New Insight into Gap Electrospinning: Toward Meter-long Aligned

Gap electrospinning is a facile technique to produce aligned nanofibers useful for many applications, but its potential has not yet been fully exploit...
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Article Cite This: Langmuir 2018, 34, 13788−13793

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New Insight into Gap Electrospinning: Toward Meter-long Aligned Nanofibers Tingping Lei,*,†,⊥ Zhenjin Xu,†,⊥ Xiaomei Cai,*,‡ Lei Xu,§ and Daoheng Sun∥ †

College of Mechanical Engineering and Automation, Huaqiao University, Xiamen 361021, China School of Science, Jimei University, Xiamen 361021, China § School of Mechanical and Electric Engineering, Jingdezhen Ceramic Institute, Jingdezhen 333403, China ∥ Department of Mechanical and Electrical Engineering, Xiamen University, Xiamen 361005, China ‡

Langmuir 2018.34:13788-13793. Downloaded from pubs.acs.org by UNIV OF WINNIPEG on 01/20/19. For personal use only.

S Supporting Information *

ABSTRACT: Gap electrospinning is a facile technique to produce aligned nanofibers useful for many applications, but its potential has not yet been fully exploited in nature, leading to the fiber length still limited to several tens of centimeters at present. In this work, we report a breakthrough in the production of well-aligned nanofibers with record length and efficiency. Using a suitable poly(vinylidene fluoride) solution and a pair of parallel plates that are substrate-free and negatively connected, we demonstrate the ease of this technique to prepare length-controllable aligned fibers in a wide range (≤125 cm). Because of the crucial roles of both the jet whipping instability that continuously drives the jet to span across the static plates and the negative voltage on the plates that effectively attracts the positively charged jet, the jet can be made to move back and forth over the superlarge gap to form ultralong aligned nanofibers. By introducing a projection method, we also redefine fiber alignment in a broader sense. This work is believed to provide a new insight into the nature of gap electrospinning, which will greatly expand the versatility of this technique to create devices for a myriad of applications.

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authors have collectors negatively connected,18,22,23,28 too little or no attention has ever been given to the influence of collector configuration. Very recently, we have experimentally demonstrated very long (over 50 cm) and highly aligned electrospun fibers using U-shape collectors that are negatively connected; however, fibers exceeding 60 cm are found to become progressively more problematic with the length.27 This pushes us to contemplate the hidden rules in more concrete terms and wonder whether the terrific challenge in the preparation of ultralong (>100 cm) aligned electrospun fibers could be completely overcome if an in-depth investigation of gap electrospinning is conducted. In the study reported herein, we provide a new insight into aligned fibers produced from gap electrospinning. For this purpose, we systematically investigate the negatively connected two parallel plate collecting system and some key electrospinning parameters. We show that when the collecting system is with a proper configuration and a suitable negative potential, well-aligned poly(vinylidene fluoride) (PVDF) fibers over a meter long can be produced with no difficulty. The calculated

INTRODUCTION Electrospinning is a powerful and straight-forward technique to fabricate aligned micro/nanostructures in polymer nanofibers, with all polymer chains oriented along the fiber axis1 and molecular dipoles preferably oriented along the electric field.2 Through control of the electric field (between the spinneret and the collector) that is usually by employing a special fiber collecting system, such as a pair of parallel conducting electrodes3−5 or a rotating mandrel,6,7 nanofibers can be made macroscopically aligned with more extensive use. Researchers have used them to fabricate high-performance devices,7,8 to enhance charge transport,9,10 to aid regeneration of highly organized structures (tendons, nerve cells, and ligaments),11 and many others.12−17 In terms of “gap electrospinning” (electrospinning using two parallel conducting electrodes) first reported by Li et al.,3 although much effort has been devoted to increasing the length of aligned fibers, the length at present is still limited to several tens of centimeters (normally 100 cm), the whipping and electrostatic forces (F1 & F2) will cause the jet to hit the inside wall of the plates and move back and forth between the plates if the terminal amplitude of the jet is greater than or equal to (or sometimes even a little smaller than) the gap width. The suspended fibers would be located in position ②−⑤ (Figure 4A,B) depending on the whipping and electrostatic forces (F1 & F2). In general, with the duration of electrospinning increased, the upcoming jet tends to move forward (towards the top, e.g., ⑤ → ④ or even to ③ or ②) to form multilayer aligned fibers because of repulsion of the already deposited fibers. As mentioned above, the degree of fiber alignment could be further promoted because the two ends of the highly charged suspended fibers are likely to be pulled to the negative plates to straighten the suspended fibers (Figure 4B), as evidenced by the arrows marked in Figure 4D. At the same time, the repulsive forces of fibers (F3, Figure 4C) and possible collisions with upcoming fibers may also cause fibers to become crooked (Figure 4B) as the fibers also retain the solvent(s) more or less.33

process. The corresponding SEM image of the fibers is shown in Figure 3B, revealing good fiber alignment for the above sample. By substituting the plates with a large thickness (8 cm), aligned fibers over 1 m long can be repeatedly prepared with ease: a 125 cm-long fiber sample prepared within 20 min (including the adjustment of spinning) is shown in Figure 3D. However, for a fiber length less than about 10 cm, the bigwidth plates are not as efficient as the small-width ones, and sometimes it is very difficult to produce aligned fibers of some tens of centimeters in length (Figure S2 in the Supporting Information). The above data show unambiguously that we are now able to prepare ultralong and length-controllable aligned nanofibers via gap electrospinning. Because of the fact that (1) the jet whipping instability was always observed during all our experiments and (2) elimination of the whipping motion makes it impossible for the jet to continuously span across static plates to form aligned fibers, a new understanding of fiber alignment is proposed from the viewpoint of jet whipping, which can be broadly divided into two categories based on the following definition. Here, the whipping amplitude for the jet just before entering the space in between the parallel plates is defined as “pre-entering amplitude.” Category I (pre-entering amplitude ≥ gap width): When the pre-entering amplitude ≥ the gap width (typically gap width ≤10 cm), there is no difficulty to make the whipping jet hit on the top of plates in theory (provided the plates’ dimension is large enough). Under such a condition, by applying a negative voltage on the plates, the positively charged jet will be effectively attracted and move back and forth over the two plates to form aligned fibers (e.g., position ①, Figure 4A,B) because of the whipping and electrostatic forces. During this process, there are many forces acting on the fibers, including the most important F1−F3 (Figure 4C) that affect the fiber alignment much, adhesion to the plates, their own weight, collisions with other fibers, and electrostatic attraction to the substrate (if the plates have a substrate). As the suspended fibers are still highly charged and retain the solvent(s) after deposition,33 the degree of fiber alignment would be further promoted because the electrostatic attraction force (F1) resulting from positively charged fibers and negative plates

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CONCLUSIONS In summary, we have conducted an in-depth investigation of gap electrospinning and made a breakthrough in the production of well-aligned nanofibers with record length and efficiency. Using a suitable PVDF solution and a pair of parallel plates that are substrate-free and negatively connected, we have demonstrated the ease of this technique to prepare lengthcontrollable aligned fibers with a wide range of workable WD and applied voltages (V+, V−). On the basis of our experimental results and the published research reports in the literature, we propose a new understanding of fiber alignment from the viewpoint of jet whipping (the inherent 13791

DOI: 10.1021/acs.langmuir.8b03114 Langmuir 2018, 34, 13788−13793

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Langmuir nature of electrospinning) and redefine fiber alignment in a broader sense using the projection method.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.langmuir.8b03114. Additional photos and tables for experimental conditions of electrospun fibers (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (T.L). *E-mail: [email protected] (X.C.). ORCID

Zhenjin Xu: 0000-0002-4567-3079 Author Contributions ⊥

T.L. and Z.X. contributed equally.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (nos. 61404059, 11762007, and U1505243).

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DOI: 10.1021/acs.langmuir.8b03114 Langmuir 2018, 34, 13788−13793

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DOI: 10.1021/acs.langmuir.8b03114 Langmuir 2018, 34, 13788−13793