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Optimization of the Electrospinning Processing-Window to Fabricate Nanostructured PE-b-PEO and Hybrid PE-b-PEO/EBBA Fibers.

INTRODUCTION

Nowadays, the need to produce novel materials, for example for packaging, tiny electronic devices and biomedical applications, has helped to develop innovative techniques and improve others for this purpose.

Electrospinning is the main technique that allows, through axial stretching of the viscoelastic solution, the obtaining of micro and nanofibers. These fiber sizes permit the discovery of unique features, such as high surface-area-to-volume and large length-to-diameter ratios [1].

First, in the electrospinning process, a polymer solution is pushed by a pump through a capillary up to a needle. Then, a strong electric field is applied between the needle and the collector which provokes the stretching of this solution and the solvent evaporation allowing the fabrication of polymeric fibers [1-4].

The main difficulty of this technique lies in the number of parameters that are required to control the final properties of the fabricated fibers. The parameters involved in the generation of these fibers are the concentration of the polymer solution, surface tension, solution conductivity, voltage, outflow, distance needle-collector and relative humidity, among others [3, 5].

The low production rate and the solvents used are the most important problems in the electrospinning process since their can limited the possible application leading to significant safety problem during manufacture due to their flammability, toxicity or carcinogenic. Moreover, another handicaps related to the solvents used can be also cost and recovery [6-8].

Table 1 summarizes the most important parameters that take part in the electrospinning process and their influence on the final properties of the designed fibers [9-11].

The aim of this technique is to collect continuous single fibers with regular and controllable diameters and defect-free surface by playing with the electrospinning parameters.

The electrospinning technique can be considered an easy and versatile technique since a wide range of polymers, such as polyoxymethylene (POM) [12], poly(lactic acid) (PLA) [13, 14], poly([epsilon]-caprolactone) (PCL) [15], polyethylene oxide (PEO) [16], polyglycolic acid (PGA) [17], polyvinylalcohol (PVA) [18], and different block copolymers [19-21] can be used to fabricate electrospun fibers.

These kinds of fibers can found in applications in different fields such as medical prosthesis (grafts, vessels, and tissues), filtration systems, or electrical and optical devices [22-25].

Recently, the fabrication of hybrid electrospun nanofibers has been performed using a coaxial electrospinning technique [26, 27]. Co-electrospinning is a modification of the conventional technique, based on two concentric needles. This technique is able to simultaneously electrospun different polymers obtaining core-shell structure nanofibers [2, 28-31]. Several publications can be found related to co-electrospun nanofibers based on different polymers, such as, PEG [32] or PLA [33-35], block copolymers [36] and based on hybrid nanofibers with chitosan [37], cellulose [38, 39], multiwalled carbon nanotubes (MWCNTs) [40], or inorganic nanoparticles [41, 42],

In addition, liquid crystal molecules have also been used to design hybrid fibers with conductive properties, to fabricate electronic devices or sensors. Until now, developed liquid crystal devices have been produced using a layer of nanofibers permeated with a liquid crystal [43], or using the coaxial electrospinning technique to confine liquid crystal inside a polymer shell [44-47].

In this work, the processing-window to obtain poly(ethyIeneb-ethylene oxide) block copolymer (PE-b-PEO) fibers by the electrospinning technique was optimized. Moreover, using the co-electrospinning technique, hybrid fibers based on the same block copolymers and N-(4-ethoxybenzylidene)-4-butylaniline nematic liquid crystal (EBBA) were successfully fabricated and investigated on macro and nanoscale by different advanced microscopy techniques.

EXPERIMENTAL

Materials

Poly(ethylene-b-ethylene oxide) block copolymer (PE-b-PEO) with a molecular weight of 2,250 g/mol and 80 wt% PEO block content and N-(4-ethoxybenzylidene)-4-butylaniline nematic liquid crystal (EBBA) with 98% purity, were purchased from Sigma-Aldrich.

Chloroform and dimethylformamide (DMF) were also purchased from Sigma

Aldrich and were used as solvents during the process.

Sample Preparation

Electrospun PE-b-PEO block copolymer fibers and hybrid PE-b-PEO/EBBA fibers were performed using an Electrospinner Yflow 2.2.D-350 (Nanotechnology Solutions) with a coaxial vertical standard configuration and connected to a high voltage power (see Scheme 1). In particular, the polymer solution flows through the inner needle and the same solvent solution used for the polymer solution flows through the outer one. In this case, the solvent solution was used to avoid the obstruction of the inner needle during the electrospun process.

To fabricate PE-b-PEO block copolymer fibers by electro-spinning process, two groups of parameters were mainly considered, one related to the polymer solution and the other one with the processing conditions.

Regarding the polymer solution, several PE-b-PEO block copolymer concentrations (Cp [wt%]), between 20 and 50 wt%, were prepared in a mixture of chloroform/DMF with different ratios from 3:1 to 1:0. Polymeric solutions were stirred during 24 h at room temperature.

During the electro-spinning process, the block copolymer solution was pushed using a pump through a capillary up to the inner needle of the concentric needle, while solvents, flowed through the outer one. For comparison, both PE-b-PEO fibers and PE-b-PEO/EBBA fibers were obtained using a coaxial nozzle.

In addition, processing conditions were optimized, playing with solvent flow rate, Q% (mL/h), block copolymer solution flow rate, Qp (mL/h), and with the electric field applied between needle and the collector. The needle-collector distance was fixed to 18 cm.

Solvent and block copolymer solution flow rates were varied from 0 to 0.5 mL/h and from 0.1 to 5 mL/h, respectively. In the case of electric field, the positive voltage (V+) applied over the double needle was between 3 and 14 kV and the negative voltage ([V.sup.-]) applied over the collector ranged between 3 and 14 kV.

Table 2 summarized the preparation conditions employed to fabricate PE-b-PEO block copolymer fibers.

On the other hand, for the development of hybrid PE-b-PEO/ EBBA fibers, block copolymer solution flowed through the inner needle of the equipment and a solution of 5 wt% of EBBA nematic liquid crystal in chloroform through the outer one.

For this purpose, the best conditions for the fabrication of block copolymer fibers have been chosen as starting point. Thus, as explained below in results and discussion section, the PE-b-PEO block copolymer concentration was 46 wt% in a mixture of chloroform/DMF.

As well as in the case of PE-b-PEO block copolymer electrospun fibers, for hybrid PE-b-PEO/EBBA electrospun fibers the solvent used for the block copolymer solution was a mixture of chloroform/DMF with different solvents ratio. Table 3 summarized the preparation conditions employed to fabricate hybrid PE-b-PEO/EBBA electrospun fibers.

Characterization Techniques

Optical microscopy (OM) images were captured using a Nikon Eclipse E600W microscope at room temperature. To determine the average diameters and lengths of the fibers, 10 independent fibers for each OM image were taken into account and the diameter and length of each fiber was determined using AnalySIS Auto 3.2 software (Soft Imaging System GmbH).

Morphology and structure of designed electrospun fibers were observed using a PHILIPS XL30 scanning electron microscope (SEM).

Fibers surface morphology was investigated by atomic force microscopy (AFM) technique under ambient conditions. AFM images were obtained operating in tapping mode (TM-AFM) with a scanning probe microscope (Nanoscope Ilia, Multimode from Digital Instruments) equipped with an integrated silicon tip/cantilever having a resonance frequency of 300 kHz from the same manufacturer.

RESULTS AND DISCUSSION

Optical microscopy technique was used to study the appearance of fabricated fibers. First of all, it should be mentioned that for block copolymer concentrations below 40 wt% electrospray was obtained (results not shown here) since these concentrations were too low for fiber fabrication.

The electrospray drops were closer and became bigger with the increasing of block copolymer concentrations, around 40 wt%, which led to the formation of very short electrospun fibers with beads. The formation of fibers was easier employing chloroform/DMF mixture as solvent than chloroform.

The optical microscopy technique illustrates that the best conditions to fabricate PE-b-PEO block copolymer fibers were with concentrations between 45 and 47 wt% of PE-b-PEO block copolymers and a chloroform/DMF mixture ratio of 4:1 and 5:1, as shown in Figs. 1-4.

Here, it should be pointed out, that for PE-b-PEO block copolymer concentrations higher than 47 wt% the needle was blocked hindering the flow of the block copolymer solution and therefore the formation of fibers.

PE-b-PEO block copolymer fibers obtained from 45 wt% of PE-b-PEO block copolymers in a chloroform/DMF (4:1) solution, applying a voltage difference of 22 kV, a chloroform flow rate of 0.1 raL/h and a block copolymer solution flow rate of 5 mL/h are shown in Fig. 1.

These electrospinning conditions allowed for the fabrication of fibers with an average diameter of 5 [+ or -] 1 [micro]m and the average length of 350 [+ or -] 40 [micro]m. It should be noted that the fiber length was homogeneous; in contrast, the diameters were different along each fiber and between them. This heterogeneity was related to the beads that appeared under these processing conditions.

In the case of the PE-b-PEO block copolymer fibers prepared using 46 wt% of block copolymers in a chloroform/DMF (5:1) solution, applying a voltage difference of 22 kV and a chloroform flow rate of 0.1 mL/h, different sizes of PE-b-PEO block copolymer fibers were obtained playing with the block copolymer flow rate.

Figure 2a shows the PE-b-PEO block copolymer fibers fabricated with a block copolymer flow rate of 1 mL/h. The average diameter was 5 [+ or -] 2 [micro]m and the average length was 270 [+ or -] 90 [micro]m. For the PE-b-PEO block copolymer fibers generated with a block copolymer flow rate of 3 and 5 mL/h, in Fig. 2b and c, respectively, similar sized fibers were produced with an average diameter of 4 [+ or -] 1 [micro]m and an average length of 200 [+ or -] 50 pm. Thus, the increase in the block copolymer flow rate led to a decrease in both fiber diameter and length.

Moreover, as shown in Fig. 2, for the block copolymer flow rate of 1 mL/h, the fibers were more abundant than in the case of the higher block copolymer flow rate.

In addition, the PE-b-PEO block copolymer fibers were also prepared maintaining both, the same PE-b-PEO block copolymer concentration (46 wt%) and chloroform flow rate of 0.1 mL/h, and changing the chloroform/DMF ratio to 4:1. To obtain PE-b-PEO block copolymer fibers under these experimental conditions, both the block copolymer flow rate and applied voltage difference were varied.

Figure 3 shows the PE-b-PEO block copolymer fibers obtained from an applied voltage difference and a block copolymer flow rate of 22 kV and 0.1 mL/h, and 21 kV and 5 mL/h, respectively. On the one hand, the PE-b-PEO block copolymer fibers prepared with 22 kV and 0.1 mL/h (Fig. 3a) resulted in an average diameter of 3 [+ or -] 2 pm and an average length of 320 [+ or -] 50 [micro]m. Moreover, employing 21 kV and 5 mL/h (Fig. 3b), the size of the PE-b-PEO block copolymer fibers was ~5 [+ or -] 2 [micro]m in diameter and ~290 [+ or -] 50 [micro]m in length. Consequently, as show in Fig. 3a, in the first preparation conditions, thinner and larger fibers were observed if compared with the second preparation conditions as visualized in Fig. 3b.

Increasing the block copolymer concentration up to 47 wt%, using solvents ratio of 5:1 and a block copolymer flow rate of 0.5 mL/h, PE-b-PEO block copolymer fibers can be also achieved by varying the electrospinning parameters. Figure 4a shows PE-b-PEO block copolymer fibers employing a chloroform flow rate of 0.1 mL/h and applying a voltage difference of 22 kV. The fabricated fibers possessed an average diameter of 5 [+ or -] 1 [micro]m and an average length of 410 [+ or -] 90 [micro]m.

In the case of Fig. 4b, the chloroform flow rate was 0.5 mL/ h and applying a voltage difference of 21 kV. These experimental conditions was responsible for a decrease in the average fiber diameter as well as in the corresponding length, being 4 [+ or -] 1 and 270 [+ or -] 90 [micro]m, respectively.

Comparing the results achieved using different preparation conditions, it can be concluded that the longest and widest fibers were obtained with a 47 wt% of PE-b-PEO block copolymer concentration in a chloroform/DMF (5:1) solution, a block copolymer solution flow rate of 0.5 mL/h, a chloroform flow rate of 0.1 mL/h and by applying a voltage difference of 22 kV.

Taking into account the ideal target stabilized for fibers designed by the electrospinning technique, a PE-b-PEO block copolymer concentration of 46 wt% in a chloroform/DMF (5:1) solution showed the best results since, on the one hand, the diameter of fibers obtained under these conditions was consistent and controllable, and on the other hand, the variation of the other electrospinning parameters (block copolymer, solvent flow rates, and applied voltage differences) did not significantly affect the final fiber properties providing a permissible widespread electrospinning processing-window.

Taken into account the promising results obtained for PE-b-PEO fibers, a 46 wt% of PE-b-PEO block copolymer in a chloroform/DMF (5:1) solution was used to design the hybrid PE-b-PEO/EBBA fibers.

In this regard, is interesting to mention the work developed by Rajgarhia et al. [31] in which the relationship between solvent evaporation rate and the morphology of the obtained hybrid fibers was studied. Authors find strong influence of the evaporation rate and the solubility parameter of the solvents on the morphology.

Different hybrid PE-b-PEO fibers were developed by playing with the block copolymer flow rate (1,3, and 5 mL/h). As well as in the case of the 46 wt% of PE-b-PEO block copolymers in a chloroform/DMF (5:1) solution, applied voltage difference and chloroform flow rate remained constant at 22 kV and 0.1 mL/h, respectively.

Figure 5a shows the hybrid PE-b-PEO/EBBA fibers fabricated with a block copolymer flow rate of 1 mL/h. The average diameter of the hybrid fibers was 4[+ or -]1 ([micro]m and the average length was 200 [+ or -] 30 [micro]m.

By increasing the block copolymer How rate up to 3 mL/h, in Fig. 5b, the average length of the hybrid fibers increased up to 230 [+ or -] 30 nm conversely, the average diameter remained the same, at 4 [+ or -] 1 [micro]m.

For the hybrid PE-b-PEO/EBBA fibers created with a block copolymer flow rate of 5 mL/h, in Fig. 5c, the average diameter was 4 [+ or -] 1 ([micro]m and the average length was 260 [+ or -] 80 ([micro]m. Therefore, this block copolymer flow rate resulted in an increase in both, fiber diameter and length.

In addition, as shown in Fig. 5, for the block copolymer flow rate of 5 mL/h, the hybrid PE-b-PEO/EBBA fibers were more abundant than in the case of a lower block copolymer flow rate.

A comparison between the PE-b-PEO block copolymer fibers and the hybrid PE-b-PEO/EBBA fibers obtained following the same electrospinning processing-window (Figs. 2 and 5) showed a completely opposite behavior. In the case of the PE-b-PEO fibers without EBBA, an increase in the block copolymer flow rate led to a decrease in fiber diameter and length. On the contrary. for hybrid PE-b-PEO/EBBA fibers an increase in the block copolymer flow rate allowed for obtaining longer and wider hybrid fibers. This phenomenon could be explained by observing the EBBA nematic liquid crystal chemical structure. Its configuration is based on two aromatic rings and an imine group, both of them with delocalized electrons which facilitate electrical conduction. Thus, these electrons contributed to the applied voltage difference and resulted in an increase of fiber length. Consequently, the addition of EBBA liquid crystal improved the electrospinning process.

Scanning electron microscopy (SEM) technique was employed to perform a deeper study of fibers obtained using the broader electrospinning processing-window, which corresponds to a 46 wt% of block copolymers in a chloroform/DMF (5:1) solution. SEM images of the PE-b-PEO fibers and the hybrid PE-b-PEO/EBBA fibers based on the 46 wt% of block copolymers in a chloroform/DMF (5:1) solution are shown in Figs. 6 and 7, respectively.

In the case of the PE-b-PEO block copolymer fibers, once again the fiber diameter was regular and controllable. Moreover, the decrease in the abundance of the fibers due to the increase in the block copolymer flow rate was also observed.

As concluded by means of the OM technique the best conditions for fiber production were achieved with 46 wt% of PE-b-PEO block copolymers in a chloroform/DMF (5:1) solution, therefore this sample was employed to fabricate the hybrid PE-b-PEO/EBBA fibers.

Figure 7 shows the SEM images of the hybrid PE-b-PEO/ EBBA fibers obtained from the 46 wt% of PE-b-PEO block copolymers in a chloroform/DMF (5:1) solution, applying a voltage difference of 22 kV, an EBBA solution flow rate of 0.1 mL/h and a block copolymer solution flow rate of 1 mL/h (Fig. 7a), 3 mL/h (Fig. 7b), and 5 mL/h (Fig. 7c).

In agreement with the behavior observed using the OM technique, the SEM images confirmed that an increase in the block copolymer flow rate caused an increase in the fiber diameter and in the abundance of the fibers. Thus, the behavior is completely opposite if compared with the results obtained for the PE-b-PEO fibers without the EBBA liquid crystal.

With the aim of analyzing the surface morphology of the fabricated PE-b-PEO and hybrid PE-b-PEO/EBBA fibers, AFM measurements were also carried out.

Figure 8a shows an AFM phase image of the PE-b-PEO fibers based on the 46 wt% of PE-b-PEO block copolymers in a chloroform/DMF (5:1) solution, applying a voltage difference of 22 kV, a chloroform flow rate of 0.1 mL/h and a block copolymer solution flow rate of 1 mL/h.

Under these electrospinning conditions, the PE-b-PEO block copolymer fibers showed a microphase separation. Taking into account that the modulus of PE is higher than that for PEO at room temperature [48, 49], the darker areas correspond to PEO block domains and the brightest areas correspond to PE block domains. As can be observed in Fig. 8a, the electrospun PE-b-PEO fibers exhibit a cylindrical structure with parallel and perpendicularly oriented cylinders. Moreover, the PEO block nanocrystals (~15 nm in size) within the cylindrical PEO domains can be also detected.

In the case of hybrid PE-b-PEO/EBBA fibers fabricated under the same processing conditions, the addition of EBBA liquid crystal led to a significant change in the morphology from a cylindrical structure to a lamellar one (Fig. 8b).

This fact can be explained, on the one hand, by taking into account that as it is known from previous work [50], EBBA liquid crystal is more miscible with PE blocks than with PEO blocks, consequently EBBA liquid crystal was positioned in the PE block domains. On the other hand, the EBBA nematic liquid crystal chemical structure could have also an influence on the self-assembly process, since, as pointed out before, it affects the electrospinning processing conditions.

CONCLUSIONS

Nanostructured PE-b-PEO block copolymer fibers were fabricated using electrospinning technique. Block copolymer solution concentration and in consequence their viscosity, have strong influence on the characteristics of obtained fibers. Concentrations of around 40 wt% of block copolymer in chloroform or chloroform/DMF mixture lead to electrospun fibers. According to the optical microscopy images, these fibers were better formed in solutions with chloroform/DMF solvent than in chloroform. Consequently, one can conclude that DMF improved the electrospun fibers formation. According to obtained results the best PE-b-PEO block copolymer fibers were fabricated using a 46 wt% of PE-b-PEO block copolymer in chloroform/DMF (5:1) solution, applying a voltage difference of 22 kV, solvent flow rate of 0.1 mL/h and block copolymer solution flow rate of 1 mL/h.

Moreover, hybrid PE-b-PEO/EBBA fibers were also developed following the same electrospinning processing window.

EBBA nematic liquid crystal improved hybrid PE-b-PEO/ EBBA fibers formation in width and length for higher block copolymer flow rate (5 mL/h). In addition, the addition of EBBA provokes changes on fiber morphology resulting in well-ordered lamellar structure.

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Sheyla Carrasco-Hernandez, (1) Junkal Gutierrez, (1) Laura Peponi, (2) Agnieszka Tercjak (1)

(1) Group 'Materials + Technologies' (GMT), Department of Chemical and Environmental Engineering, Engineering College of Gipuzkoa, University of the Basque Country (UPV/EHU), Donostia-San Sebastian 20018, Spain

(2) Institute of Polymer Science and Technology, Spanish National Research Council ICTP-CSIC, Madrid 28006, Spain

Correspondence to: A. Tercjak; e-mail: agnieszka.tercjaks@ehu.eus Contract grant sponsor: Spanish Ministry of Economy and Competitiveness and European Commission; contract grant numbers: MAT2013-48059-C2-1R, MAT2015-66149-P; contract grant sponsor: The Basque Government for Grupos Consolidados; contract project number: IT776-13; contract grant sponsor: Spanish Ministry of Economy and Competitiveness; contract grant number: BES-2013-066734; contract grant sponsor: Ramon y Cajal; contract grant number: RyC-2014-15595.

DOI 10.1002/pen.24492

Caption: FIG. 1. Optical micrographs of PE-b-PEO block copolymer fibers obtained from 45 wt% of PE-b-PEO block Copolymer in chloroform/DMF (4:1), applying a voltage difference of 22 kV, chloroform flow rate of 0.1 mL/h and block copolymer solution flow rate of 5 mL/h.

Caption: FIG. 2. Optical micrographs of PE-b-PEO block copolymer fibers obtained from 46 wt% of PE-b-PEO block copolymer in chloroform/DMF (5:1), applying a voltage difference of 22 kV, chloroform now rate of 0.1 mL/h and block copolymer solution flow rate of (a) 1 mL/h, (b) 3 mL/h, and (c) 5 mL/h.

Caption: FIG. 3. Optical micrographs of PE-b-PEO block copolymer fibers obtained from 46 wt% of PE-b-PEO block copolymer in chloroform/DMF (4:1), chloroform flow rate of 0.1 mL/h applying a voltage difference and block copolymer solution flow rate of (a) 22 kV and 0.1 mL/h and (b) 21 kV and 5 mL/h.

Caption: FIG. 4. Optical micrographs of PE-b-PEO block copolymer fibers obtained from 47 wt% of PE-b-PEO block copolymer in chloroform/DMF (5:1), block copolymer solution flow rate of 0.5mL/h, applying a voltage difference and chloroform flow rate of (a) 22 kV and 0.1 mL/h and (b) 21 kV and 0.5 mL/h.

Caption: FIG. 5. Optical micrographs of hybrid PE-b-PEO/EBBA fibers obtained from 46 wt% of PE-b-PEO block copolymer in chloroform/DMF (5:1), applying a voltage difference of 22 kV, EBBA solution flow rate of 0.1 mL/h and block copolymer solution flow rate of (a) 1 mL/h, (b) 3 mL/h, and (c) 5 mL/h.

Caption: FIG. 6. SEM images of PE-b-PEO block copolymer fibers obtained from 46 wt% of PE-b-PEO block copolymer in chloroform/DMF (5:1), applying a voltage difference of 22 kV, chloroform How rate of 0.1 mL/h and block copolymer solution flow rate of (a) 1 mL/h, (b) 3 mL/h, and (c) 5 mL/h.

Caption: FIG. 7. SEM images of hybrid PE-b-PEO/EBBA fibers obtained from 46 wt% of PE-b-PEO block copolymer in chloroform/DMF (5:1), applying a voltage difference of 22 kV, EBBA solution flow rate of 0.1 mL/h and block copolymer solution flow rate of (a) 1 mL/h, (b) 3 mL/h. and (c) 5 mL/h.

Caption: FIG. 8. AFM phase images of (a) PE-b-PEO fiber and (b) hybrid PE-b-PEO/EBBA fiber
TABLE 1. The effect of the electrospinning
parameters on the fibers formation.

Parameter               High values             Low values

Concentration of        Hinders the passage     Fiber breakage,
the polymer solution    of the solution         droplet formation
                        through the
                        capillary

Surface tension         Defects (beads)         Smooth fibers
                        in fibers

Solution                Thin fibers             Thick fibers
conductivity

Voltage                 Thick fibers, the       The solution does
                        appearance of beads     not reach the
                                                collector

Outflow                 Thick fibers, beads     Fibers without
                        with large sizes        defects

Distance                This fibers and         Appearance of beads
needle-collector        appearance of beads
                        (the fibers may
                        break due to its
                        own weight)

Relative humidity       Appearance of pores     No effect

TABLE 2. Summary of the experimental conditions used during
electrospinning process of PE-b-PEO block copolymer.

Number of     PE-b-PEO      Solvent ratio    PE-b-PEO flow
samples     concentration   chloroform/DMF   rate [Q.sub.p]
                (wt%)                            (mL/h)

330          24 (10-50)      5 (3:1-1:0)       6 (0.1-5)

Number of   Chloroform flow   Positive voltage   Negative voltage
samples     rate [Q.sub.s]     [V.sup.+] (kV)     [V.sup.-] (kV)
                (mL/h)

330            4 (0-0.5)          8 (3-14)           7 (3-14)

TABLE 3. Summary of the experimental conditions used during
co-electrospinning process of hybrid PE-b-PEO/EBBA fibers.

Number of   Solvent ratio       EBBA flow      PE-b-PEO flow
samples     chloroform/DMF   rate [Q.sub.LC]   rate [Q.sub.p]
                                 (mL/h)            (mL/h)

12               5:1            (0.1-0.5)         (0.1-5)
5                4:1            (0.1-0.5)         (0.1-5)

Number of   Positive voltage   Negative voltage
samples      [V.sup.+] (kV)     [V.sup.-] (kV)

12                 11                 11
5                  11                 11
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Title Annotation:polyethylene-b-ethylene oxide
Author:Carrasco-Hernandez, Sheyla; Gutierrez, Junkal; Peponi, Laura; Tercjak, Agnieszka
Publication:Polymer Engineering and Science
Article Type:Report
Date:Nov 1, 2017
Words:5003
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