polymers

Article Bubble Melt for Production of Polymer Microfibers

Ye-Ming Li 1,†, Xiao-Xiong Wang 1,†, Shu-Xin Yu 1, Ying-Tao Zhao 1, Xu Yan 2 , Jie Zheng 2, Miao Yu 1,3, Shi-Ying Yan 1 and Yun-Ze Long 1,*

1 Collaborative Innovation Center for Nanomaterials & Devices, College of Physics, Qingdao University, Qingdao 266071, China; [email protected] (Y.-M.L.); [email protected] (X.-X.W.); [email protected] (S.-X.Y.); [email protected] (Y.-T.Z.); [email protected] (M.Y.); [email protected] (S.-Y.Y.) 2 Industrial Research Institute of Nonwovens & Technical Textiles, College of Textiles & Clothing, Qingdao University, Qingdao 266071, China; [email protected] (X.Y.); [email protected] (J.Z.) 3 Department of Mechanical Engineering, Columbia University, New York, NY 10027, USA * Correspondence: [email protected] or [email protected]; Tel.: +86-139-5329-0681 † These authors contributed equally to this work.

 Received: 8 October 2018; Accepted: 7 November 2018; Published: 10 November 2018 

Abstract: In this paper, we report an interesting bubble melt electrospinning (e-spinning) to produce polymer microfibers. Usually, melt e-spinning for fabricating ultrafine fibers needs “Taylor cone”, which is formed on the tip of the spinneret. The spinneret is also the bottleneck for mass production in melt e-spinning. In this work, a metal needle-free method was tried in the melt e-spinning process. The “Taylor cone” was formed on the surface of the broken polymer melt bubble, which was produced by an airflow. With the applied voltage ranging from 18 to 25 kV, the heating temperature was about 210–250 ◦C, and (TPU) and (PLA) microfibers were successfully fabricated by this new melt e-spinning technique. During the melt e-spinning process, polymer melt jets ejected from the burst bubbles could be observed with a high-speed camera. Then, polymer microfibers could be obtained on the grounded collector. The fiber diameter ranged from 45 down to 5 µm. The results indicate that bubble melt e-spinning may be a promising method for needleless production in melt e-spinning.

Keywords: bubble melt electrospinning; polymer microfibers; polyurethane; polylactic acid

1. Introduction In the last 20 years, electrospinning (e-spinning) technology has become one of the main methods for producing ultrafine fibers [1,2], and this technology also has a rapid development both in fabricating functional fibers and potential applications. The electrospun (e-spun) fibers have been applied in various fields, such as biomedicine, tissue engineering, filtration, textiles, and nanodevices, because of their many excellent characteristics, such as large surface area to volume ratio, high porosity, and microscale to nanoscale diameter [3–6]. As is well known, the needle is easily blocked sometimes during solution e-spinning. Hence, many designs of needleless e-spinning have been proposed to solve the problem. For instance, a two-layer system, with a ferromagnetic suspension at the bottom and a polymer solution above it, can produce nanofibers [7]. A conical metal wire can be used for producing nanofibers, and cylinder and disk nozzles can also replace the needle [8]. He et al. reported a bubble solution e-spinning, where the output of fibers is higher than in traditional e-spinning. The diameter of fibers fabricated by this method is usually below 100 nm. He et al. also produced some different shapes, such as helix fibers and even nanoparticles [9–13]. Bubble solution e-spinning

Polymers 2018, 10, 1246; doi:10.3390/polym10111246 www.mdpi.com/journal/polymers Polymers 2018, 10, 1246 2 of 11 has its obvious advantages compared with traditional e-spinning, such as the spinning process with no blockage, the throughput is high and producing fiber membrane of high porosity [14,15]. When the e-spun fibers were used in biomedicine and tissue engineering, the shortcomings in traditional e-spinning, such as toxic solvent and low productivity, obviously restrict further applications in these fields [16–19]. Traditional solution e-spinning has its inherent defects which may hinder the way to solve these problems, particularly those of toxic solvent, solvent evaporation, and residual solvent [20–22]. Melt e-spinning, another branch of e-spinning, has attracted widespread attention in recent years. Many researchers have paid much attention to melt e-spinning, particularly the morphology adjustment and the potential application of microfibers. In 1981, Larrondo and Manley published the first paper on melt e-spinning. However, the second study on melt e-spinning was not published until 2001, by Reneker and Rangkupan [23,24]. Since the devices for melt e-spinning experiments are more complicated, and the fiber diameter is larger than the conventional solution e-spun fibers, the research work on melt e-spinning is relatively less than that on solution e-spinning, although melt e-spinning is a technique without toxic and solvent evaporation [25–27]. Melt e-spinning controls the fiber membrane more precisely than meltblown, but the needle is also frequently and easily blocked, which is the main shortcoming for high productivity [28–31]. In order to increase productivity, some progress has been made in the past several years, for example, Yang et al. used umbellate spinneret for mass production of ultrafine fibers [32]. Fang et al. produced (PP) fibers by a rotary metal disc spinneret without needle block [33]. However, no research about bubble melt electrospinning has been reported. Detailed research on this topic would provide a reference for mass production using melt electrospinning. Here, we propose a bubble melt e-spinning for fabricating polymer microfibers. Since the viscosity of polymer melt is much larger than polymer solution, this is a challenge for the bubble melt e-spinning. When compressed air was passed into polymer melt, many bubbles were formed on the surface of the polymer melt. When these bubbles were exposed to the electrostatic field between the polymer melt and the collector, the bubbles burst, and some jet flows were generated from the “Taylor cones” of the burst bubbles. Then, polymer microfibers were deposited on the collector. The effects of different parameters on the fiber morphologies were also investigated.

2. Experimental Section

2.1. Materials Polylactic acid (PLA) with a molecular weight of ~20,000 and polyurethane (TPU) with a molecular weight of ~100,000 were purchased from Dongguan Thriving Plastic Raw Materials Co., Ltd. (Dongguan, China). All the chemicals were directly used without further processing.

2.2. Melt E-Spinning Setup The bubble melt e-spinning device was assembled by ourselves, which consisted of a gas pump, a DC high voltage generator (DW-P4-3-1ACCC, Tianjin, China), a thermostatic heating platform (200 × 200 mm2, 0–400 ◦C, 800 W, Shenzhen, China), a metal container (150 × 150 mm2) and an aluminum foil as collector. A metal conduit connected the air pump at one end, and the other end was fixed at the bottom of the metal container, which was placed on the thermostatic heating platform. The aluminum foil collector was over the top of the container, and the distance between collector and container could be adjusted. The positive and negative electrodes connected with the collector and the metal container, respectively. Figure1 shows the schematic illustration of the bubble melt e-spinning setup. Polymers 2018, 10, 1246 3 of 11 Polymers 2018, 10, x FOR PEER REVIEW 3 of 11

Figure 1. Schematic drawing of the bubble melt e e-spinning-spinning setup.

2.3. E E-Spinning-Spinning P Processrocess The polymer (TPU or PLA) particles were put into the metal container which was placed on the thermostatic heating heating platform. platform. Then Then,, turning turning on on the the heating heating platform, platform, the theheating heating process process would would last forlast several for several minutes minutes until until the the polymer polymer particles particles became became a a melt. melt. The The air air pump pump was was turned turned on gradually to create bubbles from the surface of polymer melt. Onc Oncee the bubbles were generated on the surface of polymer melt, the DC high voltage generator was turned on to form an electrostatic fieldfield between betweenthe the collector collector and and the the metal metal container. container. Through Through the high-speed the high-speed camera, camera, we could we observe could observedirectly thatdirectly multiple that multiple jets were je ejectedts were from ejected the from burst the bubble burst towards bubble thetowards collector the collector during the during melt thee-spinning melt e-spinning process. process. 2.4. Characterization 2.4. Characterization The morphologies and structures of the melt e-spun polymer fibers were measured and The morphologies and structures of the melt e-spun polymer fibers were measured and characterized by a scanning electron microscope (SEM, Hitachi TM-1000, Tokyo, Japan) and a characterized by a scanning electron microscope (SEM, Hitachi TM-1000, Tokyo, Japan) and a Fourier transform infrared (FTIR) spectrometer (Thermo Scientific Nicolet iN10, Shanghai, China). Fourier transform infrared (FTIR) spectrometer (Thermo Scientific Nicolet iN10, Shanghai, China). These samples were coated with an evaporated Au thin film before SEM characterization. A rheometer These samples were coated with an evaporated Au thin film before SEM characterization. A (MCR301, Anton Paar, Shanghai, China) was used to test the rheological properties of the polymer melt rheometer (MCR301, Anton Paar, Shanghai, China) was used to test the rheological properties of the at different temperatures. The melt e-spinning process was recorded by a high-speed camera (Photron, polymer melt at different temperatures. The melt e-spinning process was recorded by a high-speed UX50, Tokyo, Japan). X-ray diffraction (XRD) was tested using a Rigaku SmartLab X-ray diffractometer camera (Photron, UX50, Tokyo, Japan). X-ray diffraction (XRD) was tested using a Rigaku SmartLab using Cu-Kα radiation (λ = 1.54178 Å) with an acceleration voltage of 40 kV. Thermogravimetric X-ray diffractometer using Cu-Kα radiation (λ = 1.54178 Å) with an acceleration voltage of 40 kV. analysis (TGA) of the fiber membranes was performed from on a TGA2 (Mettler Toledo, Zurich, Thermogravimetric analysis (TGA) of the fiber membranes was performed from on a TGA2 Switzerland), from 25 to 500 ◦C. (Mettler Toledo, Zurich, Switzerland), from 25 to 500 °C. 3. Results and Discussion 3. Results and Discussion When the bubble melt e-spinning started, several bubbles were generated on the polymer melt surfaceWhen (please the bubble see the melt video e- inspinning the Supplementary started, several Materials). bubbles Inwere order generated to see the on e-spinning the polymer process melt surfacemore clearly, (please Figure see the2 only video shows in the a Supplementary big bubble. In Materials fact, there). wereIn order many to see bubbles the e- generatedspinning process in the morespinning clearly, process. Figure The 2 only bubbles shows gradually a big bubble. became In a fact, cone there while were the appliedmany bubbles voltage generated was close in to the spinningthreshold process. value (Figure The bubbles2a). Then, gradually increasing became the gas a pressurecone while and the the applied voltage voltage to exceed was the close threshold to the value,threshold the valuebubbles (Figure became 2a). unstable Then, andincreasing then burst the with gas pressure multiple jetsand ejected the voltage toward tothe exceed collector. the threIn theshold process, value, several the bubbles bubbles became and conical unstable tips whichand then played burst the with role multiple of Taylor jets cone ejected in the toward traditional the collector.e-spinning In process the process, were several uplifted bubbles after the and bubble conical burst tips (Figurewhich played2b–d). Thethe role number of Taylor of bubbles cone in and the traditionaljets mainly edepends-spinning on process the viscosity were ofuplifted polymer after melts, the gas bub pressureble burst and (Fig theure air 2b flow–d). rate, The the number applied of voltage,bubbles and thejets distancemainly depends between on the the surface viscosity and collector.of polymer These melts, parameters gas pressure can be and adjusted the air in flow the rate,e-spinning the applied process voltage to produce, and the more distance bubbles between and jets. the surface and collector. These parameters can be adjusted in the e-spinning process to produce more bubbles and jets.

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FigFigureure 2.2. TheThe photos photos of of the the unstable unstable bubble bubble and and the burst the burst bubble bubble formed formed fibers: fibers: (a) the bubble(a) the formed; bubble formed(b) the bubble; (b) the burst; bubble (c )burst the conical; (c) the tip conical uplifted; tip uplifted and (d); the and fibers (d) the formed. fibers formed.

Figure3 a,b 3a,b shows shows some some typical typical SEM SEM images images of the of melt the e-spun melt e TPU-spun microfibers, TPU microfibers, taken at different taken at differentpositions positionsof the collector. of the As collector. shown in As the shown picture, in most the picture, of the fibers most are of disordered. the fibers Besidesare disordere straightd. Besidesfibers, some straight helical fibers, fibers, some thick helical fibers fibers, (~45 µm) thick and fibers thin fibers(~45 µm (~5)µ andm) also thin can fibers be observed. (~5 µm) also Figure can3c,d be observed.shows the Figure diameter 3c, distributiond shows the maps.diameter Most distribution of the fibers maps. have Most a diameter of the offibers about have 20 µam. diameter However, of weabout can 20 see µm from. However, the pictures we that can a see few from fibers the are pictures thinner that than a others. few fibers According are thinner to the than experiment, others. weAccording analyzed to that the it experiment, was simply we because analyzed the broken that it bubbleswas simply were because different the from broken each bubbles other both were in differentsize, viscosity, from each and distanceother both to in the size, collector. viscosity, Multiple and distance jets generated to the collector. from bubbles Multiple with jets a generated small size fromand thinner bubbles bubble with a wall small were size more and thinner likely to bubble formthinner wall were fibers. more In likely addition, to form the distancethinner fibers. between In addition,the bubble the tip distance and the between collector the was bubble also different tip and forthe bubbles collector with was different also different sizes. Hence,for bubbles it is stillwith a differentchallenge sizes. to produce Hence, stable it is still uniform a challenge fibers. to In produce this experiment, stable unifor the difficultiesm fibers. In are this obvious experiment, compared the difficultiesto the common are obvious solution compared e-spinning, to because the common the viscosity solution of e the-spinning, polymer because melt is the larger. viscosity At the of same the polymertime, the melt bottom-up is larger. way At for the e-spinning same time, needs the bottom to overcome-up way the for gravity. e-spinning As a needs result, to the overcome diameter the of gravity.the fabricated As a fibers result, by the this d methodiameter is of larger. the fabricated However, throughputfibers by this of the method fibers is produced larger. However,by bubble throughputmelt e-spinning of the is higher fibers than produced the traditional by bubble melt melt e-spinning e-spinning with is a higherneedle. than Then, the fibers traditional fabricated melt by ebubble-spinning are morewith a scattered needle. thanThen, the fibers traditional fabricated one; by at bubble the same are time, more the scattered porosity than is higher. the traditional one; atFigure the same4 shows time, the the infrared porosity spectra is higher. of the TPU fibers that were e-spun at different heating times duringFigure the experiment4 shows the (heatinginfrared temperaturespectra of the was TPU 240 fibers◦C, that heating were time e-spun was at 20, different 40, and heating 60 min times from duringthe start the of theexperiment experiment). (heating From temperature the infrared was test 240 results, °C, heating we could time also was see 20, that 40, the and TPU 60 wasmin stable from thein the sta meltrt of the e-spinning experiment). process. From The the peak infrared at 3330 test cm results,−1, which we could indicates also thesee N–Hthat the group TPU in was urethane stable in(–NHCOO–), the melt e-spinning and the peaks process. at 2958 The cm peak−1, at which 3330 belongs cm−1, which to the indicates asymmetric the andN–H symmetric group in urethane vibration ( of– NHCOOthe –CH2–group,), and the respectively, peaks at 2958 are thecm characteristic−1, which belongs peaks to ofthe TPU asymmetric [34,35]. However, and symmetric in our vibration experiment, of the color–CH of2 group, the TPU respectively, melt gradually are changed the characteristic into light yellow peaks with of increasing TPU [34,35 heating]. However, time, which in may our experiment,be the result the of partial color of thermal the TPU degradation melt gradually of the changed polymer into [36,37 light]. yellow with increasing heating time,Figure which5 may shows be the rheologicalresult of partial properties thermal of degradation the TPU at different of the polymer temperatures. [36,37]. The results also exhibitFigure the mechanism5 shows the ofrheological this melt properties e-spinning of experiment. the TPU at different At the same temperatures. temperature, The the results viscosity also exhibitcurve is the stable. mechanism The shear of this viscosity melt e- decreasesspinning experiment. obviously with At the the same temperature temperature, rising the at theviscosity same curveshear rate.is stable. Hence, The atshear the beginningviscosity decreases of the experiment, obviously wewith need the temperature to heat the materials rising at forthe asame long shear time. rate.Otherwise, Hence, the at viscosity the beginning is too high of the to experiment, prevent the formation we need to of heatthe bubbles. the materials Therefore, for a keeping long time. the Otherwise,heating temperature the viscosity at about is too 210 high◦C to is prevent necessary the for forma the fibers’tion of formation.the bubbles. When Therefore, the temperature keeping the is heating temperature at about 210 °C is necessary for the fibers’ formation. When the temperature is above 270 °C, the viscosity of the polymer melt is too low to obtain stable bubbles. We would see

Polymers 2018,, 10,, xx FORFOR PEERPEER REVIEWREVIEW 5 of 11 many jets directly formed from the surface of the polymer at the same time. However, these jets are not stable, and the resultant fibers are too thick. When these fibers are collected at the collector, it Polymersnot stable,2018 ,and10, 1246 the resultant fibers are too thick. When these fibers are collected at the collector,5 of 11it will prevent the normal process of the e-spinning. Keeping a proper temperature during the experiment is the key factor to the success of the experiment. ◦ aboveFigure 270 C,6 shows the viscosity the XRD of patterns the polymer of TPU melt fibers is toofabricated low to obtainby bubble stable melt bubbles. electrospinning. We would From see manythe picture, jets directly we can formed see that from the the TPU surface had of a theclear polymer crystalline at the peak same after time. electrospinning. However, these Ajets broad are notamorphous stable, and band the peaked resultant at fibersaround are 2θ too = thick.20°. This When indicates these fibersthat the are TPU collected remains at the stable collector, after itbeing will preventspun into the fibers.normal This process result of the also e-spinning. proves thatKeeping the a prepared proper temperature TPU fibrous during membrane the experiment has low is thecrystallinity. key factor to the success of the experiment.

FigureFigure 3.3.. ((a,,b)) SEM photograph and (c,,d) diameter diameter distribution distribution of the the melt melt e-spun e-spun polyurethanepolyurethane (TPU)(TPU) fibers.fibers.

Figure 4. Infrared spectra of TPU with different heating times during the experiment. Figure 4.. Infrared spectra of TPU with different heating times during the experiment.

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FigFigureure 5.5. Rheological properties of TPU at different temperatures.

Figure6 shows the XRD patterns of TPU fibers fabricated by bubble melt electrospinning. From the picture, we can see that the TPU had a clear crystalline peak after electrospinning. A broad amorphous band peaked at around 2θ = 20◦. This indicates that the TPU remains stable after being spun into fibers. This result also provesFig thature 5 the. Rheological prepared properties TPU fibrous of TPU membrane at different has temperatures. low crystallinity.

Figure 6. XRD Patterns of TPU fibers.

Figure 7 shows the TGA test of the TPU fibers produced by bubble melt electrospinning. we can see that the TPU fibers stay stable before temperature reaches 300 °C. Our experiment was usually

carried out between 200 and 300 °C, therefore, the properties of TPU are not changed. PLA microfibers were also fabricatedFigureFigure 6.6. XRDXRD by this PatternsPatterns method, ofof TPUTPU as fibers.fibers.shown in Figure 8a,b. The e-spun PLA microfibers are fragile, which can be clearly seen from the broken fibers in the picture [38]. Figure FigureFigure7 7 shows shows the the TGA TGA test test of of the the TPU TPU fibers fibers produced produced by by bubble bubble melt melt electrospinning. electrospinning. we we can can 8c,d shows the diameter distribution maps. The average fiber diameter◦ is about 30 µm. Compared to seesee thatthat thethe TPUTPU fibersfibers staystay stablestable beforebefore temperaturetemperature reaches 300300 °CC.. Our experimentexperiment waswas usuallyusually the TPU fibers e-spun by this method,◦ the average diameter of PLA fibers is larger. However, PLA carriedcarried outout betweenbetween 200200 andand 300300 °CC,, therefore,therefore, thethe propertiesproperties ofof TPUTPU areare notnot changed.changed. microfibers were also successfully produced by the bubble melt e-spinning, indicating that it is an PLAPLA microfibersmicrofibers were were also also fabricated fabricated by by this this method, method, as asshown shown in Figure in Figure 8a,8b.a,b. The The e-spun e-spun PLA effective method to fabricate various polymer microfibers. PLAmicrofibers microfibers are fragile, are fragile, which which can be can clearly be clearly seen from seen the from broken the broken fibers in fibers the picture in the picture[38]. Figure [38]. Figure 9 shows the XRD test of the PLA fibers produced by bubble melt electrospinning. The Figure8c,d shows8c,d showsthe diameter the diameter distribution distribution maps. The maps. average The fiber average diameter fiber is diameterabout 30 µm. is about Compared 30 µ m.to patterns show us that the electrospun PLA fibers had a clear crystalline peak. The broad amorphous Comparedthe TPU fibers to the e-spun TPU by fibers this e-spunmethod, by th thise average method, diameter the average of PLA diameter fibers is of larger. PLA fibersHowever, is larger. PLA band peaked at around 2θ = 17° and matches the common PLA XRD patterns. The result means that However,microfibers PLA were microfibers also successfully were also produced successfully by producedthe bubble by melt the e bubble-spinning, melt indicating e-spinning, that indicating it is an the properties of electrospun PLA fibers stay stable. The PLA did not change during the bubble melt thateffective it is anmethod effective to fabricate method tovarious fabricate polymer various microfibers. polymer microfibers. e-spinning process. Figure 9 shows the XRD test of the PLA fibers produced by bubble melt electrospinning. The Figure 10 shows the TGA test of the PLA fibers produced by bubble melt electrospinning. We patterns show us that the electrospun PLA fibers had a clear crystalline peak. The broad amorphous can see that the PLA fibers stay stable before the temperature reaches 280 °C. Our experiment was band peaked at around 2θ = 17° and matches the common PLA XRD patterns. The result means that the properties of electrospun PLA fibers stay stable. The PLA did not change during the bubble melt e-spinning process. Figure 10 shows the TGA test of the PLA fibers produced by bubble melt electrospinning. We can see that the PLA fibers stay stable before the temperature reaches 280 °C. Our experiment was

PolymersPolymers 2018 2018, 10, 10, x, FORx FOR PEER PEER REVIEW REVIEW 7 7of of 11 11

Polymersusuallyusually2018 carried carried, 10, 1246 out out between between 200 200 and and 250 250 °C °C, ,indicating indicating that that our our experiment experiment can can be be carried carried7 of out 11out successfully.successfully.

FigureFigFigureure 7.7 .7 TGA. TGA patterns patterns of of TPU TPU fibers.fibers. fibers.

Figure 8. (a,b) SEM photograph and (c,d) diameter distribution of the melt e-spun PLA fibers. FigFigureure 8 .8 (.a (,ab,)b SEM) SEM photograph photograph and and (c (,dc,)d diameter) diameter distribution distribution of of the the melt melt e -espun-spun PLA PLA fibers. fibers. Figure9 shows the XRD test of the PLA fibers produced by bubble melt electrospinning. The patterns show us that the electrospun PLA fibers had a clear crystalline peak. The broad amorphous band peaked at around 2θ = 17◦ and matches the common PLA XRD patterns. The result means that the properties of electrospun PLA fibers stay stable. The PLA did not change during the bubble melt e-spinning process.

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Figure 9.9. XRD Patterns of PLA fibers.fibers.

Figure 10 shows the TGA test of the PLA fibers produced by bubble melt electrospinning. We can see that the PLA fibers stay stable before the temperature reaches 280 ◦C. Our experiment was usually ◦ carried out between 200 and 250 C,Figure indicating 9. XRD that Patterns our experiment of PLA fibers. can be carried out successfully.

Figure 10. TGA patterns of PLA fibers.

Figure 11 shows the infrared spectra of the PLA fibers that were melt e-spun at different temperatures (230, 250, and 270 °C) with the heating time of about one hour. The peak at 1758 cm−1 indicates the exist of a carbonylFigure group,Figure 10. 10 theTGA. TGA peak patterns patterns at 1132 of of PLA PLA cm fibers.−1 fibers. is assigned to the C–O symmetric telescopic vibration, and the bending vibration peak of CH3 is at 1455 cm−1. Here, it is noted that FigureFigure 11 11 shows shows the the infrared infrared spectra spectra of of the the PLA PLA fibers fibers that that were were melt melt e-spun e-spun at at different different during our experiment, the color◦ of the PLA melt also gradually changed into light yellow over time,−1 temperaturestemperatures (230, (230, 250, 250, and and 270 270C) °C with) with the the heating heating time time of of about about one one hour. hour. The The peak peak at 1758at 1758 cm cm−1 just like the TPU melts. Hence, we want to know whether the− color1 change of PLA melts has some indicatesindicates the the exist exist of of a carbonyl a carbonyl group, group, the the peak peak at at 1132 1132 cm cm−1is is assigned assigned to to the the C–O C–O symmetric symmetric relationship with heating temperature. The results in Figure 11 clearly show−1 that the PLA melts telescopic vibration, and the bending vibration peak of CH is at 1455 cm .−1 Here, it is noted that remainedtelescopic stable vibration, at different and the temperatures bending vibration (230, 250,peak andof CH 3 2703 is°C at). 1455 Namely, cm . noHere, obvious it is noted therma thatl during our experiment, the color of the PLA melt also gradually changed into light yellow over degradationduring our ofexperiment, PLA melts the was color observed of the evenPLA atmelt 270 also °C forgradually a heating changed time of into one light hour. yellow over time, time,just justlike likethe theTPU TPU melts. melts. Hence, Hence, we want we want to know to know whether whether the color the color change change of PLA of PLA melts melts has hassome somerelationship relationship with with heating heating temperature. temperature. The resultsThe results in Figure in Figure 11 clearly 11 clearly show show that that the PLA the PLA melts ◦ meltsremained remained stable stable at different at different temperatures temperatures (230, (230, 250, 250, and and 270 270 °CC).). Namely, Namely, no no obvious obvious thermal thermal ◦ degradationdegradation of of PLA PLA melts melts was was observed observed even even at at 270 270C °C for for a heatinga heating time time of of one one hour. hour.

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FigFigureure 11 11.. InfraredInfrared spectra of melt e e-spun-spun PLA fibers fibers at different temperatures. 4. Conclusions 4. Conclusions A new method, bubble melt e-spinning, has been proposed in this paper, which is an effective A new method, bubble melt e-spinning, has been proposed in this paper, which is an effective way to fabricate fibers in melt e-spinning. It may be a promising needleless e-spinning method way to fabricate fibers in melt e-spinning. It may be a promising needleless e-spinning method for for mass production in melt e-spinning. The device of the experiment, as shown in our work, mass production in melt e-spinning. The device of the experiment, as shown in our work, was easy was easy to assemble and operate in certain conditions. Obviously, the bubble melt e-spinning to assemble and operate in certain conditions. Obviously, the bubble melt e-spinning method is an method is an environmentally friendly and efficient technique. It overcomes some defects, like needle environmentally friendly and efficient technique. It overcomes some defects, like needle congestion, congestion, low yield, and toxic and mutual interference in the common solution e-spinning process. low yield, and toxic and mutual interference in the common solution e-spinning process. This This e-spinning system may be applied for mass production in melt e-spinning. The quality of fibers e-spinning system may be applied for mass production in melt e-spinning. The quality of fibers and and the experimental device will be developed in further research. the experimental device will be developed in further research. Supplementary Materials: The following are available online at http://www.mdpi.com/2073-4360/10/11/1246/s1. Supplementary Materials: The following are available online at www.mdpi.com/xxx/s1. Author Contributions: Conceptualization, Y.-M.L. and X.-X.W.; Methodology, S.-Y.Y.; Validation, S.-X.Y., Y.-T.Z.; FormalAuthor Analysis,Contributions: X.Y.; Data Conceptualization, Curation, J.Z.; Writing—OriginalY.-M.L. and X.-X. DraftW.; Methodology, Preparation, Y.-M.L.;S.-Y.Y.; Writing—Review Validation, S.-X.Y &., Editing,Y.-T.Z.; X.-X.W.; Formal Supervision, Analysis, X M.Y.;.Y.; Project Data Administration, Curation, J.Z.; S.-Y.Y.; Writing Funding—Original Acquisition, Draft Y.-Z.L.Preparation, Y.-M.L.; Funding:Writing—ThisReview research & Editing, was funded X.-X.W by.; theSupervision, National Natural M.Y.; Project Science Administration, Foundation of China S.-Y. (51673103),Y.; Funding the Acquisition, Shandong ProvincialY.-Z.L. Natural Science Foundation, China (ZR2016EMB09), and the Postdoctoral Scientific Research Foundation of Qingdao (2016014). Funding: This research was funded by the National Natural Science Foundation of China (51673103), the ConflictsShandong of Provincial Interest: The Natural authors Science declare Foundation, no conflict of China interest. (ZR2016EMB09), and the Postdoctoral Scientific Research Foundation of Qingdao (2016014). References Conflicts of Interest: The authors declare no conflict of interest. 1. Reneker, D.H.; Chun, I. Nanometre diameter fibres of polymer, produced by electrospinning. Nanotechnology References1996, 7, 216. [CrossRef] 2. Norton, C.L. Method of and Apparatus for Producing Fibrous or Filamentary Material. U.S. Patent 2,048,651, 1. Reneker, D.H.; Chun, I. Nanometre diameter fibres of polymer, produced by electrospinning. 21 July 1936. Nanotechnology 1996, 7, 216. 3. Ramakrishna, S.; Fujihara, K.; Teo, W.E.; Yong, T.; Ma, Z.; Ramaseshan, R. Electrospun nanofibers: solving 2. Norton, C.L. Method of and Apparatus for Producing Fibrous or Filamentary Material. U.S. Patent global issues. Mater. Today 2006, 9, 40–50. [CrossRef] 2,048,651, 21 July 1936. 4. Ying, T.H.; Ishii, D.; Mahara, A.; Murakami, S.; Yamaoka, T.; Sudesh, K.; Samian, R.; Fujita, M.; Maeda, M.; 3. Ramakrishna, S.; Fujihara, K.; Teo, W.E.; Yong, T.; Ma, Z.; Ramaseshan, R. Electrospun nanofibers: solving Iwata, T. Scaffolds from electrospun polyhydroxyalkanoate copolymers: fabrication, characterization, global issues. Mater. Today 2006, 9, 40–50. bioabsorption and tissue response. Biomaterials 2008, 29, 1307–1317. [CrossRef][PubMed] 4. Ying, T.H.; Ishii, D.; Mahara, A.; Murakami, S.; Yamaoka, T.; Sudesh, K.; Samian, R.; Fujita, M.; Maeda, M.; 5. Munir, M.M.; Nuryantini, A.Y.; Mikrajuddin, A.; Iskandar, F.; Okuyama, K. Preparation of polyacrylonitrile Iwata,nanofibers T. Scaffolds with controlled from morphology electrospun usingpolyhydroxyalkanoate a constant-current electrospinning copolymers: fabrication, system for filter characterization, applications. bioabsorptionMater. Sci. Forum and2013 tissue, 737 response., 159–165. Biomaterials [CrossRef] 2008, 29, 1307–1317.

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