key: cord-0816361-dlf2sifh
authors: Kwak, Byeong Eun; Yoo, Hyo Jeong; Lee, Eungjun; Kim, Do Hyun
title: Large-Scale Centrifugal Multispinning Production of Polymer Micro- and Nanofibers for Mask Filter Application with a Potential of Cospinning Mixed Multicomponent Fibers
date: 2021-02-17
journal: ACS Macro Lett
DOI: 10.1021/acsmacrolett.0c00829
sha: d34676f0c7bb41f8df6ae2961d77b23ba1b32d77
doc_id: 816361
cord_uid: dlf2sifh

[Image: see text] Recently, the polymer nanofiber web is in high demand as a strong barrier against harmful particles due to its high capture efficiency and strong droplet-blocking ability. As an advanced spinning technique, the centrifugal multispinning system was designed by sectioning a rotating disk into triple subdisks, showing mass producibility of polymer nanofibers with cospinning ability. Using the system, gram-scale production of polystyrene (PS), poly(methyl methacrylate), and polyvinylpyrrolidone (PVP) was demonstrated, showing a possibility for versatile use of the system. Moreover, a high production rate of ∼25 g/h for PS nanofibers was achieved, which is ∼300× higher than that of the usual electrospinning process. Utilizing the cospinning ability, we controlled the contact angle and electrostatic charge of the multicomponent nanofiber web by adjusting the relative amounts of PS and PVP fibers, showing a potential for functional textile application. With the fabricated PS nanofiber-based filters, we achieved high capture efficiency up to ∼97% with outstanding droplet-blocking ability.

T he importance of a mask filter has been highlighted because of fine dust and coronavirus disease-2019 (COVID- 19) , which are urgent issues in human health. 1−3 A polymer nanofiber-based filter can act as an effective barrier against dust and virus-containing droplets since it can show high capture efficiency and excellent blocking ability against water droplets arising from their fine network and hydrophobicity. 4−6 Electrospinning is a routine way to prepare nanofiber webs, but in terms of mass production, it has several drawbacks, such as low throughput, the requirement of an electrically conductive target, a safety problem resulting from high voltage, and the difficulty of scale-up. 7−9 Centrifugal spinning is a strong candidate for the mass production of polymer micro-and nanofibers because of its advantages, including high production rate, cost effectiveness, and no need for high voltage and an electrically conductive target. 10, 11 With this useful feature, centrifugal spinning has the potential to be employed not only for mass production of polymer micro-and nanofibers, but also for versatile processes such as aligned fiber spinning, 12−18 but still with room for improvement. The recent trend in centrifugal spinning research is mainly focused on the integration between centrifugal spinning and electrospinning processes, using centrifugal force as the supporting force rather than the main driving force to prepare fibers. 12,19−21 This limited research trend restricts further enhancement of the productivity of centrifugal spinning since a high-voltage electrical system in this research limits the scalability of the system. The spinneret is a key component that determines the morphology and production rate of fibers in a centrifugal spinning system. The use of multiple spinnerets can directly lead to the scalability of the spinning system, but most research employed a single spinneret. 22−25 Employment of multiple spinnerets can also allow us to form multicomponent nanofiber webs with fine control of their physical and chemical properties for various applications such as drug delivery, 12 tissue engineering, 26 and functional textile applications. 27, 28 Inspired by those strengths, we designed a centrifugal multispinning system for the large-scale production of polymer micro-and nanofibers with a potential of cospinning mixed multicomponent fibers. As a strategy to multiplicate the spinneret, we sectioned a rotating disk into three subdisks that are vertically integrated together, and the subdisks were rotated simultaneously with a single axis.

With the novel spinneret, we set the spinning system as shown in Figure 1A . The inset displays the detailed structure of the multispinning disk consisting of three subdisks. Polymer solutions were ejected through the gaps at the side wall of each subdisk. Before the multispinning, which is more complicated than normal centrifugal spinning, we first optimized the polymer concentration using a single subdisk for simplicity. We prepared polystyrene (PS) polymer solutions by dissolving PS in chloroform with different concentrations of 3, 5, 7, 10, and 15 wt %, followed by centrifugal spinning. Scanning electron microscopy (SEM) images show that PS fibers were successfully prepared ( Figure 1B−F) . Since the fiber diameter is the most important factor for the mass production of nanofibers, we focused on it as a necessary criterion to optimize the concentration. Additionally, the production rate is another consideration, so the two values at each concentration are summarized in Figure 1G .

Average diameters of the PS fibers were measured as 263 ± 247, 558 ± 475, 888 ± 855, 3688 ± 3186, and 8372 ± 6134 nm at the concentrations of 3, 5, 7, 10, and 15 wt %, respectively. Fiber diameter distributions were shown in Figure  S1 . At concentrations higher than 5 wt %, the prepared fibers have extremely large diameters compared to other electrospinning processes (100−650 nm; Table S1), 29−50 while the average diameter at concentrations of 3 and 5 wt % is similar to that of electrospun fibers. The results clearly show that a higher concentration induced the formation of fibers with larger diameters. This increase in diameter along the concentration is due to the high viscosity and reduced contents of solvent, which are evaporated during the spinning, further limiting the elongation of the polymer jets. 51, 52 Moreover, merged fibers indicated as dashed circles in SEM images are more frequently observed in higher concentrations larger than 5 wt %, which leads to dramatic increase of average fiber diameter. Thus, a lower concentration is more appropriate for preparing nanofibers. Meanwhile, the production rate of the PS fibers was calculated as 3. 38, 7.368, 8.832, 9. 984, and 1.67 g/h at the concentrations of 3, 5, 7, 10, and 15 wt %, respectively, showing an increase along the concentration and a sudden decrease at the concentration of 15 wt %, which is a different trend with the fiber diameter. In higher concentrations, since the resultant polymer jets have a smaller surface area, the solvent evaporation was further retarded, forming concentrated droplets and imperfectly solidified fibers rather than a fibrous network. Those solvent-containing structures can destroy the preformed fibers at the collector by fusion, so more formation of those structures may limit the further enhancement of the production rate at higher concentrations. Considering both factors, we selected 5 wt % PS solution as a base solution since it can dominantly provide nanofibers rather than microfibers with a high production rate.

To demonstrate the applicability of our system for the fiber preparation with various polymers, poly(methyl methacrylate) (PMMA) and polyvinylpyrrolidone (PVP), which are frequently employed in various applications, 53−58 were also spun in the same way. PMMA and PVP were dissolved in chloroform and ethanol, respectively, with various concentrations. SEM images show that PMMA and PVP fibers were successfully prepared ( Figures S2 and S3 ). The prepared PMMA fibers showed a similar trend with PS, producing fibers with a larger diameter at a higher concentration, while the results of PVP spinning demonstrate a narrower concentration window. PMMA and PVP have the highest production rates of 6.516 and 1.77 g/h at 5 and 7 wt % concentrations, respectively, showing a potential of centrifugal spinning for gram-scale production of polymer nanofibers. These results also demonstrate that the centrifugal spinning system can produce various polymer micro-and nanofibers, regardless of their chemical and physical properties.

Before expansion from normal centrifugal spinning to centrifugal multispinning, the performance of each subdisk should be cross-checked in terms of average diameter and morphology of fibers since all the aforementioned results are obtained using the single subdisk. Therefore, we prepared PS nanofibers by each subdisk and examined whether all the subdisks can produce the same results or not ( Figure S4 ). Consequently, there is no morphology change of the resultant nanofibers, and only small deviation of average fiber diameter was observed for each subdisk, showing negligible influence of different injection locations. It may be due to that polymer solutions in subdisk are accelerated for extremely short time with negligible difference until ejection. Thus, we could expect the same performance of centrifugal multispinning with that of single spinning, just increasing the production rate of the nanofibers.

To assess the effectiveness of the centrifugal multispinning system for mass production of the nanofiber, we adopted an electrospinning process as a conventional spinning method and compared its production rate with that of our system. We used the PS solution with the same concentration range from 3 to 15 wt %, and we varied the feed flow rate from 0.5 to 8 mL/h ACS Macro Letters pubs.acs.org/macroletters Letter to find the maximum production rate at each concentration ( Figures S5−S9) . Based on the results, we selected the adequate condition for the production of electrospun fibers by removing cases that produce only beads without fibers or a molten area. The fiber diameter and the production rate for each case are summarized in Figure S10 . Since we selected the 5 wt % PS solution as a base solution for centrifugal spinning, we compared the nanofiber production rate between the two systems employing this concentration (Figure 2A ). At 5 wt % concentration, the maximum production rate was measured as 0.078 g/h for a single-syringe electrospinning system, while the centrifugal spinning system produced 7.864 g of nanofibers per hour. Moreover, with an increase in the number of subdisks, the production rate was further enhanced by 16.514 and 24.965 g/h for double-and triple-subdisk cases, respectively ( Figure 2B ). As a result, a centrifugal multispinning system showed an extremely enhanced production rate of over ∼300× compared to the electrospinning system with the same solution condition. Although the case of 7 wt % concentration with a feed flow rate of 5 mL/h recorded the maximum production rate of 0.606 g/h for the electrospinning system, the nanofiber production rate of the centrifugal multispinning system is still 41× higher than that value. We also compared the production rate with that of other multinozzle or needleless electrospinning processes (Table S1 ). While most mass-producible electrospinning systems showed low productivity of a few grams per hour, some processes showed a high productivity of ∼20−40 g/h, 36, 40 which is comparable to that of the centrifugal multispinning process. However, in the comparison of productivity per unit spinneret, our centrifugal multispinning system still has a relatively high productivity of ∼8.3 g/h·subdisk, compared to that of the processes (∼0.20−4.6 g/ ACS Macro Letters pubs.acs.org/macroletters Letter h·spinneret). This overwhelmingly higher value in the production rate than that of the electrospinning system clearly demonstrates a potential of our system for producing nanofibers. Moreover, analogous to multinozzle electrospinning system, the productivity of the centrifugal multispinning system can be further enhanced by an integration of more subdisks, showing easy scalability of our system. Besides mass production, we employed centrifugal multispinning for cospinning different polymer species to expand the application of polymer nanofibers. Figure 3A shows a schematic illustration for the preparation of a multicomponent nanofiber web. Three types of polymer solutions were injected to each subdisk, and each polymer solution can be ejected by centrifugal force. After elongation and solvent evaporation, polymer jets become nanofibers, and consequently, a multicomponent nanofiber web can be fabricated. Since the multicomponent fiber web consists of different types of polymers, its physical and chemical properties can be finely tuned by controlling the relative amounts of different polymer fibers. To demonstrate the practical preparation of multicomponent nanofiber webs, we selected two polymer solutions, 5 wt % PS in chloroform and 7 wt % PVP in ethanol, which are hydrophobic and hydrophilic, respectively. The 7 wt % concentration of PVP solution was chosen to prepare nanofibers having similar diameters with that of the PS nanofibers. For fine control of physical and chemical properties of bicomponent nanofiber web, relative amounts of two polymers should be controlled. To produce different amounts of nanofibers without changing fiber quality, we controlled the feed flow rate of PVP solution. Since the injected solution drops experienced the same centrifugal force, it does not affect the average diameter and morphology of the prepared nanofibers ( Figure S11 ). Thus, we varied the feed flow rate of the PVP solution from 0 to 5 mL/min to control the ratio of PS and PVP nanofibers within multicomponent fiber webs. To visualize each type of nanofiber, rhodamine B and blueemitting carbon dots were mixed with PS and PVP solutions, respectively, and each solution was injected to different subdisks.

Without an injection of the PVP solution, only PS fibers (red color) were spun as shown in Figure 3B . As the flow rate of the PVP solution increased, the amount of PVP fibers (blue color) increased ( Figure 3C,D) . The confocal images demonstrate that two polymer solutions were not mixed during the spinning process forming the bicomponent nanofiber webs. When two polymer solutions were injected into the same subdisk, most fibers showed a purple-color emission ( Figure S12) , implying that the mixing of two solutions occurred during the spinning. It shows that each chamber for the spinning solutions should be sectioned to prevent mixing of different solutions for the preparation of multicomponent fiber webs. Among many characteristics of the webs, the contact angle and electrostatic charge were measured (Figures 3E and S13) . Due to the hydrophilicity of the PVP nanofibers, the contact angle gradually decreased as the amount of PVP nanofibers increased within bicomponent fiber webs. Moreover, larger amounts of PVP nanofiber led to a lower electrostatic charge because of its hygroscopicity. 59, 60 These results show that the fine control of wettability and electrostatic charge, which are important factors for drug delivery, wound dressing, and functional textiles, can be achieved by introducing different polymer species within fiber webs. 12,26−28 Since this combinatorial synergy of various polymer species can be further tailored using centrifugal multispinning, our system would open the potential of the multicomponent fiber webs for those applications.

To demonstrate the practicability of our process, we fabricated mask filters using PS nanofibers from our process considering the high demand of mask filters in recent years. First, we fabricated PS nanofiber-based filters for capturing fine dust with different amounts of PS nanofibers per unit area. The fabricated filter was easily integrated in the mask, as shown in Figure 4A . Capture efficiency and resistance of the filters were analyzed using an automated filter tester. Also, the filter performance was compared with that of filters from the literature and commercial products that are surgical mask, filters with grades of KF80 and KF94, and a mask fabricated by electrospun nanofibers (Mask-ES). Capture efficiency of the fabricated filters gradually increased from 82.97% to 96.63% as the weight of nanofiber increased (Figure 4B) , showing a slightly higher performance than that of filters consisting of nanofibers with a similar average diameter (∼500 nm; Table  S2 ). 61−66 Moreover, the performance is almost equal to that of commercial filters with KF80 and KF94 grades, implying excellent performance of our filter made of fibers spun by a centrifugal multispinning system. For the particle removal, inertial impaction and interception effect may be dominant among the well-established mechanisms ( Figure 4C ) considering that the average aerosol size corresponds to submicroscale. However, we cannot completely exclude the contribution of other filtration mechanisms that account for the removal of particulate matters of O(100) nm to O(10) μm because the test aerosol is not perfectly monodisperse. Meanwhile, resistance of the filters was relatively high compared to that of most filters in the literature, which may be due to the high flow velocity of the test condition. Actually, at the same face velocity, our filters demonstrated similar pressure drops with those of commercially available products, which shows a potential for the commercial use of PS nanofiber-based filters. Compared to our filters and other products, Mask-ES showed extremely high resistance, and it may be due to the highly compact nanofiber network. Since the PS nanofiber-based filters have a small fraction of merged fibers with microscale, those large structures may lead to reduced resistance by introducing the cavity within the filter media. 67, 68 We also examined the water droplet-blocking ability of the fabricated filters together with commercial products to demonstrate a potential of nanofiber-based filter to block viruses which spread by droplets (Figures 4D and S14 ). The test was performed under KF-antidroplet test conditions. As a result, no water droplet reached the bottom after 30 min, showing the outstanding droplet-blocking ability of our filters, like most of the other commercial products, except a surgical mask. It is due to the fact that the PS nanofiber web has a fine network and a nearly superhydrophobic nature, displaying a large contact angle of 145°. The results clearly demonstrate that the PS nanofiber-based filter can be employed for a mask filter to capture fine dust and infectious viruses, which spread by droplets, such as coronavirus.

For the mass production of a polymer nanofiber and an extension of nanofiber applications, a centrifugal multispinning system has been developed for the first time. Using a single subdisk of the novel spinneret, various polymer species, including PS, PMMA, and PVP, were successfully spun to micro-and nanofibers with a gram-scale production. Mass producibility and scalability of the system for the preparation of nanofibers were demonstrated using multiple subdisks by comparing the electrospinning process. Multicomponent fiber webs using PS and PVP solutions were also prepared, and the contact angle and electrostatic charge of the webs were finely controlled by varying the relative amount of PS to PVP. To demonstrate the applicability of produced fibers, the PS nanofibers were employed for mask filter application, and the fabricated filters showed high capture efficiency and outstanding blocking ability against water droplets, which show the potential of a nanofiber-based mask filter to capture fine dust and infectious viruses.

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsmacrolett.0c00829.

Experimental section: materials, centrifugal spinning of polymer nanofiber, electrospinning of polymer nanofiber, fabrication of mask filter and water blocking test, and characterization. 

This paper is based on research that has been conducted as part of the KAIST-funded Global Singularity Research Program for 2020.

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