of shell layer was chitosan or chitosan oligosaccharide. Generally, with the increase of the electrospun solution viscosity and the polymer chain entanglement, the diameter of resulting fibers would also increase.31,32 However, that is not the case for the PEO/CS blending system.29 Fiber diameter from Figure 3A was the largest based on the fixed blending ratio and total polymer concentration, which shows a reverse trend with the chitosan molecular weight, as shown in Figures 3A, 3C, and 3D. The reason for this interesting phenomenon, we speculated, might be due to the fact that the chitosan is a polyelectrolyte. The viscid electrospun solution would result in more charge accumulation on the surface of the Taylor cone, which would enhance the stretching effect during the electrospinning process. X-ray Diffraction (XRD) and Differential Scanning Calorimetry (DSC). Figure 4A shows the XRD patterns and indexations33 of pure PEO powder and nonwoven fiber. Pure PEO powder showed a strong reflection at 23.20 and a relative weak reflection at 19.03. However, reflection of PEO nonwoven was strong at 19.52 while relative weak at 23.94, which was contrary to PEO powder. After blending
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Figure 3. SEM images of the PEO/CS nanofiber before and after treated with alcohol or water. (A), (C), and (E) were the samples before rinsing; (B) and (D) were rinsed by alcohol; (F) was rinsed by deionized water. (A) and (B) were prepared by PEO and chitosan oligosaccharide (Mw=3000 g/mol), (C) and (D) were prepared by PEO and chitosan oligosaccharide (Mw = 10 000 g/mol), and (E) and (F) were prepared by PEO and chitosan (Mw= 50 000 g/mol). All of the samples were prepared with the PEO/CS weight ratio of 3/1.
with chitosan, reflection of PEO at 23 decayed even more weakly presenting with a flat pattern. The reasons for this variation could be attributed to the following factor. PEO crystals of electrospun nanofiber were orientated for the tension effect during electrospinning process. Crystal planes of 032 and 112 had weak diffraction intensity in equator as the PEO chains were paralleled to fiber axes.33 However, reciprocal vectors of PEO powder were random, the diffraction pattern was presented as diffraction ring. Furthermore, comparing the full width at half-maximum (fwhm) of PEO powder and nonwoven (Table 3), it could be considered that the crystallinity degree of PEO decreased by the electrospinning process for the rapid evaporation of solvent. The DSC results (Table 4) also confirmed the above conclusion; the melting enthalpy of nanofibers electrospun at room temperature was less than powder. Figures 4B, 4C, 4E, and 4F are the XRD and DSC curves of the electrospun nonwoven prepared by PEO and CS blending system. Figures 4B and 4E show the change in the crystallinity of PEO and CS (Mw=10 000 g/mol) with blend ratio. The fwhm of the reflection at 032 (Table 3) shows the downtrend with the increase of PEO mass, while the melting enthalpy (Table 4) presents an increased tendency. The variation suggested that the mass of chitosan had a strong influence on the crystallinity degree of PEO. PEO and chitosan would separate into core and shell layer during the electrospinning process. However, the two polymers could not separate completely because of polymer chain entanglement and rapid fiber formation process. The crystal
lattice of PEO was inhibited and disturbed by chitosan, presenting peak decrease and disappearance at 032 and 112. Therefore, the fact that the core to fiber diameter ratio was not consistent with the blend ratio could be understood. However, a more perfect crystal formed when the electrospinning temperature was elevated to 70 C. The fwhm of PEO/CS nonwoven at 032 and 112 reduced by 0.04 and 0.07, respectively (Table 3), and the melting enthalpy was beyond PEO powder (Table 4), compared with room temperature. These significant differences might be that the higher electrospinning atmospheric temperature (which is higher than PEO melting temperature30) would lead to no crystallization of PEO during the fiber formation, while a further crystallization of PEO carried out after removing the nanofibers from the high-temperature atmosphere. Because the metal collector was cooled by radiation, the crystallization could proceed further. As the XRD patterns show (Figure 4C), the molecular weight of chitosan also had a strong effect on the crystallization of PEO. Nanofibers electrospun from the PEO and chitosan (Mw=10 000 g/mol) mixture had the smallest fwhm value of 0.52 with the reflection at 032 and 0.65 at 112 (Table 3), meaning a more prefect crystal. The reflection at 112 could not be detected by MDI Jade in the curves of PEO/ CS (Mw =3000 g/mol) and PEO/CS (Mw =200 000 g/mol). The absence of the reflection peak at 112 could be due to the inhibition of crystallization by the polymer chain interaction. In fact, too strong or weak chitosan chain mobility would be harmful for the crystallization of PEO.
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Figure 4. XRD and DSC curves of PEO/CS fibers. (A) XRD patterns of PEO nonwoven and powder. (B) Changes in XRD patterns of PEO/CS nanofibers with different weight ratio and electrospinning temperature. (C) Changes in XRD patterns of PEO/CS nanofibers with CS molecular weight. (D) DSC curves of PEO and CS powder. (E) DSC curves of PEO/CS nanofiber with different weight ratio. (F) DSC curves of PEO/CS nanofibers with different CS molecular weight and electrospinning temperature. Table 3. XRD Data for PEO Powder, Nonwoven, and PEO/CS Nanofibers 120 component PEO powder PEO nonwoven PEO/CS (Mw = 3000 g/mol) PEO/CS (Mw = 10 000 g/mol) PEO/CS (Mw = 10 000 g/mol) PEO/CS (Mw = 10 000 g/mol) PEO/CS (Mw = 10 000 g/mol, T = 70 C) PEO/CS (Mw = 50 000 g/mol) PEO/CS (Mw = 200 000 g/mol)
032, 112
blend ratio
crystallization peak (2θ/deg)
full width at halfmaximum (fwhm/deg)
crystallization peak (2θ/deg)
full width at halfmaximum (fwhm/deg)
0.38 0.50 0.60 0.58 0.55 0.52 0.48
23.20 23.94
0.66 0.87
3/1 1/3 1/1 3/1 3/1
19.03 19.52 19.11 19.01 19.14 19.03 19.05
23.04 23.03
0.65 0.59
3/1 3/1
18.96 18.93
0.56 0.62
23.04
0.69
The reflection at 032 and 112 of PEO/CS (Mw=50 000 g/mol) nanofibers could be detected; however, the fwhm was larger than that of PEO/CS (Mw=10 000 g/mol) nanofiber. The DSC result, displayed in Figure 4F and Table 4, confirmed the
results obtained from XRD, which showed a fact that the PEO/CS (Mw =10 000 g/mol) nanofiber had a higher crystallinity degree. The reason could be the mobility of chitosan chain induced by acetic acid. The easily movement of the
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Table 4. Enthalpy (ΔHm) of PEO Powder and PEO/CS Nanofibers component PEO powder PEO/CS (Mw = 10 000 g/mol) PEO/CS (Mw = 10 000 g/mol) PEO/CS (Mw = 10 000 g/mol) PEO/CS (Mw = 10 000 g/mol, T = 70 C) PEO/CS (Mw = 50 000 g/mol)
blend ratio
ΔHm (J/g) (PEO)
1/3 1/1 3/1 3/1
202.6 53.20 70.78 164.13 266.8
3/1
102.3
chitosan polymer chain would block the crystallization of PEO. Shell Layer Component Analysis by EDS. The shell layer composition of the nanofibers was investigated by using a transmission electron microscopy with energy dispersive spectroscopy (EDS) in order to corroborate that the major composition of shell was chitosan. The EDS spectrum and TEM image are shown in Figure 5. The inset red circle within the confines of shell layer in the TEM image was for EDS study. The elements C, N, O, and Cu were detected. However, the strong signal of C and Cu arose from the carboncoated copper which was the substrate carrying PEO/CS nanofibers. The signal of N and O demonstrated that chitosan was detected in the shell layer, which accorded with the above conclusion. The reason for the accumulation of chitosan in the shell layer might ascribe to the amino cation of the chitosan. Positive charged amino groups of chitosan were generated in aqueous solution, since the chitosan was polycation. During the electrospinning, the charged groups preferred to move out with the effect of electrical field and electrostatic repulsion, which might be another important factor for the formation of core-shell structure. Correlationship of Binodal Phase Separation and CoreShell Structure. In an effort to elucidate the formation of core-shell structure, it is essential to establish the ternary phase diagram of polymer-polymer-solvent system to guide the track of the proportion of ingredients as the evaporation of solvent. Depending on the evaporation and cooling into unstable or metastable regions, phase separation is known to occur by nucleation and growth (NG, binodal phase separation) or by spinodal decomposition (SD, spinodal phase separation). This task may be implemented in the context of Gibbs free energy of the blending system, which had been discussed by Scott34 ΔGmix ¼ RTV=Vs ½jS ln jS þðjA =xA Þ ln jA þðjB =xB Þ ln jB þχAB jA jB þχAS jA jS þχBS jB jS ð1Þ where R is a constant of 8.314 J/(K mol), T is absolute temperature, V is the total volume of the blending, Vs is equal to the molar volume of the solvent, j is the volume fraction of each component, xA and xB are the degrees of polymerization of polymer A and B, and χAS, χAB, and χBS are the binary interaction parameters, which could be written in the terms of Hildebrand solubility parameters:34 χ12 ¼
Vr ðδ1 -δ2 Þ2 RT
ð2Þ
where χ is the Flory-Huggins interaction parameter, Vr is a reference volume which is taken as close to the molar volume of the smallest polymer repeat unit as possible, and δ1 and δ2 are the Hildebrand solubility parameters for the each component of the blending. The solubility parameter δi of substance i is defined as34 P F Fi δ ¼ ð3Þ M
Figure 5. EDS spectrum and TEM image of PEO/CS nanofiber; the inlet circle within the confines of shell layer was for the EDS study. Table 5. Group Molar Attraction Constants32 group
molar attraction ((J cm3)1/2/mol)
-CH2æCH-O- ether -O- acetal -OH-H bonded -NH2 ring, nonaromatic 6-member
269.0 176.0 235 236 485 464 -48
where F is the density of the polymer at the temperature of interest, M is the Pmolecular weight of the repeat group in the polymer, and Fi is the sum of all the molar attraction constants of all the chemical groups in the polymer repeat unit. The group molar attraction constants are exhibited in Table 5.35 Therefore, χPEO-CS could be calculated from eqs 2 and 3 with the value of 0.75 (T = 298.15 K). The basic interpretative concept of Flory-Huggins interaction parameter is that a large positive value indicates unfavorable interaction, a low value indicates little interaction, and a negative value indicates a rather strong specific interaction.36 Since the χPEO-CS was larger than 0.5, PEO and chitosan were apt to separate into two phases. However, the FloryHuggins interaction parameter of polymer-solvent, χPEOH2O and χCS-H2O, could not be calculated as χPEO-CS because the strong hydrogen bond in the polymer solution might make χ smaller than 0.5 or even negative.37 An enormous variety of ternary phase diagrams can be calculated from eq 1. A typical phase diagram of the blending system (polymer-polymer-solvent) is often shown at a single temperature on an equilateral triangle phase diagram.34 Since the χPEO-CS = 0.75, χPEO-H2O = 0.4,30 and χCS-H2O < 0.5 (T=298.15 K), such a phase diagram could be predicted (Figure 6) for the PEO-CS-H2O blending system.34 The phase diagram was separated by a binodal curve into single-phase region and two-phase region. The initial concentration of PEO and chitosan is situated in the stable region outside the binodal curve (point X). Solvent removal during electrospinning would take the blending system to point Y. When the ternary blending system approaches the binodal curve, the phase separation process was by nucleation and growth (NG). PEO is first precipitated from the blending system as the nucleus for PEO crystal. As the further increase of polymer concentration, PEO crystals grow, and phase separation proceeded further simultaneously. In addition, the charged chitosan is driven acting under the electrical field and electrostatic repulsion; the rheological processes will occur simultaneously with the
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References and Notes
Figure 6. Phase diagrams for the blending system of PEO-CS-H2O.
formation of the phases to affect their morphology.36 Therefore, the core-shell structure was generated. The blend ratio of PEO and CS and electrospinning temperature would also influence the phase behaviors during the electrospinning process. With the variation of PEO/CS blending ratio, the initial and terminal system point in the phase diagram changed. Therefore, the track traveling from stable to metastable region alter, resulting in a different structure. If the temperature is higher than Tc (critical temperature), phase separation might not proceed because there is only a stable region in the phase diagram. Conclusion In the present work, a simple and versatile method was demonstrated for the fabrication of core-shell structure PEO/ CS nanofiber. Core-shell structure and sharp interface of the nanofiber could be observed easily in the TEM images. SEM images of the PEO/CS nanofiber before and after treated with alcohol or water were induced to verify that chitosan was the major component of the shell layer. The blending ratio of PEO and chitosan, the molecular weight of chitosan, and the electrospinning temperature had a significant influence on the coreshell structure. With the increase mass of CS, the thickness of shell layer would be enlarged, and the core-shell structure would vanish at a higher temperature of 70 C. Binodal phase separation of the ternary system was the basic mechanism of the formation of core-shell structure. Because the nanofiber was prepared from homogeneous polymer solution and the chitosan constructed the shell layer, it has several additional advantages: easily preparation without any complex electrospinning setup; desirable biocompatibility without any post-treatment for the chitosan outer layer; most of all, controllable core-shell structure by process parameters. Correlation of shell layer thickness and biodegradation time might contribute to the biomedical applications. Acknowledgment. The Program for Changjiang Scholars and Innovative Research Team in University is gratefully acknowledged. We gratefully thank Professor Hangquan Li (Beijing University of Chemical Technology, Beijing, 100029, China) for the helpful discussions.
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