chitosan core-shell nanofibers by a stable emulsion system

chitosan core-shell nanofibers by a stable emulsion system

Colloids and Surfaces A 583 (2019) 123956 Contents lists available at ScienceDirect Colloids and Surfaces A journal homepage: www.elsevier.com/locat...

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Colloids and Surfaces A 583 (2019) 123956

Contents lists available at ScienceDirect

Colloids and Surfaces A journal homepage: www.elsevier.com/locate/colsurfa

Electrospinning of polycaprolacton/chitosan core-shell nanofibers by a stable emulsion system

T



Liang Ma, Xuejuan Shi, Xiaoxiao Zhang, Lili Li

Key Laboratory of Automobile Materials, Ministry of Education, and College of Materials Science and Engineering, Jilin University, Changchun, 130022, China

G R A P H I C A L A B S T R A C T

The PCL/CS nanofibers are electrospun by a surfactant-free emulsion system, and CS fibers are obtained by removing PCL shell.

A R T I C LE I N FO

A B S T R A C T

Keywords: Emulsion electrospinning Polycaprolactone Chitosan Core-shell Fiber

Electrospun core-shell structured nanofibers have got wide attention due to their excellent properties and potential applications. Encapsulated CS in PCL to form the core-shell structured fibers would be a festival way to expanding the applications of biopolymers. In this work, the electrospun core-shell polycaprolacone/chitosan (PCL/CS) composite nanofibers were prepared by a stable emulsion system. Different from other reported works, the surfactant-free and low toxic water-in-oil emulsion was selected to form the electrospinning precursor. The high stability of the emulsion was achieved by adjusting the volume ratio of the solvents and the concentration of the polymers. PCL/CS fibers with different core to shell ratios were obtained via the adjustment of the concentration of CS and PCL in emulsion system. The distribution of the composition in the fibers was analyzed by FT-IR, XRD, EDS, XPS and WCA. Further, intact ultrafine CS fibers with a mean diameter of 143 ± 49 nm could be obtained after the removal of PCL shell, indicating the low diffusivity between core and shell solutions during emulsion electrospinning process. This study provided a promising method to fabricate natural polymer fibers which were difficult to electrospin.

1. Introduction Polymer nanofibers have predominant physicochemical properties due to their interconnected pore structure, high specific surface area



and high porosity. Hence, they can be applied in various fields, such as water purification [1,2], self-cleaning material [3], biomedical engineering [4], drug delivery [5], filtration [6,7], etc. To further expand the applications of the fiber materials, core-shell structured fibers are

Corresponding author. E-mail address: [email protected] (L. Li).

https://doi.org/10.1016/j.colsurfa.2019.123956 Received 2 August 2019; Received in revised form 5 September 2019; Accepted 9 September 2019 Available online 10 September 2019 0927-7757/ © 2019 Elsevier B.V. All rights reserved.

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2. Experimental section

favored by researchers in recent years. Core-shell structured nanofibers exhibit both core and shell functions independently, which greatly expand their applications in controlled release of drugs, tissue engineering, supercapacitors, adsorbents and filtration [8–14]. In addition, the polymer nanofibers that are difficult to process can be obtained by post-treatment of core-shell fibers [15,16]. Several methods for fabrication of nanofibers including drawing, template synthesis, self-assembly, phase separation and electrospinning have been reported [17–20]. Among these methods, electrospinning, a versatile and more reliable method, has been widely utilized for its merits of simple operation, controllable structure and relatively high production efficiency. Coaxial electrospinning and emulsion electrospinning are two major techniques to fabricate the core-shell nanofibers [21,22]. Emulsion electrospinning is a convenient technology which can obtain the stable core-shell fibers by using a single nozzle compared with coaxial electrospinning [23,24]. The stability of forming emulsion is an important factor for emulsion electrospinning. The addition of the surfactant as emulsifier is a general method to prepare electrospun emulsion. The morphology and the properties of the fibers are affected by the species and the concentration of the surfactants. Hu et al. studied the influences of non-ionic, anionic and cationic surfactants on the morphology of core-shell fibers [25]. The results indicated that these surfactants had different electrostatic and hydrogen bonding effects due to their different chemical structures, which acted on the fiber morphology. Wang et al. added anionic surfactant (sodium dodecyl sulfate) and non-ionic surfactant (Span80) into W/O emulsions respectively and studied the influence in the core-shell structure of fibers [26]. Results showed that it was more likely to form continuous core in fibers with Span80 as the surfactant. While the addition of surfactant may result in some problems, such as the surfactant in fibers would affect the application of core-shell fibers [27]. Currently, some works have focused on the stable surfactant-free emulsion electrospinning. Li et al. mixed polycaprolactone (PCL) solution (dichloromethane/hexafluoroisopropanol (DCM/HFIP)) and silkfibroin (SF) solution (HFIP) to form a stable emulsion, which was electrospun to obtain PCL/SF coreshell nanofibers [28]. A. Camerlo et al. prepared a stable limonene/poly (vinyl alcohol) (PVA) emulsion by adding limonene dropwise to PVA aqueous solution, and fabricated core-shell fibers via emulsion electrospinning [29]. Biopolymers have been studied in recent years for their non-toxicity, good biocompatibility and biodegradation. Chitosan (CS) is a multifunctional biopolymer, which has a wide application prospect in bioengineering, antibacterial and other fields [30–36]. However, electrospinning pure CS is difficult on account of solvent limitation and low solubility [37]. And CS as cationic polymer has high charge density in solution, which can cause the inhomogeneous splitting of jets and nonuniform diameter distribution [38–40]. For making the best use of CS, the preparation of the CS fibers with uniform diameter and high specific surface area has attracted attention. To improve the spinnability, CS is often blended with synthetic polymers, such as PCL and poly(lactic acid) (PLA) [41–43]. PCL is one of wildly used biopolymer materials. It has good mechanical property, excellent processability and biological compatibility [44,45]. While it shows poor compatiblity with hydrophilic polymer such as CS, alginate and gelatin [46–48]. Thus, encapsulated CS in PCL to form the core-shell structured fibers may be a festival way to expanding the applications of biopolymers. In this work, a stable and surfactant free emulsion system was established. PCL/CS core-shell structured nanofibers were obtained without any additives via emulsion electrospinning. The immiscible formic acid/dichloromethane solvents were selected as electrospun solvent system first time due to the low toxicity in the biological field [43,49]. The morphologies and properties of the core-shell fibers were investigated in detail by altering the content of polymers. Meanwhile, the intact CS fibers were also expected to obtain by washing the shell component of the core-shell fibers with THF.

2.1. Materials PCL (Mw =50 kDa) and CS (viscosity < 200 mPa·s) were purchased from Aladdin Industrial Corporation, China. Formic acid (FA), dichloromethane (DCM), methyl orange (MO) and tetrahydrofuran (THF) were supplied by Beijing Chemical Works, China. Deionized water was obtained from the laboratory. The chemicals were all of analytical grade and used without further purification. 2.2. Preparation of the electrospinning emulsion PCL and CS (PCL/CS) with different weight ratios of 6/2, 8/2 and 10/2 (w/w) were dissolved in FA/DCM, respectively. FA and DCM consisted the co-solvent system by varying the volume ratio of 7/3, 5/5 and 3/7 (v/v). The prepared blends were continually stirred for 8 h at room temperature. 2.3. Fabrication of nanofibers via electrospinning Fig. 1 showed the schematic diagram of the electrospinning setup. The polymer emulsion was loaded in a 5 mL capacity of glass syringe connected with a 7 gauge metal needle. The electrospinning experiments were carried out at the collecting distance of 18 cm from needle tip to collector with the applied voltage of 13 kV. The feeding speed was 0.3 mL/h. These electrospinning parameters for core-shell structured fibers were achieved from a large number of experiments. The nanofibers were collected by aluminium foil collector and then dried in an oven at 35 °C to evaporate the solvent for the further research. 2.4. Measurements and characterization The viscosity of the electrospun emulsions were measured using a viscosimeter (NDJ-1, Yutong, China). The electrical conductivity of the emulsions were measured using a conductometer (DDS-307, Leici, China). The surface tension of emulsions were tested by a surface tension detector (JYW, Chengde, China). All measurements mentioned above were repeated 3 times. A light microscopy (BK-POL, OPTEC, China) was taken to survey the formation of the emulsion. The emulsion was dropped to a glass slide by a dropper. Then the image was focused and captured by the installed software. The composition and morphology of the electrospun fibers were observed by field emission transmission electron microscopy (FE-SEM) (JSM-6700 F, JEOL, Japan) equipped with the energy dispersive spectroscopy (EDS). The electrospinning fibers were sputtered with platinum under vacuum before taking digital photomicrographs from randomly selected areas at an acceleration voltage of 5 kV. The fiber diameter distributions were calculated by measuring fiber diameters of electrospun samples at 100 random positions via Image J software. The interior structure of nanofibers was observed by transmission electron microscope (TEM, JEM-2100 F, JEOL, Japan). The fibers of varied proportions of PCL and CS were directly electrospun on carbon coated grids and observed under 80 kV. Surface area, pore volume and pore size were characterized by N2 adsorption and desorption using a surface area and pore size analyzer (BET, Autosorb-iQ2, Quantachrome Instruments, China). The composition of the fibers was observed by Fourier transform infrared spectroscopy (FT-IR, FTIR-4100, JASCO, Japan) ranged from 4000 to 400 cm−1 spectral ranges. The resolution was 4 cm−1 and the number of scans was 32. The XRD patterns of the samples were recorded on a wide-angle Xray diffractometer (XRD, D/Max 2500 PC, Rigaku, Japan) under an area detector operating at a voltage of 40 kV and a current of 40 mA using Cu/Kα radiation (λ = 1.5405 Å). The scanning speed was 4° min−1 in 2

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Fig. 1. Schematic diagram for electrospinning of the PCL/CS core-shell nanofibers.

Fig. 2. The observation of emulsion miscibility.

The wetting behavior of membranes was evaluated using a Drop Shape Analyzer (DSA100, Kruss, Germany). 3 μL deionized water droplets were dropped on the membrane and the equilibrium angle was determined using Protractor software. Water contact angle (WCA) was an average value obtained from at least three measurements.

3. Results and discussion 3.1. Preparation and characterization of PCL/CS emulsion 3.1.1. Solvent system The FA/DCM co-solvent with different volume ratios of 3/7, 5/5 and 7/3 (v/v) were pre-prepared first (Fig. 2). FA was dyed with MO and DCM was colorless for easy observation. The obvious stratification took placed in FA/DCM system with volume ratios of 3/7 and 5/5 (v/ v). When PCL or CS was added in pre-prepared layered solvents respectively, the clear boundary between FA and DCM could still be found. Homogeneous solution with wine color was observed at FA/ DCM volume ratio of 7/3 (v/v). When PCL or CS was added in miscible FA/DCM with a volume ratio of 7/3 (v/v), the color of both stratified solutions changed. And there was no obvious stratification after standing for a while. The stability of the emulsions using FA/DCM with different volume ratios of 7/3, 5/5 and 3/7 (v/v) as co-solvent after stirring 8 h were

Fig. 3. The phase separation of solvent system (WPCL/WCS = 8/2).

the 2θ range from 0 to 60°. The chemical compositions on surface of the fiber were examined by X-ray photoelectron spectroscopy (XPS, ESCALab 250Xi, VG Scientific, USA) with an Al/Kα X-ray source. The tube electric pressure and tube electric current were 40 kV and 30 mA, respectively. 3

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Fig. 4. Optical micrographs and the diameter distribution of the droplets for polymer emulsions of various PCL/CS concentrations: (a) 6/2, (b) 8/2 and (c) 10/2 (w/ w).

Table 1 Electrical conductivity, viscosity and surface tension of the electrospun emulsions. WPCL/WCS

Electrical conductivity (mS·cm−1)

Viscosity (mPa·s)

Surface tension (mN·m−1)

6/2 8/2 10/2

4.42 3.80 2.60

1150 1225 1313

50.0 48.2 57.3

Table 2 BET results for the three fibrous samples. WPCL/WCS

Specific surface area (m2/g)

Pore volume (cc/ g)

Average pore diameter (nm)

6/2 8/2 10/2

328 299 257

0.362 0.346 0.317

4.41 4.63 4.94

volume and miscibility of solvents, but also to the addition of polymers [50,51]. So the 7/3 (v/v) of FA/DCM co-solvent was used to dissolve different proportions of polymers for further study.

observed (Fig. 3). The stratification was observed for emulsion with 3/7 (v/v) of FA/DCM after 10 min, and the emulsion with 5/5 (v/v) of FA/ DCM after standing for 6 h. While the emulsion with 7/3 (v/v) of FA/ DCM still kept the initial state and no obvious stratification was observed. This indicated the emulsion with 7/3 (v/v) of FA/DCM had the best stability. The stability of emulsion was not only related to the

3.1.2. The observation of the emulsions Fig. 4 showed the emulsion drop size and distribution with various weight ratios of PCL and CS in FA/DCM with volume of 7/3 (v/v). PCL

Fig. 5. FE-SEM images (a–f) and diameter distributions (g–i) of PCL/CS electrospun nanofibers. Horizontal: different magnifications (a–c) ☓2k and (d–f) ☓10k; vertical: different PCL/CS weight ratios (a, d, and g) 6/2 (w/w), (b, e and h) 8/2 (w/w), (c, f and i) 10/2 (w/w). 4

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Fig. 6. TEM images of core-shell structured nanofibers with different PCL/CS weight ratios: (a) 6/2 (w/w), (b) 8/2 (w/w), and (c) 10/2 (w/w).

in emulsion. This was due to the decreased content of aqueous phase, would generate smaller droplets. These results were consistent with the reported work [52]. The size of dispersed phase droplets in emulsions decreased along with the decrease of dispersed phase volume fractions. The affection of the droplet size on the internal structure of the electrospun nanofibers was further studied in detail. The electrical conductivity, viscosity and surface tension of PCL/CS emulsions with different polymer concentrations in the FA/DCM cosolvent system (7/3, v/v) were summarized in Table 1. Because of the protonation of the amine groups on CS, the conductivity decreased when the content of CS in emulsions decreased. While the viscosity increased with the increase of PCL content. The viscosity of the emulsion was mainly determined by the viscosity of the PCL continuous phase. An increase in PCL content led to an increase in the degree of molecular chains entanglement [53]. Moreover, the surface tension of the emulsions did not change significantly.

3.2. Surface morphology and internal structure of electrospun fibers Fig. 7. FT-IR spectra of (a) pure PCL membrane, composite membranes with different PCL/CS weight ratios ((b) WPCL/WCS = 10/2, (c) WPCL/WCS = 8/2 and (d) WPCL/WCS = 6/2) and (e) pure CS fibrous membrane.

The surface morphologies of PCL/CS electrospun nanofibers were shown in Fig. 5. All the fibers had relatively uniform diameter, smooth surface and no apparent beaded structure. As shown in Fig. 5(g–i), the average diameter of the nanofibers increased from 214 ± 90 nm to 715 ± 330 nm and the distribution of diameters widened with the reduction of CS content. This could be explained that the conductivity decreased and the viscosity of the emulsions increased as the content of CS decreased (Table 1), which led to increase of the average diameter. Table 2 listed the specific surface area and pore structure of three fibrous samples which were analyzed by the nitrogen adsorption-desorption method. When the content of CS decreased, surface area and pore volume decreased. This could be due to the increase of average diameter of fibers with the decrease of CS content. In order to verify the core-shell structure, a single nanofiber was observed by TEM (Fig. 6). All the PCL/CS nanofibers exhibited distinct core-shell structure. The diameter ratio of the core to shell was calculated by measuring the inner and outer diameters of the fibers. When the content of CS as the core component decreased, the core-shell diameter ratios were 0.86, 0.69, and 0.59, which exhibited a downward trend. This was attributed to the property of the emulsion which influenced by the composition. In the emulsion, PCL as continuous phase tended to form the shell, and CS as dispersed phase formed the core. The droplet size decreased with the dispersed phase content of CS decreased (Fig. 4). During the electrospinning progress, the emulsion was stretched by the electric field force, which decreased with the content of CS decreased. The outer PCL shell became thicker with the decrease of the content of CS, leading to the decrease of the core to shell diameter ratio. Therefore, the electrical conductivity of core component played an important role in emulsion electrospinning. The controllable core to shell ratio of fiber could be achieved by adjusting the content of core component in emulsion.

Fig. 8. XRD spectra of (a) pure PCL membrane, composite membranes with different PCL/CS weight ratios ((b) WPCL/WCS = 10/2, (c) WPCL/WCS = 8/2 and (d) WPCL/WCS = 6/2) and (e) pure CS fibrous membrane.

dissolved in DCM and CS dissolved in FA constituted the oil phase and aqueous phase in the emulsion, respectively. Meanwhile, aqueous phase was dispersed in the oil phase. As seen from the inserted images in Fig. 4, when the weight ratios of PCL/CS were 6/2, 8/2 and 10/2 (w/ w), the average diameters of the droplets were 7.2 ± 3.8, 5.8 ± 1.8 and 2.8 ± 1.1 μm, respectively. The diameter of the dispersed aqueous phase droplets decreased progressively with the decrease of CS content 5

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Fig. 9. EDS (a, b and c) and XPS spectra (d and e) for the core-shell structured PCL/CS nanofibers.

increased. This might be due to the formation of the hydrogen bond between the ester group of PCL and the amine group of CS, which would form an interaction between the core (CS) and the shell (PCL). This interaction could also promote the stability of emulsion. The XRD analyses of all the fibrous samples represented in Fig. 8. The pure CS membrane exhibited a broad diffraction peak indicating its amorphous state. And the peaks located at 2θ = 21.4°, 22.6° and 23.8° corresponded to (110), (111) and (200) crystal planes of PCL, respectively [54]. These sharp and strong peaks were attributed to PCL semicrystalline structure. Compared with the pure PCL membrane, when the content of CS increased, the intensity of these three sharp peaks in coreshell fibrous membranes weakened. This could be attributed to the increase content of CS inhibited the crystallization of PCL. An observation of a slight left shift (∼0.2°) for (110) lattice plane was found in the core-shell fibers. This might be ascribed to the interaction between PCL and CS. This result was consistent with FT-IR analysis.

3.4. EDS and XPS characterization

Fig. 10. The SEM morphology of PCL/CS core-shell nanofibers after immersing in THF for 3 h (WPCL/WCS = 6/2).

The shell layer of the fiber mats was analyzed by EDS spectra (Fig. 9a, b and c). It could be seen that only carbon and oxygen elements existed on the fibrous surface. The XPS test (Fig. 9d and e) illustrated the elements existing within 10 nm of the fibers surface. It could be observed that two peaks with the binding energy of 532 and 284 eV appeared, which were assigned to O1s and C1s, respectively. These observations proved the shell of all three fibers were PCL component. The high resolution scans of N1s (399 eV) for core-shell electrospun fibers were examined in Fig. 9e. N element appearing in the core-shell nanofibers corresponded to the amino group (-NH2) in the CS molecule. When the weight ratio of PCL and CS was 10/2, there was almost no N1s peak. As the CS increased, the content of N element on fiber surface increased gradually. The presence of N1s peak indicated the diffusion of inner CS to the shell layer of the fibers when the CS content increased. As mentioned in TEM analysis, the shell thickness decreased and the core-shell diameter ratio of electrospun fibers increased as the CS

3.3. FT-IR and XRD analysis Fig. 7 showed the FT-IR spectra of electrospun fibers. The characteristic peaks at 3000-3600 cm−1 (OH and NH stretching of the primary amino groups), 1590-1670 cmee−1 (NH stretching of Amide I and II), 1400 cme−1 (CH), 1164 cme−1 (COCee stretching vibration) existed in the CS curve. For FTIR spectra of PCL, C]O stretching vibrations at 1730 cm−1 was assigned as a characteristic peak of ester. And the stretching vibration band of CH2 and COCee groups were located at 2868-2949 cm−1 and 1172-1240 cm−1, respectively. The characteristic peaks of PCL and CS could be observed for all core-shell fibers. Compared with the pure PCL membrane, when the content of CS increased, the intensity of bandwidth at 3000-3600 cm−1 and 15901670 cm−1 increased observably. The peak intensity of ester decreased and the position of ester peak shifted slightly to left as the content of CS 6

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Fig. 11. SEM (a and b) and EDS spectrum (c and d) of PCL/CS core-shell nanofibers after immersing in THF overnight (WPCL/WCS = 6/2).

content increased. Therefore, the intensity of the N1s peak became strong as the CS content increased. 3.5. The acquisition of CS fibers THF was a good solvent for PCL. PCL/CS fiber with a weight ratio of 6/2 was chosen to perform the dissolving experiment of shell due to its uniform diameter distribution. After the immersion of fibrous membrane in THF for three hours, the shell morphology of the fibers changed because of the dissolution of PCL (Fig. 10). The inserted picture was an amplified morphology of PCL/CS core-shell fiber which circled in the red area. The internal structure was observed after the dissolution of PCL shell. The fibers adhesion was also observed, which might be attributed to the incomplete dissolution of PCL. Then the fiber mats were treated by THF for 10 h and washed several times to remove the PCL shell of the nanofibers. As shown in Fig. 11a, the fibers diameter reduced greatly after washing by THF. The PCL/CS nanofibers after washing by THF was shown in Fig. 11b. EDS test was performed on the regions 1 and 2 in Fig. 11b, which corresponded to Fig. 11c and 11d, respectively. It could be seen that both two areas contained the four elements of C, O, N and Pt. The Pt element was observed because of the spray platinum treatments for the fiber mats before taking the photo. Comparing with PCL/CS nanofibers before treatment in Fig. 9, the appearance of N element indicated that CS was on the surface of the fiber. In Fig. 11c and 11d, it could be seen that the contents of all the elements in two regions were similar, which confirmed the uniform distribution of composition in treated fibrous membrane. And the weight of N element in the fibers was close to the theoretical value in CS. The average diameter of fibers decreased after THF treatment. It was worth mentioning that the ultrafine CS fibers with a mean diameter of 143 ± 49 nm were successfully fabricated by this facile method. The addition of PCL facilitated the formation of stable fibers in this work. The diameter distribution of pure CS fibers that obtained by stripping the shell of core-shell fibers was relatively uniform comparing with electrospinning pure CS in reported works [31,55–57]. The specific surface area and pore structure of the pure CS fiber were measured. The specific surface area, pore volume and average pore diameter were 346 m2/g, 0.389 cc/g and 4.50 nm, respectively. Compared with the as-spun core-shell fibers, both the specific surface area and pore volume increased. This was due to the decrease of fiber diameter after the dissolution of shell. To confirm the formation of the obtained pure chitosan fibers, FT-IR

Fig. 12. FT-IR spectrum of the obtained pure chitosan fibers.

Fig. 13. The water contact angle of PCL/CS core-shell nanofibrous membranes before and after THF immersion.

7

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spectrum was tested. As shown in Fig. 12, the characteristic peaks of CS were similar to those in Fig. 7. The OeH and NHe stretching vibrations in amino groups were located at 3000-3600 cm−1. The NHe stretching of Amide I and II appeared at 1590-1670 cm−1. 1400 cm−1 and 1165 cm−1 corresponded to CH and COCeee stretching vibration, respectively. In addition, there was no characteristic peak of PCL in the spectrum, which proved the PCL shell completely stripped. This results agreed with EDS analysis.

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3.6. The hydrophilicity measurement To further confirm the surface component for fibers, the wetting properties of the membranes before and after THF treatment were tested. The insert picture in Fig. 13 showed the water contact angle (WCA) of the PCL and CS polymers at different ratios. It could be seen that all three core-shell fibers were hydrophobic (WCA > 130°), which proved that the composition of the shell was PCL. The results were in accordance with the observations through XPS analysis (Fig.9). After THF treatment, all the fibrous membranes were hydrophilic (WCA < 50°). This indicated PCL as shell component was successfully stripped and CS component exposed to the outer layer of the fibers. The wettability of the fibrous membrane increased drastically due to containing -OH and -NH2 groups in CS fibers. 4. Conclusion In this work, a stable surfactant-free water-in-oil emulsion as a novel solvent system was used to fabricate PCL/CS nanofibers with core-shell structure. The high stability of the emulsion system could be achieved by the miscibility of solvents and the interaction between PCL and CS. As the content of CS increased in emulsion, the electrical conductivity increased and the viscosity decreased, generating the small fiber diameter. The diameter of aqueous dispersed droplets increased by increasing CS content, which led to the increase of the core to shell diameter ratio for fibers. CS as core component was also proved by EDS and XPS analysis. The shell component was stripped and CS fibers were obtained when immersing the fibrous membrane in THF. This work provided a method to prepare core-shell nanofibers of natural polymers which were difficult to electrospin. By adjusting the content of core component in emulsion, the controllable core to shell ratio of fibers could be obtained. Furtherly, for the preparation of controllable shellcore structured fiber by emulsion electrospinning, the core component with electrical conductivity was suggested to choose. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgement This research was supported by National Natural Science Foundation of China (No. 51103058). References [1] Y. Wang, B. Wang, Q. Wang, J. Di, S. Miao, J. Yu, Amino-functionalized porous nanofibrous membranes for simultaneous removal of oil and heavy metal ions from wastewater, ACS Appl. Mater. Interfaces 11 (2019) 1672–1679. [2] C. Wang, J. Yin, R. Wang, T. Jiao, H. Huang, J. Zhou, L. Zhang, Q. Peng, Facile preparation of self-assembled polydopamine-modified electrospun fibers for highly effective removal of organic dyes, Nanomaterials 9 (2019) 116–133. [3] M.Q. Khan, D. Kharaghani, S. Ullah, M. Waqas, A.M.R. Abbasi, Y. Saito, C. Zhu, I.S. Kim, Self-cleaning properties of electrospun PVA/TiO2 and PVA/ZnO nanofibers composites, Nanomaterials 8 (2018) 1–11. [4] O. Suwantong, Biomedical applications of electrospun polycaprolactone fiber mats, Polym. Adv. Technol. 27 (2016) 1264–1273.

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