Ioanna Savva*a,
Eftychia Evaggelou†
a,
Georgia Papaparaskeva†a,
Theodoros Leontioub,
Triantafyllos Stylianopoulosa,
Fotios Mpekrisa,
Kypros Stylianoua and
Theodora Krasia-Christoforou*a
aUniversity of Cyprus, Department of Mechanical and Manufacturing Engineering, P. O. Box 20537, 1678, Nicosia, Cyprus. E-mail: krasia@ucy.ac.cy; joan.savva@gmail.com
bFrederick University, General Department, School of Engineering and Applied Sciences, 7, Y. Frederickou Str. Pallouriotissa, 1036, Nicosia, Cyprus
First published on 7th December 2015
During recent years, electrospinning has become a powerful technique for the cost-effective production of fibrous materials with diameters ranging from a few nanometers up to a few micrometers. In a conventional electrospinning system the produced fibers are collected on a flat grounded collector in a random manner, resulting in isotropic non-woven fibrous mats. Many researchers have been focusing on the modification of the electrospinning collectors for inducing fiber orientation since aligned fibrous mats exhibit unique mechanical, electrical and optical properties rendering them highly attractive in many fields. Unlike other reported collector modification approaches developed for inducing fiber alignment via electrospinning, a very simple concept for producing aligned polymer fibers is presented herein, based on the modification of the electric field profile by replacing the flat metallic collector employed in a typical electrospinning set-up, with a concave one. The electric field profile developed in the case of the flat and the concave collectors was simulated performing a finite elements analysis. Most importantly electrospun meshes were produced and quantification of fiber alignment with a Fourier transform method on different deposition sites of the concave collector showed an up to 70% fiber alignment in the center area. This work creates new prospects towards the design of static collectors employed in electrospinning that could enable the fabrication of highly aligned electrospun fibers.
Herein, a very simple concept is employed for the first time for obtaining aligned fibers via electrospinning. This new approach involves the use of a concave metallic grounded collector instead of a flat one employed in a conventional electrospinning set-up. Compared to the previously reported methodologies developed towards the same purpose, the approach presented herein is easy, straightforward and cost-effective since it does not require the use of mechanical motion, magnetic field or complicated system modification modes for inducing fiber orientation.
Two types of polymer solutions were used in this study in order to investigate the applicability of the concave collector in the alignment of both polymer and polymer-based nanocomposite fibers. More precisely, polyvinylpyrrolidone (PVP) and PVP-capped Pd nanoparticle solutions prepared in methanol were electrospun on flat and on concave collectors. Quantification of the fiber orientation on different locations on the surface of the concave collector was performed by applying a Fourier transform analysis on images obtained by scanning electron microscopy.22,23
In the case of the concave collector, fiber alignment was found to vary depending on the position where the fibers were deposited (center area, left and right) with the highest degree of alignment observed in the center of the collector, reaching up to 70%. The present work creates new prospects in the design of static collectors used in the electrospinning process that could enable the production of highly oriented electrospun nano- and microfibers.
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Fig. 1 (a) Schematic presentation of the modified electrospinning set-up with a concave collector. (b) Photographs of the concave and flat collectors used as collectors during electrospinning. |
PVP (Mn = 1300
000 g mol−1, Sigma-Aldrich) fibrous mats were fabricated by electrospinning starting from polymer solutions prepared in methanol (MeOH) (Scharlau). For investigating the applicability of our approach in the alignment of polymer-based electrospun nanocomposite fibers, Pd nanoparticles with average diameters of approximately 5 nm (ref. 24) were introduced in the polymer solution precursor prior to electrospinning. The PVP-capped Pd nanoparticles (mols vinyl pyridine units per mols palladium salt = 9
:
1) were synthesized according to a previously reported synthetic methodology.25 Briefly, in a round bottom flask equipped with a magnetic stirrer, PVP (1.0 g, 9 mmol of vinylpyrridine units) was dissolved in MeOH (10 mL). Subsequently, palladium(II) acetate (222.2 mg, 1 mmol) was added to the polymer solution and the reaction mixture was heated at reflux (65 °C) for 2 h. During this period, the colour of the solution changed from yellow to dark brown indicating the formation of Pd nanoparticles. After the completion of the reaction, the brown-coloured solution was allowed to cool down to room temperature and it was then stored in a sealed glass vial. The solution was highly stable and no precipitation was observed even after several months.
All electrospinning experiments were performed at room temperature. Equipment included a controlled flow four-channel volumetric microdialysis pump (KD Scientific Model: 789252), a syringe with specially connected spinneret needle electrode, a high-voltage power source (10–50 kV) and custom designed, grounded collector collectors (either flat or concave), inside an interlocked Faraday enclosure safety cabinet.
The electrospinning conditions employed during the experiments were 10% w/v polymer solution concentration, 2 mL h−1 flow (delivery) rate, 16 G needle diameter, 25 cm needle-to-collector distance and 20 kV applied voltage. In order to investigate whether the fiber orientation induced in the presence of the concave collector was retained for long electrospinning periods, the electrospinning time was varied from 1 to 5 minutes.
The morphology of the fibers was investigated by scanning electron microscopy (SEM) (Vega TS5136LS-Tescan). The samples were gold-sputtered (sputtering system K575X Turbo Sputter Coater – Emitech) prior to SEM inspection.
The dependence of the degree of orientation of the fibers on the deposition site was also investigated in the case of the concave collector. More precisely, sample specimens were collected from different positions on the collector i.e. left (L), center-left (CL), center (C), center-right (CR) and right (R) as shown in Fig. 2 to be further analyzed by SEM. Additionally, in order to investigate the effect of the collector geometry on the fiber diameters, the latter were determined from the SEM images using digimizer image analysis software.
Transmission electron microscopy (TEM) analysis was also performed by using a TECNAI F30 G2 S-TWIN microscope operated at 300 kV equipped with energy dispersive X-ray spectrometer (EDX). Samples were placed into a double copper grid (oyster) to be visualized by TEM.
FTIR spectra were recorded using a Shimadzu FTIR-NIR Prestige-21 spectrometer bearing an ATR accessory.
Thermal gravimetric analysis (TGA) measurements were performed on a Q500 TA Instruments under nitrogen flow at a heating rate of 10 °C min−1.
Mechanical testing was performed using a high precision mechanical testing system (Instron 5944, Norwood, MA, USA). The specimens were cut in an orthogonal shape with dimensions 12 × 8 × 1 mm (length × width × thickness) and placed between two parallel grips. Tensile stress–strain experiments were carried out to study the elastic response of the material. The specimens were stretched to 30% strain with a strain rate of 0.5 mm min−1. The stress was calculated as the force measured on the load cell divided by the initial area of the specimen (i.e., 1st Piola-Kirchhoff stress) and the strain was calculated as the displacement, Δl divided by the initial length of the specimen. The Young's modulus was calculated from the slope of the linear part of the stress–strain curves for low strains. Six specimens were tested for each group (n = 6).
For the experimental verification of the aforementioned, electrospinning of both PVP and PVP/Pd solutions was carried out under identical electrospinning conditions with the only difference being the geometry of the metallic collector. Upon introducing the conventional flat metallic fiber collector, randomly oriented PVP fibers were generated which were uniformly distributed on the collector's surface. The same behavior was also observed in the case of the nanocomposite PVP/Pd system, indicating that the presence of the Pd NPs within the polymer solution had no effect on the fiber orientation. Fig. 4a provides a photograph of the PVP/Pd fibers that were randomly deposited on the flat collector. PVP/Pd fiber alignment was observed along the concave collector as seen in the photograph provided in Fig. 4b. The brown color of the fibers is attributed to the presence of the Pd nanoparticles embedded within the fibers.24
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Fig. 4 Photographs of the PVP/Pd electrospun nanocomposite fibers deposited on the flat (a) and concave (b) metallic collectors. |
The obtained fibers were characterized in terms of their thermal stability, composition and morphology by thermal gravimetric analysis (TGA), FTIR and electron microscopy (SEM, TEM) respectively.
The decomposition temperatures of the pristine PVP and the PVP/Pd electrospun fibers were determined by TGA. As seen from the thermograms provided in Fig. 5, PVP degrades at approximately ∼460 °C in line with previously reported findings. The remaining residue observed in the case of the Pd-containing fibers at T > 500 °C, corresponds to the metallic palladium content.
FTIR analysis was performed to obtain information on the chemical composition of the fibrous mats. Fig. 6 illustrates the FTIR spectra of the PVP and the PVP/Pd nanocomposite fibrous mats. Both spectra show a strong band at ∼1660 cm−1 characteristic of the stretching vibration of the CO group that is present in PVP. This band is shifted to slightly lower wavenumber in the PVP/Pd nanocomposite whereas it also exhibited a decrease in the width. According to C. Evangelisti et al.26 the broadness of the carbonyl stretching vibration band in PVP decreases in the presence of Pd due to the development of PVP/Pd intermolecular interactions in which the C
O group is involved, acting competitively to the PVP–PVP intermolecular interactions.
Fig. 7 provides indicative SEM images of the PVP and PVP/Pd electrospun fibers deposited on the flat metallic collector.
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Fig. 7 SEM images of the PVP (a and b) and the PVP/Pd (c and d) fibers deposited on the flat metallic collector. Deposition time: (a) and (c): 1 minute; (b) and (d): 3 minutes. Scale bar: 100 μm. |
The PVP/Pd nanocomposite fibers were also visualized by TEM (Fig. 8). The TEM bright field images reveal that the Pd nanoparticles are homogeneously distributed within the fibers and they are spherical in shape with average diameters of around 5 nm.24 Moreover, as seen in the HRTEM image (Fig. 8, right image) the Pd nanoparticles are nanocrystals disclosing the crystalline planes (111) and (200) with characteristic interplanar distances of 2.27 and 1.97 Å respectively.
The morphologies and orientations of the PVP and PVP/Pd electrospun fibers deposited at different sites (L, CL, C, CR, R) on the concave collector were investigated by SEM. Most importantly, the degree of fiber orientation was quantified using FTM. In the results, Fig. 9 and 10, the vectors on the SEM images show the direction and relative degree of orientation of the fibers. For isotropic fiber networks that have no preferred orientation the direction of the vectors is random and their length is equal. For anisotropic fiber networks the longer vector points towards the direction of the preferred alignment. The figures also present the analysis of the FTM that shows the distribution of the angles θ, as well as the components of the fiber orientation tensor Ω. As shown in the figures, when a flat collector was used the resulting electrospun fiber networks (both PVP and PVP/Pd) had a random orientation with the diagonal components of the orientation tensor to be almost equal to each other (Fig. 9). Upon replacing the flat collector with the concave one, the fibers deposited on the sides of the concave collector (positions L and R) still had a random orientation (Fig. 10b and d). However, at the centre of the collector the fibers exhibited a high degree of alignment of approximately 0.6 to 0.7 for both the PVP and PVP/Pd electrospun fibers (Fig. 10a and c).
Table 1 summarizes the degree of orientation of the fibers deposited on the concave and on the flat collector, as determined from the SEM images, by employing a Fourier transform method (FTM).
Concave collector | Flat collector | ||||
---|---|---|---|---|---|
Center (C) | Center-left (CL) | Center-right (CR) | Left/right (L, R) | Center area | |
PVP | 70% | 68% | 60% | Random orientation | Random orientation |
PVP/Pd | 68% | 70% | 70% | Random orientation | Random orientation |
Besides processing parameters including electrical potential, solution flow rate, needle diameter, and needle-to-collector distance, the electric field distribution may have an influence on the fiber diameters. As previously noted, the concave geometry induces an electric field inhomogeneity which is more pronounced at the collector's center area. This phenomenon not only induces fiber orientation but also results in the reduction of the fiber diameters in comparison to the ones deposited on the flat collector at the center area, as seen from the data provided in Table 2.
Concave collector | Flat collector | ||
---|---|---|---|
Center area | Left/right area | Center area | |
PVP | 0.58 ± 0.11 μm | 0.88 ± 0.18 μm | 0.85 ± 0.12 μm |
PVP/Pd | 0.74 ± 0.19 μm | 1.06 ± 0.15 μm | 1.02 ± 0.29 μm |
Basic statistical analysis of the experimental data (t-test) demonstrated that the differences observed in the diameters of the fibers when comparing PVP and PVP/Pd deposited on the flat and the concave collector (center area) are statistically significant (p < 0.05).
In line with the above, C. J. Angammana and S. H. Jayaram have studied the electric field profile in the multi-jet electrospinning process and its influence on the fiber morphology. Using COMSOL Multiphysics, they have calculated the electrostatic-field distribution between the needle and the collector, in order to obtain the results for single and multi-jet arrangements. From their studies they found that the diameter of the fibers decreased with the increase in the number of needles in the arrangement due to the increase of the electric field's distortion and the deterioration of the local electric field.27
Considering the electric field profile differences observed between the flat and the concave collector geometry, the above results experimentally verified that the electric field inhomogeneity which was more pronounced within the collector's central cavity, promoted fiber orientation to a relatively high extent for both systems whereas it also led to a decrease in the diameters of the fibers.
Based on previous literature reports, both the fiber diameter and the fiber orientation are influencing factors on the mechanical properties of the fibers.28–30 In order to verify this experimentally, we have studied the mechanical properties of both, the PVP and the PVP/Pd fibrous mats deposited in the center areas of the flat and concave collectors.
The experimentally obtained stress–strain curves are illustrated in Fig. 11a. The random PVP and PVP/Pd meshes generated using the flat target exhibited a similar mechanical response and both meshes became stiffer for the concave target, when they were stretched in the direction of the preferred alignment. Additional the meshes with the preferred alignment reached a yield point at 20–25% strain, while the stress developed in the random networks reached a plateau at these strain levels. The Young's moduli depicted in Fig. 11b, were calculated from the slope of the linear part of the stress–strain curves for low strains (<10%).
Footnote |
† Equal contribution. |
This journal is © The Royal Society of Chemistry 2015 |